The spatial distribution and temporal variation of desert riparian forests and their influencing factors in the downstream Heihe River basin, China

Desert riparian forests are the main restored vegetation community in Heihe River basin. They provide critical habitats and a variety of ecosystem services in this arid environment. Since desert riparian forests are also sensitive to disturbance, examining the spatial distribution and temporal variation of these forests and their influencing factors is important to determine the limiting factors of vegetation recovery after long-term restoration. In this study, field experiment and remote sensing data were used to determine the spatial distribution and temporal variation of desert riparian forests and their relationship with the environmental factors. We classified five types of vegetation communities at different distances from the river channel. Community coverage and diversity formed a bimodal pattern, peaking at the distances of 1000 and 3000 m from the river channel. In general, the temporal normalized difference vegetation index (NDVI) trend from 2000 to 2014 was positive at different distances from the river channel, except for the region closest to the river bank (i.e. within 500 m from the river channel), which had been undergoing degradation since 2011. The spatial distribution of desert riparian forests was mainly influenced by the spatial heterogeneity of soil properties (e.g. soil moisture, bulk density and soil particle composition). Meanwhile, while the temporal variation of vegetation was affected by both the spatial heterogeneity of soil properties (e.g. soil moisture and soil particle composition) and to a lesser extent, the temporal variation of water availability (e.g. annual average and variability of groundwater, soil moisture and runoff). Since surface (0–30 cm) and deep (100–200 cm) soil moisture, bulk density and the annual average of soil moisture at 100 cm obtained from the remote sensing data were regarded as major determining factors of community distribution and temporal variation, conservation measures that protect the soil structure and prevent soil moisture depletion (e.g. artificial soil cover and water conveyance channels) were suggested to better protect desert riparian forests under climate change and intensive human disturbance.

The distribution pattern of desert riparian forests and its relationship with soil moisture and soil properties in the low reaches of Heihe River Basin, China

Desert riparian forests are critical habitats that provide a variety of ecosystem services in arid environments. They are also endangered ecosystem types that are sensitive to disturbance and threatened by desertification. Despite of previous studies stressed on the interactions between desert riparian forests and water availability, the lack of comprehensive information on the forests distribution range and their relationship with soil properties constraints further conservation efforts of this community under a changing climate. In this study, vegetation community characteristics, soil moisture and soil properties were investigated within a 3000 m radius around the river channel in the low reaches of Heihe River Basin, northwest China to determine the distribution pattern of desert riparian forests and their relationship with environmental factors. We found that desert riparian forests mainly distributed within the range of 2500 m from the river channel and the first 1000 m was regarded as the optimum range. Five types of vegetation communities were identified based on Two-way Indicator Species Analysis (TWINSPAN) and they gradually shifted from the riparian tree-shrub-herb communities to riparian-desert shrubs with increasing distance from the river channel. Vegetation community coverage and diversity indices formed bimodal patterns while community height and density declined significantly as the distance from the river increased. Soil moisture, soil physical properties, and soil nutrition explained 53.6 % of the variance in community characteristics and different environment variables influenced different community characteristics. Soil moisture, accounting for 62.7 % of the total explanation, mainly influenced the community coverage and density. Soil physical properties (e.g., bulk density, soil particle composition) exerted influence on shrub layer, while soil nutrition mainly affected community richness. With surface (0–30 cm) and deep (100–200 cm) soil moisture, bulk density and total phosphorus regarded as major determining factors in the community structure and diversity, conservation measures that protect the soil structure and prevent soil moisture deficiency (e.g., artificial soil cover and water conveyance channel) were suggested to better protect the desert riparian forests under climate change and intensive human disturbance.