Riparian plant litter quality increases with latitude

Abstract

Plant litter represents a major basal resource in streams, where its decomposition is partly regulated by litter traits. Litter-trait variation may determine the latitudinal gradient in decomposition in streams, which is mainly microbial in the tropics and detritivore-mediated at high latitudes. However, this hypothesis remains untested, as we lack information on large-scale trait variation for riparian litter. Variation cannot easily be inferred from existing leaf-trait databases, since nutrient resorption can cause traits of litter and green leaves to diverge. Here we present the first global-scale assessment of riparian litter quality by determining latitudinal variation (spanning 107°) in litter traits (nutrient concentrations; physical and chemical defences) of 151 species from 24 regions and their relationships with environmental factors and phylogeny. We hypothesized that litter quality would increase with latitude (despite variation within regions) and traits would be correlated to produce 'syndromes' resulting from phylogeny and environmental variation. We found lower litter quality and higher nitrogen:phosphorus ratios in the tropics. Traits were linked but showed no phylogenetic signal, suggesting that syndromes were environmentally determined. Poorer litter quality and greater phosphorus limitation towards the equator may restrict detritivore-mediated decomposition, contributing to the predominance of microbial decomposers in tropical streams.

Figure 1

Principal component analysis (PCA) of litter traits [nitrogen (N) and phosphorus (P) concentration, N:P ratio, magnesium (Mg) and tannin (Tan) concentration, and specific leaf area (SLA); in bold letters] and environmental and spatial variables (mean annual temperature, MAT; mean annual precipitation, MAP; precipitation of the driest quarter, PDQ; temperature seasonality, TS; precipitation seasonality, PS; latitude, Lat; and altitude, Alt). Open and closed circles represent species from tropical and non-tropical regions (i.e., temperate, Mediterranean and boreal), respectively.

Figure 2

Variation of litter traits [nitrogen (N) and phosphorus (P) concentrations, log-transformed N:P ratio, magnesium (Mg) and tannin (Tan) concentrations, and log-transformed specific leaf area (SLA)] in relation to mean annual temperature (MAT), mean annual precipitation (MAP) and absolute degrees of latitude. Significant and non-significant relationships are depicted by solid and dotted lines, respectively. Fits for MAT (a–e) and MAP (f–i) derive from linear models that included multiple predictors; some graphs are omitted because MAT or MAP had been excluded from the final model; estimates and p-values are shown in Table 1. Fits for latitude (j–o) derive from additive models, which allowed analyses of non-linear relationships; r ² and p-values are the following: N (r ² = 0.18, p = 0.0024); P (r ² = 0.06, p = 0.064); N:P (r ² = 0.05, p = 0.0057); Mg (r ² = 0.05, p = 0.212); Tan (r ² = 0.04, p = 0.0156); SLA (r ² = 0.32, p < 0.0001). Open and closed circles represent species from tropical and non-tropical regions, respectively.

Figure 3

Location of 24 riparian litter collection sites; open and closed circles represent tropical and non-tropical regions, respectively. The map was created in the maps R package (Original S code by Richard A. Becker, Allan R. Wilks. R version by Ray Brownrigg. Enhancements by Thomas P Minka and Alex Deckmyn. (2016). maps: Draw Geographical Maps. R package version 3.1.1. https://CRAN.R-project.org/package=maps).