Relationships between Riparian Forest Fragmentation and Biological Indicators of Streams

Abstract

Anthropogenic activities, such as land use and land cover modifications in riparian areas, can alter the degree of fragmentation of riparian vegetation, lead to the degradation of stream habitats, and affect biological communities in the streams. The characteristics of the riparian forests can modify the condition of stream environments and the transporting mechanisms of materials, sediments, nutrients, and pollutants loaded from the watersheds. This study aimed to examine the relationships between forest fragmentation and three biological indicators of trophic diatom, benthic macroinvertebrate, and the fish assessment in the Nakdong River, Korea. Eighty-nine biological assessment sampling sites in the National Aquatic Ecological Monitoring Program of South Korea were identified. For each sampling site, riparian forest data within a 500 m radius were extracted from national LULC using GIS to compute fragmentation metrics using FRAGSTATS software. Four fragmentation metrics—number of forest patches, percentage of riparian forest cover (PLAND), largest riparian forest patch index (LPI), and riparian forest division index (DIVISION)—were correlated with the biological indicators. Also, due to severe spatial autocorrelation among observations, the fragmentation metrics and stream environmental variables were regressed to biological indicators using regression tree analysis. Our results indicate that the biological indicators were significantly associated with most forest fragmentation metrics. We found positive correlations of PLAND and LPI with biological indicators, whereas DIVISION was negatively correlated with biological indicators. Both correlation and regression tree analyses revealed that the biological conditions of streams were likely to be better if riparian forests are less fragmented. Particularly, stronger relationships were revealed between macroinvertebrates and fish with the fragmentation metrics of riparian forests than with benthic diatoms. However, these relationships varied with elevation, stream size, and slope conditions. The results of this study reinforced the importance of including riparian forests in the planning, restoration, and management of stream environments. These results also suggested that planners and managers may need to consider different strategies for different stream environments and topographic characteristics in managing riparian forests.

Multi-Scale Assessment of Relationships between Fragmentation of Riparian Forests and Biological Conditions in Streams

“…These results suggested that biological conditions in streams were better when riparian areas were covered with more forested area, contained larger forest patches, and the patches were located near riparian areas. These results confirmed the findings of a previous study, which suggested better biological condition with less fragmentation of riparian forests at a 500-m scale [8]. Thus, fragmentation of riparian forests was clearly negatively associated with biological condition of streams.…”

“…Thus, fragmentation of riparian forests was clearly negatively associated with biological condition of streams. The biological condition of a stream was generally better if riparian forests were less fragmented, regardless of riparian scale, in accordance with recent studies [1,3,8]. As discussed by Yirigui et al (2019) and others (e.g., References [1,120]), more fragmented riparian forests may not provide the benefits of intercepting rainfall, lowering run-off speed, and increasing infiltration into soils and uptake time by plants typical of riparian areas.…”

“…Land use patterns with strongly fragmented forests or no forests located in stream riparian areas have significant negative impacts on water quality and aquatic ecological communities [1][2][3][4] due to alteration of stream environments and sediment run-off mechanisms, pollution, and nutrient loading [5][6][7][8][9]. Thus, land use within riparian areas has become a key concern for stream management and restoration [10].…”

“…In addition, we examined the variation in relationships between riparian forest fragmentation and biological indicators for streams over different spatial scales (i.e., extents). The degree of forest fragmentation in riparian areas was assumed to impact the effectiveness of various mechanisms (e.g., filtering, absorbing, and up-taking) of riparian forests, as well as hydrological and biochemical runoff processes [8,[65][66][67][68], resulting in degraded stream environments (e.g., high levels of pollutants, nutrients, and sediments) and poor biological indicators [37,[69][70][71][72][73][74][75]. Additionally, we hypothesized that the negative influence of forest fragmentation on biological conditions in streams may vary with riparian buffer size due to the proximity of streams.…”

“…Furthermore, forest fragmentation within riparian areas has been directly linked with degraded water quality and stream ecosystem health [8,11,[20][21][22], and spatiotemporal changes in land use, logging, intensive forest management, and rapid economic development have played significant roles in accelerating forest fragmentation [23][24][25][26][27]. Human activities in forested areas affect various stream characteristics, such as the microclimate, local air temperature, stream water temperature, humidity, wind speed [28,29], and concentrations of nutrients, sediments, and pollutants in streams, as well as ecological conditions [8,11,[30][31][32][33][34][35][36]. However, the main characteristics of the relationship between forest fragmentation and stream ecosystems remain poorly understood, because they are associated through complex mechanisms involving numerous other factors (e.g., climate, geology, topography, and hydrological processes) [37][38][39][40].…”