Spatial and temporal functional changes in alpine summit vegetation are driven by increases in shrubs and graminoids.

Susanna Venn,Ken Green,Catherine Pickering

doi:10.1093/aobpla/plu008

Abstract

Classical approaches to investigating temporal and spatial changes in community composition offer only partial insight into the ecology that drives species distribution, community patterns and processes, whereas a functional approach can help to determine many of the underlying mechanisms that drive such patterns. Here, we aim to bring these two approaches together to understand such drivers, using an elevation gradient of sites, a repeat species survey and species functional traits. We used data from a repeat vegetation survey on five alpine summits and measured plant height, leaf area, leaf dry matter content and specific leaf area (SLA) for every species recorded in the surveys. We combined species abundances with trait values to produce a community trait-weighted mean (CTWM) for each trait, and then combined survey results with the CTWMs. Across the gradient of summits, more favourable conditions for plant growth (warmer, longer growing season) occurred at the lower elevations. Vegetation composition changes between 2004 and 2011 (according to non-metric multi-dimensional scaling ordination) were strongly affected by the high and increasing abundance of species with high SLA at high elevations. Species life-form categories strongly affected compositional changes and functional composition, with increasing dominance of tall shrubs and graminoids at the lower-elevation summits, and an overall increase in graminoids across the gradient. The CTWM for plant height and leaf dry matter content significantly decreased with elevation, whereas for leaf area and SLA it significantly increased. The significant relationships between CTWM and elevation may suggest specific ecological processes, namely plant competition and local productivity, influencing vegetation preferentially across the elevation gradient, with the dominance of shrubs and graminoids driving the patterns in the CTWMs.

Highlights

In the context of environmental change, linking quantitative measures of plant species physical characteristics, for example their functional traits or life form, with species distributions and local environmental factors can reveal the processes that drive patterns in vegetation composition (Dıaz and Cabido 1997; Tilman et al 2001; Lavorel and Garnier 2002; McGill et al 2006)
Mountainous regions are ideal for testing the role of plant functional traits across environmental gradients in determining community composition and shifts therein; over relatively short spatial scales, rapid changes in elevation interact with local topography to create steep gradients in temperature and precipitation (Korner 1999)
The variation in species abundance across an environmental or elevation gradient will impact on community composition and ecosystem functioning (Tilman et al 1997), as different processes are affected by different species via changes in the representation of species functional traits (Chapin et al 1996)

Summary

Introduction

In the context of environmental change, linking quantitative measures of plant species physical characteristics, for example their functional traits or life form, with species distributions and local environmental factors can reveal the processes that drive patterns in vegetation composition (Dıaz and Cabido 1997; Tilman et al 2001; Lavorel and Garnier 2002; McGill et al 2006). The variation in species abundance across an environmental or elevation gradient will impact on community composition and ecosystem functioning (Tilman et al 1997), as different processes are affected by different species via changes in the representation of species functional traits (Chapin et al 1996). The interrelation between a species life form and its functional traits is exemplified across elevation gradients in alpine regions; for example, shrubs (taller, long-lived, woody plants) dominate in favourable environments. Through their morphological and physiological traits, shrub species can modify a wide range of ecosystem processes, including alteration of local snow depths and associated hydrological dynamics, nutrient exchange and associated net carbon balance (Myers-Smith et al 2011). Graminoids, namely grasses and sedges, may compete with shrubs in the more favourable environments, recruit within the canopy of senescing shrubs (Williams and Ashton 1988) and are well adapted to a range of environmental conditions; many C3 grasses and sedges can rapidly increase in abundance after sufficient rainfall and improved abiotic conditions (Jarrad et al 2009)

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