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Keywords:

  • community-weighted traits;
  • effect trait;
  • intraspecific variation;
  • leaf economics spectrum;
  • litter decomposition;
  • plant–soil (below-ground) interactions;
  • response trait;
  • soil fertility gradients;
  • temperate rain forest;
  • trait plasticity

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information
  1. Despite recent progress in characterizing the within-species variability (WSV) of plant functional traits, the importance of this WSV in driving ecological processes such as leaf litter decomposability within species or at the whole community level is poorly understood.
  2. We ask whether leaf and litter functional traits vary within species to form a spectrum of variability analogous to the leaf economics spectrum that occurs among species. We also ask whether this spectrum of trait variation within species is an important driver of leaf litter decomposability. To address these questions, we quantified both WSV and between-species variation of leaf and litter traits and litter decomposability of 16 co-occurring temperate rain forest plant species along soil toposequences characterized by strong shifts in soil nutrient status in New Zealand.
  3. We found considerable WSV of both leaf and litter traits for all species, and a within-species spectrum of coordinated trait variation for 11 species. The WSV of leaf and to a lesser extent foliar litter C to N and C to P values were often strongly related to soil C to N and C to P ratios across plots. Further, in many cases, WSV and its covariation with species turnover contributed significantly to the community-level aggregate trait response to variation in soil fertility.
  4. Contrary to our expectations, the WSV in leaf and litter traits did not generally predict within-species variation in leaf litter mass loss, nor N and P release, during decomposition. Further, inclusion of WSV did not improve predictions of leaf litter decomposability using community-level trait measures.
  5. Synthesis. Our findings support the view that WSV of plant functional traits is an important component of plant community responses to environmental factors such as soil fertility. However, the apparent decoupling of WSV of leaf economic traits from WSV of ecological processes such as litter decomposability suggests that consideration of WSV may not be necessary to understand the contributions of trait variation to determining the breakdown of plant litter and therefore, potentially, ecosystem processes.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Plant functional traits are indicators of both plant community responses to environmental factors and their effects on processes contributing to ecosystem functioning (Lavorel & Garnier 2002) and can thus be used to predict the likely impacts of global change drivers on terrestrial ecosystems (Suding et al. 2008). Until recently, literature on plant functional traits focused on differences in mean values of traits among individual species, and therefore primarily considered between-species variability (BSV) (e.g. Grime 2001; Diaz et al. 2004). Further, regional and global trait data sets have been used to identify the leaf economic spectrum (LES) across species (Wright et al. 2004; see also Grime et al. 1997; Diaz et al. 2004), which represents a major axis of specialization along which vascular plant species trade off rapid resource acquisition against resource conservation. Although analyses of mean trait values for species have provided insights into many ecological patterns, the quantification and understanding of within-species variation (WSV) is less well developed. However, there is a rapidly emerging view that quantification of WSV is necessary for understanding the ecological role of functional traits as drivers of processes occurring at the community level (Messier, McGill & Lechowicz 2010; Albert et al. 2011; Violle et al. 2012).

Recent literature suggests that WSV provides additional insights into trait-based measures of functional diversity (Albert et al. 2012), community assembly (Jung et al. 2010) and community responses to environmental conditions (Albert et al. 2010b; Lepš et al. 2011). However, the importance of WSV in influencing the relationships between functional traits and processes driven by these traits, such as litter decomposability, are poorly understood. In two recent studies, community-weighted mean values of effect traits have been shown to be relatively insensitive to WSV (Lavorel et al. 2008; Albert et al. 2012). This has led to the prediction that WSV will not contribute substantially to how trait variation among plant communities contributes to ecosystem processes (Albert et al. 2011), although there is a dearth of formal tests of this idea. Nonetheless, significant WSV in litter decomposibility has been observed among plant genotypes (Schweitzer et al. 2004; LeRoy et al. 2007; Crutsinger, Sanders & Classen 2009), geographically distinct populations (Lecerf & Chauvet 2008) and plant populations from high versus low fertility sites (Sariyildiz & Anderson 2003; Wardle et al. 2009). These studies have considered WSV of only one or few species, and no studies have evaluated whether WSV in leaf or litter traits at the whole plant community level drives decomposability in the manner shown at the across-species level for multiple species (e.g. Santiago 2007). Furthermore, although several functional traits are potentially able to predict decomposition across sites at the whole community level (Fortunel et al. 2009; Sundqvist, Giesler & Wardle 2011; Lagerström, Nilsson & Wardle 2013), the relative contributions of WSV versus BSV are untested. Here, we apply recently developed methodological and analytical approaches (e.g. Albert et al. 2010b; de Bello et al. 2011; Lepš et al. 2011; Violle et al. 2012) to determine the relative contributions of WSV and BSV of leaf and litter traits in driving litter decomposability.

In the present study, we address two questions. Firstly, at the within-species level, is there a spectrum of trait covariation representing a trade-off between resource acquisition and conservation analogous to the LES at the among-species level? Secondly, does this trait spectrum govern variation in decomposability within species in the same manner as has been shown across species (e.g. Santiago 2007)? To address these questions, we compared WSV with BSV for leaf and litter functional traits and leaf litter decomposability for each of 16 common co-occurring temperate rain forest species across 32 sites differing in relative nutrient availability in the Westland region of New Zealand. We hypothesized that (i) a spectrum of trait variability occurs within individual species that represents a shift from resource conservation to resource acquisition strategies as soil fertility increases, and which is equivalent to the LES among species, (ii) within individual species, leaf litter decomposability can be predicted by this spectrum of trait variability in a manner similar to that observed across species; and (iii) WSV of leaf and litter traits is sufficiently large relative to BSV to contribute significantly to relationships between soil nutrient status and community-level trait measures and to relationships between community-level trait measures and community-level measures of litter decomposability. By addressing these hypotheses, we evaluate the importance of WSV in functional traits for understanding relationships between soil fertility, plant trait spectra and litter decomposability.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Sites and plot selection and vegetation surveys

Sampling was conducted in the Westland region of the South Island of New Zealand between 42°29′ S and 43°21′ S and between 169°52′ E and 171°23′ E. The region has a wet temperate climate with an annual average temperature of 11 °C (Hessell 1982) and an average annual rainfall ranging from 3.5 m to 8.0 m. There is considerable spatial variability of basal substrate in this system; the main substrate is schist derived from glacial outwashes, other basal substrates also commonly occur including greywacke, granite and limestone. Landform or topography is also highly variable and is a major driver of soil nutrient status in these types of forests (Richardson, Allen & Doherty 2008). Vegetation was sampled from 32 unbounded plots, each between 0.25 and 0.5 ha, and located within a uniform vegetation type, geological substrate and landform. All plots were situated within broadleaf-conifer temperate rain forests that are characterized by a mixed canopy of evergreen angiosperms and emergent podocarps (Wardle 1991), with plots selected to represent the range of dominant basal soil substrates and landforms occurring in the region. For each of four basal soil substrates, namely greywacke, schist, granite and limestone, two plots were selected for each of four landforms as follows: ridge crests, slope faces, gullies and terraces (i.e. 4 basal substrates × 4 landforms × 2 plots). Previous work has demonstrated that these soil toposequences represent strong fertility gradients, from low fertility on ridges to high fertility in gullies (Richardson, Allen & Doherty 2008). Significantly disturbed vegetation was avoided. We measured the diameter at breast height (dbh, 135 cm) of all stems > 2 cm dbh and identified each stem to species within a 20 × 10 m subplot centred within each unbounded plot. In general, tree and understorey species in these forests are characterized as annual evergreens (i.e. having leaves persisting about 1 year but senescence occurs throughout the year) and are relatively slow growing (Wardle 2012). Few direct measurements of tree or leaf longevity have been made on these species, but the evidence from a previous study in the region indicates that leaf turnover is not strongly linked to soil fertility (Richardson et al. 2010). Reliable data on other life-history traits for most of these species are lacking (but see McGlone, Richardson & Jordan 2010 for tree and sapling heights).

Soil sampling

Soil sampling was conducted from November 2009 to March 2010. To characterize the differences in soil chemical properties across plots, 3–5 soil cores, each 10 cm in diameter and of sufficient depth to ensure that the top 10 cm of mineral soil was included, were taken randomly from within each 10 × 20 m subplot. From each core, the organic and mineral soil layers were separated, and the top 10 cm of each layer was retained and pooled across cores; the litter layer and mosses and lichens were excluded. Where the organic layer was < 10 cm thick, the entire organic layer was retained, and for one gully plot, the organic layer was entirely absent, so only mineral soil was sampled. The mineral and organic soils were sieved to 4 mm before subsampling for analysis. Organic and mineral soil samples from each plot were analysed separately following standard procedures through the Landcare Research laboratory for environmental chemistry in Palmerston North, New Zealand (Blakemore, Searle & Daly 1987). Soils were analysed for pH (in water), total C and N (FP2000 CN analyzer; LECO Corp., St Joseph, MI, USA), mineral N (NO3 and NH4+ extracted in 100 mL 2 M KCl; quantified by colorimetry), total P (Kjeldahl acid digestion; quantified by colorimetry), available P (Olsen extraction; quantified by colorimetry), exchangeable bases (Ca, Mg K and Na; atomic absorbtion spectrometry from a 1M ammonium acetate extract), cation exchange capacity and base saturation (NH4+ ions displaced by 1M molar NaCl; quantified by colorimetry).

Leaf and litter sampling and trait measurements

All plant material was collected between November 2008 and March 2009. For each plot, leaf and litter material were collected from at least 5 mature individuals for each of 16 commonly occurring plant species in the region (Table S1 in Supporting Information). These species included 13 angiosperm trees and three fern species, two of which are tree ferns. From each individual, fully emerged leaves were collected; sunlight leaves for canopy species and the leaves of subcanopy and ground layer species were collected from the highest light environments in which they occurred. The primary photosynthetic units of the fern species were selected as structures equivalent to leaves, that is, pinnules from the tree ferns and pinnae from the fern Blechnum discolor. Leaf material was collected using an orchid pruner or, where necessary, a shotgun. To avoid water loss from leaf material during transport, whole branchlets, fronds or pinnae were sampled with leaves or leaf equivalents still attached. Leaf material was immediately sealed in plastic bags and pooled by species within each plot. All leaf material was transported in ice-cooled boxes and stored in the laboratory at 4 °C prior to analysis. Fresh senescent leaf litter (i.e. the most recently fallen litter on the soil surface in a physically intact form and without signs of fragmentation) for each target species within each plot was collected from underneath the same individuals as those from which the foliage was collected. In total, 345 leaf and litter samples were taken, with each species occurring on between 9 and 30 of the 32 plots (median = 22 plots).

The leaf and litter traits we measured were selected because they are known to be part of, or closely linked to, the LES; these include foliar and litter C, N and P, specific leaf area (SLA) and leaf dry matter content (LDMC). As such, we expect that they reflect the trade-off between resource acquisition and conservation, with concentrations of N and P, and SLA increasing, and concentrations of C and LDMC decreasing, with increasing soil fertility (see Diaz et al. 2004; Wright et al. 2004). Once in the laboratory, leaves, pinnae and pinnules were excised from their parent material and representative subsamples were composed, consisting of a minimum of 10 leaves. Green leaf and litter subsamples from each species from each plot were measured for total C, N and P. Subsamples were oven-dried at 65 °C for 48 h, and the concentrations of C were quantified by dry combustion; those of N and P were measured by the Kjeldahl acid digestion method. Further, for each green leaf sample (petioles included), leaf area and fresh weight were determined for 20–35 fresh leaves; these samples were then oven-dried to constant mass for 48 h and weighed again. These values were used to determine LDMC as the ratio of dry weight to fresh weight (mg g−1) and SLA as the ratio of leaf area to dry weight (m2 kg−1) as described by Cornelissen et al. (2003).

Decomposition assay

The decomposability of leaf litter for each species from each plot was determined using a standardized laboratory bioassay (Wardle, Bonner & Barker 2002; Wardle et al. 2009; Jackson, Peltzer & Wardle 2013). For each species from each plot, 3.9-cm-diameter Petri dishes were each two-thirds filled with a standardized humus substrate (1.97% N, pH 3.7; collected from a Metrosideros umbellataWeinmannia racemosa-dominated forest near Otira, New Zealand; 42°50′ S, 171°37′ E) and amended to 300% moisture content (dry-weight basis); a disc of nylon mesh with 1 mm holes was placed on the humus surface. A subsample of leaf litter (1 g, air-dried), cut into 2 cm2 fragments (when individual leaves were larger than that), was placed on the surface of the mesh of each Petri dish; the dish was then sealed with tape to minimize water loss and incubated for 4 months at ~22 °C. Following this incubation period, all remaining leaf litter was removed from the Petri dish, picked or brushed clean of soil particles and fungal hyphae and oven-dried at 65 °C for 48 h before measurement of the remaining dry mass. A subsample of the remaining (undecomposed) material for each litter was then analysed for N and P concentration as described above. Decomposition was determined as the percentage of the initial mass (corrected for water content) lost during incubation. Net loss of N and P from the litter was calculated as (the total mass × nutrient concentration prior to incubation) − (the remaining mass × nutrient concentration after incubation) (Wardle, Bonner & Barker 2002). The proportion of total initial N and P lost from the litter during decomposition was calculated from these values.

Statistical analyses

Coefficients of variation (i.e. SD/mean; CVs) were calculated to quantify the variability of leaf and litter traits and litter mass loss both within and between species (Albert et al. 2010a; Fajardo & Piper 2011). For each trait or litter mass loss variable, within-species CVs were determined across plots. Between-species CVs for these variables were calculated using the mean trait or mass loss values for each species across all plots. For N and P loss from leaf litter, some negative values (indicating net immobilization of N or P) were observed, and thus, CVs were calculated from gross and not net values, that is, relative to an initial concentration of 100%. The coordination in the variation of traits across species was examined by principle component analysis (PCA) using the mean trait values across all plots for each species so that each species was represented by a single value (Diaz et al. 2004; Wright et al. 2004). Furthermore, to explore whether within-species changes in leaf trait values were coordinated, a separate PCA was performed for each species using the trait values for all plots in which it was present.

For each species, regression analyses were performed to examine the relationships of leaf and litter traits across plots as the response variables, with two measures of soil nutrient status of the plots as the explanatory variables, that is, the soil C/N and C/P ratios (Richardson et al. 2005; Richardson, Allen & Doherty 2008). Although several additional measures of soil fertility showed some relation to trait variation (data not shown), the soil C/N and C/P ratios were the most consistently related to the leaf and litter traits both within and between species. Within species, regression analyses were also used to assess the relationship of litter mass loss and N and P release with leaf and litter trait values across plots.

To assess the relationships at the whole plot level of the leaf and litter traits with the soil C/N and C/P ratios and with the loss of litter mass, N and P during decomposition, we used a weighted averages approach (Garnier et al. 2007; Fortunel et al. 2009), weighting species by their relative basal area in each plot:

  • display math(eqn 1)

where traitagg is the aggregate (or weighted average) value of that trait (or decomposition variable) for all tree species collected in that plot, pi is the basal area of a tree species i as a proportion of the total basal area for all tree species collected in that plot and traiti is the value of the trait (or decomposition variable) for tree species i. To ensure that the weighted trait values were representative of the whole plant communities, those plots for which the 16 species we measured collectively accounted for < 70% of the total basal area were excluded from community-level analyses, consistent with the recommendations of Garnier et al. (2004); here, 24 of the 32 plots were retained.

We first determined the relationship of aggregated trait values with soil C/N and C/P ratios across the 24 plots using linear regression. In addition, the relative contributions of the turnover of species among plots, intraspecific variation of species, and their covariation, to the relationships between community trait values and soil nutrient status were estimated following the sum of squares decomposition procedure of Lepš et al. (2011). In this method, species turnover refers to trait variation attributable to variation in tree species' basal areas among plots, whereas intraspecific variation refers to trait variation within species among plots, both of which contribute to differences in overall aggregate trait values between plots. Aggregate mean trait values for the tree species community at each plot were calculated by firstly using the trait values specific to each plot (Specific) and secondly using the fixed species means from across all plots (Fixed). The difference between the specific mean and the fixed mean represents the effect of intraspecific variability (Intrasp.) and was retained as a third community parameter. Using linear regression, these three community parameters were then modelled as a function of the measures of soil fertility across the plots. By decomposition of the sums of squares (SS) across the three linear models, the total SS of each parameter represents how much of the variability is accounted for by each component: SSSpecific = total variance (V); SSFixed = species turnover (T); and SSIntrasp = intraspecific variation (I). Further, if V = T + I (eqn. 2), then turnover and intraspecific variability effects are independent, whereas if the total V is larger or smaller than the sum of T and I, then there is positive or negative covariation (C) between the species turnover and intraspecific effects (for details on SS decomposition procedure see Lepš et al. 2011).

Using the same variance partitioning approach, the contributions of WSV, BSV and their covariation to the total observed variation in the aggregated measures of decomposability across the plots were calculated. We then assessed the power of the aggregated leaf and litter trait measures to predict the aggregated measures of the loss of litter mass, N and P during decomposition across the 24 plots, using linear regression. Both aggregated leaf and litter trait measures inclusive of WSV and exclusive of WSV (calculated as above) were regressed against loss of mass, N and P, and the goodness-of-fit of each pair of models was compared using Akaike's information criteria (AIC). We recognize that, like other studies that have determined aggregated measures of litter decomposability (Wardle et al. 2009; Sundqvist, Giesler & Wardle 2011), our methodology does not take into account the effects of litter mixing on decomposition. However, mixing effects are likely to be negligible; it has been shown both for the type of forest and species considered in the present study (Wardle et al. 2006) as well as more widely (Srivastava et al. 2009) that these mixing effects are generally small and non-directional compared with the substantial overriding effects of plant species identity and plant traits (Cornwell et al. 2008).

All statistical analyses were performed in r version 2.12.2 (R Foundation for Statistical Computing, Vienna, Austria). Sum of squares decomposition was performed using the ‘trait.flex.anova’ macro (Lepš et al. 2011).

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Variability of soils, traits and litter decomposability

The soils from the 32 plots showed considerable variation in all measures of soil fertility; total P varied more than fourfold, and total N varied threefold, among plots (Table 1). Considerable within-species variation was also observed for all leaf and litter traits, but in all cases, this variation was less than the variation observed among species (Fig. 1 and Table S2). The median within-species CVs for most traits was between 45 and 55% of the variation observed among species, with the only exceptions being litter N content and C/N ratio where the median CV was < 40% that of the among-species CV. In contrast, the ranges of within-species CVs differed between traits. Across species, litter P content and C/P ratios showed the largest ranges of within-species variation with some species having CVs that were < 40% and others > 80% of the CV among species. Leaf dry matter content showed the most constrained range in values of within-species CV and was overall the least variable trait both within and among species.

image

Figure 1. Coefficients of variation (CV) for litter mass loss and leaf and litter traits for 16 temperate rain forest species. Here, each species represents an independent data point. Open bars represent total interspecific variability (the range of trait values across all species), and box plots within bars represent intraspecific variability (the range of values for CVs within species). Boxes and whiskers represent 50% and 95% of intraspecific CVs, respectively.

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Table 1. Summary of soil properties across 32 plots, located along topographical sequences on each of the four major basal rock types in the Westland region of New Zealand
Soil characteristicRangeMean (SD)
pH3.3–5.494.34 (0.64)
C (%)12.9–54,034.5 (9.7)
N (%)0.68–1.971.31 (0.32)
C/N18.4–39.826.6 (5.4)
NO3 (mg kg−1)0.12–136.4017.4 (34.6)
NH4+ (mg kg−1)0.6–212.732.7 (46.7)
Total P (mg kg−1)331–1473820 (322)
P – Olsen (mg kg−1)6.1–56.425.3 (11.5)
Ca (cmol+ kg−1)0.4–59.620.4 (17.2)
Mg (cmol+ kg−1)3.4–14.17.7 (3.1)
K (cmol+ kg−1)0.51–7.551.4 (1.22)
Na (cmol+ kg−1)0.15–2.590.7 (0.52)
Cation exchange capacity (cmol+ kg−1)33.3–137.780.5 (25.4)
Base saturation (%)12.2–86.438.6 (23.1)

The decomposability of leaf litter also showed considerable variation within species. The CVs for litter mass loss within species ranged from 8.8% to 26.6%, whereas the CV among species for litter mass loss was 44.5% (Fig. 1). For N loss, the CV among species was 15.0%, while the ranges of values for the CVs for within-species variation varied from 6.9% to 25.3%. For P loss, the CV among species was 7.7%, while the ranges of values for the CVs for within-species variation varied from 6.5% to 21.2%.

The primary axis derived from the PCA captured 74% of the total variability of leaf trait data across species (Fig. 2a). Four of the five leaf traits loaded strongly along this axis while the fifth one, leaf C, loaded along both the first and second axes. At the within-species level, 11 of the 16 species showed similar patterns of trait covariation to that found among species, that is, Carpodetus serratus, Dicksonia squarrosa and Pseudowintera colorata (Fig. 2b–d) and Blechnum discolor, Coprosma lucida, Cyathea smithii, Griselinia littoralis, Hedycarya arborea, Pseudopanax crassifolius, Raukaua simplex and Schefflera digitata (data not presented). For each of these species, LDMC, SLA and leaf N and leaf P also loaded strongly along a common primary axis of variation. However, in contrast to the pattern at the between-species level, leaf C varied orthogonally to the primary axis. The remaining five species either showed contrasting patterns to that observed among species, or weak trait covariation, that is, Quintinia aqutifoilia, Weinmannia racemosa (Fig. 2e–f) Aristotelia serrata, Melicytus ramiflorus and Metrosideros umbellata (data not presented).

image

Figure 2. Principle component analyses of relationships between leaf functional traits across all 16 species (a) and within-species (b–f) for five representative species (with each point representing a single plot).

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Relationships between traits and decomposability

For the majority of species, within-species leaf C/N and C/P ratios were positively linearly related to soil C/N and C/P ratios, respectively, across plots (Tables 2 and S3). For the 13 species for which these relationships were statistically significant, shifts in soil C/N and C/P ratios across plots explained 17–58% of the within-species variation in leaf C/N and C/P ratios. Notably, the few species that did not show significant relationships between these variables were those species that had the highest SLA, and leaf N and P concentrations and the lowest LDMC and C content, that is, A. serrata, M. ramiflorus and S. digitata (Fig. 2a).

Table 2. Coefficients of determination (R2, P-values in brackets) for relationships between leaf and litter C/N and C/P ratios and soil C/N and C/P ratios, within each of 16 temperate rain forest species. Relationships for additional litter and leaf traits are in Table S3
Species n Soil C/NSoil C/P
Leaf C/NLitter C/NLeaf C/PLitter C/P
  1. n, the number of plots individual species were recorded in. Values in bold indicate significance at < 0.05.

Aristotelia serrata 90.233 (0.273) 0.589 (0.044) 0.025 (0.737)0.000 (0.978)
Blechnum discolor 28 0.163 (0.037) 0.078 (0.159) 0.443 (< 0.001) 0.496 (< 0.001)
Carpodetus serratus 22 0.229 (0.033) 0.023 (0.533)0.131 (0.117)0.080 (0.242)
Coprosma lucida 14 0.555 (0.002) 0.206 (0.119) 0.284 (0.049) 0.232 (0.096)
Cyathea smithii 28 0.368 (< 0.001) 0.239 (0.011) 0.462 (< 0.001) 0.505 (< 0.001)
Dicksonia squarrosa 24 0.412 (< 0.001) 0.130 (0.091) 0.396 (0.001) 0.164 (0.055)
Griselinia littoralis 27 0.269 (0.007) 0.046 (0.295) 0.346 (0.002) 0.191 (0.026)
Hedycarya arborea 21 0.462 (0.001) 0.245 (0.026) 0.293 (0.017) 0.365 (0.005)
Melicytus ramiflorus 180.193 (0.068)0.091 (0.223)0.198 (0.065)0.089 (0.229)
Metrosideros umbellata 18 0.275 (0.037) 0.023 (0.557) 0.560 (< 0.001) 0.226 (0.054)
Pseudowintera colorata 21 0.585 (< 0.001) 0.633 (0.001) 0.363 (0.006) 0.293 (0.056)
Pseudopanax crassifolius 22 0.584 (< 0.001) 0.075 (0.228) 0.443 (0.001) 0.358 (0.004)
Quintinia acutifolia 29 0.183 (0.026) 0.028 (0.411) 0.363 (< 0.001) 0.220 (0.016)
Raukaua simplex 13 0.347 (0.044) 0.133 (0.301) 0.468 (0.014) 0.184 (0.216)
Schefflera digitata 110.311 (0.119)0.008 (0.814)0.007 (0.829)0.007 (0.826)
Weinmannia racemosa 32 0.363 (< 0.001) 0.305 (0.001) 0.522 (< 0.001) 0.457 (< 0.001)

In contrast, only about a third of the species showed significant linear relationships between litter C/N or C/P ratios and soil C/N or C/P ratios, respectively (Table 2). Further, there was no apparent association between which species showed significant relationships and their positions along the principal traits × species ordination axis (Fig 2a). For most species, within-species leaf and litter P concentrations were also significantly correlated with both the soil C/N and C/P ratios (Table S3) across plots. Similarly, for most species, within-species leaf N concentration was significantly correlated with the soil C/N ratio across plots (Table S3). Conversely, only a minority of species showed significant correlations between their litter N concentration and the soil C/N ratio and between their leaf and litter N concentrations and the soil C/P ratio (Table S2). For about half of the species, within-species SLA and LDMC were significantly correlated with soil C/N across plots and while for about a third of the species, within-species SLA and LDMC were significantly correlated with C/P ratios across plots (Table S3).

Despite the considerable within-species variation in leaf and litter traits and in the decomposability of the leaf litter, there were few significant relationships of litter mass loss at the within-species level with leaf or litter C/N and C/P ratios (Table 3) or with other leaf or litter traits (Table S4). Only 3 of the 16 species showed significant relationships between leaf or litter traits (i.e. litter C, and leaf LDMC and SLA) and the N or P loss from their litter (data not shown).

Table 3. Coefficients of determination (R2, P-values in brackets) for the relationships between decomposition (litter mass loss) and C/N and C/P ratios of leaf and litter within each of 16 temperate rain forest species. Relationships for additional litter and leaf traits are given in Table S4
Species n Leaf C/NLitter C/NLeaf C/PLitter C/P
  1. n, the number of plots individual species were recorded in. Values in bold indicate significance at < 0.05.

Aristotelia serrata 90.031 (0.679)0.156 (0.333)0.179 (0.296)0.200 (0.267)
Blechnum discolor 28 0.231 (0.010) 0.279 (0.004) 0.202 (0.017) 0.181 (0.024)
Carpodetus serratus 220.064 (0.280)0.023 (0.527)0.121 (0.133)0.032 (0.451)
Coprosma lucida 140.000 (0.956)0.000 (0.958)0.004 (0.832)0.034 (0.545)
Cyathea smithii 280.097 (0.107) 0.361 (0.001) 0.032 (0.366)0.050 (0.261)
Dicksonia squarrosa 240.040 (0.348)0.019 (0.533)0.102 (0.128)0.007 (0.705)
Griselinia littoralis 270.006 (0.709)0.004 (0.750)0.033 (0.361)0.043 (0.301)
Hedycarya arborea 210.027 (0.501)0.169 (0.091)0.011 (0.671)0.006 (0.756)
Melicytus ramiflorus 180.143 (0.122)0.134 (0.135)0.031 (0.486)0.076 (0.270)
Metrosideros umbellata 180.206 (0.067)0.011 (0.675)0.003 (0.827)0.209 (0.056)
Pseudowintera colorata 210.041 (0.486)0.069 (0.363)0.201 (0.108) 0.300 (0.043)
Pseudopanax crassifolius 220.118 (0.118) 0.252 (0.017) 0.360 (0.003) 0.542 (< 0.001)
Quintinia acutifolia 290.054 (0.242)0.075 (0.166)0.040 (0.317)0.012 (0.589)
Raukaua simplex 130.001 (0.928)0.022 (0.663)0.033 (0.575)0.007 (0.810)
Schefflera digitata 110.037 (0.593)0.168 (0.240)0.263 (0.130)0.017 (0.719)
Weinmannia racemosa 320.009 (0.601)0.013 (0.528)0.000 (0.941)0.006 (0.675)

Relationships between community-level trait variation and decomposability

Aggregate mean trait values for a number of leaf and litter traits, notably the leaf and litter C/N and C/P ratios, were positively linearly related to changes in soil C/N and C/P ratios across the plots (Fig. 3, Table S5). Changes in the soil C/N and C/P ratios explained between 32 and 61% of the variation in leaf C/N and C/P ratios at the whole community level (Fig. 3). The sum of squares decomposition of these relationships revealed that intraspecific variation (I) contributed significantly to the relationship between the aggregate traits and the soil C/N or C/P ratios in only two cases, whereas species turnover (T) contributed significantly in all cases (Fig. 3). Nonetheless, for six of the eight relationships, a large proportion of the explained trait variation was due to strong covariation (C) between species turnover and intraspecific variability.

image

Figure 3. Relationships between aggregate mean leaf and litter C/N and C/P ratios of the tree species community (weighted by basal area) and soil C/N and C/P ratios across 24 plots, with each plot represented by a separate data point. For R2 values, **< 0.01; ***P < 0.001. Contributions of species turnover (T), intraspecific variation (I) and their covariation (C) to each model are given, with bold text indicating significant (< 0.05) contributions of T and I (testing of statistical significance of C is not possible). Contributions of the different sources of variation were derived from a sum of squares decomposition (see 'Materials and methods') in which species turnover refers to variation in tree species' basal areas among plots, while intraspecific variation refers to variation within species among plots, both of which contribute to differences in overall aggregate trait values between plots.

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Partitioning of the total variance in the aggregated measures of decomposability across the plots revealed that the variance due to species turnover was much larger (T = 1.063) than the variance due to intraspecific variation (I = 0.058). In addition, there was moderate negative covariation between the two variance components (C = −0.12). Mass, N and P loss of leaf litter during decomposition at the community level was usually linearly related to aggregate leaf and litter traits across plots (Fig 4, Table S6). However, models based on aggregate trait measures exclusive of within-species trait variation had higher R2 values and better model fits (lower AIC values) than models that included within-species trait variation (Fig. 4). Further, for N and P loss from the litter, models inclusive of within-species trait variation performed no better than models exclusive of intraspecific trait variation (Fig. 4).

image

Figure 4. Relationships between aggregate (weighted by basal area) leaf and litter traits and aggregate loss of mass, N and P from leaf litter during decomposition, across 24 forest plots. Relationships are shown for aggregate trait measures both inclusive and exclusive of within-species trait variation (WSV). The goodness-of-fit of each model is indicated by R2 coefficients and Akaike's information criteria (AIC). *< 0.05 **< 0.01; ***P < 0.001.

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Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Our results demonstrate considerable within-species trait variation in the live leaves and leaf litter of co-occurring temperate rain forest species and that a within-species economic spectrum of trait covariation occurs for most species. Further, we demonstrate that within-species trait variation contributes meaningfully to community-level responses to soil nutrient status. However, within-species trait variation was a poor predictor of within-species variation in litter decomposability. Further, the inclusion of within-species trait variation did not improve the ability of leaf or litter traits to predict decomposability at the whole community level. We discuss our findings and their implications in the context of the response of plant traits to soil fertility and their effects on decomposition processes.

For 11 of the 16 plant species in this study, within-species covariation of leaf economic traits was structured similarly to that observed among species. A common axis of variation explained 53–70% of the total trait variation within each of these 11 species; LDMC scaled in opposition to SLA, leaf N and leaf P along this axis, whereas leaf C consistently varied orthogonally to this axis (Fig. 2). Similar within-species trait covariation has been observed in alpine perennial shrubs (Albert et al. 2010b), but the patterns are more pronounced in the present study. This finding indicates that the trade-offs between functional traits that operate among species and give rise to the LES (Wright et al. 2004; Freschet et al. 2010) also occur within species, consistent with our first hypothesis. However, this pattern of within-species trait variation was not universal because five of the species we studied showed contrasting or weak within-species trait covariation. These species did not appear to be a distinct subset of the community: they were from different plant families, included both canopy and subcanopy species, and fast-growing species with soft wood and slow-growing species with hard wood. Further work is needed to predict which species show a clear within-species LES and under what circumstances.

Generally, there was about half as much variability in leaf and litter traits within individual species as was observed among species (Fig. 1), and this is of a comparable magnitude to previous studies (e.g. Albert et al. 2010a; Messier, McGill & Lechowicz 2010). For the majority of species, we also observed that within-species changes in leaf or litter traits across plots corresponded closely to changes in soil nutrient status among plots. Notably, the within-species C to N and C to P ratios of foliage and to a lesser extent of litter were often strongly related to the soil C to N and C to P ratios, in line with some studies but not others (see Ordoñez et al. 2009). Our results are consistent with previous studies showing that leaf functional traits can vary predictably within species in response to temperature (Albert et al. 2010b), rainfall (Messier, McGill & Lechowicz 2010; Fajardo & Piper 2011), disturbance (Jung et al. 2010; Lepš et al. 2011) and soil nutrients (Wardle et al. 2009; Lagerström, Nilsson & Wardle 2013). Additionally, although species turnover made the strongest contribution to the relationships between community-level trait measures and soil nutrients across plots, within-species variation also made an important contribution in many cases (Fig. 3). Our findings are largely consistent with previous studies (Albert et al. 2010b; Jung et al. 2010; Messier, McGill & Lechowicz 2010) in showing that within-species trait variation can be an important component of plant community responses to environmental gradients.

We also observed substantial WSV in litter decomposability, although this was generally less than the variation in leaf and litter traits within species. Despite this, we observed few significant relationships between WSV of decomposability and WSV of leaf or litter traits (Table 3, Fig. 4). This result does not support our second hypothesis, but rather suggests that some degree of decoupling between litter decomposability and leaf or litter traits occurs within species. Although the magnitude of within-species variation in litter decomposition in our study is comparable with that of other studies (Sariyildiz & Anderson 2003; LeRoy et al. 2007; Lecerf & Chauvet 2008; Crutsinger, Sanders & Classen 2009), the lack of consistent relationships between litter decomposition rates and measures of litter quality within species contrasts with several previous studies (Madritch & Hunter 2003; Sariyildiz & Anderson 2003; Lecerf & Chauvet 2008; Crutsinger, Sanders & Classen 2009). However, Haase (2009) also noted the absence of a relationship between the litter traits of Fagus sylvativa and litter decomposition, despite strong trait variation within populations from several geographic provenances. Our data shows that although leaf traits can covary along an economic spectrum at the within-species level in a manner similar to the between-species LES, this within-species LES does not drive decomposition in the way that the between-species LES does (e.g. Santiago 2007; Cornwell et al. 2008; Freschet, Aerts & Cornelissen 2012). One reason may be that at the within-species level, decomposability and leaf traits both have a much narrower range of variation than that observed across species, thereby weakening potential relationships between the two. Another possibility is that traits not typically associated with the LES at the across-species level and which we therefore did not measure might instead be important drivers of within-species decomposability; these could include the ratio of lignin to N (Taylor, Parkinson & Parsons 1989), as well as the amounts and types of secondary compounds such as phenolics present (LeRoy et al. 2007), which can show species-specific responses along environmental gradients (Sundqvist et al. 2012).

Our observation that either WSV or covariation of WSV with species turnover often contributed to the variation of aggregate trait means among plots is not consistent with prior observations of the insensitivity of aggregate trait values to WSV (Lavorel et al. 2008, Albert et al. 2011). Nonetheless, this contribution of WSV did not translate into an improved ability of aggregate trait values to predict decomposability at the community level. Aggregate mean trait values for the whole community exclusive of WSV were equally good or better predictors of community-level decomposability than were aggregate mean trait values that incorporated WSV (Table 3, Fig. 4). This finding, which is inconsistent with the predictions of our third hypothesis, appears to be due to the decoupling of WSV in the measured traits with the WSV variation in decomposability, and because of the minor contribution of WSV to the aggregate measures of decomposability among plots. Overall, our results support the prediction by Albert et al. (2011) that at least for the plots and species that we considered, variation in community-level decomposability across plots is little influenced by WSV and can be adequately predicted using species mean trait values. As such, consideration of within-species variation in these traits does not improve this prediction. We also note that our study design did not account for interactive effects among litters (i.e. litter-mixing effects) on decomposition, which may occur in the field, but such effects are likely to be small and non-directional (Srivastava et al. 2009) as has previously been shown for several of the same species used in this study and in a similar type of forest (Wardle et al. 2006). As such, the effects of plant traits on decomposability (Cornwell et al. 2008), such as we considered in our study, should far outweigh any litter-mixing effects.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

Our finding of an intraspecific economic trait spectrum for the majority of species indicates that the functional trade-offs generating the leaf economic spectrum among species (Wright et al. 2004) also occur within species. Identification of such intraspecific leaf economics spectra provides an important basis for understanding WSV and how it can be incorporated with BSV in plant trait research. However, we did not find evidence for a within-species LES for about a third of the species, and further exploration will be required to understand or predict for which species and circumstances it does occur. Further, our data strongly support prior findings highlighting the importance of WSV as a component of how individual plant species as well as whole plant communities respond to environmental gradients (Albert et al. 2010b; Jung et al. 2010; Fajardo & Piper 2011; Auger & Shipley 2013). However, contrary to our expectations, the WSV in leaf and litter traits was largely decoupled from within-species variation of litter decomposability, indicating that at the within-species level, traits associated with the LES do not have the same power to predict litter decomposability as they do among species (Santiago 2007; Freschet, Aerts & Cornelissen 2012). Overall, our data suggest that while explicit consideration of WSV of leaf economics traits can benefit functional trait research focused on understanding plant community responses to environmental drivers, it may be less important to consider WSV when attempting to understand the contributions of trait variation to driving processes like litter decomposition and nutrient release at the whole plant community level.

Acknowledgements

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information

We thank Chris Morse, Simon Burrows, Sophie Walker, Jessica Thorne and Anna Heller for help with field work and Karen Boot and Gaye Rattray for laboratory assistance. Thanks also to Jan Lepš, Gregoire Freschet and Andy Siegenthaler for statistical advice and Xavier Morin and three anonymous referees for comments on an earlier draft. This work was funded by a SLU Excellence Grant and a Wallenberg Scholars award to DAW and by Core funding for Crown Research Institutes from the New Zealand Ministry of Business, Innovation and Employment's Science and Innovation Group.

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  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information
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Supporting Information

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  10. Supporting Information
FilenameFormatSizeDescription
jec12155-sup-0001-TableS1-S6.docxWord document100K

Table S1. Species names, abbreviations and families of the 16 New Zealand forest species used in the study.

Table S2. Ranges in mass, N and P loss and leaf and litter traits for the 16 forest plant species included in the study.

Table S3. Coefficients of determination (R2, P-values in brackets) for the relationships of additional leaf and litter traits not presented in Table 2 with soil C/N and C/P ratios at the within-species level.

Table S4. Coefficients of determination (R2, P-values in brackets) for the relationships of additional leaf and litter traits not presented in Table 3 with the mass loss of the leaf litter at the within species level.

Table S5. Coefficients of determination (R2, P-values in brackets) for the relationships of aggregated (by basal area) mean leaf and litter traits of the tree community with the soil N and P contents and C/N and C/P ratios, across forest plots.

Table S6. Coefficients of determination (R2, P-values in brackets) for the relationships of the aggregated (by basal area) mean trait values of the tree species community with the aggregated (by basal area) mass, N and P loss from the litter, across forest plots.

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