The leaf economic and plant size spectra of European forest understory vegetation

Josep Padullés Cubino,Milan Chytrý,Wolfgang Willner,Ute Jandt,Florian Jansen,Xavier Font,Valerijus Rašomavičius,Željko Škvorc,Idoia Biurrun,Tatiana Braslavskaya,Gianmaria Bonari

doi:10.1111/ecog.05598

Abstract

Forest understories play a vital role in ecosystem functioning and the provision of ecosystem services. However, the extent to which environmental conditions drive dominant ecological strategies in forest understories at the continental scale remains understudied. Here, we used ~29 500 forest vegetation plots sampled across Europe and classified into 25 forest types to explore the relative role of macroclimate, soil pH and tree canopy cover in driving abundance‐weighted patterns in the leaf economic spectrum (LES) and plant size spectrum (PSS) of forest understories (shrub and herb layers). We calculated LES using specific leaf area (SLA) and leaf dry matter content (LDMC) and PSS using plant height and seed mass of vascular plant species found in the understories. We found that forest understories had more conservative leaf economics in areas with more extreme mean annual temperatures (mainly Fennoscandia and the Mediterranean Basin), more extreme soil pH and under more open canopies. Warm and summer‐dry regions around the Mediterranean Basin and areas of Atlantic Europe also had taller understories with heavier seeds than continental temperate or boreal areas. Understories of broadleaved deciduous forests, such asFagusforests on non‐acid soils, or ravine forests, more commonly hosted species with acquisitive leaf economics. In contrast, some coniferous forests, such asPinus,LarixandPiceamire forests, orPinus sylvestrislight taiga and sclerophyllous forests, more commonly hosted species with conservative leaf economics. Our findings highlight the importance of macroclimate and soil factors in driving trait variation of understory communities at the continental scale and the mediator effect of canopy cover on these relationships. We also provide the first maps and analyses of LES and PSS of forest understories across Europe and give evidence that the understories of European forest types are differently positioned along major axes of trait variation.

Highlights

Determining the mechanisms driving plant community functions has been a central goal in vegetation ecology during the last decades (Ackerly and Cornwell 2007, Violle et al 2007)
Taller plants with heavier seeds tended to occur in the Mediterranean Basin, France and Ireland, while plants significantly shorter than under random expectation rarely dominated forest understories, occurring sporadically at latitudes higher than 50°N
Our findings suggest that future warming could lead to forest understories with taller species and more resource-conservative leaves in temperate and boreal Europe, both directly and indirectly through the structure of the tree layer

Summary

Introduction

Determining the mechanisms driving plant community functions has been a central goal in vegetation ecology during the last decades (Ackerly and Cornwell 2007, Violle et al 2007). These mechanisms have usually been investigated using plant functional traits, defined as any morphological, physical or phenological features of individuals that affect their fitness (Violle et al 2007). The leaf economic spectrum (LES) runs from species with an acquisitive resource-use strategy (i.e. rapid resource capture and a high relative growth rate) to those with a more conservative resource-use strategy (i.e. long-lived leaves and a low photosynthetic rate) (Wright et al 2004, Reich 2014). Larger plants tend to have heavier seeds and seed size is known to affect plant regeneration, for example, through seed production (Jakobsson and Eriksson 2000), seed survival in the soil (Thompson et al 1993) or seed dispersal distance (Thomson et al 2011)

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