Life after death: the role of litter in ecosystems.

Edited by Ken Thompson

Plant organs die, and ultimately whole plants die, but dead plant material, or litter, continues to have powerful effects on ecosystems, driving nutrient turnover, soil formation and atmospheric composition. Soil properties in turn have strong impacts on plant community composition, diversity and productivity. This feedback system is commonly interpreted in terms of the leaf economics spectrum (Wright et al. 2004), the major axis of plant evolutionary trade-offs, from fast-growing, nutrient-acquisitive species of fertile soils, with palatable, rapidly decomposing leaves and litter at one extreme, to slow-growing, nutrient-conservative species of infertile soils, with unpalatable, tough leaves and litter at the other. Our understanding of the leaf economics spectrum, and specifically its relationship to litter decomposability, comes mainly from temperate regions, but Bakker, Carreno-Rocabado & Poorter (2010) recently confirmed that it is also a good predictor of decomposition in a range of plants from Bolivia: litter from tough, high-density, low-nutrient leaves decomposed faster than that from cheap, productive, high-nutrient leaves. There was also a powerful effect of land use, with species of agricultural land being at the ‘fast’ end of the spectrum, with rapidly-decomposing litter. Species of primary and secondary forest did not differ in leaf traits or in decomposability.

It is assumed that the leaf economics spectrum is merely one aspect of a whole-plant economics spectrum (Grime et al. 1997). In other words, that consistent patterns of traits and economic strategies apply not only to leaves, but also to roots, stems and flowers. Recently Freschet, Aerts & Cornelissen (2011) conducted the first large multi-organ test of this idea, using 40 species from the subarctic flora around Abisko Research Station in northern Sweden. The results confirm a very high level of consistency between the decomposability of all plant organs, with some measure of physical toughness (lignin, dry matter content, or carbon content) being the best predictor of decomposition across all organs. Different organs varied in the intercept, rather than the slope, of trait-decomposability regressions; at a common lignin content of 20 %, leaves decomposed 1.6 times faster than fine stems, 2.1 times faster than fine roots and 3.1 times faster than coarse stems. These differences probably reflect both morphological and chemical differences between organs, although surprisingly nitrogen content was a good predictor of decomposition only in leaves. The suggested explanation is that N content is only a major control on decomposition of organs (i.e. leaves) that are relatively high in N; if decomposers have to rely on exogenous N, litter toughness exerts the major control.

A priori assignment of plant species to functional groups is not necessarily useful for predicting key functional traits related to decomposition. In a study of litter decomposability for 41 co-occurring woody species from a New Zealand floodplain, including native and non-native N-fixers, and native and non-native non N-fixers, Kurokawa, Peltzer & Wardle (2010) found that decomposition is driven by a small number of key traits, such as C : P ratio and total phenolic concentrations. Invasive status (unsurprisingly) and N fixation ability (perhaps more surprisingly) did not help to predict decomposability of litter.

The leaf economics spectrum undoubtedly represents the main evolutionary trade-off in plants, but as with all generalisations, it also conceals much interesting detail. For example, although litter tends to be described as either ‘nutrient-rich’ or ‘nutrient-poor’, what is the effect of N:P ratio? Güsewell and Gessner (2009) tackled this question by looking at the decomposition of cellulose doped with different N:P ratios and litter of Carex spp. grown at different ratios. They found that decomposition tended to be N-limited at low ratios and P-limited at high ratios, but also depended on total nutrient supply. Fungi and bacteria also responded differently to N:P ratios, with fungi becoming P-limited at higher N:P ratios than bacteria. An interesting finding was that as long as P was not in short supply and conditions were conducive to bacterial growth, the bacterial community was dominated by N-fixing species. Under these conditions, treatments with high C:N and low N:P ratio ended up with N levels exceeding the amount originally added.

Another trait that is perhaps not strictly part of the leaf economics spectrum, but is nevertheless strongly linked to it, is leaf pH. Low-pH leaves, either because of high concentrations of organic acids or low levels of base cations, or both, tend to produce acidic litter that decomposes particularly slowly. But is leaf pH a genuine species trait, or is it influenced by soil pH? Cornelissen et al. (2011) have shown that, at least in a collection of 23 temperate herbs, leaf pH is a highly conservative species trait, scarcely affected at all by soil chemistry. They suggest that leaf pH, although not commonly found in existing trait databases, is cheap and easy to measure and has a useful role to play in predicting the biogeochemistry of ecosystems.

Although litter ‘quality’ and climate are the major determinants of decomposition, there is evidence that, to some extent, the microbial community can adapt to different litter types. Strickland et al. (2009) found that the microbial community from a forest habitat perceived ‘low quality’ Rhododendron litter to be of higher quality than the microbes from a grass-dominated community. This was a highly controlled laboratory study, and an interesting question is how far such effects operate in the field.

Two recent Functional Ecology papers dealt with fungal endophytes present in litter. Infection by Neotyphodium occultans had complex effects on recruitment in the annual grass Lolium multiflorum (Omacini et al. 2009). Lolium seedlings emerge through a dense layer of litter of the previous generation, and endophyte-infected litter delayed emergence and reduced the number of emerging seedlings. At the same time, seedlings from infected seeds performed better than uninfected seedlings. Omacini et al. suggest that infected plants may enjoy a relative advantage over non-infected conspecifics, in terms of holding the ground occupied by their predecessors, even if the litter deposited by those predecessors creates generally poorer recruitment microsites. The actual mechanisms of the impacts of endophytes remain obscure, and a paper by Siegrist et al. (2010) only serves to deepen the mystery. In the tall fescue (Schedonorus arundinaceus) – fungal endophyte (Neotyphodium coenophialum) system, endophytes in litter or in the incubation microenvironment both slow the rate of decomposition. Alkaloids produced by endophytes are often invoked as the cause of such effects, but Siegrist et al. showed that alkaloid levels in litter are very low. Furthermore, when decomposition of fresh, alkaloid-laden leaves was compared with (alkaloid-free) litter, alkaloids were quickly lost from the fresh leaves, and differences in decomposition were only slight. We clearly still have a lot to learn about the effects of endophytes on decomposition (and on other ecological processes).

Given its pivotal role in nutrient cycling and soil formation, it’s not surprising that the quantity and composition of litter have important consequences for ecosystems. Tropical forest litter is home to much of the world’s biodiversity, so what determines how much there is? Litter depth varied 16-fold across a sample of 28 lowland forest stands in Peru and Panama, and was very closely correlated with C:P ratio of the litter, consistent with litter breakdown being limited by P; essentially, less phosphorus equals more litter (Kaspari & Yanoviak 2008). Thus, since litter is not only habitat for soil animals, but also food, this means ‘more food, less habitat’. Litter depth may also act as a negative feedback mechanism limiting leaching of P.

Succession is normally associated with increasing biomass and productivity, but in the prolonged absence of further disturbance retrogressive succession takes place, in which productivity, nutrient availability, litter quality and decomposition rates all diminish. The question asked by Wardle et al. (2009) is whether these changes are driven by species turnover or variation within species. A study of retrogressive successions in New Zealand, Hawaii, Sweden, Alaska and Australia found that although within-species variation was important in some instances, between-species variation was a consistently important ecological driver, suggesting that decomposition processes are most likely to be highly responsive to gradients of soil fertility when those gradients are accompanied by significant species turnover. The synchronized flowering and dieback of the native bamboo, Chusquea culeou, over more than 200,000 ha in Patagonia in 2001 illustrates the key role of litter in ecosystem functioning (Austin & Marchesini 2011). Bamboo litter decomposes much more slowly than tree litter, and the massive input of recalcitrant litter reduced both soil mineral nitrogen and net nitrogen mineralisation.

How is litter decomposition affected by mixing litter from two or more species? De Oliveira, Haettenschwiler & Handa (2010), in a study of decomposition in four Mediterranean woody species, found no significant non-additive effects on litter mass loss in the absence of animals. There were significant non-additive effects of litter mixing on mass loss in the presence of the gastropod Pomatias elegans and/or the diplopod Glomeris marginata, but these were complex and sometimes positive, sometimes negative. Bruder, Chauvet & Gessner (2011), studied litter decomposition in an intermittent stream in the Pyrenees in south-western France. They expected that during dry periods, recalcitrant (oak) leaf litter with high waterholding capacity would allow decomposition to continue in less recalcitrant (alder) litter when both litter types were mixed, leading to a positive litter diversity effect on decomposition, but no such effect was detected. Scherer-Lorenzen (2008) buried both litter mixtures and experimental materials (wood and cotton) in experimental plant communities of varying species and functional-group richness. He found significant positive effects of functional-group richness on decomposition, but these effects were strongly influenced by a large positive effect of legumes, i.e. a sampling effect. Wardle et al. (2009) also looked at litter mixing in their retrogressive successions, and found extremely mixed results, from uniformly positive to uniformly negative interactions in different chronosequences, while the sign of the interaction varied according to site age in others.

In short, the question of the importance of litter diversity in decomposition, especially under natural conditions, remains unresolved. On the other hand, the key role of litter quality in decomposition and in ecosystem functioning generally seems clear, although much important detail remains to be discovered.

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Austin, A.T. & Marchesini, V.A. (2011) Gregarious flowering and death of understorey bamboo slow litter decomposition and nitrogen turnover in a southern temperate forest in Patagonia, Argentina. Functional Ecology.
Bakker, M.A., Carreno-Rocabado, G., & Poorter, L. (2010) Leaf economics traits predict litter decomposition of tropical plants and differ among land use types. Functional Ecology 25, 473-483.
Bruder, A., Chauvet, E., & Gessner, M.O. (2011) Litter diversity, fungal decomposers and litter decomposition under simulated stream intermittency. Functional Ecology 25, 1269-1277.
Cornelissen, J.H.C., Sibma, F., Van Logtestijn, R.S.P., Broekman, R.A., & Thompson, K. (2011) Leaf pH as a plant trait: species-driven rather than soil-driven variation. Functional Ecology 25, 449-455.
De Oliveira, T., Haettenschwiler, S., & Handa, I.T. (2010) Snail and millipede complementarity in decomposing Mediterranean forest leaf litter mixtures. Functional Ecology 24, 937-946.
Freschet, G.g.T., Aerts, R., & Cornelissen, J.H.C. (2011) A plant economics spectrum of litter decomposability. Functional Ecology.
Grime, J.P., Thompson, K., Hunt, R., et al. (1997) Integrated screening validates primary axes of specialisation in plants. Oikos 79, 259-281.
Guesewell, S. & Gessner, M.O. (2009) N : P ratios influence litter decomposition and colonization by fungi and bacteria in microcosms. Functional Ecology 23, 211-219.
Kaspari, M. & Yanoviak, S.P. (2008) Biogeography of litter depth in tropical forests: evaluating the phosphorus growth rate hypothesis. Functional Ecology 22, 919-923.
Kurokawa, H., Peltzer, D.A., & Wardle, D.A. (2010) Plant traits, leaf palatability and litter decomposability for co-occurring woody species differing in invasion status and nitrogen fixation ability. Functional Ecology 24, 513-523.
Omacini, M., Chaneton, E.J., Bush, L., & Ghersa, C.M. (2009) A fungal endosymbiont affects host plant recruitment through seed- and litter-mediated mechanisms. Functional Ecology 23, 1148-1156.
Scherer-Lorenzen, M. (2008) Functional diversity affects decomposition processes in experimental grasslands. Functional Ecology 22, 547-555.
Siegrist, J.A., McCulley, R.L., Bush, L.P., & Phillips, T.D. (2010) Alkaloids may not be responsible for endophyte-associated reductions in tall fescue decomposition rates. Functional Ecology 24, 460-468.
Strickland, M.S., Osburn, E., Lauber, C., Fierer, N., & Bradford, M.A. (2009) Litter quality is in the eye of the beholder: initial decomposition rates as a function of inoculum characteristics. Functional Ecology 23, 627-636.
Wardle, D.A., Bardgett, R.D., Walker, L.R., & Bonner, K.I. (2009) Among- and within-species variation in plant litter decomposition in contrasting long-term chronosequences. Functional Ecology 23, 442-453.
Wright, I.J., Reich, P.B., Westoby, M., et al. (2004) The worldwide leaf economics spectrum. Nature 428, 821-827.

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