Latitude dictates plant diversity effects on instream decomposition

Abstract

Running waters contribute substantially to global carbon fluxes through decomposition of terrestrial plant litter by aquatic microorganisms and detritivores. Diversity of this litter may influence instream decomposition globally in ways that are not yet understood. We investigated latitudinal differences in decomposition of litter mixtures of low and high functional diversity in 40 streams on 6 continents and spanning 113° of latitude. Despite important variability in our dataset, we found latitudinal differences in the effect of litter functional diversity on decomposition, which we explained as evolutionary adaptations of litter-consuming detritivores to resource availability. Specifically, a balanced diet effect appears to operate at lower latitudes versus a resource concentration effect at higher latitudes. The latitudinal pattern indicates that loss of plant functional diversity will have different consequences on carbon fluxes across the globe, with greater repercussions likely at low latitudes.

Fig. 1. Predictions about latitudinal variation of the litter diversity effect on decomposition (LDED) resulting from differences in plant and detritivore diversity at low and high latitudes.

At low latitudes, the high diversity (and continuous availability, not shown in the figure) of litter provides a wide variety of resources and favors a balanced diet for detritivores. At high latitudes, the low diversity (and seasonal or periodic availability, now shown) of litter favors detritivore specialization in the use of concentrated resources. Gray arrows represent the movement of detritivores (represented by brown drawings) between different types of litter (green drawings).

Fig. 2. Graphical summary of our experimental design.

We combined litter of nine plant species belonging to three families (represented by different shades of green) in three low-diversity mixtures (each containing three species of the same family) and three high-diversity mixtures (containing three species of different families. Each treatment was incubated in each stream in coarse- and fine-mesh litterbags, replicates of which were placed in five consecutive pool habitats in pairs. After 23 to 46 days of incubation, we quantified decomposition [as litter mass loss (LML)] for each species in each mixture. We then calculated the LDED (our response variable) as the difference between LML in the high-diversity and the low-diversity mixture from the same pool.

Fig. 3. Global distribution and photos of study sites and variation of the LDED across latitudes and biomes in coarse- and fine-mesh litterbags.

Study sites were 43 streams (3 of which were excluded from analyses due to loss of replicates; represented by broken circles) that spanned 113° of latitude and were located in 26 countries in all inhabited continents (A). Colors correspond to terrestrial biomes included in the study, with absent biomes represented by gray color. The LDED decreased with latitude for coarse-mesh litterbags (B) and showed no latitudinal pattern for fine-mesh litterbags (C) and no differences among biomes for both types of litterbag (D and E); see table S3 for whole model results. Photographs show one stream site from each biome (from left to right: tropical savanna, TrS; tropical wet forest, TrWF; xeric shrubland, XeS; Mediterranean forest, MeF; temperate broadleaf forest, TeBF; temperate coniferous forest, TeCF; and tundra, Tu). Photo credit: GLoBE consortium.

Fig. 4. Results of linear mixed-effects models testing the effect of the interaction between absolute latitude and mesh type on LDED for each species.

Mesh types were coarse (dark brown lines) and fine (light brown lines), which allowed or excluded detritivores from litterbags, respectively (see table S3 for whole model results).