23 Dec 2014: The PLOS ONE Staff (2014) Correction: Nine Years of Irrigation Cause Vegetation and Fine Root Shifts in a Water-Limited Pine Forest. PLOS ONE 9(12): e116642. https://doi.org/10.1371/journal.pone.0116642 View correction
Scots pines (Pinus sylvestris L.) in the inner-Alpine dry valleys of Switzerland have suffered from increased mortality during the past decades, which has been caused by longer and more frequent dry periods. In addition, a proceeding replacement of Scots pines by pubescent oaks (Quercus pubescens Willd.) has been observed. In 2003, an irrigation experiment was performed to track changes by reducing drought pressure on the natural pine forest. After nine years of irrigation, we observed major adaptations in the vegetation and shifts in Scots pine fine root abundance and structure. Irrigation permitted new plant species to assemble and promote canopy closure with a subsequent loss of herb and moss coverage. Fine root dry weight increased under irrigation and fine roots had a tendency to elongate. Structural composition of fine roots remained unaffected by irrigation, expressing preserved proportions of cellulose, lignin and phenolic substances. A shift to a more negative δ13C signal in the fine root C indicates an increased photosynthetic activity in irrigated pine trees. Using radiocarbon (14C) measurement, a reduced mean age of the fine roots in irrigated plots was revealed. The reason for this is either an increase in newly produced fine roots, supported by the increase in fine root biomass, or a reduced lifespan of fine roots which corresponds to an enhanced turnover rate. Overall, the responses belowground to irrigation are less conspicuous than the more rapid adaptations aboveground. Lagged and conservative adaptations of tree roots with decadal lifespans are challenging to detect, hence demanding for long-term surveys. Investigations concerning fine root turnover rate and degradation processes under a changing climate are crucial for a complete understanding of C cycling.
Citation: Herzog C, Steffen J, Graf Pannatier E, Hajdas I, Brunner I (2014) Nine Years of Irrigation Cause Vegetation and Fine Root Shifts in a Water-Limited Pine Forest. PLoS ONE 9(5): e96321. https://doi.org/10.1371/journal.pone.0096321
Editor: Gil Bohrer, The Ohio State University, United States of America
Received: January 28, 2014; Accepted: April 5, 2014; Published: May 6, 2014
Copyright: © 2014 Herzog et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This study was funded by the Swiss National Science Foundation (SNF), Grant number: SNF 31003A_149507, additional funding was received from COST Action FP0803 'Belowground C turnover in European Forests'. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing interests: The authors have declared that no competing interests exist.
In the inner-Alpine valleys of Switzerland, increased temperatures and drought periods have become more frequent and have reached a greater extent during the last few decades , . The most recent predictions reveal no improvement of the situation, contrariwise an increase of severe drought events is expected . The most abundant tree species, Scots pine (P. sylvestis L.), has suffered from limited water and from increasing competition from pubescent oak (Q. pubescens Willd.), leading to a drastic increase in tree mortality , . Recent publications concerned with drought induced tree mortality, explaining the interrelation of loss of hydraulic function and carbohydrate depletion, and concluding that the direct lack of water exceeds the carbon failing , . Tree defoliation and dieback is not restricted to the Swiss alpine region. In all southern Europe forests, degeneration due to severe droughts has been recorded . In a review, Anderegg et al.  depicted the global significance of forest dieback as a result of drought and temperature stress. Assessment of drought events are still insufficient and demand for adequate study length . In addition definition of severe drought events and standardization of climate extreme studies are needed .
In order to reduce tree mortality, an experiment was started in 2003 with the installation of an irrigation set-up in a mature Scots pine stand in Pfynwald (Valais). After a few years of irrigation treatment, therefore reducing drought events, the forest showed explicit changes in Scots pine growth patterns. Adaptations in needle morphology, and shoot and stem growth were detected after four years of irrigation , as well as in the δ13C signal in tree rings .
Water uptake in trees is regulated by their roots (e.g. ). However, only a limited number of studies deal with fine root adaptations in forests –. A few studies have focused on root adaptation after irrigation combined with a fertilization treatment (e.g. –). In the irrigation study of the forest Pfynwald, Scots pine fine root morphology changed after four years of irrigation only slightly , more precisely, an increase in specific root length (SRL)  and a significant decrease in root tissue density (RTD)  were noted. In 2007, Ostonen et al.  reviewed the importance of SRL as a morphological parameter in fine roots to address transformations induced by changing environmental factors. Root Area Index (fine root area relative to soil area, RAI) was promoted as a useful morphological parameter to detect changes between soil types  or approach the effect of irrigation and fertilization on root morphology . In order to detect structural changes, biochemical approaches are feasible. Several fine root compounds are known to increase with parasitic pressure , .
After death, fine roots become fine root litter. Nowadays, with global warming plant litter plays a crucial role in the carbon (C) cycle. The turnover of C in soils highly depends on the residence time of the litter. The underlying factors for litter degradation are biotic (decomposer community) and abiotic (e.g. litter quality, lignin contents, soil temperature, soil water content, pH) . Litter is mostly considered to be foliar litter input on the forest floor. The fine roots of trees are an often overlooked part of litter, even though they account for 13% of the net primary production (NPP) but only account for 3% of trees biomass . Fine root turnover tends to be slower than pine needle litter . Importantly, it should be considered that in contrast to foliar litter, fine root litter is less exposed to weathering (e.g. water, temperature). In any case, Fujii & Takeda  showed that the position of the litter above or below the organic layer is of minor importance. The major factors of slow degradability of fine root litter are low water content  and higher lignin content , ; . Rasse et al.  stated a faster degradation of fine root litter than shoot-derived litter in an isotope incubation experiment. The lower degradation rate is biased in nature by the physico-chemical protective properties of the soil and protection by mycorrhiza and root-hair activity. These protective mechanisms highly depend on environmental conditions such as moisture. Nevertheless, the decomposition rate of plant litter material correlates positively with nutrients but negatively with CN ratio and lignin content . Hence, the question remains: can a changing environment (e.g. increased rainfall) affect the chemical structure of roots directly, or does the moisture only act as a degradation stimulant without modifying root chemical composition beforehand? The C storage potential of temperate forests therefore depends on degradability and mean lifespan of fine roots.
Natural radioactive isotope of C is a cosmogenic isotope produced in the atmosphere in reaction of thermal neutrons and 14N. A steady-state condition of atmospheric concentrations is maintained between production and decay. However, due to the above ground nuclear test in 1950s superficial 14C was produce and its concentration doubled. Presence of this so called ‘bomb peak’ global 14C tracer in the atmosphere has been monitored and applied during the last 50 years (for review see ). Broecker et al.  were pioneers in using this tracer to analyse C cycling in aquatic environments. Later, Trumbore  adapted the 14C approach used in the terrestrial ecosystem to analyse soil organic matter (SOM) dynamics. Gaudinski et al.  introduced a new application possibility for 14C isotope measurement to investigate mean fine root age. Further investigation and testing showed that not only new assimilated C is used for root growth but also stored C which can be older. This can lead to a mean age of C of 0.4 yr at the point of integration . Several recent studies detected discrepancies in mean fine root age and postulated two pools of fine roots: one, a fast but smaller turnover pool with a mean turnover time of <1 yr, the other, larger, with a decadal turnover time , . In many studies, fine root thickness is or was arbitrary . In root age estimation and turnover studies this is not the case and is suggested for partitioning . Recently, Sah and co-workers  tested the reliability of the radiocarbon method for determining root age whereby fine roots from ingrowth cores with a known maximum age were analyzed. Only for fine roots (<0.5 mm), the measured 14C age was in agreement with the ingrowth core age, thicker roots tended to be older with 14C measurement. Fine root age can vary largely among stands and tree species and there is a tendency for older fine root ages to be in less fertile soil . Alongside increasing fine root diameters, increasing soil depth as well reveals a positive correlation with fine root age .
Benefiting from our nine-year irrigation study site, we attempt to fill the research gap regarding long-term adaptations of Scots pine fine root structure and composition. Is the associated vegetation of the mature pine forest affected by irrigation? Furthermore, our results will contribute to the ongoing discussion on fine root biomass increase or decrease in the topsoil after excessive water addition. Finally, is the longevity of fine roots, measured by radiocarbon dating, influenced by the water availability?
Materials and Methods
The irrigation experiment was situated in the Rhone Valley near Leuk (Valais, Switzerland, 46°18′ N, 7°37′ E, 615 m a.s.l.) in a Scots pine (P. sylvestris) forest with occasional interspersed pubescent oak (Q. pubescens). Permission for the field experiment was issued by the forest service of the canton Wallis (CH) (Kantonaler Forstdienst, Kreis Oberwallis, Kantonsstrasse 275, 3902 Brig-Glis). Additionally, the permission for use of the forest for research purpose was approved by the owner of the forest, the Burgerschaft Leuk (http://www.burgerschaft-leuk.ch). The geological properties are dominated by gravel input from the Rhone river and from the Illgraben alluvial cone. A more pristine pedogenic event was the landslide from Siders. The mean annual precipitation measured in Sion (20 km) was 518 mm and the mean annual temperature 10.7°C from 2003 to 2012 . The irrigation experiment had 8 plots (25 × 40 m) of which four were randomly chosen for irrigation, whereas the remaining four served as control. The plots were separated by a buffer zone of 5 m (Fig. 1). From 2003 to 2012, the irrigation system was activated in rainless nights during the vegetation period (May-October), doubling the annual rainfall amount. Water from the Rhone-channel situated along the experiment site (Fig. 1) was used for irrigation. Nutrient input through irrigation was minor: phosphate was below the detection threshold (PO4 <0.15 kg ha−1 yr−1) and the input of nitrogen (2.4–3.3 kg ha−1 yr−1) was less than the amount that could be expected to be deposited by a doubling of rainfall (N ≤ 3.5 kg ha−1 yr−1) , . Three identical trees per plot with the lowest crown transparency value, which refers to trees with the highest foliation, were chosen for our study . In the first two plots, the volumetric soil water content was monitored hourly at a soil depth of 10 cm at four different locations using time domain reflectometry (Tektronix 1502B cable tester, Beaverton, OR).
Irrigated plots (grey) and control plots (white) with the nearby water channel which was used for irrigation.
Vegetation assessment was executed to identify the vegetation shift after the nine years of irrigation. Sampling was done from 23th/24th May 2012. The coverage of vascular plant species of forbs, of shrubs and of the tree layer, were estimated in each plot using the Londo scale . The nomenclature used to describe the vegetation is based on Aeschimann et al. . The Landolt ecological indicator values for Swiss flora were calculated . The indicator values ranged from 1 (low) to 5 (high). The nine indicators are D (aeration value), F (moisture value), H (humus value), K (continentality value), L (light value), N (nutrient value), R (reaction value (pH)), T (temperature value), W (moisture variability value). More detailed information about the indicator values is listed in Table S1.
Fine root sampling
Fine root sampling was executed before the irrigation experiment started, on the 24th April 2003  and a second time after 9 years of irrigation on the 10th May 2012. For the 10th May 2012 sampling four soil cores at a distance of 1 m from each of the three trees per plot, were taken with an incremental borer (Ø 45 mm). The soil cores were kept refrigerated until further processing. Soil cores were washed in a sieve (mesh size 0.5 mm), and the roots were collected and separated into pine-roots, oak-roots and remaining root types. The separation of the fine roots (Ø ≤ 2 mm) was executed after following criteria: lignification (shrubs and tree roots), dichotomic branching of root tips (oak and pine only), mycorrhizal root tip size (oak < 1 mm, pine > 1 mm). This separation is merely valid for the forest site at Pfynwald. Fine roots were used for all examinations, exclusively.
Fine root morphological and chemical analyses
The fine roots of P. sylvestris and Q. pubescent were scanned using the WinRhizo version 4.1b software package (Régent Instruments, Inc., Quebec, Canada) to obtain data regarding fine root morphology. All collected fine roots were dried (72 h, 60°C), weighted and grinded for 2 min at 80% intensity using a Retsch Mixer Mill (MM 2000, Haan, Germany).
Klason lignin of Scots pine fine roots was extracted using an adaptation of a Hiltbrunner et al.  protocol. Briefly described, our procedure was as follows: 200 mg of grinded fine root was weighed in falcon tubes (50 ml). Water extraction was run three times with 80°C water and 15 min incubation time per run. Centrifugation (5000 rpm) was done for 10 min. A fourth water washing step was done with ambient MilliQ water. The supernatant of the four water washing steps was then collected in order to determine the presence of phenolic substances. The washing steps were the same for the ethanol extraction. Ethanol (96%) was used at room temperature for all three rounds. Pellets were resolved in ethanol and filtered (paper filter Ø 70 mm, Schleicher Schuell, Blauband 5893) and dried overnight (105°C). Acid soluble lignins were extracted using H2SO4 (72%) for 1 h in a shacking bath (30°C). After having added 16.8 ml MilliQ water, the samples were autoclaved at 120°C for 1 h. Samples were filtered through porcelain filter caps (40 mL, Ø 40 mm). After weighing, the retained solid phase was burned in a muffle kiln (4 h, 550°C). The non-acid-soluble lignins were equal to the difference between the retained sample and the ash content. The acid-soluble lignins were measured with a photometric approach . Therefore, the filtrate was measured at a wavelength of 205 nm in a Varian Cary 50 UV-visible spectrophotometer (Varian, USA). Overall lignin content was equal to the sum of the percentages of the dry mass acid-soluble and the non-acid-soluble lignins.
Cellulose was extracted using an adaptation of a protocol implemented by Endrulat et al. . Our procedure is described here briefly: 50 mg of dried and powdered Scots pine fine roots were sealed into Teflon filter bags (F57; ANKOM Technology, Macedon, NY, USA), followed by an incubation for 2 h in 5% NaOH at 60°C to extract fats, oils, tannins and hemicelluloses, and three washing steps with deionised boiling water. To remove the lignin, a washing step at 60°C with a 7% NaClO2 solution for 30 h was conducted. Another three washing steps with deionised boiling water were executed before drying over night at 50°C. Using the water solubles of the lignin extraction, the phenolic substances could be quantified , . In a 10 ml test tube, a 900-µl water extract or calibration solution was mixed with 300 µl Folin-Denis reagent (purum, Fluka, 47742) and incubated at room temperature for 3 min. 600 µl of sodium carbonate (waterfree, puriss p.a. Fluka, 71350) was added and set to rest for 3 h. With a 2 ml syringe, the 1 ml solution was filtered with a 0.2 µm syringe filter directly in a cuvette. The absorption was measured at 725 nm with a Varian Cary 50 UV spectrophotometer (Varian Com. US). δ13C of the extracted P. sylvestris fine root cellulose and bulk material was analyzed with an elemental analyser–continuous flow isotope ratio mass spectrometer (Euro-EA, Hekatech GmbH, Germany, interfaced with a Delta-V Advanced IRMS, Thermo GmbH, Germany) . The 13C/12C sample values were divided by the Vienna Pee Dee Belemnite international standard (VPDB), resulting in the ratio of the 13C/12C ratio of the sample relative to the preindustrial standard VPDB (δ13C). C and nitrogen (N) content and CN ratios were analyzed with gas chromatography (NC-2500, Carlo Erba Instruments, Wigan, UK). Similar to the procedure used by Richter et al.  the extracted P. sylvestris fine root cellulose was combusted and graphitized  The 14C/12C ratio was measured using the dedicated acceleretor mass spectrometry AMS system of MICADAS at the ETH facility .
All statistical analyses were executed with the open source tool R (R Development Core Team, 2011). For multivariate analyses, the add-in package VEGAN  and BiodiversityR  were required. A non-metric multidimensional scaling (NMDS, NMSrandom function) ordination technique was used for visualization of the similarity of equally treated plots. The indicator values were chosen as environmental variables to the NMDS. Their explanatory significance was tested with a permutation test comparing variables separately. A PERMANOVA (adonis function) was used for testing the variation of the vegetation in differently treated plots. To test for treatment effects, a mixed model was computed with n = 12 and plot as random effect by using the add-in package lme4 . A likelihood ratio test was performed using the ANOVA function. Residuals were checked for normal distribution with the Kolmogorov-Smirnov test . A homoscedasticity test based on Levene (1960)  was computed. For the statistical tests the difference between 2012 and 2003 was used (T0 reduction), thus reducing individual tree disparity. In case of vegetation assessment, one-way ANOVA variance analysis was executed to detect treatment-induced shifts. To estimate the age of roots based on 14C measurements, the data was compared to the 14C data measured in the atmosphere by Levin and Kromer  which were fitted using a polynomial regression (y = −476.5×2+1'222.5x−754.4, R2 = 0.977) (Table 1).
Volumetric water content in soil
The amount of water added for each irrigation period was on average 587 mm, which corresponds to a doubling of the annual precipitation amount. This resulted in a significant effect of irrigation on the volumetric water content (VWC) of the soil at 10 cm depth (p = 0.022). Figure 2 illustrates the monthly mean (VWC) during the nine years of irrigation. The mean value of VWC over the nine-year period was 27.8% in the control plot and 34.3% in the irrigated plot. In some cases, the VWC were similar in both plots during failures of the irrigation system. In the wintertime and before irrigation starts in the spring, the VWC was similar in both control and irrigated plots.
The annual precipitation (light blue bars) and the applied annual irrigation (blue bars) in millimetres are plotted on the second y-axis. Irrigation periods are indicated as grey bars.
The vegetation type of the pine forest belongs to the Erico-Pinetum caricetosum albae Br.-Bl. . The species richness and the coverage rate of vegetation assessment are listed in table 2 and the full plant list and plant species mean abundance is illustrated in Table S2. Species richness did not differ between control and irrigated plots. The mean number of species in irrigated plots was 38.8 ± 1.9, whereas control plots showed a mean number of species of 37.5 ± 0.9. Coverage of vegetation differed significantly between the two treatments. Mean vegetation cover of irrigated sampling site was 50.5% ± 11.1% and 29.3% ± 9.3% in control plots. Coverage of trees showed significant differences between the two treatments. Irrigated sites showed significantly higher Scots pine coverage than controlled sites and a significant decrease of pubescent oak coverage. Coverage of dead wood, herb and moss was significantly higher in control sites than irrigated sites. Shrub cover showed no significant differences between the treatments. The calculated Landolt indicator values were also significantly higher for moisture (p = 0.004), moisture variability (p = 0.037) and nutrient value (p = 0.041), but significantly lower for continentality (p = 0.019), and the reaction value (p = 0.032). These five values were significantly explanatory for the NMDS distribution of the sampling plots (Figure 3). Using a NMDS technique, the plots that experienced identical treatment showed significant clustering (PERMANOVA: p = 0.023, Figure 3).
The plant species are plotted with logarithmic abundance indicated by font size and orientation of their highest abundance in the plots indicated by the arrows (dark green; Ps = Pinus sylvestris, Ca = Carex alba, Qp = Quercus pubescens, Ch = Carex humilis, Sa = Sorbus aria, Vl = Viburnum lantana, Pt = Populus tremula, Bp = Betula pendula, Au = Arctostaphylos uva-ursi, Bv = Berberis vulgaris). The Landolt indicator values are fitted to the plot (light green arrows; significant = solid; non-significant = dotted) for the following properties: D (aeration value), F (moisture value), H (humus value), K (continentality value), L (light value), N (nutrient value), R (reaction value (pH)), T (temperature value), and W (moisture variability value).
Fine root morphology
Fine root morphology was altered after the nine-year irrigation period (Table 3). The dry weight per soil volume of fine roots developed in the irrigated plots increased for P. sylvestris and Q. pubescens. The remaining roots derived from other species were not affected by irrigation in their dry weight per soil volume (data not shown). Fine root tips were less frequent under irrigation, significantly for pubescent oak and slightly for Scots pine. Regarding SRL and RTD, we detected contradictory results for the two tree species. For Scots pine, SRL increased and RTD decreased whereas pubescent oak tended to react in the opposite way. Agreement in the trends between the two species was detected in the RAI. Both pine and oak increase their RAI with irrigation, though both trends are not significant.
Fine root chemical properties
Treatment failed to induce significant differences in cellulose, phenol or lignin (Table 4). Therefore, coarse structure and the composition of the fine root chemistry persisted. There was a slight increase in cellulose under irrigation, but significant differences due to irrigation in root composition was limited to the amount of N in bulk roots. The amount of N in fine roots was reduced by irrigation. The CN ratio appeared to increase as a result of the N increase under irrigation. A highly significant result occurred in the 13C signal of extracted cellulose, as well as in the bulk fine root data. The 13C/12C ratio was diminished by the induced irrigation.
The correlation of the δ13C values of cellulose and bulk fine root material showed a strong overall correlation (p < 0.001; Figure 4). However, the individual treatments control (p < 0.001) and irrigation (p = 0.023) showed a positive δ13C values correlation of cellulose and bulk likewise. The obvious treatment effect on the δ13C was visible as a shift to a lower δ13C ratio of bulk and cellulose (Figure 4, Table 4).
Irrigated values (open squares) show a reduction of δ13C compared to the control (closed triangles). Dotted linear regression shows the overall correlation (p < 0.001), the dashed line is the linear regression of the irrigated ratios (p = 0.023) and the solid regression line belongs to the control (p < 0.001), (n = 4).
Results of 14C analysis performed on roots collected in 2003 estimated fine root ages reflected similar results between the irrigation and the control plots (one-way ANOVA p = 0.674; Table 1). After the nine-year irrigation period, the results showed a significant difference in fine root ages (mixed model p = 0.044). The variation in the data was larger in 2012 than the one in 2003. The mean fine root age was 11.2 yr in 2003 and 7.9 yr in 2012. In 2012, the mean fine root age of the irrigated plots was strongly diminished (5.5 yr) in contrast to the control (10.4 yr).
Water shortage in inner-Alpine valleys force trees to reduce their crowns, shorten their needles, and cause hydraulic failure in extreme drought events , , . Simultaneously, damage caused by nematodes, insects, fungi, and mistletoes increase because the ability to resist such pests is reduced –. As a consequence, Scots pines die at a higher rate than the average . It has thus been predicted that there will be a shift from sub-boreal Scots pine forests towards the sub-Mediterranean pubescent oak forests in the long-term . However, the alleviation of water shortage by water addition alters the competition situation among all the plants within the Scots pine forest.
After nine years of irrigation, an increase in vegetation cover, mainly due to the better closure of Scots pine crowns, was observed. In contrast, pubescent oak and other drought adapted shrubs such as Ligustrum vulgare, Berberis vulgaris, and Arctostaphylos uva-ursi decreased. Subsequently, with the increase in tree cover, a decrease of the herb (e.g., Carex alba) and moss cover was observed, whereas the shrub cover remained unaffected. A change in species richness, however, was not observed, although new species came in and a few species disappeared. Among the new species, some are known to predominantly prefer wet or variably moist conditions, e.g., Populus tremula, Clematis vitalba, or Acer spp. , . Species such as Viola pyrenaica, Trifolium montanum, Ligustrum vulgare, and Teucrium chamaedris were naturally present in the control but strongly reduced in the irrigation plots. The reason for this exclusion is most likely caused by fast changes between wet and dry conditions, and competition with better adapted pioneers (e.g. ). These effects are limited to annual or herbaceous perennial plants with a short lifespan. Nevertheless, the irrigation also affected, in addition to crown transparency, the mortality of the Scots pine , . Rather surprising is the fact that there is a lack of literature using plant indicator values to demonstrate shifts in vegetation upon environmental change, as has been shown earlier for mires or alpine meadows , . Undoubtedly, the water availability indicated by moisture variability and moisture values are consistent. However, the assessment of the plant community additionally reveals subtle changes like a decrease in the light value or an increase in nutrient availability. The light value is reduced by the effect of a closing canopy in the irrigated plots, whereas the nutrient availability seems to be triggered directly by the increase in soil moisture content . The decrease in the light value was also detected earlier by Dobbertin et al.  having recorded a decrease in crown transparency and an increase in needle length in the irrigated plots.
Nine years after the start of the irrigation, the treatment resulted in a significant increase in the fine root biomass to a near-to-significant increase of the fine root length for P. sylvestris. Such a trend was observed as well in hardwood forests in the US , , indicating that water seems to stimulate root elongation after suffering from long lasting dry periods . The measured decrease in fine root tips, which was significant for pubescent oak and a tendency for Scots pine, appears to be caused by the detected elongation of the fine roots. The total increase in fine root dry weight per soil volume after irrigation is unsurprising (e.g. ). Interestingly this change wasn't detected immediately. In the initial years of the irrigation experiment no significant change could be detected . Comparing the results from Brunner et al.  with ours evidently demonstrates the slow adaptation capacity of the Scots pine fine root system in the drought afflicted Pfynwald (Fig. 5). Bakker et al.  demonstrated a shift of fine roots to shallower soil layers with increasing soil moisture, and consequently, the overall biomass was increased in the top layers (0–10 cm). Moreover, Leuschner et al.  found in their driest stand of Fagus sylvatica (precipitation < 520 mm yr−1), which is equivalent to our study site, the lowest fine root biomass. In a comparable study in a semi-deciduous forest in Panama, similar results were observed with irrigation in a water limited situation, with an increase of the fine root biomass in the upper most soil layers . This drought effect often referred to as deep rooting strategy (e.g. ) appears to be suspended by the irrigation treatment, resulting in the detected fine root biomass increase.
 (error bars = standard error), (n = 4). (a) Fine root dry weight (g/m2) are shown as bar plots from 2003, 2004 and 2005  compared to the 2012 detected. P-values < 0.05 calculated with a mixed model followed by ANOVA are marked with *. (b) A significant positive linear correlation of the Scots pine fine roots increased means could be shown (R2 = 0.9958, p-values = 0.0014).
With an increase of root length and root biomass, a change in the SRL obviously cannot be expected. Indeed, Ostonen et al. , in their meta-analysis of SRL, did not record a change in SRL, either due to irrigation or due to drought. While only a few studies have detected changes in SRL mediated by irrigation, they were also combined with fertilization (e.g. ). However, SRL is best known for differentiating between species or plant strategies  as it is known for leaf and needle traits such as specific leaf area or leaf dry matter content (e.g. –). The comparison of the two dominating tree species in our study revealed large differences, with the pubescent oak having a much higher SRL (2.37±0.23 cm mg−1) than the Scots pine (0.73±0.04 cm mg−1). Similar values were recorded by Ostonen et al.  for Q. robur (1.29±1.79 cm mg−1) and P. sylvestris (0.75±1.05 cm mg−1), leading to the conclusion that oaks with 2–3× longer roots per weight unit might have an advantage in water absorbance compared to pines. Overall, this evidence supports the notion that the pubescent oak has greater competitiveness compared to the Scots pine under a drought situation . Hertel et al.  recently showed a positive correlation of the RAI to annual precipitation in beech forests. In our study the RAI reacts positively to the increased water availability. These trends indicate a marginal and tardy morphological plasticity of mature pine roots with the same evolutionary background.
In contrast, air warming or drought treatments are known to decrease the root length of Quercus sp. –. These results are potentially triggered by an accompanying nutrient deficiency , , taking into consideration the fact that nutrients and water availability are often highly linked. Due to known drought tolerance following a deep rooting strategy , the fine roots of Q. pubescens in Pfynwald perhaps neglect the upper soil layer (0–10 cm).
Root chemical properties
The fine root structure of P. sylvestris reacted strongly to the irrigation treatment. On the one hand, the amount of N in bulk roots was reduced even if the irrigation increased the N input slightly. This phenomenon is known and is described in the literature, where the increase in N results in plant growth stimulation (e.g. –). Eilmann et al.  clearly demonstrated the increased pine growth at our study site. The N in the tree fine roots is therefore diluted by the boost of fixated C under irrigation. In addition, the priority of biomass accumulation, including N integration, is based on aboveground structures (regenerative organs, needle production, stem growth) . Meanwhile, the increased irrigation in our study potentially enhanced N leaching. These results agree with the assumption of a water limited forest in the Pfynwald rather than N limitation. On the other hand, the isotopic ratio of δ13C is strongly influenced by additional water supply. The comparison of fine root bulk with cellulose showed a significant correlation. This indicates, similar to the results of Eilmann et al. , that photosynthates were used in a coupled ratio. The shift to overall reduced δ13C merely demonstrates an increased photosynthetic activity of Scots pine by irrigation due to the discrimination of the 13C isotope . There is no enhanced cellulose production in relation to other C-associated root compounds. The fine root composition of cellulose, lignin, and phenols under irrigation was unchanged. The fine root rough structure was preserved, and revealed a conservative nature similar to King et al.  who detected marginal changes with altered nutrient availability. In addition, it is possible that, due to the slow turnover of tree roots, adaptations are still lagging after nine years of irrigation. This is rather surprising, keeping in mind that phenolic compounds  or lignin  can increase with stress caused by pathogens.
Radiocarbon analysis revealed a decrease in mean fine root age by different treatments of P. sylvestris after nine years of irrigation. Up to now, a treatment induced alteration of the mean fine root age has not been demonstrated using this new technique of 14C dating. Nonetheless, appropriate caution is needed here. First, the total root age varies considerably depending on the root lifespan assessment method , and second, considering the increased fine root biomass with irrigation, it is possible that we have detected more young and newly produced roots in relation to old roots. Black et al.  and Chapin et al.  observed in their studies that root elongation tends to be negatively correlated with their longevity. Interestingly, water increase as well as decrease can have negative effects on fine root lifespan , . However, both studies are barely comparable to our findings; not only was the detection period very short (100 days and 1 year, respectively), but also additional factors such as competition or logging could have had crucial effects on the fine root lifespan . A similar trend, as in our results, was observed by Yuan and Chen  and Finér et al. , who recorded an increase in the turnover rate caused by an increase in the mean annual precipitation. It also appears that, using the fine root lifespan as a fine root turnover approximation, is risky in a temperate zone due to phenology . In our case, the samples were collected in the same spring period, which, accordingly, reduces the phenological influence. Subsequently, we postulate an increase in fine root turnover rate in the presence of increasing water availability. Furthermore, it is obvious that the 14C bomb peak model is more uncertain the more recent the samples are because the bomb peak effect is flattening out due to the ocean uptake and recently fossil fuel combustion . Moreover, the turnover rate of fine roots is a highly discussed topic and represents a major factor for C sequestration which will trigger future climatic conditions , . Precipitation and drought, amongst others, are relevant factors which will directly influence future climate and CO2 feedback cycles , .
In conclusion our data imply that the responses belowground to irrigation are less conspicuous than the more rapid adaptations aboveground. Lagged and conservative adaptations of tree roots with decadal lifespans are challenging to detect, hence demanding for long-term surveys. Furthermore, interactions of treatments with biogeochemical processes operate on longer time scales and cannot be detected in short-term studies, which highlights the importance of long-term experiments at natural forest sites (e.g. –). Investigations concerning fine root turnover rate and degradation processes under a changing climate are crucial for a complete understanding of C cycling.
Translation and specified classification for the indicator values by Landolt (1977).
Species list with the mean abundance (%) in the control and irrigated plots. One-way ANOVA: p < 0.05 = significant, denoted; p ≤ 0.05–0.1 = non-significant, denoted; p > 0.1 = non-significant (ns).
We thank Andreas Rigling for being responsible for the whole treatment experiment in the ‘Pfynwald’, Peter Bleuler and Werner Landolt for regulating the irrigation and providing the striking image, Magdalena Nötzli for the help with lignin and cellulose extraction, the team of the Central Laboratory of WSL for the elemental analyses, and Curtis Gautschi for correcting the English language. The ’Pfynwald’-site belongs to the WSL Long-Term Forest Ecosystem Research Programm LWF.
Conceived and designed the experiments: IB. Performed the experiments: CH. Analyzed the data: CH. Wrote the paper: CH IB. Vegetation assessment: JS. Volumetric water content data: EGP. 14C measurements: IH .
- 1. Rebetez M, Dobbertin M (2004) Climate change may already threaten Scots pine stands in the Swiss Alps. Theoretical and Applied Climatology 79: 1–9.
- 2. Bigler C, Bräker OU, Bugmann H, Dobbertin M, Rigling A (2006) Drought as an inciting mortality factor in Scots pine stands of the Valais, Switzerland. Ecosystems 9: 330–343.
- 3. Forzieri G, Feyen L, Rojas R, Flörke M, Wimmer F, et al. (2014) Ensemble projections of future streamflow droughts in Europe. Hydrology and Earth System Sciences 18: 85–108.
- 4. Dobbertin M, Mayer P, Wohlgemuth T, Feldmeyer-Christe E, Graf U, et al. (2005) The decline of Pinus sylvestris L. forests in the Swiss Rhone valley-a result of drought stress? Phyton-Horn 45: 153.
- 5. Dobbertin M, Rigling A (2006) Pine mistletoe (Viscum album ssp. austriacum) contributes to Scots pine (Pinus sylvestris) mortality in the Rhone valley of Switzerland. Forest Pathology 36: 309–322.
- 6. Mitchell PJ, O'Grady AP, Tissue DT, White DA, Ottenschlaeger ML, et al. (2013) Drought response strategies define the relative contributions of hydraulic dysfunction and carbohydrate depletion during tree mortality. New Phytologist 197: 862–872.
- 7. Hartmann H, Ziegler W, Kolle O, Trumbore S (2013) Thirst beats hunger - declining hydration during drought prevents carbon starvation in Norway spruce saplings. New Phytologist 200: 340–349.
- 8. Carnicer J, Coll M, Ninyerola M, Pons X, Sánchez G, et al. (2011) Widespread crown condition decline, food web disruption, and amplified tree mortality with increased climate change-type drought. Proceedings of the National Academy of Sciences 108: 1474–1478.
- 9. Anderegg WRL, Kane JM, Anderegg LDL (2013) Consequences of widespread tree Mortality triggered by drought and temperature stress. Nature Climate Change 3: 30–36.
- 10. Smith MD (2011a) The ecological role of climate extremes: current understanding and future prospects. Journal of Ecology 99: 651–655.
- 11. Smith MD (2011b) An ecological perspective on extreme climatic events: a synthetic definition and framework to guide future research. Journal of Ecology 99: 656–663.
- 12. Dobbertin M, Eilmann B, Bleuler P, Giuggiola A, Graf Pannatier E, et al. (2010) Effect of irrigation on needle morphology, shoot and stem growth in a drought-exposed Pinus sylvestris forest. Tree Physiology 30: 346–360.
- 13. Eilmann B, Buchmann N, Siegwolf R, Saurer M, Cherubini P, et al. (2010) Fast response of Scots pine to improved water availability reflected in tree-ring width and δ13C. Plant, Cell & Environment 33: 1351–1360.
- 14. Steudle E (2000) Water uptake by roots: effects of water deficit. Journal of Experimental Botany 51: 1531–1542.
- 15. Pronk A, De Willigen P, Heuvelink E, Challa H (2002) Development of fine and coarse roots of Thuja occidentalisBrabant'in non-irrigated and drip irrigated field plots. Plant and Soil 243: 161–171.
- 16. Genenger M, Zimmermann S, Hallenbarter D, Landolt W, Frossard E, et al. (2003) Fine root growth and element concentrations of Norway spruce as affected by wood ash and liquid fertilisation. Plant and Soil 255: 253–264.
Konôpka B, Nogushi K, Takahashi M, Sakata T, Konôpková Z (2008) In: Sánchez JM, editor. Droughts: Causes, Effects, and Predictions. New YorkNova Science Publishers, Inc167–189.
- 18. Montagnoli A, Terzaghi M, Di Iorio A, Scippa GS, Chiatante D (2012) Fine-root morphological and growth traits in a Turkey-oak stand in relation to seasonal changes in soil moisture in the Southern Apennines, Italy. Ecological Research 27: 1015–1025.
- 19. Kätterer T, Fabião A, Madeira M, Ribeiro C, Steen E (1995) Fine-root dynamics, soil moisture and soil carbon content in a Eucalyptus globulus plantation under different irrigation and fertilisation regimes. Forest Ecology and Management 74: 1–12.
- 20. Coleman M (2007) Spatial and temporal patterns of root distribution in developing stands of four woody crop species grown with drip irrigation and fertilization. Plant and Soil 299: 195–213.
- 21. Bakker M, Jolicoeur E, Trichet P, Augusto L, Plassard C, et al. (2009) Adaptation of fine roots to annual fertilization and irrigation in a 13-year-old Pinus pinaster stand. Tree Physiology 29: 229–238.
- 22. Cubera E, Moreno G, Solla A, Madeira M (2012) Root system of Quercus suber L. seedlings in response to herbaceous competition and different watering and fertilisation regimes. Agroforestry Systems 85: 205–214.
- 23. Brunner I, Pannatier EG, Frey B, Rigling A, Landolt W, et al. (2009) Morphological and physiological responses of Scots pine fine roots to water supply in a dry climatic region in Switzerland. Tree Physiology 29: 541–550.
- 24. Ostonen I, Püttsepp Ü, Biel C, Alberton O, Bakker MR, et al. (2007) Specific root length as an indicator of environmental change. Plant Biosystems 141: 426–442.
- 25. Hacke UG, Sperry JS, Ewers BE, Ellsworth DS, Schafer KVR, et al. (2000) Influence of soil porosity on water use in Pinus taeda. Oecologia 124: 495–505.
- 26. Ewers BE, Oren R, Sperry JS (2000) Influence of nutrient versus water supply on hydraulic architecture and water balance in Pinus taeda. Plant Cell and Environment 23: 1055–1066.
- 27. Cahill DM, McComb JA (1992) A comparison of changes in phenylalanine ammonia-lyase activity, lignin and phenolic synthesis in the roots of Eucalyptus calophylla (field resistant) and E. marginata (susceptible) when infected with Phytophthora cinnamomi. Physiological and Molecular Plant Pathology 40: 315–332.
Oβwald W, Fleischmann F, Treutter D (2012) Host–Parasite Interactions and Trade-offs Between Growth-and Defence-Related Metabolism Under Changing Environments. In: Matyssek R, Schnyder H, Oβwald W, editors. Growth and Defence in Plants. Heidelberg: Springer. pp. 53–83.
- 29. Prescott CE (2010) Litter decomposition: what controls it and how can we alter it to sequester more carbon in forest soils? Biogeochemistry 101: 133–149.
- 30. Ostonen I, Lõhmus K, Pajuste K (2005) Fine root biomass, production and its proportion of NPP in a fertile middle-aged Norway spruce forest: Comparison of soil core and ingrowth core methods. Forest Ecology and Management 212: 264–277.
- 31. Bird JA, Torn MS (2006) Fine roots vs. needles: a comparison of 13 C and 15 N dynamics in a ponderosa pine forest soil. Biogeochemistry 79: 361–382.
- 32. Fujii S, Takeda H (2010) Dominant effects of litter substrate quality on the difference between leaf and root decomposition process above- and belowground. Soil Biology and Biochemistry 42: 2224–2230.
- 33. De Santo AV, Berg B, Rutigliano FA, Alfani A, Floretto A (1993) Factors regulating early-stage decomposition of needle litters in five different coniferous forests. Soil Biology and Biochemistry 25: 1423–1433.
- 34. Berg B (1984) Decomposition of root litter and some factors regulating the process: Long-term root litter decomposition in a scots pine forest. Soil Biology and Biochemistry 16: 609–617.
- 35. Rasse DP, Rumpel C, Dignac M-F (2005) Is soil carbon mostly root carbon? Mechanisms for a specific stabilisation. Plant and Soil 269: 341–356.
- 36. Zhang D, Hui D, Luo Y, Zhou G (2008) Rates of litter decomposition in terrestrial ecosystems: global patterns and controlling factors. Journal of Plant Ecology 1: 85–93.
- 37. Hajdas I (2008) Radiocarbon dating and its applications in Quaternary studies. Eiszeitalter und Gegenwart Quaternary Science Journal 57: 2–24.
Broecker WS, Peng T-H, Beng Z (1982) Columbia UniversityTracers in the Sea: Lamont-Doherty Geological Observatory
- 39. Trumbore S (2000) Age of soil organic matter and soil respiration: radiocarbon constraints on belowground C dynamics. Ecological Applications 10: 399–411.
- 40. Gaudinski J, Trumbore S, Davidson E, Cook A, Markewitz D, et al. (2001) The age of fine-root carbon in three forests of the eastern United States measured by radiocarbon. Oecologia 129: 420–429.
- 41. Gaudinski JB, Torn MS, Riley W, Swanston C, Trumbore SE, et al. (2009) Use of stored carbon reserves in growth of temperate tree roots and leaf buds: analyses using radiocarbon measurements and modeling. Global Change Biology 15: 992–1014.
- 42. Riley WJ, Gaudinski JB, Torn MS, Joslin J, Hanson PJ (2009) Fine-root mortality rates in a temperate forest: Estimates using radiocarbon data and numerical modeling. New Phytologist 184: 387–398.
- 43. Gaudinski JB, Torn M, Riley W, Dawson T, Joslin J, et al. (2010) Measuring and modeling the spectrum of fine-root turnover times in three forests using isotopes, minirhizotrons, and the Radix model. Global Biogeochemical Cycles 24: GB3029.
- 44. Pregitzer KS, DeForest JL, Burton AJ, Allen MF, Ruess RW, et al. (2002) Fine root architecture of nine North American trees. Ecological Monographs 72: 293–309.
- 45. Sah SP, Jungner H, Oinonen M, Kukkola M, Helmisaari H-S (2011) Does the age of fine root carbon indicate the age of fine roots in boreal forests? Biogeochemistry 104: 91–102.
- 46. Sah S, Bryant C, Leppälammi-Kujansuu J, Lõhmus K, Ostonen I, et al. (2013) Variation of carbon age of fine roots in boreal forests determined from 14C measurements. Plant and Soil 363: 77–86.
- 47. Gaul D, Hertel D, Leuschner C (2009) Estimating fine root longevity in a temperate Norway spruce forest using three independent methods. Functional Plant Biology 36: 11–19.
Meteo Schweiz Website, online data sheet for the measured years from 1864 until 2013. Available: http://www.meteoschweiz.admin.ch/web/de/klima/klima_heute/homogene_reihen.html. Accessed 2014 Apr 14.
- 49. Thimonier A, Schmitt M, Waldner P, Rihm B (2005) Atmospheric deposition on Swiss long-term forest ecosystem research (LWF) plots. Environmental Monitoring and Assessment 104: 81–118.
- 50. Thimonier A, Pannatier EG, Schmitt M, Waldner P, Walthert L, et al. (2010) Does exceeding the critical loads for nitrogen alter nitrate leaching, the nutrient status of trees and their crown condition at Swiss Long–term Forest Ecosystem Research (LWF) sites? European Journal of Forest Research 129: 443–461.
- 51. Londo G (1976) The decimal scale for relevés of permanent quadrats. Plant Ecology 33: 61–64.
Aeschimann D, Lauber K, Moser DM, Theurillat J-P (2004) Flora alpina: Haupt Bern.
Landolt E (1977) Ökologische Zeigerwerte zur Schweizer Flora. Veröffentlichung des Geobotanischen Institut Rübel 64.
Hiltbrunner D, Zimmermann S, Hagedorn F (2013) Afforestation with Norway spruce on a subalpine pasture alters carbon dynamics but only moderately affects soil carbon storage. Biogeochemistry: 1–16.
Dence C (1992) The determination of lignin. Methods in lignin chemistry Springer33–61.
- 56. Endrulat T, Saurer M, Buchmann N, Brunner I (2010) Incorporation and remobilization of 13C within the fine-root systems of individual Abies alba trees in a temperate coniferous stand. Tree Physiology 30: 1515–1527.
- 57. Swain T, Hillis W (1959) The phenolic constituents of Prunus domestica. I.—The quantitative analysis of phenolic constituents. Journal of the Science of Food and Agriculture 10: 63–68.
Kakáč B, Vejdělek ZJ (1977) Handbuch der photometrischen Analyse organischer Verbindungen: Verlag Chemie.
- 59. Richter A, Hajdas I, Frossard E, Brunner I (2013) Soil acidity affects fine root turnover of European beech. Plant Biosystems 147: 50–59.
- 60. Wacker L, Němec M, Bourquin J (2010) A revolutionary graphitisation system: Fully automated, compact and simple. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 268: 931–934.
- 61. Synal H-A, Stocker M, Suter M (2007) MICADAS: a new compact radiocarbon AMS system. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 259: 7–13.
Oksanen J, Blanchet FG, Kindt R, Legendre P, Minchin P, et al. (2013) vegan: Community Ecology Package. R package version 2.0-2. Available: http://cran.r-project.org/web/packages/vegan/index.html. Accessed 2014 Apr 12.
Kindt R, Coe R (2008) BiodiversityR: GUI for biodiversity and community ecology analysis.
- 64. Bates D (2005) Fitting linear mixed models in R. R news 5: 27–30.
- 65. Massey Jr FJ (1951) The Kolmogorov-Smirnov test for goodness of fit. Journal of the American statistical Association 46: 68–78.
Levene H (1960) Robust tests for Equality of Variances. In: Olkin I, editor. Contributions to probability and statistics: essays in honor of Harold Hotelling. Stanford, California: Stanford University Press. pp. 278–292.
- 67. Levin I, Kromer B (2004) The tropospheric 14CO2 level in mid latitudes of the northern hemisphere (1959–2003). Radiocarbon 46: 1261–1271.
Werner P (1985) La végétation de Finges et de son Rhône sauvage.
- 69. Bréda N, Huc R, Granier A, Dreyer E (2006) Temperate forest trees and stands under severe drought: a review of ecophysiological responses, adaptation processes and long-term consequences. Annals of Forest Science 63: 625–644.
- 70. McDowell N, Pockman WT, Allen CD, Breshears DD, Cobb N, et al. (2008) Mechanisms of plant survival and mortality during drought: why do some plants survive while others succumb to drought? New Phytologist 178: 719–739.
- 71. Polomski J, Schönfeld U, Braasch H, Dobbertin M, Burgermeister W, et al. (2006) Occurrence of Bursaphelenchus species in declining Pinus sylvestris in a dry Alpine valley in Switzerland. Forest Pathology 36: 110–118.
- 72. Rouault G, Candau J-N, Lieutier F, Nageleisen L-M, Martin J-C, et al. (2006) Effects of drought and heat on forest insect populations in relation to the 2003 drought in Western Europe. Annals of Forest Science 63: 613–624.
- 73. Wermelinger B, Rigling A, Schneider Mathis D, Dobbertin M (2008) Assessing the role of bark-and wood-boring insects in the decline of Scots pine (Pinus sylvestris) in the Swiss Rhone valley. Ecological Entomology 33: 239–249.
- 74. Giordano L, Gonthier P, Varese G, Miserere L, Nicolotti G (2009) Mycobiota inhabiting sapwood of healthy and declining Scots pine (Pinus sylvestris L.) trees in the Alps. Fungal Diversity 38: 69.
- 75. Rigling A, Eilmann B, Koechli R, Dobbertin M (2010) Mistletoe-induced crown degradation in Scots pine in a xeric environment. Tree Physiology 30: 845–852.
- 76. Heiniger U, Theile F, Rigling A, Rigling D (2011) Blue-stain infections in roots, stems and branches of declining Pinus sylvestris trees in a dry inner alpine valley in Switzerland. Forest Pathology 41: 501–509.
- 77. Zweifel R, Bangerter S, Rigling A, Sterck FJ (2012) Pine and mistletoes: how to live with a leak in the water flow and storage system? Journal of Experimental Botany 63: 2565–2578.
- 78. Dobbertin M, Wermelinger B, Bigler C, Bürgi M, Carron M, et al. (2007) Linking increasing drought stress to Scots pine mortality and bark beetle infestations. The Scientific World Journal 7: 231–239.
- 79. Rigling A, Bigler C, Eilmann B, Feldmeyer-Christe E, Gimmi U, et al. (2013) Driving factors of a vegetation shift from Scots pine to pubescent oak in dry Alpine forests. Global Change Biology 19: 229–240.
- 80. Worrell R (1995) European aspen (Populus tremula L.): a review with particular reference to Scotland I. Distribution, ecology and genetic variation. Forestry 68: 93–105.
- 81. Lemoine D, Peltier J-P, Marigo G (2001) Comparative studies of the water relations and the hydraulic characteristics in Fraxinus excelsior, Acer pseudoplatanus and A. opalus trees under soil water contrasted conditions. Annals of Forest Science 58: 723–731.
Glaser PH, Janssens JA, Siegel DI (1990) The response of vegetation to chemical and hydrological gradients in the Lost River peatland, northern Minnesota. The Journal of Ecology: 1021–1048.
- 83. Peter M, Gigon A, Edwards PJ, Lüscher A (2009) Changes over three decades in the floristic composition of nutrient-poor grasslands in the Swiss Alps. Biodiversity and Conservation 18: 547–567.
- 84. Graf U, Wildi O, Küchler M, Ecker K (2010) Five-year changes in Swiss mire vegetation. Botanica Helvetica 120: 15–27.
Barber SA (1995) Soil nutrient bioavailability: a mechanistic approachSonsJohn Wiley
- 86. Joslin J, Wolfe M, Hanson P (2001) Factors controlling the timing of root elongation intensity in a mature upland oak stand. Plant and Soil 228: 201–212.
- 87. Tierney GL, Fahey TJ, Groffman PM, Hardy JP, Fitzhugh RD, et al. (2003) Environmental control of fine root dynamics in a northern hardwood forest. Global Change Biology 9: 670–679.
- 88. Joslin JD, Wolfe MH (1998) Impacts of water input manipulations on fine root production and mortality in a mature hardwood forest. Plant and Soil 204: 165–174.
- 89. Cavelier J, Wright SJ, Santamaría J (1999) Effects of irrigation on litterfall, fine root biomass and production in a semideciduous lowland forest in Panama. Plant and Soil 211: 207–213.
- 90. Leuschner C, Hertel D, Schmid I, Koch O, Muhs A, et al. (2004) Stand fine root biomass and fine root morphology in old-growth beech forests as a function of precipitation and soil fertility. Plant and Soil 258: 43–56.
- 91. Schenk HJ (2008) Soil depth, plant rooting strategies and species' niches. New Phytologist 178: 223–225.
- 92. De Visser PH, Beier C, Rasmussen L, Kreutzer K, Steinberg N, et al. (1994) Biological response of five forest ecosystems in the EXMAN project to input changes of water, nutrients and atmospheric loads. Forest Ecology and Management 68: 15–29.
- 93. Cornelissen JHC, Lavorel S, Garnier E, Diaz S, Buchmann N, et al. (2003) A handbook of protocols for standardised and easy measurement of plant functional traits worldwide. Australian Journal of Botany 51: 335–380.
Cornelissen J, Diez PC, Hunt R (1996) Seedling growth, allocation and leaf attributes in a wide range of woody plant species and types. Journal of Ecology: 755–765.
- 95. Wilson PJ, Thompson K, Hodgson JG (1999) Specific leaf area and leaf dry matter content as alternative predictors of plant strategies. New Phytologist 143: 155–162.
- 96. Hertel D, Strecker T, Muller-Haubold H, Leuschner C (2013) Fine root biomass and dynamics in beech forests across a precipitation gradient - is optimal resource partitioning theory applicable to water-limited mature trees? Journal of Ecology 101: 1183–1200.
Collet C, Colin F, Bernier F (1997) Height growth, shoot elongation and branch development of young Quercus petraea grown under different levels of resource availability;. 65–81.
- 98. Saxe H, Cannell MG, Johnsen Ø, Ryan MG, Vourlitis G (2001) Tree and forest functioning in response to global warming. New Phytologist 149: 369–399.
- 99. Arend M, Kuster T, Günthardt-Goerg MS, Dobbertin M (2011) Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens). Tree Physiology 31: 287–297.
- 100. Trubat R, Cortina J, Vilagrosa A (2010) Nursery fertilization affects seedling traits but not field performance in Quercus suber L. Journal of Arid Environments 74: 491–497.
- 101. Grossman JD, Rice KJ (2012) Evolution of root plasticity responses to variation in soil nutrient distribution and concentration. Evolutionary Applications 5: 850–857.
Kuster TM, Arend M, Günthardt-Goerg MS, Schulin R (2012) Root growth of different oak provenances in two soils under drought stress and air warming conditions. Plant and Soil: 1–11.
- 103. Magnani F, Mencuccini M, Borghetti M, Berbigier P, Berninger F, et al. (2007) The human footprint in the carbon cycle of temperate and boreal forests. Nature 447: 849–851.
- 104. Pregitzer KS, Burton AJ, Zak DR, Talhelm AF (2008) Simulated chronic nitrogen deposition increases carbon storage in Northern Temperate forests. Global Change Biology 14: 142–153.
- 105. Thomas RQ, Canham CD, Weathers KC, Goodale CL (2009) Increased tree carbon storage in response to nitrogen deposition in the US. Nature Geoscience 3: 13–17.
- 106. Lacointe A (2000) Carbon allocation among tree organs: a review of basic processes and representation in functional-structural tree models. Annals of Forest Science 57: 521–533.
- 107. Farquhar GD, O'leary M, Berry J (1982) On the relationship between carbon isotope discrimination and the intercellular carbon dioxide concentration in leaves. Functional Plant Biology 9: 121–137.
- 108. King J, Pregitzer K, Zak DR, Holmes WE, Schmidt K (2005) Fine root chemistry and decomposition in model communities of north-temperate tree species show little response to elevated atmospheric CO2 and varying soil resource availability. Oecologia 146: 318–328.
Eissenstat DM, Yanai RD (2002) Root life span, efficiency, and turnover. In: Waisel Y, editor. Plant roots: the hidden half. 3rd ed. New York: Marcel Dekker. pp. Online-Ressource.
- 110. Black K, Harbron C, Franklin M, Atkinson D, Hooker J (1998) Differences in root longevity of some tree species. Tree Physiology 18: 259–264.
Chapin III FS, Matson PA, Vitousek PM (2012) Plant Nutrient Use. Principles of Terrestrial Ecosystem Ecology Springer229–258.
- 112. Pregitzer KS, Hendrick RL, Fogel R (1993) The demography of fine roots in response to patches of water and nitrogen. New Phytologist 125: 575–580.
- 113. Jones RH, Mitchell RJ, Stevens GN, Pecot SD (2003) Controls of fine root dynamics across a gradient of gap sizes in a pine woodland. Oecologia 134: 132–143.
Eissenstat DM, McCormack ML, Du Q (2013) Global Change and Root Lifespan. In: Eshel A, Beeckman T, editors. Plant roots: the hidden half. 4th ed. Boca Raton: CRC Press. pp. 1–13.
- 115. Yuan Z, Chen HY (2010) Fine root biomass, production, turnover rates, and nutrient contents in boreal forest ecosystems in relation to species, climate, fertility, and stand age: literature review and meta-analyses. Critical Reviews in Plant Sciences 29: 204–221.
- 116. Finér L, Ohashi M, Noguchi K, Hirano Y (2011) Fine root production and turnover in forest ecosystems in relation to stand and environmental characteristics. Forest Ecology and Management 262: 2008–2023.
- 117. Levin I, Hammer S, Kromer B, Meinhardt F (2008) Radiocarbon observations in atmospheric CO2: Determining fossil fuel CO2 over Europe using Jungfraujoch observations as background. Science of the Total Environment 391: 211–216.
- 118. Brunner I, Bakker MR, Björk RG, Hirano Y, Lukac M, et al. (2013) Fine-root turnover rates of European forests revisited: an analysis of data from sequential coring and ingrowth cores. Plant and Soil 362: 357–372.
- 119. Heimann M, Reichstein M (2008) Terrestrial ecosystem carbon dynamics and climate feedbacks. Nature 451: 289–292.
- 120. Reichstein M, Bahn M, Ciais P, Frank D, Mahecha MD, et al. (2013) Climate extremes and the carbon cycle. Nature 500: 287–295.
- 121. Niklaus P, Alphei J, Ebersberger D, Kampichler C, Kandeler E, et al. (2003) Six years of in situ CO2 enrichment evoke changes in soil structure and soil biota of nutrient-poor grassland. Global Change Biology 9: 585–600.
- 122. Dawes MA, Hättenschwiler S, Bebi P, Hagedorn F, Handa IT, et al. (2011) Species-specific tree growth responses to 9 years of CO2 enrichment at the alpine treeline. Journal of Ecology 99: 383–394.
- 123. Bader MKF, Leuzinger S, Keel SG, Siegwolf RT, Hagedorn F, et al. (2013) Central European hardwood trees in a high-CO2 future: synthesis of an 8-year forest canopy CO2 enrichment project. Journal of Ecology 101: 1509–1519.