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High species diversity in tropical forest canopies results from environmental filtering, biotic interactions, and neutral processes, played out over timescales of evolution and biogeographic migration. However, the functional variation among tropical forest canopies is often expressed in plant traits (Wright et al., 2006), which may or may not track patterns in species composition and diversity (Kraft et al., 2009). Among these traits, leaf chemicals regulate light capture, growth, respiration, longevity and defense. Leaf chemicals are also key mediators of ecosystem processes such as decomposition and nutrient cycling (Vitousek, 1984). But the relationship between phylogeny and leaf chemistry is only starting to be explored in humid tropical forests (Fyllas et al., 2009), and the relevance of high canopy diversity to canopy chemistry or ecosystem processes remains unclear (Townsend et al., 2008).
Leaf chemical variation occurs among many important elements and molecular compounds. Nitrogen (N), phosphorus (P), base cations (calcium (Ca), potassium (K), and magnesium (Mg)), and micronutrients (manganese (Mn), zinc (Zn), boron (B), and iron (Fe)) vary in concentration based on investments in processes ranging from CO2 fixation to protection against toxic metals (Johnson & Todd, 1983; Aber & Melillo, 1991). Polyphenols including tannins play a lead role in the defense against herbivores and other pests (Coley et al., 1993; Rothstein et al., 2004). Chlorophylls and carotenoids regulate light harvesting, while lightweight, soluble carbon fractions form the high-energy products of photosynthesis (Evans et al., 1988). Larger and heavier secondary metabolites including cellulose and lignin require greater energetic investment by plants, but yield increased leaf toughness, longevity, and defense capability (Hikosaka, 2004). This chemical portfolio is maintained in a structure of varying leaf mass per unit area (LMA) (Poorter et al., 2009).
Traditionally, studies of leaf chemical variation have focused on soil fertility and climate controls, which differentially influence concentrations of N, P, base cations, and other foliar constituents (e.g. Vitousek & Sanford, 1986; Raich et al., 1996; Aerts, 1997). Others have focused on chemical differences among plant functional types (PFTs) as a means to generalize patterns at large biogeographic scales (Bonan et al., 2002; Wright et al., 2004). However, recent studies have highlighted the potential importance of species-level diversity and taxonomic organization of several leaf chemical properties in tropical forest canopies. Townsend et al. (2007) reported that species exert a dominating influence on variation in leaf N and P concentrations in Costa Rican and Brazilian lowland forests. Hattenschwiler et al. (2008) found pronounced inter-specific variation in leaf and litter chemical properties in Guyana, noting that such high chemical diversity weakens the role of general PFT-based rules in predicting canopy function or ecosystem processes. Fyllas et al. (2009) and Asner et al. (2009) documented taxonomic organization among leaf chemicals, against a backdrop of varying climate and soils, in Amazonian and Australian tropical forest canopies, respectively. In a lowland Borneo rainforest, Paoli (2006) found a differential effect of environment and phylogeny on leaf chemical variation: within the genus Shorea, variation in leaf P and specific leaf area (LMA−1) was influenced more by soil fertility than was leaf N, which more closely tracked phylogeny. These and other studies give us a sense that leaf chemical attributes are indeed strongly influenced by species composition. However, the role of soil fertility and climate in mediating the connection between phylogeny and leaf chemical traits is not well understood.
A major barrier to linking canopy diversity and chemistry, and to understanding the role of this linkage to ecosystem processes, rests in measurement and tracking of the taxa at geographic scales commensurate with community dynamics and demographic change. Field measurements cannot easily resolve changes in forest canopy composition because the pertinent demographic dynamics occur at scales larger than most plots. In humid tropical forests, this limitation is evidenced by the fact that the spatial co-occurrence of species or even congeners is often relatively low and many singletons exist in a given plot (Condit et al., 2005). To understand the ecological importance of varying canopy composition, we need a way to observe and quantify plant traits that may indicate the presence of species and their functional role over relatively large areas. The potential observables are a challenge to identify, yet the regional perspective is proving critical to understanding ecological change for conservation and management decision-making.
Recent work demonstrates that remotely sensed optical spectroscopy provides a window into the composition and diversity of tropical forests. In Hawaiian forests, Asner et al. (2008) developed airborne spectroscopic signatures to identify native and invasive species, while Carlson et al. (2007) employed the concept of spectral variance to map canopy species richness. Castro-Esau et al. (2004) and Sanchez-Azofeifa et al. (2009) used leaf-level spectroscopy to delineate liana and tree species in Panamanian forests. Clark et al. (2005) used airborne spectroscopy to classify several canopy tree species in a Costa Rican forest. These and other studies provide novel links between spectral data and species information, but none have developed the general approach required to broadly understand the interconnection between canopy composition and spectroscopy. We believe that this interconnection can be made robustly and generally via the chemical properties of the canopies.
Asner & Martin (2009) introduced the concept of spectranomics to link a specific type of remote sensing – high-fidelity spectroscopy – of foliage to canopy taxa via their detailed chemical signatures. Variation in spectroscopic properties of canopies is determined by multiple molecular compounds ranging from pigments to secondary metabolites, along with variation in leaf area and volume, and canopy architecture (Curran, 1989; Asner, 1998). In highly foliated canopies of the humid tropics, leaf chemical traits are primary determinants over high-fidelity spectra (Asner, 2008; Asner & Martin, 2008). The spectranomics approach suggests that a spectral–chemical link would allow taxonomic analysis of tropical forest canopies from aircraft, yet community composition may be disconnected from the spectral-to-chemical linkages needed to indicate species presence and functional status. This disconnect may occur if intraspecific variation in chemical attributes is high and/or if phenotypic plasticity trumps phylogenetic patterns among chemical traits. Even if spectral measurements yield quantitative information on the chemical signatures of canopy foliage, closely related species may have similar chemical portfolios (phylogenetically conserved), making it difficult to differentiate taxa. To our knowledge, the link between community composition and spectral properties through chemical traits has not been broadly demonstrated.
We sought to integrate phylogenetic, chemical and spectral properties of canopies in a lowland tropical forest spanning low- and high-fertility soils in the Peruvian Amazon. Our goals were to determine the degree to which canopy chemical traits, and combinations of traits termed ‘chemical signatures’, are phylogenetically organized within and across contrasting soil types, and to assess which chemical constituents can be quantitatively linked to canopy spectroscopy. This study is the first major test of the spectranomics concept, carried out in a very high diversity forest. Here we present data on 21 leaf chemical traits and high-fidelity spectroscopic signatures of 594 individuals of tree, palm, vine and liana growth habits. We carefully controlled for full-sunlight, upper canopy position to ensure that the light environment was relatively constant, thereby avoiding unwanted variation in chemical traits resulting from shade, and because upper canopy foliage plays a dominant role in determining the spectroscopic remote sensing signatures of tropical forests (Asner, 2008).
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Fig. S1 Coefficients of variation (CVs) within species for leaf chemical properties and leaf mass per unit area (LMA) by soil type.
Methods S1 Leaf collections and mobile processing, chemical assays, leaf and canopy spectroscopy, and statistical analyses.
Table S1 List of laboratory assays conducted to develop the chemical portfolio for each sample
Table S2 Descriptive statistics for canopy leaf samples collected from less common growth habits
Table S3 Adjusted r2 values relating leaf properties to taxonomic family, genus and species on Ultisols, Inceptisols, and the two substrates combined
Table S4 Pearson correlation coefficients among leaf properties of all individuals
Table S5 Pearson correlation coefficients among leaf properties of individuals located on Inceptisols
Table S6 Pearson correlation coefficients among leaf properties of individuals located on Ultisols
Table S7 Chemical variables associated with stepwise linear discriminant analysis with taxa containing two or more representatives
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