From Leaf Chemistry to Microbial Crews: Tree Size Rewrites the Rules of Decomposition

Every autumn, temperate forests bury themselves under a fresh carpet of fallen leaves, and the pace at which that carpet decays helps set how fast carbon and nutrients cycle back into the ecosystem. For decades, ecologists have treated the controls on this process as broadly predictable: leaf chemistry, climate, and the decomposer community together determine how quickly litter breaks down. A new field experiment in a temperate forest, published in Plant and Soil, adds a surprising twist to that picture. The study shows that the size of the tree a leaf came from—measured simply by trunk diameter—can fundamentally reshuffle which of those controls matters most, shifting the dominant driver from the chemical makeup of the litter to the composition of the microbial community that colonizes it.

The research team, led by Songye Pei, Bing Li, Guangze Jin, and Zhili Liu of Northeast Forestry University in Harbin, China, focused on four dominant tree species in a temperate forest, dividing individuals of each species into two size classes based on diameter at breast height, the standard forestry measure taken at roughly 1.4 meters above the ground. Tree size is far more than a cosmetic variable. As trees grow from saplings to canopy giants, their leaf functional traits change systematically, and so do the communities of bacteria and fungi living on the leaf surface—the phyllosphere. What remained unknown until now was whether those size-driven changes carry through into the dead leaf, altering both the litter’s chemistry and its initial microbial passengers, and whether those changes then translate into measurable differences in decomposition on the forest floor.

To find out, the researchers collected freshly fallen leaf litter from small and large trees of the four species, ran a full chemical characterization of each litter type, and used high-throughput DNA sequencing to profile both the bacterial and fungal communities carried in the material. They then deployed the litter in a field decomposition experiment and, after six months on the forest floor, measured how much mass each litter batch had lost. Six months captures the early stage of decomposition, the phase when easily digested compounds disappear first and the recalcitrant skeleton of the leaf—lignin and structural carbohydrates—begins to dominate what remains.

The chemical results revealed a clear ontogenetic signature. As tree size increased, litter concentrations of total carbon, starch, the carbon-to-nitrogen ratio, and the lignin-to-nitrogen ratio all rose, while total nitrogen declined. In other words, leaves from larger trees were, on paper, lower-quality substrate: more carbon-rich, more nitrogen-poor, and proportionally more reinforced with lignin relative to nitrogen, a combination that classically predicts slower decay. Yet when the team measured actual mass loss, the opposite happened. Litter from large trees decomposed faster than litter from small trees, and the factor most closely tied to that acceleration was an unassuming one: starch. This labile, easily metabolized carbon reserve apparently gave microbial decomposers an immediate energy source, overriding the nominally poorer overall chemistry of the large-tree leaves.

The microbial data added a second layer to the story. Using sequencing-based community profiling, the researchers found that microbial community assembly in litter from larger trees was governed more strongly by deterministic processes—the non-random, environmentally driven sorting of species—than in litter from smaller trees, where stochastic processes play a comparatively larger role. Alongside that shift in assembly rules, larger-tree litter hosted altered community compositions, reduced fungal richness, and simpler co-occurrence networks for both bacteria and fungi. Network complexity, often read as a proxy for the number and strength of ecological interactions within a community, thus declined with tree size, suggesting that the decomposer assemblages arriving on large-tree litter were structurally different in kind, not just in species identity.

The most consequential finding emerged when the team formally partitioned the relative contributions of litter chemistry and microbial community traits to variation in decomposition rates. For litter from small trees, initial chemistry explained more of the variation in mass loss, with starch and cellulose standing out as the key chemical predictors. That is the textbook expectation: substrate quality rules early decay. But in litter from large trees, the importance of chemistry diminished, while the relative importance of the microbial community rose, with shifts in fungal community composition taking center stage. The primary control on decomposition, in effect, migrates from the resource to the decomposers as the source trees grow larger.

Why would fungal composition matter more for large-tree litter? The study’s authors connect this to the deterministic assembly and compositional shifts they documented. Fungi are the principal agents of lignin and cellulose degradation, so which fungal species establish first on a litter batch can strongly condition how the tougher structural fractions are attacked. If larger trees consistently export a distinct, more deterministically assembled fungal community with their litter, the identity of that community—not merely the amount of starch or cellulose in the leaf—becomes the variable that best predicts decay. The finding also underscores the role of labile carbon fractions such as starch in regulating the early stages of decomposition, a phase during which microbes are often carbon-limited and respond rapidly to accessible energy sources.

The broader implications reach into carbon cycle modeling and forest management. Global vegetation models typically represent litter decomposition as a function of climate and coarse litter quality indices, without accounting for tree size as a driver of either substrate chemistry or decomposer community structure. If the same size-dependent shift in controls holds across other forests, then stands dominated by large, mature trees may process their litter through fundamentally different microbial pathways than regenerating stands of the same species, altering rates of carbon return to the soil and nutrient availability for regeneration. The result also dovetails with a growing body of work on phyllosphere microbiomes, which recognizes that the leaf surface is not a passive waiting room for decomposers but an active inoculum that shapes what happens after leaf fall.

Methodologically, the study is notable for treating bacteria and fungi as parallel, equally tracked communities rather than folding all microbes into a single category, and for combining chemical assays of carbon fractions, lignin, cellulose, and nitrogen with network-based analyses of microbial co-occurrence. The raw sequencing data have been deposited in the Genome Sequence Archive under accession CRA030649, allowing other researchers to reanalyze the community profiles. The work was supported by the National Natural Science Foundation of China, the Heilongjiang Postdoctoral Fund, the China Postdoctoral Science Foundation, and the China Scholarship Council.

For now, the message is a humbling one for anyone who assumed decomposition could be predicted from leaf chemistry alone. The same species, growing in the same forest, can produce litter whose decay is governed by different rules depending simply on how big the parent tree was. As forests worldwide age, are harvested, or shift in size structure under management and climate change, the hidden hand of tree size on the microbial crews that recycle dead leaves may prove to be a variable that ecosystem science can no longer afford to ignore.

Subject of Research: How tree size alters litter chemistry and microbial communities to control leaf litter decomposition in temperate forests

Article Title: Tree size shifts the relative importance of litter chemical properties and microbial community traits to litter decomposition

Article References: Pei, S., Li, B., Jin, G., & Liu, Z. (2026). Tree size shifts the relative importance of litter chemical properties and microbial community traits to litter decomposition. Plant and Soil. https://doi.org/10.1007/s11104-026-09052-y

Image Credits: AI Generated

DOI: 10.1007/s11104-026-09052-y

Keywords: tree size, litter decomposition, litter chemistry, microbial community, fungal composition, phyllosphere, starch, temperate forest, community assembly, carbon cycling, high-throughput sequencing, Plant and Soil