Abstract
Riparian habitats provide organic matter inputs that influence stream biota and ecosystem processes in forested watersheds. Over a 13-yr period, we examined the effects of litter exclusion, small- and large-wood removal, and the addition of leaf species of varying detrital quality on organic matter standing crop and export of organic and inorganic particles in a high-gradient headwater stream. Using eight pretreatment years of export data and two pretreatment years of particulate organic matter (POM) standing crop data, we report on 21 and 15 years of continuous export and POM standing crop results, respectively. Litter exclusion resulted in the elimination of leaf standing crop by the end of year three. Wood and fine benthic organic matter (FBOM) standing crops declined significantly during the exclusion and wood removal periods, but never completely disappeared. Following the introduction of artificial wood structures for retention, the addition of fast, slow, and mixed breakdown leaves in the treatment stream resulted in significantly increased mean annual leaf standing crops. After five years of leaf addition, FBOM standing crop and fine particulate organic matter (FPOM) export remained below pre-treatment levels. The reduction in leaf standing crop in the treatment stream resulted in significant increases in FPOM (23), fine inorganic particulate (33), and gravel export (103). After small wood removal we observed significant increases in export of fine inorganic particulates (23) and gravel (73) from the treatment stream. A greater proportion of coarse and FBOM standing crop was exported from the treatment stream during the litter exclusion and small wood removal periods than from the reference stream. Following the addition of slow and mixed leaves this trend was reversed, demonstrating the importance of leaf standing crop in the retention of POM. Our long-term experiment demonstrates that the quantity and type of riparian inputs to forested headwater streams will affect POM standing crop and export of POM and sediments to downstream ecosystems, and that small wood is more critical to retaining sediments and POM in small streams than previously recognized.
Highlights
Linkages between adjacent ecosystems are ubiquitous and are often extremely important to one or both systems (e.g., Polis et al 1997)
fine benthic organic matter (FBOM) standing crop declined in the last year of LWR in the treatment stream (Appendix A), but leaf addition reversed the trend of declining FBOM standing crop in the treatment stream
During the LE through PVC periods, the treatment stream lost an average of 151 g ash-free dry mass (AFDM)/m2 of FBOM per wetted stream area per year; whereas, the reference stream gained an average of 114 g AFDM/m2 of FBOM
Summary
Linkages between adjacent ecosystems are ubiquitous and are often extremely important to one or both systems (e.g., Polis et al 1997). Large and small scale physical processes, landscape topography, ecotonal complexity, and migrations of biota can all regulate the flux of materials across ecosystems (e.g., Cadenasso et al 2004, Witman et al 2004). Understanding these linkages and the effects of variation in space and time is crucial for mitigating and predicting future environmental impacts on these linkages (e.g., Freeman et al 2007, Williamson et al 2008) and improving management and policy decisions. While CPOM constitutes the major organic matter input to headwater streams, FPOM (Wallace et al 1995, 1997a, Webster et al 1999), DOM (Meyer et al 1998), and biological respiration account for the bulk of organic matter export from these ecosystems
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