Conceived and designed the experiments: LS MA. Performed the experiments: LS ML. Analyzed the data: LS ML. Contributed reagents/materials/analysis tools: MA. Wrote the paper: LS MA ML.
The authors have declared that no competing interests exist.
The synergetic effects of recent rising atmospheric CO2 and temperature are expected to favor tree growth in boreal and temperate forests. However, recent dendrochronological studies have shown site-specific unprecedented growth enhancements or declines. The question of whether either of these trends is caused by changes in the atmosphere remains unanswered because dendrochronology alone has not been able to clarify the physiological basis of such trends.
Here we combined standard dendrochronological methods with carbon isotopic analysis to investigate whether atmospheric changes enhanced water use efficiency (WUE) and growth of two deciduous and two coniferous tree species along a 9° latitudinal gradient across temperate and boreal forests in Ontario, Canada. Our results show that although trees have had around 53% increases in WUE over the past century, growth decline (measured as a decrease in basal area increment – BAI) has been the prevalent response in recent decades irrespective of species identity and latitude. Since the 1950s, tree BAI was predominantly negatively correlated with warmer climates and/or positively correlated with precipitation, suggesting warming induced water stress. However, where growth declines were not explained by climate, WUE and BAI were linearly and positively correlated, showing that declines are not always attributable to warming induced stress and additional stressors may exist.
Our results show an unexpected widespread tree growth decline in temperate and boreal forests due to warming induced stress but are also suggestive of additional stressors. Rising atmospheric CO2 levels during the past century resulted in consistent increases in water use efficiency, but this did not prevent growth decline. These findings challenge current predictions of increasing terrestrial carbon stocks under climate change scenarios.
According to the principles of plant physiology, higher CO2 concentrations generally increase the ratio between carboxilation and water transpired - water use efficiency (WUE) - enhancing productivity
Combined with isotopic analyses, dendrochronology can be used to address the limitations of the above-mentioned approaches. For example, ratios of carbon isotopes in tree rings (δ13C) allow us to determine atmospheric effects on stomatal conductance and plant gas exchange through time, while tree growth, measured by tree-ring width converted into basal area increment (BAI), is a reliable proxy for total carbon uptake
Increases in WUE may be caused by either greater photosynthetic rates (carbon uptake) or by reductions in stomatal conductance and, consequently, lower transpiration
Here we determine whether systematic changes in tree growth and WUE have been occurring in temperate and boreal forests in Ontario, Canada. We sampled mature individuals of two deciduous (red oak -
Trees typically showed a period of early growth suppression before a release phase, which was followed by growth decline (
(
P values represent significant correlations between BAI and temperature (red), precipitation (blue) or their interaction (black).
Carbon isotope abundances in the tree rings confirmed that in the vast majority of cases, changes in BAI were significantly correlated with changes in WUE (
WUE values were obtained from 5-year pooled rings and BAI values represent 5-year increment averages. Solid lines show significant correlation for old trees and dashed lines significant correlations for young trees (P<0.05). Positive linear relationships correspond to greater carbon uptake, while polynomial functions correspond to water stress.
Atmospheric concentrations of CO2 summarize actual measurements and estimated values from ice cores in Antarctica from 1850 to the present
We found recent declines in BAI for all study species and latitudes in spite of consistent long-term increases in WUE. Initial tree growth is strongly affected by stand level variations in light, nutrients, water availability and disturbance regimes
Changes in isotope ratios with ontogeny linked to developmental and microclimatic effects, also known as “age related effects”, have received attention in recent studies
It is possible that changes in BAI are not related to CO2 fertilization effects, but driven by warmer temperatures. Strong increases in tree growth in colder conditions above (but not below) the tree line and growth declines associated with warmer climates at lower altitudes have been recently reported
In some sites WUE and BAI were linearly and positively related, suggesting that growth decline is not always related to water stress. These were the very same sites where growth trends were not negatively related with warmer conditions and/or positively related with precipitation. In these cases BAI decline would be necessarily linked with reductions in WUE, meaning loss of sensitivity to CO2. Secondary effects of changes in climate, such as changes in seasonality, snowmelt time and differential growth/climate relationships could explain this loss of sensitivity
There are several non-climatic explanations for tree decline, some of which we can rule out for our study. Autogenic succession and endogenous increasing light competition are unlikely explanations because we sampled canopy trees and the species studied represent diverse successional groups. Rising tropospheric ozone concentrations have been shown to damage the photosynthetic apparatus, reducing plant sensitivity to CO2 and promoting growth decline, but this would require extremely high concentrations of O3 near the canopy. Additionally, deleterious effects of O3 on carboxilation rates seem to be restricted to young seedlings
A possible explanation for growth decline in the cases where we did not find evidence of water stress is progressive nutrient limitation. Recent ecosystem level experiments have demonstrated this process in US temperate forests
Most of our study sites are located within the Canadian Shield that is characterized by very thin organic soil lying on granite bedrock with many bare rock outcrops formed from glacial retreat. Trees growing on these soils should have higher sensitivity to drought stress and nutrient limitation than on deeper, well-structured soils found elsewhere. For instance, the common subsurface water runoff immediately after rainfall events within the Shield can lead to water stress. Additionally, despite high levels of inorganic nutrients in throughfall, subsurface runoff losses are nearly inexistent, showing that biological demand for these nutrients is greater than that available in soils
In conclusion, we suggest that either drought stress or nutrient limitation could explain the recent growth decline reported here. Demand for other resources may have a greater effect on tree growth than a positive influence of new atmospheric conditions such as elevated CO2. Long-term experiments to test soil-plant feedbacks and the effects of atmospheric changes on tree physiology should provide a more in-depth understanding of why expected increases in tree growth are not found. This, in combination with new forest models that take into account multiple stressors will improve our knowledge of forest dynamics as well as our estimates of terrestrial carbon stocks under climate change scenarios.
We chose mature stands in preserved areas with no recent history of anthropogenic disturbance. We sampled between twenty and thirty-five living trees of black spruce (
We obtained two to four wood cores for each tree with an increment borer (5.1 mm). We mounted the cores on supports where they were air-dried and polished with sandpaper from 60 to 600 grains. We counted and measured the growth rings and dated the cores using the software Windendro. We assigned the calendar age of the growth rings according to Schulman's criteria
In mature trees, ring width declines with age and therefore cannot be considered an accurate measure of tree growth
We further investigated all individual trees for changes in WUE as shown by changes in wood carbon isotope composition through time. For isotopic analysis we used the five youngest and five oldest trees of each species at each site, resulting in a total of 130 trees. For each of these trees we pooled rings (5-year blocks) using a microscope and separated them using a thin sharp blade. Combining rings for isotopic analysis is a common practice that allows adequate sample homogenization, yields enough material for the analysis and reduces processing time. We coarsely ground wood samples in a microgrinder until the particles achieved a thickness of 1.5 mm and transferred them to Eppendorf tubes. We took an aliquot from each of the Eppendorf tubes and weighed each on a microbalance (weight range 2–3 mg). We transferred each aliquot to a tin stain capsule (3 mm diameter and 8 mm height, Elemental Micro-analysis, Milan, Italy), sealed by compressing the cup and then stored in a plate ready for analysis. We prepared one standardized aliquot at every 10th aliquot and a “blank” (empty tin cup) every 40th aliquot. We prepared standards in the same way as the wood samples by weighing between 1 and 2 mg of semolina. We placed all samples in an automated elemental analyzer (Euro-EA-Elemental Analyzer, Eurovector, Milan, Italy) connected to a continuous flow isotope ratio mass spectrometer (Isoprime, GV, Manchester, England) generating results with 0.1‰ of accuracy.
To investigate WUE using this type of analysis we rely on the natural existence of both carbon 12 (C12) and carbon 13 (C13) stable isotopes in the atmosphere. The CO2 molecules contain these isotopes in the proportion of 98.89% C12 and 1.11% C13; however in all plant tissues carbon isotope ratios are variable and the C13 abundance relative to C12 is usually expressed as δ13C:
We compiled historic atmospheric δ13C levels, and estimated values for the recent atmospheric concentration of CO2, from McCarroll & Loader
Carboxilation rates (total carbon uptake) and plant water loss (transpiration) are, like carbon isotope discrimination, controlled by photosynthesis and stomatal conductance (see Fick's first law for further explanations). Since the ratio of carbon fixed to water loss is the definition of water use efficiency, we expressed the carbon ratio values in terms of changes in water use efficiency
We calculated the average annual BAI and standard deviation values for each species at each site. Growth patterns were consistent regardless of changes in tree age and/or tree size. The five youngest and five oldest trees of each species at each site showed similar radial growth (BAI) patterns through time. We used multiple regression models and analyses of variance (ANOVAs) to identify significant correlations between BAI (dependent variable) and temperature, precipitation and their interaction (independent variables) since 1950, when most trees had already reached maturity. As typically used to interpret climate data, changes in temperature and precipitation are presented as the deviation from the historical mean (1901–2007) (
We observed that changes in BAI were linked with changes in WUE and tested the significance (P<0.05) of these relationships for the youngest and oldest trees of each species at each site during the past century using least-squares and non-linear regressions. Using non-linear regressions we determined BAI and WUE responses to increasing atmospheric CO2 for young and old trees for all species across sites. We performed all regressions using 5-year BAI average values to match the WUE values calculated form pooled rings (5-years blocks) as previously described. We performed all statistical analysis using JMP software for Macintosh, version 8.0.
We thank Ze'ev Gedalof (director of the Climate and Ecosystem Dynamic Research Lab, University of Guelph), Daniel McKenney and Wayne Bell (Ministry of Natural Resources, Canada) and Andrew Gordon and Paul Sibley (School of Environmental Sciences, University of Guelph) for logistic support and/or valuable comments.