Leaf-litter decomposition is a vital process in forest environments: this is how nutrients stored in leaves are recycled, returning to the soil and becoming available for the trees to take up again, thereby sustaining their growth and survival. Leaf litter is broken down by physical factors (i.e., dissolution by water), as well as by living organisms, such as soil microbes and insects, but the rate by which litter breaks down also depends in part on the characteristics of the litter itself.
A lot of research has revealed that the chemical make-up of litter influences how fast it breaks down and becomes incorporated into the forest floor, but not quite as much work has been done to look at the role of physical traits of leaves in this process. When we look at the chemical composition of leaf litter, do those differences we observe in chemistry reflect different physical structures and characteristics that influence how fast litter breaks down? Knowing more about this might help us better understand the mechanisms by which litter decomposes.
We also know now that many leaf traits vary together along a spectrum that on one end reflects trait values commonly associated with acquiring resources (e.g., high leaf nitrogen concentrations, greater specific area relative to mass (specific leaf area), and other trait values found in plants that tend to grow more quickly, producing nutrient-rich, short-lived leaves) and the other end reflecting trait values commonly associated with conserving resources (e.g., low leaf nitrogen concentrations, high leaf dry matter content, and other trait values often found in plants producing longer-lived, less nutrient-rich leaves). Several studies have shown that decomposition rates vary along this spectrum in fresh leaves. Do relationships among traits hold true in leaf litter, though? Can they be used to infer how fast litter will decompose?
We conducted our study to investigate how well different physical characteristics would predict how fast leaf litter breaks down, and whether leaf litter traits would be related to each other in a way that would produce a spectrum that we could use to infer how fast litter from different plant species would decompose.
We measured an assortment of physical and chemical traits in both fresh leaves and leaf litter of twelve different woody plant species native to British Columbia, Canada. We looked at broadleaf species such as paper birch (Betula papyrifera), red alder (Alnus rubra), and bigleaf maple (Acer macrophyllum), and coniferous species such lodgepole pine (Pinus contorta), Douglas fir (Pseudotsuga menziesii), and western hemlock (Tsuga heterophylla). We also included western larch (Larix occidentalis), a conifer that sheds its needles annually, and salal (Gaultheria shallon), an evergreen broadleaf shrub, which we thought would both likely lie in the middle of a spectrum produced by leaf traits. We measured chemical traits, such as percent nitrogen, pH, and carbohydrate concentrations, and included physical traits such as toughness, and water uptake. We also measured the thickness of the cuticle (waxy coating) of fresh leaves; to our knowledge, cuticle thickness has not been included in a study like this.
We then measured litter mass loss rates of these twelve species over one year, measuring changes in mass of litter we had placed in mesh bags (litter bags) that we placed on the forest floor. We found that almost all these species decomposed most quickly after the first three months and slowed after then, so we decided to look at the proportion of mass lost from 0 to 3 months, and then from 3 to 12 months.
We did find that the litter traits we measured varied with each other, creating a spectrum with high leaching loss, water uptake, concentration of water-soluble extractables, and specific leaf area on one end, and high values of thickness, toughness and concentration of acid-unhydrolyzable residues on the other; this spectrum may relate to the degree of leaching that the litter undergoes. We also found that cuticle thickness was correlated with mass loss during the first three months, suggesting that thicker cuticles might slow the rate of mass loss. In short, some of the physical traits we measured like cuticle thickness and toughness explained decomposition just as well as some of the chemical traits.
Cuticle thickness was strongly correlated with concentrations of acid-unhydrolyzable residues, which would include cutin (a major chemical building block of cuticles); perhaps the use chemical and physical traits in predicting decomposition rates reflect different ways to express the same phenomenon, but measuring physical traits can provide additional insights and help us create more hypotheses about how this litter actually breaks down that we can explore further.
The traits that were more strongly related to mass loss during the first three months were traits that could relate to leaching loss, such as litter water uptake, leaching loss, specific leaf area, and concentrations of water-soluble extractables. Litter traits related to mass loss from 3 to 12 months include toughness, specific leaf area, and a carbon-to-nitrogen ratio (C:N), with tough litter with a low specific leaf area and high C:N decomposing more slowly. Because we also found relationships with decomposition and fresh leaf nitrogen but not with litter nitrogen, we wonder if maybe the relationship between leaf nitrogen and decomposition reflects a relationship between the proportion of mesophyll tissue in leaves and decomposition. Maybe leaves with higher proportions of mesophyll, which contains nitrogen-rich chlorophyll molecules that break down during senescence (the process in which a tree absorbs nutrients from them leaves before dropping them as litter), as opposed to tougher tissues like leaf veins, decompose faster because they are more accessible to decomposers. This hypothesis, though, needs to be tested.
It’s important to note that our findings are observed patterns; they are not the result of an experiment testing the influence of these traits on decomposition. From our observations, we hypothesize that litter traits are related to each other and so a spectrum of values might help us predict decompositions rates. We also hypothesize that it may be worth looking further into the physical traits of leaf litter and their relationships with litter chemistry so that we can better understand the mechanisms by which litter breaks down and nutrients are recycled in forests.
These findings are described in the article entitled Relationships among leaf functional traits, litter traits, and mass loss during early phases of leaf litter decomposition in 12 woody plant species, recently published in the journal Oecologia. This work was conducted by