# Drought and Carbon Cycling of Grassland Ecosystems under Global Change: A Review
## 3 Syntheses and Perspectives
### A Framework for Assessment and Application of Combined Effects  
Practical fluctuations with the background of global changes would result in an intricate blend of internal processes and external forces on ecosystem, which can trigger a regime shift in grasslands [173]. According to the 'fluctuating resources hypothesis', ecosystems become more susceptible to biological encroachment whenever the amount of idled resources increases in two basic ways: declines in the supply and use of ecosystem or external resources. A disturbance such as drought, grazing, or other land-use change could damage or kill resident vegetation and reduce resource uptake (light, water, and nutrients) [174]. Drought coupled with global change drivers or with favorable weather conditions can induce plant mortality or promote seed germination and ecological invasion, respectively [175]. It is clear that several interacting global change drivers and droughs trigger shifts in ecological thresholds [12]. However, there is not enough knowledge about how drought affects biodiversity and ecosystem function. The framework describes a hierarchy of mechanisms that climate change uses to impact ecosystem \(C\) dynamics, generating three levels of response, and also provides a new approach to the studying climate change impacts on \(C\) cycling [176]. The expression of functional thresholds is affected by many factors including droughs, other global change drivers, and ecosystem characteristics. Combined multi-factor effects may make ecosystems surpass ecological thresholds, resulting in a reconfigured ecosystem structure and function and a profound influence on \(C\) cycling. Rapid ecosystem shifts are triggered by droughs occurring within the background of gradual global change, overgrazing, land-use change, and invasive species [177], and is projected to increase in the future because ecosystem resilience is corroded by chronic global changes [12]. The interactions between drought and global change drivers influence and accelerate the trajectories of ecosystem response [178]. An ecotone shift caused by drought has severe consequences on ecological and accelerates soil erosion, which induces large losses of soil \(C\)[173]. The shifts of grassland composition in response to seasonal or annual precipitation regimes represent drought-induced alterations of vegetation dynamics that may trigger threshold development [12]. Ecological thresholds must further evaluate and develop to effectively manage grassland \(C\) cycling and fully assess the ecological consequences of climate extremes for ecosystem structure and function [179]. Threshold recognition and prediction is significant to help managers prevent the emergence of undesirable states and promote the management of grassland \(C\) cycling [180]. We do not know the ecological mechanisms trigger the ecological threshold, or the speed and degree of response to the triggering threshold. Therefore, a systematic framework that assesses ecosystem dynamics in response to drought under current and future global change and human activity is urgently needed to focus on the combined effects that drought and global change drivers have on an ecosystem and to assess the hierarchical response of that ecosystem [181; 182]. More research is needed to create a framework that meets the following needs:1. Defines the intensity and duration of drought at which ecological thresholds are triggered;
2. Represents ecological mechanisms of response to drought under current and future global change scenarios at different spatial-temporal scales;
3. Assesses the hierarchical responses of an ecosystem to drought and global change, including individual, species, and ecosystem responses;
4. Quantifies the contribution of other global change drivers that prevent ecological thresholds triggered by droughts;
5. Provides a baseline to assess the impact of drought under global change;
6. Assesses the contribution of ecological thresholds to the fate of grassland \(C\) cycling;
7.