Change in grass hill size can signal species diversity changes and ecosystem state transitions during alpine wetland degradation

Abstract

The prospect of likely catastrophic transitions in ecosystems requires the development of early warning indicators that will provide advance warnings for undesired transitions. A grass hill is a unique microtopography that plays a crucial role in the functioning of wetland ecosystems. However, it is unclear whether grass hill size could serve as an important warning indicator to predict vegetation dynamics and ecosystem transitions during wetland degradation. Six degradation levels were selected along a grazing gradient in the alpine wetlands on the eastern Tibetan Plateau. General linear and segment regression were used to analyze whether grass hill area (GHA) can be used as an important warning indicator to predict biotic and abiotic factors and ecosystem transitions from typical alpine wetlands to meadows. The species diversity and biomass of aboveground vegetation increased slowly before the threshold point but decreased sharply after the threshold point (GHA (grass hill area) = 0.13 m2). The species diversity of both persistent and transient seed banks and the seed density of persistent seed banks also increased slowly but increased sharply before and after the threshold point (GHA = 0.2 m2). Soil moisture (SM), soil organic carbon (SOC) and total nitrogen (TN) decreased slowly but decreased sharply before and after the threshold point (GHA > 0.2 m2). Surprisingly, we also found the same threshold point (GHA = 0.13–0.2 m2) for the ecosystem transition from alpine wetlands to meadows as that for the other factors. GHA accurately predicted biotic (species diversity of vegetation and seed bank) and abiotic (SM, SOC, and TN) changes and the ecosystem transition from alpine wetlands to meadows. Thus, grass hill area is an effective warning indicator that could be used in future alpine wetland restoration and management.