Decomposition is a key ecosystem process that connects all trophic levels. Through the activity of decomposers and trophic transfer, nutrients like nitrogen and phosphorus are made available to primary producers and higher trophic levels (Campbell & Reece 2002; Wardle et al. 2004). Litter decomposition is controlled by three main factors: environmental conditions, the decomposer community and substrate quality (Pérez-Harguindeguy et al. 2000; Toledo Castanho & Adalardo de Oliveira 2008). Environmental conditions such as climate and soil have occasionally been found to be the best predictors of litter decomposition (Aerts 1997). However, Cornwell et al. (2008) showed in a global meta-analysis that the traits of plant species exert a dominant control over litter decomposition rates.
Both litter and fresh leaf traits have successfully been used to predict litter decomposition rate, and both sets of traits have advantages. Where litter traits shed light on the initial quality of decomposing leaves, fresh leaf traits are more closely linked to the plant’s growth strategy and are more widely available. Decomposition rate has been found to correlate positively with litter nitrogen (Kurokawa & Nakashizuka 2008; Parsons & Congdon 2008), phosphorus (Alvarez-Sánchez & Becerra Enríquez 1996; Cornwell et al. 2008; Parsons & Congdon 2008) and cation concentrations (Mg, K and Ca; Alvarez-Sánchez & Becerra Enríquez 1996), while it correlates negatively with molecules consisting of large carbon chains, such as lignin and cellulose (Vaieretti et al. 2005; Kurokawa & Nakashizuka 2008; Parsons & Congdon 2008).
Taking a step further away from the decomposition process, fresh leaf N, Mg, K and Ca concentration, and total base content (Cornelissen & Thompson 1996; Santiago 2007; Parsons & Congdon 2008) turned out to be good predictors of decomposition rate. Leaf nitrogen concentration (LNC) was better at predicting decomposition rate than litter nitrogen concentration (Cornwell et al. 2008). Physical leaf properties are also related to decomposition rate. Specific leaf area (SLA; leaf area divided by leaf dry mass) has a positive effect on litter decomposition rate (e.g. Cornelissen et al. 1999; Vaieretti et al. 2005; Santiago 2007; Kurokawa & Nakashizuka 2008), while leaf dry matter content (LDMC; ratio leaf dry : fresh mass; Kazakou et al. 2006; Cortez et al. 2007; Cornwell et al. 2008; Kurokawa & Nakashizuka 2008) and leaf toughness (Cornelissen & Thompson 1997; Cornelissen et al. 1999) have a negative effect.
There is no unambiguous answer to the question whether chemical or physical traits determine decomposition rate – and in fact, both groups are closely associated, because they are both the result of the plant’s strategy. Plants follow different physiological strategies that lead to roughly the same fitness levels for coexisting species. They produce either low-quality leaves at low energy costs, or high-quality leaves at high structural costs, thus showing a trade-off between either fast growth and high photosynthesis or slow growth and persistent, long-lived leaves (Wright et al. 2004; Poorter & Bongers 2006; Santiago 2007). This continuum is referred to as the leaf economics spectrum (LES). In tropical forests, for example, the LES ranges from slow-growing shade-tolerant tree species to fast-growing pioneer species with high light requirements for regeneration (Poorter & Bongers 2006).
The ‘economic’ value of a leaf influences its afterlife, because many of the physiological and protective features of green leaves persist through senescence and after shedding. For example, traits that make leaves resistant to physical damage and herbivores (such as high leaf toughness, LDMC and low SLA and nutrient concentrations) are at the same time effective barriers against soil decomposers. Leaf palatability and litter decomposition rates are therefore positively correlated (Grime et al. 1996).
So far, leaf economic value has never been related directly to litter decomposition rate, although several previous studies have shown the relevance of individual leaf traits associated with the LES to decomposability (Kazakou et al. 2006; Cortez et al. 2007; Fortunel et al. 2009). Also, for tropical rain forest trees, there is a striking lack of published studies on the relation between individual leaf traits and litter decomposition rate. Although some work has been done on the influence of environmental conditions like soil, climate, water availability and decomposer organisms on decomposition rate (e.g. Sherman 2003; Rueda-Delgado, Wantzen & Tolosa 2006; Powers et al. 2009), knowledge of the relation between leaf traits and decomposition rate is scarce in this part of the world (but see Santiago 2007, 2010; Kurokawa & Nakashizuka 2008).
Furthermore, no study has explicitly evaluated the consequences of land use change on leaf decomposition rates in the tropics. Human-induced changes on land use lead to major changes in plant community composition (Boyle & Boyle 1994; Huntley et al. 1997) and ecosystem processes (Chapin et al. 2000; Díaz & Cabido 2001) such as litter decomposition and nutrient cycling (Vitousek 1997; Vitousek et al. 1997). The relation between land use, green leaf traits of the plant community and decomposability has been shown for herbaceous communities across Europe (Fortunel et al. 2009), Chinese grasslands (Zheng et al. 2010), and Australian grasslands and forests (Dorrough & Scroggie 2008), but never for slash-and-burn agriculture and secondary forest succession in tropical ecosystems. Yet, it is important to observe changes in plant communities and ecosystem processes in the tropics, because especially in poor, largely rural tropical countries people rely directly on ecosystem services that plant communities provide, like food, shelter and water regulation (Díaz et al. 2006).
Here we present the results of a decomposition study with 23 plant species with different growth strategies from a range of common land use types in the moist tropics of lowland Bolivia. The following questions were addressed: (i) How are green leaf and litter traits associated? (ii) Which leaf and litter traits are good predictors of decomposition rate? (iii) How do leaf traits and decomposition rates differ between species that are typical for different land use types? We had the following corresponding hypotheses: (i) not only leaf traits but also litter traits show a slow–fast continuum, in line with the LES; (ii) litter traits are better predictors of decomposition than green leaf traits because they directly affect decomposers; P is a better predictor than N because tropical soils are P-limited, and not only individual leaf and litter traits predict decomposition rate, but also the position of a leaf on the LES; and (iii) leaf nutrient concentrations and decomposition rates are lowest for mature forest species, intermediate for secondary forest species, and highest for agricultural species.