Plant interactions as biotic drivers of plasticity in leaf litter traits and decomposability of Quercus petraea

Abstract

The importance of plant litter traits and decomposability for nutrient cycling processes and plant community dynamics through plant–litter–soil feedbacks has been largely emphasized. However, the role of biotic interactions as drivers of intraspecific variability in litter traits remains surprisingly little studied. In this study, we used a large‐scale, multi‐site network of long‐term tree removal experiments manipulating the abundance of a foundation tree species, i.e., Quercus petraea, to assess how plant interactions control intraspecific variation in oak leaf litter traits and decomposability. We studied 19 plots across eight experimental sites covering a large gradient of oak abundance, stand age, and local abiotic context. Oak leaf litter quality strongly declined with tree removal in early forest successional stage. Litter became poorer in nutrients such as N and Mg and richer in secondary metabolites such as lignin and condensed tannins. This in turn slowed its decomposition. Importantly, litter N loss switched from N release to N immobilization. Variance partitioning indicated that oak abundance explained as much variation in oak leaf litter traits as oak age and twice as much as soil inherent fertility. Confirmatory path analysis revealed that the decline of oak leaf litter quality induced by tree removal was most likely driven by a shift in understory plant species composition. Plasticity of oak leaf litter traits to the shortage of nutrient supply related to the development of understory plants competitors with higher nutrient capture and retention ability could potentially explain this response pattern. Our data also give consistent but weaker support that the decline of oak leaf litter quality could be driven by alleviated competition for light among canopy trees and subsequent enhanced crown exposure to light. Overall, our study provides evidence that biotic factors such as plant interactions are major drivers of plasticity in leaf litter traits and decomposability. This finding contributes to the emerging view that phenotypic plasticity is fundamentally related to biotic interactions for sessile organisms, especially for long‐lived and large plant species such as trees. Taking this source of functional diversity into account could help us to better understand plant community dynamics and ecological processes in terrestrial ecosystems.

Plasticity in leaf litter traits partly mitigates the impact of thinning on forest floor carbon cycling

“…Interestingly, our results brings evidences that the slowdown of litter decomposition induced by thinning intensification was mainly driven by the phenotypic plasticity of the dominant tree species Q. petraea that produces leaf litter of poorer quality following thinning in the early “aggradation” but not in the late “biostatic” stage, with a leaf litter poorer in nutrients such as N and Mg and richer in secondary metabolites such as polyphenols (Henneron, Chauvat, et al, ). Oak leaf litter was dominant in both the tree litterfall and the freshly fallen litter layer of the forest floor (Figure c and S5b).…”

“…The study system consisted of a network of long‐term thinning experiments implemented in temperate sessile oak ( Q. petraea ) forests (Henneron, Chauvat, et al, ). Quercus petraea is a long‐lived, mid‐ to late‐successional deciduous tree species which is common and frequently dominant in Western European broadleaf forests.…”

“…The sites were selected across two contrasting stages of the forest development cycle sensu Oldeman (): “aggradation” (early, 15‐ to 90‐year‐old), which is characterized by intense tree height growth and plant competition; and “biostatic” (late, 90‐ to 175‐year‐old), in which trees have reached maturity (Bormann & Likens, ). The soil abiotic context was also taken into account by classifying the sites according to their dominant humus type (Ponge, Jabiol, & Gegout, ), that is “mull” with moderately acidic and “moder” with very acidic soil conditions, using data from Henneron, Chauvat, et al (). The 19 plots studied (0.36 ha in average) included three in aggradation mull, six in aggradation moder, four in biostatic mull and six in biostatic moder.…”

“…Forest floor (organic layer: O‐horizon) was sampled in spring 2013 within each of the three subplots by collecting the entire organic horizon within a 625‐cm 2 quadrat. Because understorey vegetation can sometimes represent a non‐negligible component of plant productivity and litterfall at the ecosystem scale in temperate forests (Gilliam, ) and its cover has previously been found to increase in response to thinning (Henneron, Chauvat, et al, ), we assessed the relative leaf litter production of understorey versus tree plant species. Oak, other tree species and understorey plant leaves located in the freshly fallen litter layer were thus sorted and counted in the field to estimate their proportion.…”

“…The potential mechanisms underlying this slower decomposition, such as the alteration of litter quality, microenvironment and soil decomposers, have been little studied. We recently found that the plasticity of Q. petraea to thinning can lead to a strong decline of its leaf litter quality and ex situ decomposability in temperate oak forests (Henneron, Chauvat, et al, ). Whether such plasticity in leaf litter traits of this dominant tree species could in turn affect in situ litter decomposition at the ecosystem level remains yet unknown.…”