Responses of leaf traits to climatic gradients: adaptive variation versus compositional shifts

T.-T Meng,G Wang,H Wang,J Ni,S P Harrison,I C Prentice

doi:10.5194/bg-12-5339-2015

Abstract

Abstract. Dynamic global vegetation models (DGVMs) typically rely on plant functional types (PFTs), which are assigned distinct environmental tolerances and replace one another progressively along environmental gradients. Fixed values of traits are assigned to each PFT; modelled trait variation along gradients is thus driven by PFT replacement. But empirical studies have revealed "universal" scaling relationships (quantitative trait variations with climate that are similar within and between species, PFTs and communities); and continuous, adaptive trait variation has been proposed to replace PFTs as the basis for next-generation DGVMs. Here we analyse quantitative leaf-trait variation on long temperature and moisture gradients in China with a view to understanding the relative importance of PFT replacement vs. continuous adaptive variation within PFTs. Leaf area (LA), specific leaf area (SLA), leaf dry matter content (LDMC) and nitrogen content of dry matter were measured on all species at 80 sites ranging from temperate to tropical climates and from dense forests to deserts. Chlorophyll fluorescence traits and carbon, phosphorus and potassium contents were measured at 47 sites. Generalized linear models were used to relate log-transformed trait values to growing-season temperature and moisture indices, with or without PFT identity as a predictor, and to test for differences in trait responses among PFTs. Continuous trait variation was found to be ubiquitous. Responses to moisture availability were generally similar within and between PFTs, but biophysical traits (LA, SLA and LDMC) of forbs and grasses responded differently from woody plants. SLA and LDMC responses to temperature were dominated by the prevalence of evergreen PFTs with thick, dense leaves at the warm end of the gradient. Nutrient (N, P and K) responses to climate gradients were generally similar within all PFTs. Area-based nutrients generally declined with moisture; Narea and Karea declined with temperature, but Parea increased with temperature. Although the adaptive nature of many of these trait-climate relationships is understood qualitatively, a key challenge for modelling is to predict them quantitatively. Models must take into account that community-level responses to climatic gradients can be influenced by shifts in PFT composition, such as the replacement of deciduous by evergreen trees, which may run either parallel or counter to trait variation within PFTs. The importance of PFT shifts varies among traits, being important for biophysical traits but less so for physiological and chemical traits. Finally, models should take account of the diversity of trait values that is found in all sites and PFTs, representing the "pool" of variation that is locally available for the natural adaptation of ecosystem function to environmental change.

Highlights

The plant functional type (PFT) concept has been important in the development of dynamic global vegetation models (DGVMs), which combine vegetation dynamics at the grid-cell scale with hydrological and biogeochemical processes driven by the physical environment and modulated by plant functional types (PFTs) characteristics (Prentice et al, 2007; Prentice and Cowling, 2013)
(1) To what extent are quantitative trait-environment relationships universal? Alternatively, (2) are there systematic differences in the trait-environment relationships shown by different PFTs? (3) To what extent are variations in traits along environmental gradients accounted for by variation within PFTs, as opposed to successive replacements of one PFT by another? (4) What fractions of total trait variation are linked to climate, and/or to PFT membership, as opposed to being unexplained by either climate or PFTs? We address these questions with an analysis of variations in leaf traits in plant communities sampled on long gradients of temperature and moisture availability in China (Fig. 1)
This similarity confirms that the pattern of variation in climate across the sites reflects the general pattern of climate gradients across China, and that these gradients can be summarised using two variables, representing temperature and plant moisture availability respectively

Summary

Introduction

The plant functional type (PFT) concept has been important in the development of dynamic global vegetation models (DGVMs), which combine vegetation dynamics (changes in vegetation composition, expressed as abundances of PFTs) at the grid-cell scale with hydrological and biogeochemical processes driven by the physical environment and modulated by PFT characteristics (Prentice et al, 2007; Prentice and Cowling, 2013). A fixed set of properties (parameter values) is assigned to each PFT. This expedient simplifies modelling, but it is a potential weakness because it disregards continuous adaptive variation within PFTs; the fact that trait variation within PFTs often exceeds trait differences between PFTs; and the possibility that such variation is “universal” – that is, manifested within and between species, PFTs and communities. Neglect of continuous adaptive variation in models could lead to underestimation of the potential for vegetation to adapt to environmental change and generally incorrect assessments of the response of vegetation to climate (Kleidon et al, 2009; Scheiter and Higgins, 2009) and vegetation feedbacks to climate (Alton, 2011)

Methods

Results

Conclusion