Tropical streams are increasingly threatened by climate and land-use changes over vast regions of the globe. Even in areas where riparian forests are protected, conversion of forests to croplands modifies remaining forests, which in turn may increase stream water temperatures, change the tree species that contribute litter to streams, and alter the concentrations of nutrients reaching stream waters from adjacent croplands. Additionally, regional-scale warming caused by forest conversion to agriculture has exceeded greenhouse gas-driven warming in the tropics, compounding the potential for increasing thermal stresses. These forest transitions may modify riparian inputs to streams but we know little about the stream responses under these altered conditions. To understand the implications for stream organic matter and energy flow in streams, we measured stream temperature, inputs of dissolved N and P, riparian forest LAI and litter inputs, and the standing stocks and retention of organic matter in six streams with riparian forest bufferes in cropland and forested watersheds on Tanguro Ranch in Mato Grosso, Brazil.
Results/Conclusions
Cropland riparian forests were hotter and drier than intact riparian forests, less diverse, and had different species composition. Although cropland riparian forests had lower LAI, litterfall was similar to intact riparian forests. Our data suggest that these riparian forest shifts have led to greater shifts in allochthonous than autocthonous organic matter dynamics. Forest and cropland streams both received little sunlight and support minimal algal growth (~0.4 µg Chl a cm-2). Although streams of both land uses had similar litter inputs, leaf detritus was much lower in cropland streams compared with forest streams. A litter retention experiment (leaves and 10 cm twigs) showed equal retention in forest and cropland streams suggesting that lower organic matter stocks in cropland streams were not driven by greater discharge. It is yet not clear if lower organic matter stocks were due to warmer temperatures (2-4 °C greater in cropland streams), changes in litter quality, or changes to stream invertebrate or fish communities. Future work is needed to understand how altered tree species composition and the stream biotic community affect organic matter dynamics and food webs as the climate and land use change in this region.