Seasonality and Forest Succession in Tropical Watersheds Affects Stream Export
The export and retention of leaf litter greatly influences water quality, food webs, and habitat structure of forested headwater streams. In the Bisley Experimental Watersheds, Forest Service scientists determined leaf litter exported from streams varied in terms of nutrient quality between the dry and wet seasons during a 15-year study. Variation in the quantity and quality of exported material depended on traits of particular weather events, such as storms and hurricanes, season, and the successional status of the forest.
The chemical composition of exports varied over time, with the proportion of carbon to nitrogen being highest (low quality for consumers) in the driest months and lowest (high quality for consumers) during rainy months. The differences in export quality signaled that more attention should be given to changes in seasonal rainfall in the tropics, because these changes not only affect stream discharge and timing of new leaf and flower production but also increase the seasonal range in quality of organic matter exports that reach streams.
In conclusion, the state of watershed vegetation development limited the quantity of material that was exported during a storm. This finding is independent of the level of hillslope or stream runoff. The synergy between hurricane intensity and frequency and the level of vegetation maturity defined the long-term pattern of high leaf litter export events in forested watersheds.
Headwaters under nonforested land cover conditions may not be able to retain and process a large percent of leaf litter inputs, which in turn affects water quality and resource availability for aquatic ecosystems downstream. With these findings in mind, watershed and aquatic wildlife managers should consider that changes in the quality of organic matter in streams could potentially alter ecosystem processes and the aquatic food webs that depend on them.
Forest Service Partners