Woody Encroachment in Savannah and Open Ecosystems: A Global Analysis of Patterns and Impacts

Abstract Background and Aims Woody encroachment is a growing ecological concern in savannas and grasslands, potentially disrupting the coexistence between woody and herbaceous-graminoid layers. This study aimed to assess and analyze how woody encroachment impacts the herbaceous-graminoid vegetation, identify profile traits of encroaching species, and evaluate the role of rainfall regime, soil texture and plant functional traits in modulating this impact. Methods A systematic review was conducted using Web of Science, followed by rigorous screening criteria. A meta-analysis was conducted using continuous effect sizes (Log Response Ratio and Y-index) to assess woody-herbaceous/graminoid interactions under encroachment. Using Mixed-Effects Meta-Regression (MEM) and Linear Mixed Models (LMM), we investigated the role of mean annual precipitation (MAP), soil clay content, and woody functional traits (nitrogen fixation and leaf phenology) on the impact of woody on the herbaceous-graminoid layer. Key Results Woody encroachment leads to competitive exclusion, reducing cover and mainly the diversity of the herbaceous-graminoid layer. Local soil and functional traits overrode regional climate in determining interaction outcomes. Clayey soils significantly exacerbated the competitive exclusion of the herbaceous-graminoid layer. Furthermore, evergreen and non-N-fixing species drove severe competition. Most studies were conducted in dry climates, revealing a potential observational bias in drylands. Conclusion Woody encroachment severely alters plant-plant interactions in grasslands and savannas globally, predominantly driving the competitive exclusion of the herbaceous-graminoid layer. The magnitude of this negative impact is governed by local edaphic factors and the functional profile of the encroaching species.