The Editor’s Choice paper from issue 102:1 of the Journal is The phenology–substrate-match hypothesis explains decomposition rates of evergreen and deciduous oak leaves by Pearse, Cobb & Karban. Read Associate Editor Rien Aerts’ commentary on the paper below.

Editor’s Choice 102:1

Litter decomposition is the major pathway of energy and biomass transfer in most terrestrial ecosystems and plays a key role in biogeochemical cycling.  Given its major importance in ecological processes, there is a vast literature on litter decomposition and its controls.  This started with the seminal papers of Waksman and Tenney (1928) and Tenney and Waksman (1929) on the chemical and climatic controls on litter decomposition. Surprisingly, not much further progress was made during the next 50 years. However, since the 1980’s an overwhelming number of papers has been published on the chemical controls on litter decomposition and on the effects of climate and of soil factors (both biotic and abiotic).

These studies showed that although both climate and substrate quality (litter species identity) are good predictors of litter decomposition at large spatial scales, they often fail to predict litter decomposition at small spatial scales. This is a serious problem as many decomposition studies seek to predict carbon and nutrient fluxes at those small scales and we need a proper mechanistic understanding of decomposition at small spatial scales to enable robust scaling –up to large scales in Earth System Models (ESMs) in a changing world.

It has become clear that much of the variation in litter decomposition at small scales is caused by spatial heterogeneity in the presence and abundance of guilds of decomposing organisms (bacteria, fungi, detritivores) and their interactions with particular litters. This was first described by Hunt et al. (1988) with their Home Field Advantage (HFA) hypothesis which postulates that at a particular spot there is a close match between the dominant litter type and the decomposers, leading to higher decomposition rates compared to a situation where allochtonous litter was introduced. Despite the logic behind this hypothesis, more recent studies showed that the HFA hypothesis often failed to predict litter decomposition rates and that it was very context dependent. These shortcomings have led Fréschet et al. (2012) to postulate the Substrate quality Matrix quality Interaction (SMI) hypothesis, which is in fact a generalization of the HFA hypothesis. The SMI hypothesis states that the match between the litter substrate and the soil matrix (the combination of the decomposers and the abiotic environment) affects the rate of litter decomposition. In contrast with the HFA, the SMI hypothesis can explain situations where there is a better match between allochtonous litter and the home site than between autochtonous litter and the home site, a situation which has been found in several studies.

In their very interesting paper, The phenology–substrate-match hypothesis explains decomposition rates of evergreen and deciduous oak leaves, Pearse and colleagues build forth on the HFA and SMI hypotheses by proposing that litter decomposition rates can be influenced by the timing of leaf senescence and fall, because of temporal changes in the composition of the decomposer community, which is in turn partly driven by changes in the quality of the soil matrix litter. Based on this, their Phenology-Substrate-Match (PSM) hypothesis predicts that a lagged match between litter type and soil matrix will result in an optimum decomposition environment. They conducted a decomposition experiment with litter of both a deciduous oak (leaves are shed in autumn) and an evergreen oak (leaves are shed in spring) in both autumn and spring. In agreement with their PSM hypothesis, they found that deciduous oak litter decomposed faster compared to evergreen litter when incubated in spring, and evergreen litter decomposed faster compared to deciduous litter when incubated in autumn.

Photo credit: Richard Cobb

Although the PSM hypothesis is so far only supported by a very limited dataset, the idea behind this hypothesis is very appealing. Moreover, the temporal context of the match between litter substrate and decomposer community has been largely overlooked so far and the present theory is an important addition to our understanding of the controls on litter decomposition. It also urges us to seriously consider the timing of the start of decomposition studies and to do this in agreement with the actual timing of litter fall of the studied species. From this paper it is clear that this can really make a difference. The PSM hypothesis also strongly suggests that in a rapidly warming world, where the timing of leaf senescence and litter fall is changing, this may have serious repercussions for the phenological match between decomposers and their substrate.  So, in my opinion, this paper is a ‘must read’ for all ecologists interested in the controls on litter decomposition rates.

Rien Aerts
Associate Editor, Journal of Ecology

Fréschet, G.T., Aerts, R. & Cornelissen, J.H.C. (2012) Multiple mechanisms for trait effects on litter decomposition: moving beyond home-field advantage with a new hypothesis. Journal of Ecology, 100, 619-630.

Hunt, H.W., Ingham, E.R., Coleman, D.C., Elliot, E.T. & Reid, C.P.P. (1988) Nitrogen limitation of production and decomposition in prairie, mountain meadow, and pine forest. Ecology, 69, 1009-1016.

Tenney, F. G. & Waksman, S. A. (1929) Composition of natural organic materials and their decomposition in the soil: IV. The nature and rapidity of decomposition of the various organic complexes in different plant materials, under aerobic conditions. Soil Science,  28, 55-84.

Waksman, S. A. & Tenney, F. G. (1928) Composition of natural organic materials and , their decomposition in the soil: III. The influence of nature of plant upon the rapidity of its decomposition. Soil Science, 26, 155-171.