Plant litter decomposition, a major driver of carbon and nutrient cycling in terrestrial and freshwater ecosystems, controls the provision of fundamental ecosystem services such as soil formation, nutrient availability and atmospheric composition, with feedback to vegetation composition. One major aim of ecology is to model how functional features of vegetations differing in species composition feed back to soil carbon turnover, and thereby atmospheric chemistry and climate, in different biomes (Sitch et al. 2003; Cornwell et al. 2009). Here, we make a leap forward towards this aim by taking an explicit whole-plant functional approach to assessing litter decomposition rates, more specifically by linking the ‘plant economics spectrum’ (PES, Freschet et al. 2010a) to litter decomposability.
Whilst leaf litter decomposition rates are strongly determined by climate (Berg et al. 1993; Parton et al. 2007) and community composition of soil organisms (Lavelle et al. 2006), litter quality (‘species identity’) is their predominant driver within biomes (Cornwell et al. 2008). Structural and chemical leaf traits have ‘afterlife’ effects on litter decomposability (Cornelissen et al. 2004). Indeed, interspecific variation in traits of fresh leaves and that of leaf litter tends to be strongly correlated (e.g. Freschet et al. 2010b). Thus, lignin content (Meentemeyer 1978), physical toughness (Pérez-Harguindeguy et al. 2000), polyphenol content (Coq et al. 2010) or specific leaf area and dry matter content (DMC; dry to water-saturated weight ratio) (Garnier et al. 2004; Kazakou et al. 2006) can substantially affect leaf decomposition rates. Also, the high nutrient requirement of decomposer organisms creates nutrient-limited conditions for decomposition processes (Enríquez, Duarte & Sand-Jensen 1993). Thus, nitrogen (N), phosphorus (P) and calcium (Ca) contents ((Enríquez, Duarte & Sand-Jensen 1993; Aerts 1997) and pH (as a proxy for basic cation content and antimicrobial organic acids; Cornelissen et al. 2006) are usually significant predictors of leaf litter decomposition rates.
Most litter turnover studies linking vegetation composition to decomposition have focused on the relation between leaf traits and leaf litter decomposability. Some evidence exists that interspecific variation in litter quality is also the predominant driver of root litter decomposition (Silver & Miya 2001), and the huge range in wood functional trait values (Chave et al. 2009) and within-site wood decomposition rates (Harmon et al. 1995; van Geffen et al. 2010) suggests that similar pattern may exist for plant stems too. Indeed, several chemical traits related to leaf decomposition, such as N, Ca and lignin concentrations, also impact root decomposition, (Silver & Miya 2001; Vivanco & Austin 2006), whereas N, P and tissue density affect stem decomposition (Chambers et al. 2000; Weedon et al. 2009). However, with only few studies available on interspecific variation in stem and root decomposability, we still do not know whether the traits underpinning decomposition rates, or their relative contributions, have the same effect across plant parts. Whilst it seems likely that the same traits will have broadly similar effects on litter decomposition of distinct plant organs, differences in the magnitude of their impact are likely. For instance, whilst litter N or P contents are major determinants of colonization–degradation by soil organisms (Cornwell et al. 2008), whether these nutrient pools are active (e.g. in enzymes for leaf photosynthesis, root adsorptive capacity) or passive (e.g. stem or root storage, recalcitrant defence compounds) will partly determine their chemical form after senescence and thereby modulate their availability to decomposers. Traits related to physical support functions, expressed in plant allometric relationships including organ sizes, may play important roles too; for instance, tree trunk diameter predicted variation in decomposition rates amongst 15 Bolivian tree species (van Geffen et al. 2010). Any such differences in organ function, as expressed in structural and physiological differences, might cause shifts in trait–decomposability relationships between organs.
Empirical evidence is growing that plant species possess integrated strategies across their organs with regards to C and nutrient economy (Freschet et al. 2010a), which are moreover robust to geographical scaling (Kerkhoff et al. 2006; Liu et al. 2010). In other words, (i) each vegetative plant organ (leaves, twigs, main stems, coarse roots, fine roots) seems to obey a fundamental trade-off between traits inferring rapid resource acquisition and traits leading to resource conservation, owing to direct and indirect mutual dependencies between these traits (Reich et al. 2003; Chave et al. 2009; Elser et al. 2010) and (ii) those traits seem to be generally coordinated across vegetative organs, likely due to plant physiological, ontogenetic and allometric constraints (Niklas & Enquist 2002; Wright et al. 2006; but see Baraloto et al. 2010 for decoupling of coarse wood and leaf traits amongst tropical trees). As over half the global variance in these traits is at local to plant community scale (Wright et al. 2004; Freschet et al. 2011a), many contrasting plant economic strategies are already found within a local flora (e.g. Freschet et al. 2010a).
Assuming that consistent traits and economic strategies drive decomposition across plant organs, decomposability is probably coordinated across organs too. This would indicate tight control of plant economics on the carbon and nutrient turnover in ecosystems.
At present, however, there is very little empirical evidence for this. Wang, Liu & Mo (2010) observed significant correlation between leaf and root decomposition of four tree species only, whilst Hobbie et al. (2010) did not find any consistent relationship for eleven tree species. Here, we present the first empirical evidence of a clear association between the PES and litter decomposability, in a comprehensive multispecies, multi-organ decomposition study covering a wide range of subarctic plant functional types and aquatic, riparian and terrestrial habitats. More precisely, we test the hypotheses that (i) the decomposability of distinct plant organs (leaves, fine stems, coarse stems, fine roots and reproductive parts) is controlled by the same structure-related (lignin, DMC, C) and chemical (N, P, phenolic contents, pH) traits, although the relative influence of these traits might shift across organs. Considering the strong trait coordination found across leaves, stems and roots in a subarctic flora (Freschet et al. 2010a), we further hypothesize that (ii) interspecific variation in litter decomposability is coordinated across plant vegetative organs. Finally, assuming the validity of the two previous hypotheses, we test that (iii) the locally operating PES is a good predictor of the decomposability of vegetative plant organs.