Fig 1.
Steps involved in reconstructing long-term changes in functional traits by linking plant traits to pollen data.
The modern vegetation data are firstly grouped into palynological equivalents of the modern pollen types to account for the limitations imposed by pollen taxonomic resolution. Trait values are then assigned to the modern pollen types via direct or indirect trait attribution and by weighting pollen counts by relative pollen productivity (RPP; see text). Next, modern pollen assemblages are related to the modern vegetation to establish how well the trait data of the contemporary vegetation reflect that of the modern pollen taxa to determine the extent to which pollen assemblages can be used to predict the trait composition of the vegetation. This is a vital step that has been missing from previous studies but important if extrapolating results to other sites. Pollen data from dated sediment deposits are then used to reconstruct temporal changes in trait patterns using the best fitting traits between the contemporary vegetation and the modern pollen taxa. Traits are assigned to Holocene pollen assemblages by attributing mean trait values (directly and indirectly) measured from the vegetation. Leaf trait community-weighted means (CWMs) for the Holocene pollen samples can then be calculated.
Fig 2.
Linear relationships between the CWMs of vegetation and modern pollen before (top row, panels a to f) and after (middle row, panels g to l) relative pollen productivity (RPP) adjustment (Tcw and Tacw, respectively) using the 13 pollen types for which RPP estimates were available. The bottom row (panels m to r) shows the relationship between vegetation and modern pollen including all 44 pollen types with and without RPP estimates and with trait data available. Marginal r2 and RMSEP values of linear mixed effects models (LMEs) of the standing vegetation as a function of the modern pollen data are shown in Table 1. Marginal r2 and RMSEP were used to determine how well plot-level mean trait data of the modern pollen assemblages can predict mean trait values of the modern fen vegetation.
Fig 3.
Linear relationships between the CWMs of herbaceous (green circles) and woody (brown circles) vegetation and modern pollen before (top row, panels a to f) and after (middle row, panels g to l) relative pollen productivity (RPP) adjustment (Tcw and Tacw, respectively) using the 13 pollen types (eight herbaceous and five woody types) for which RPP estimates were available. The bottom row (panels m to r) shows the relationship between vegetation and modern pollen including all 44 pollen types (34 herbaceous and 10 woody types) with and without RPP estimates and with trait data available. Marginal r2 and RMSEP values of linear mixed effects models (LMEs) of the standing vegetation as a function of the modern pollen data are shown in Table 1. Marginal r2 and RMSEP were used to determine how well plot-level mean trait data of the modern pollen assemblages can predict mean trait values of the modern fen vegetation.
Table 1.
r2 and RMSEP (root mean square error of prediction) values of the linear relationship between modern pollen and vegetation.
n pollen and n veg columns refer to the number of pollen types and vegetation species used in the analysis with count-weighted (Tcw) and adjusted count-weighted traits (Tacw). r2 values in bold are significant at p < 0.05. SLA = specific leaf area; LDMC = leaf dry-matter content.
Fig 4.
Changes in trait community weighted means (CWMs) through time (in thousands of calendar years before present) of the Holocene pollen-derived leaf trait records from three Romney Marsh sites (circles = Brookland; triangles = The Dowells; squares = Hope Farm).
Green and brown symbols indicate the predominance of herbaceous and woody taxa in the Holocene record, respectively. The means of the modern vegetation (i.e., plant-based data) are shown for herb (green vertical lines) and woody (brown vertical lines) fens. Shaded areas around the modern means depict their 95% confidence intervals. Darker-coloured symbols show Holocene samples significantly different from their equivalent (herbaceous or woody) modern vegetation means at p < 0.05 (one-observation t-tests).
Fig 5.
Changes in trait community weighted means (CWMs) through time (in thousands of calendar years before present) of the Holocene pollen-derived leaf trait records from five Fenland sites (circles = Murrow; squares = Redmere; diamonds = Swineshead; point-up triangles = Welney Washes 3rd peat; point-down triangles = Welney Washes 4th peat).
Green and brown symbols indicate the predominance of herbaceous and woody taxa in the Holocene record, respectively. The means of the modern vegetation (i.e., plant-based data) are shown for herb (green vertical lines) and woody (brown vertical lines) fens. Shaded areas around the modern means depict their 95% confidence intervals. Darker-coloured symbols show Holocene samples significantly different from their equivalent (herbaceous or woody) modern vegetation means at p < 0.05 (one-observation t-tests).