Encroachment diminishes herbaceous plant diversity in grassy ecosystems worldwide

Abstract

Woody encroachment is ubiquitous in grassy ecosystems worldwide, but its global impacts on the diversity of herbaceous plants that characterise and define these ecosystems remain unquantified. The pervasiveness of encroachment is relatively easily observed via remote sensing, but its impacts on plant diversity and richness below the canopy can only be observed via field‐based studies. Via a meta‐analysis of 42 field studies across tropical to temperate grassy ecosystems, we quantified how encroachment altered herbaceous species richness, and the richness of forbs, C3 graminoids and C4 graminoids. Across studies, the natural logarithm of the response ratio (lnRR) of herbaceous species richness ranged from −3.33 to 0.34 with 87% of encroached ecosystems negatively impacted. Assessment of the extent of encroachment, duration of encroachment, mean annual rainfall, latitude, and continent demonstrated that only extent of encroachment had relevance in the data (univariate model including a random effect of study explained 45.4% of variance). The global weighted mean lnRR of species richness decreased from −0.245 at <33% of woody cover increase, to −0.562 at 33%–66%, and to −0.962 at >66%. Continued encroachment results in substantial loss of herbaceous diversity at medium and high extents, with a loss of richness that is not replaced. Although all functional groups are significantly negatively impacted by encroachment, forb richness is relatively more sensitive than graminoid richness, and C4 graminoid richness relatively more than C3 graminoid richness. Although no geographic or climatic correlates had relevance in the data, encroachment as an emergent product of global change coalesces to decrease ground layer light availability, lead to loss of fire and grazers, and alter hydrology and soils. Encroachment is accelerating and grassy ecosystems require urgent attention to determine critical woody cover thresholds that facilitate diverse and resilient grassy ecosystems.

Seasonal community stability increased with water addition and shrub removal but reduced with nitrogen addition in semi‐arid grassland

“…This question is especially relevant in semi‐arid grasslands, which are increasingly experiencing shrub encroachment (Bai et al, 2020; Maestre et al, 2009). Shrub encroachment, that is, increases in the density, coverage and biomass of shrubs, is an important ecological and environmental problem faced by global grassland ecosystems (Eldridge et al, 2011; Ratajczak et al, 2012; Wieczorkowski & Lehmann, 2022). Studies have shown that nearly 10%–20% of the area in semi‐arid regions are experiencing shrub encroachment (Reynolds et al, 2007).…”

Socio-economic factors strongly determine megafauna biomass in African savanna protected areas

“…The percentage of open savanna, the main natural driver, likely supports higher megafauna biomass by promoting savanna heterogeneity (e.g., grassland-woodland mosaics) and, as a result, increasing megafauna functional diversity 55,56 . This result also reiterates the threat posed by bush encroachment on Africa's biodiversity and ecological functioning 57,58 , and why management actions that may favour woody encroachment for other purposes (e.g. carbon sequestration) are ill advised 59 .…”

Should we burn the Cerrado? Effects of fire frequency on open savanna plant communities

“…The composition dissimilarities between burned plots suggest that the fire legacy shapes different plant communities. Eight years of fire exclusion led to the beginning of woody encroachment, with the presence of tree species (absent in burned plots), which may lead to a loss of resilience in these plant communities, resulting in a decrease in species and functional diversity as well as changes in the state and functioning of the community (Hoffmann et al, 2012; Abreu et al, 2017; Wieczorkowski & Lehmann, 2022). Therefore, the presence of fire and how often the area is burned will influence both the maintenance of Cerrado open savannas and their diversity.…”

“…Woody species may encroach into savannas rapidly in the absence of fire: in a 4‐year period, a great increase in species richness and the density of trees and generalist species could be observed in FE savannas (Pilon, 2019). This process of encroachment can have several consequences for the system, such as alterations in the structure and diversity, thus affecting its functional composition and functioning (Abreu et al, 2017; Pilon, Cava et al, 2021; Wieczorkowski & Lehmann, 2022). Moreover, woody encroachment would alter the local fire behavior due to changes in the fuel load quantity and type, and as a consequence, the flammability of the system (Newberry et al, 2020).…”