Hai-Yang Zhang, Xiao-Tao Lü, Alan K. Knapp, Henrik Hartmann, Edith Bai, Xiao-Bo Wang, Zheng-Wen Wang, Xiao-Guang Wang, Qiang Yu, Xing-Guo Han
Woody plant encroachment, defined as an increase in abundance of indigenous woody plants in formerly grass-dominated ecosystems, has been frequently reported in arid, mesic, alpine and arctic areas worldwide. How shrubs coexist with grasses along a climatic gradient and the consequences of their interactions for ecosystem structure and function remain unclear. According to the stress-gradient hypothesis, facilitation (positive interactions) between plants increases in stressful environments, for example as rainfall declines. However, a number of studies report a collapse of facilitation under high-stress conditions, thereby challenging the stress-gradient hypothesis. The collapse of facilitation is typically driven by the reduced “nurse plant effect” under low rainfall condition, especially for legumes, since symbiotic N fixation is highly sensitive to water availability. Investigating nurse plant effects on resource availability could help assess the validity of the stress-gradient hypothesis. Here, we first conducted a meta-analysis by synthesizing a global dataset from 66 studies and evaluated how shrubs affected soil organic carbon (C) and nitrogen (N) in grasslands along a precipitation gradient. Because this global analysis included a lot of variability among different grasslands and shrubs traits, we then took new measurements in a single grassland type encroached by leguminous shrubs from a single genus (Caragana). We investigated how shrubs affected the soil N pool and how shrub-grass interactions varied along a precipitation gradient (147-342 mm) in a temperate steppe of China. At the global scale, nurse plant effects on the soil N pool increased with precipitation for leguminous but not for non-leguminous shrubs. Results from the field study showed that facilitation became relatively weaker as water stress increased, indicating a collapse of facilitation. Our results highlight that plant-plant interactions can be altered by precipitation, and that shrub functional traits (leguminous, i.e. N-fixing, or not) should be included as mechanisms in conceptual frameworks of plant facilitation.