Timing, magnitude, and sources of coarse particulate organic matter in tropical montane streams

Abstract Leaf litter in headwater streams serves as an important energy source for the aquatic food web. However, the timing, magnitude, and source dynamics of leaf litter inputs into tropical montane streams have received less attention. Here we present an 8-year study of leaf litter inputs from two tropical montane first-order headwater streams in the Luquillo Experimental Forest, Puerto Rico that include a prolonged drought period and major hurricanes. Leaf litter was collected across different landscape sources such as canopy and riparian inputs, and litter transported from upstream. The different sources of leaf litter were compared to environmental variables (precipitation, wind, solar radiation, and temperature) to identify potential drivers of inputs to streams and exports. Leaf litter from upstream was the greatest contributor (50–95%) and can vary with storms and droughts. Transitions between dry and wet periods led to pulses of leaf litter in both streams. Increasing precipitation, solar radiation, and wind led to increased leaf litter inputs but varied across sources. The variable responses across sources to climatic events suggest that the timing and magnitude of litter inputs to headwater streams may be specific by source. Moreover, the results underscore the value in considering the forest cover of the watershed landscape in its entirety when quantifying organic matter inputs as they depend on upstream conditions that go beyond the immediate reach-adjacent riparian zone.