Research gaps related to forest management and stream sediment in the United States

Impact of fine-grained sediment on mountain stream macroinvertebrate communities: Forestry activities and beaver-induced sediment management

Fine-grained sediments are a natural component of river systems. Human activities generate additional sources of fine sediment. In mountainous areas, the anthropogenic inputs of fine sediments are associated with forestry. The aim of this study was to analyse the differences in the macroinvertebrate communities between the reference and caused by forest harvesting activities increased influx of fine-sediment to mountain streams. The tested hypothesis was that the macroinvertebrate communities will differ depending on the intensity of forest harvesting practices in the stream catchment that causes excessive influx of fine sediment into the stream. The reintroduction of beavers in the study area, and the formation of in-stream beaver dams, contribute the accumulation of sediments in stream sections with slower water. Thus, it was also assumed that by capturing and storing fine sediments, may contribute to the restoration of the natural structure of the benthic communities downstream of the ponds. The study was carried out in a mountain stream catchment area (Carpathians, Poland), in which inflow of fine sediments in the stream sections varied in intensity. The study was conducted over three years (2018-2020). The extensive use of forest roads, timber skidding trails, and timber storage areas produced fine sediments that clogged the interstitial spaces between the stones in the riffles, limiting the presence of rheophilic taxa associated with coarse-grained substrates. The reduction of the number of scrapers and shredders (i.e. primary consumers) associated with the influx of fine sediments may significantly affect the entire food-web structure in stream ecosystems. The capture and deposition of fine sediments in beaver ponds may accelerate the revitalisation of the flowing sections of the stream. Beaver-induced sediment management is strongly recommended as a beneficial practice that could contribute to ecological preservation and the potential of streams, particularly in mountain areas.

Modeling forest management effects on water and sediment yield from nested, paired watersheds in the interior Pacific Northwest, USA using WEPP

The Water Erosion Prediction Project (WEPP) model was applied to seven paired, nested watersheds within the Mica Creek Experimental Watershed located in northern Idaho, USA. The goal was to evaluate the ability of WEPP to simulate the direct and cumulative effects of clear-cutting and partial-cutting (50% canopy removal) on water and sediment yield. WEPP was modified to better represent changes in the Leaf Area Index during post-harvest forest vegetative recovery. Good agreement between simulated and observed streamflow was achieved with minimal to no calibration over a 16-year (1992-2007) period. For the seven watersheds and the entire study period, the overall Nash-Sutcliffe Efficiency (NSE), Kling-Gupta efficiency (KGE), and deviation of runoff volume (D_V) between observed and simulated daily streamflow ranged 0.58-0.71, 0.67-0.81, and -4% to 9%, respectively. Good agreement between predicted and observed suspended sediment yield was achieved through the calibration of a single channel critical shear stress parameter. For sediment yield, NSE, KGE, and D_V ranged 0.62-0.97, 0.43-0.97, and -2% to 2%, respectively, for the calibration period, and 0.61-0.93, 0.42-0.95, and -24% to 13%, respectively, for the period of model performance assessment. Regression analysis of observed- and WEPP-simulated increase in water and sediment yield following clear-cut treatment was similar; however, the WEPP-simulated increase was lower compared to observations particularly from the partial-cut watershed. The variability in the critical shear parameter for different stream channels in the study watersheds was directly related to the observed mean particle size on the stream bed and suggests that applications of the WEPP model in ungauged basins could potentially set the critical shear parameter based on particle size. Overall, the simulated results demonstrate the potential of WEPP as a modeling tool for forestland watershed management, particularly for estimating the effects of forest harvest on hydrograph fluctuations and consequently, stream sediment transport.

Eucalyptus Short-Rotation Management Effects on Nutrient and Sediments in Subtropical Streams

Forested catchments generally present conserved aquatic ecosystems without anthropogenic disturbances; however, forest management operations can degrade these environments, including their water quality. Despite the potential degradation, few studies have analyzed the effects of forest management in subtropical regions, especially in forest plantations with intensive management, such as Eucalyptus plantations in Brazil. The intensive management of those plantations is characterized by fast-growing, short rotation cycles, and high productivity. This study aimed to assess the effects of Eucalyptus plantations harvesting on the concentration and exportation of nutrients and suspended solids in subtropical streams. Results showed that clear-cut harvesting and subsequent forest management operations do not alter most of the concentration of nitrate, potassium, calcium, and magnesium. The concentration of suspended solids increased during the first year after timber harvesting in all studied catchments, however, the increases were statistically significant in only two catchments. In the first year after harvest, it was observed an increment of water yield/precipitation ratio at three catchments, which also increased export of nutrients and suspended solids. Our results showed that harvesting of fast-growing Eucalyptus forest plantations partially affected sediment exports and did not compromise water quality in the studied catchments. However, the catchment land-use design, especially related to road density and land-use composition, showed significant relationship with sediment exportation.

Effect of deforestation on access to clean drinking water

Using satellite data on deforestation and weather in Malawi and linking those datasets with household survey datasets, we estimate the causal effect of deforestation on access to clean drinking water. In the existing literature on forest science and hydrology, the consensus is that deforestation increases water yield. In this study, we directly examine the causal effect of deforestation on households' access to clean drinking water. Results of the two-stage least-squares (2SLS) with cluster and time fixed-effect estimations illustrate strong empirical evidence that deforestation decreases access to clean drinking water. Falsification tests show that the possibility of our instrumental variable picking up an unobserved time trend is very unlikely. We find that a 1.0-percentage-point increase in deforestation decreases access to clean drinking water by 0.93 percentage points. With this estimated impact, deforestation in the last decade in Malawi (14%) has had the same magnitude of effect on access to clean drinking water as that of a 9% decrease in rainfall.

Historical land-use influences the long-term stream turbidity response to a wildfire

Wildfires commonly result in an increase in stream turbidity. However, the influence of pre-fire land-use practices on post-fire stream turbidity is not well understood. The Lower Cotter Catchment (LCC) in south-eastern Australia is part of the main water supply catchment for Canberra with land in the catchment historically managed for a mix of conservation (native eucalypt forest) and pine (Pinus radiata) plantation. In January 2003, wildfires burned almost all of the native and pine forests in the LCC. A study was established in 2005 to determine stream post-fire turbidity recovery within the native and pine forest areas of the catchment. Turbidity data loggers were deployed in two creeks within burned native forest and burned pine forest areas to determine turbidity response to fire in these areas. As a part of the study, we also determined changes in bare soil in the native and pine forest areas since the fire. The results suggest that the time, it takes turbidity levels to decrease following wildfire, is dependent upon the preceding land-use. In the LCC, turbidity levels decreased more rapidly in areas previously with native vegetation compared to areas which were previously used for pine forestry. This is likely because of a higher percentage of bare soil areas for a longer period of time in the ex-pine forest estate and instream stores of fine sediment from catchment erosion during post-fire storm events. The results of our study show that the previous land-use may exert considerable control over on-going turbidity levels following a wildfire.