WOOD TRANSPORT AND RETENTION IN TROPICAL HEADWATER STREAMS, COSTA RICA

We estimate wood retention rates for undisturbed headwater streams in eastern Costa Rica using data from two years of monitoring all large wood pieces in 10 stream reaches in a tropical rain forest preserve. Wood pieces at least 1 m long and 10 cm in diameter were flagged, numbered, and surveyed, and their position (unattached, buried, ramp, or bridge) and decay state (qualitative, 7-class scale) were described at 4-month intervals. Although retention rate studies in the temperate zone are rare and techniques vary, our estimated tropical mean wood residence time of 2-13 years are an order of magnitude shorter than residence times reported from the Pacific Northwest. When our estimated residence times are weighted by piece volume, mean residence time varies from 2-86 years among the reaches, which overlaps the range reported from the temperate zone. Mean residence time of wood at our 10 study sites had weak negative correlations to the gradient (R²=0.14), unit stream power (R²=0.13), and drainage area (R²=0.07) of the stream. Likelihood of transport out of the study reach was independent of decay class, suggesting that transportability is not controlled by decay, although 85% of the pieces observed were in the two most decayed classes. Unattached pieces were transported out of the study reaches at a higher rate (51%) than other types (29%) and accounted for 52% of the pieces removed from the study reaches. Stage was measured continually at one site, and peak discharge in the interval between surveys correlated well with the total number of logs removed in the study reaches over that interval (R²=0.90, n=5), but wood introduction showed no relationship with precipitation or flow. These data suggest that stream discharge is more important to individual piece mobility than is decay over the timescales observed.