Effects of Sediment Deposition on Aboveground Net Primary Productivity, Vegetation Composition, Structure, and Fine Root Dynamics in Riparian Forests

Abstract

This study examined how increased sediment deposition from anthropogenic disturbance impacts functions performed by riparian forests associated with ephemeral streams. Specifically, this study looked into aboveground net primary productivity, vegetation composition and structure, and fine root dynamics. Across a range of sediment deposition levels, nine ephemeral riparian forests were classified as highly disturbed, moderately disturbed, or reference. Paired circular treatment plots were established in each of the nine areas, with one established in the upper extremities near the stream origin where sediment was most likely to be received and another located farther down, beyond visual evidence of sediment deposition. Comparisons were made between treatments and among disturbance categories. The features observed by this study included: litterfall, woody increments, aboveground net primary productivity, litterfall nutrient contents, number of seedling and saplings, number of species, shade tolerance, presence/absence of nitrogen fixers, fine root biomass, fine root production, and nutrient contents of fine roots. Treatment effects (upper vs. lower plots) were not apparent for aboveground parameters, vegetation composition and structure. An exception was the number of seedlings and saplings measured, which was significantly greater in upper plots of highly disturbed areas. However, comparisons of the same variables among disturbance categories at upper plots indicated that litterfall, annual woody increments, and aboveground NPP were significantly lower in highly disturbed areas. No significantly differences were observed between moderately disturbed and reference areas. Vegetation composition and structure were also similar among moderately disturbed and reference areas, where relatively closed canopies, low numbers of seedling and saplings, and dominance of shade tolerant species was observed. In contrast, upper plots of highly disturbed areas exhibited open canopies with very high numbers of seedlings and saplings, dominance of shade intolerant species and presence of nitrogen fixers. Results from this study suggest that sediment deposition has a negative impact upon riparian forests, altering patterns of vegetation composition and structure, and ultimately, decreasing forest productivity. Fine root dynamics appeared to be most sensitive to high rates of sediment deposition in riparian forests. Fluctuations in fine root standing crop biomass were more pronounced on lower plots and upper plots with fewer disturbances, whereas upper plots of highly disturbed areas displayed a relatively constant biomass throughout the sample period. Comparisons between upper and lower treatments indicate that fine root productivity was also significantly less in upper plots of highly disturbed areas. No differences were observed between the other two categories. Comparisons across disturbance categories at upper plots followed a similar pattern, with the lowest productivity in highly disturbed areas. Fine root nutrient contents of live and dead roots mirrored changes in fine root biomass and detritus. Lower carbon and nitrogen contents were observed in upper plots of highly disturbed areas. These results suggest that fine roots are good indicators of environmental stress and that high levels of sedimentation may reduce levels of fine root biomass and productivity, which may lead to reductions in forest productivity.