In‐stream habitat and macroinvertebrate responses to riparian corridor length in rangeland streams

Abstract

Conservation and restoration of riparian vegetation in agricultural landscapes has had mixed success at protecting in-stream habitat, potentially due to the mismatch between watershed-scale impacts and reach-scale restoration. Prioritizing contiguous placement of small-scale restoration interventions may effectively create larger-scale restoration projects and improve ecological outcomes. We performed a multi-site field study to evaluate whether greater linear length of narrow riparian tree corridors resulted in measurable benefits to in-stream condition. We collected data at 41 sites with varying upstream tree cover nested within 13 groups in rangeland streams in coastal northern California, United States. We evaluated the effect of riparian tree corridor length on benthic macroinvertebrate communities, as well as food resources, water temperature, and substrate size. Sites with longer riparian corridors had higher percentages of invertebrates sensitive to disturbance (including clingers and EPT taxa) as well as lower water temperatures and less fine sediment, two of the most important aquatic stressors. Despite marked improvement, we found no evidence that macroinvertebrate communities fully recovered, suggesting that land use continued to constrain conditions. The restoration of long riparian corridors may be an economically viable and rapidly implementable technique to improve habitat, control sediment, and counter increasing water temperatures expected with climate change within the context of ongoing land use. Implications for Practice• Greater woody riparian corridor lengths can improve habitat conditions in rangeland streams. • In landscapes where wide corridors are impractical, coordination of the placement of small revegetation projects to produce a long corridor can improve in-stream condition. • Understanding the influences and constraints imposed by stream setting and surroundings (e.g. soil type and upstream gap in tree cover) can inform restoration goals and expectations.

Woody buffer effects on water temperature: The role of spatial configuration and daily temperature fluctuations

“…Developing woody vegetation (large trees) in a 10 m buffer directly adjacent to the river bank is most effective and already provides most of the effect of wider buffers. These results indicate that buffer width is less important compared to length, and woody buffers should be as long as possible to prevent the continuous heating in unshaded reaches, similar to the conclusions drawn by Stanford et al (2020). However, wider buffers up to 30 m are known to increase other functions like nutrient retention (Gericke et al, 2020; Sweeney & Newbold, 2014).…”

Riparian forests can mitigate warming and ecological degradation of agricultural headwater streams

“…Similarly, Stanford et al. (2019) found that only 1 km of riparian tree corridor could counteract 1.5°C water temperature warming in intermittent Mediterranean streams. In mid‐order channels in our study area (Turunen et al., 2019), the riparian forest did not affect stream water temperature, probably because of wider channels (less shading effect in forested reaches) and larger water volume causing higher thermal inertia (Quinn & Wright‐Stow, 2008; Ryan et al., 2013).…”

“…Several studies reported more silted stream beds in stream reaches that drain within agricultural fields compared to reaches that maintain riparian forests (e.g. Jones et al., 1999; Sponseller et al., 2001; Stanford et al., 2019). In comparison, mid‐order agricultural streams in the same area exhibited no physical stream habitat condition differences among forested and open reaches (Turunen et al., 2019).…”

“…bicaudata were identified as indicators for forested reaches whereas the chironomid midges O. dentifer and P. octomaculatus were indicators for open reaches. Across the streams, substratum size and fine sediment cover were more important correlates of invertebrate community structure than riparian canopy or forest cover, highlighting that excessive siltation of stream beds is one of the key stressors for benthic invertebrates (Piggot et al., 2015; Stanford et al., 2019; Turunen et al., 2017). Excessive sedimentation typically reduces EPT abundance and favours chironomids (Townsend et al., 2008; Turunen et al., 2017) as also observed for this study.…”

“…In this respect, conservation of remnant riparian forests or replanting riparian trees can be supported as a management tool to improve ecological status of agricultural headwater streams, at least in cases when the diffuse pollution is moderate. However, it is apparent that catchment land use, resulting in diffuse pollution and flow alterations are the major causes for ecological degradation in many streams and patchy riparian forests cannot completely reverse the negative impacts of agriculture in these streams (Harding et al., 2006; Stanford et al., 2019). While the benefits of local forested areas on the ecological status of streams reaches were evident in our study, forests did not have an effect on water quality, highlighting that it is mostly catchment scale land use that controls water quality variation.…”