Abandoned pastures cannot spontaneously recover the attributes of old‐growth savannas

Abstract

Active restoration strategies have been recommended to recover Neotropical savannas in abandoned lands, but no studies have investigated the trajectories and speeds of spontaneous recovery for these systems. Research into the dynamics of degraded savannas is urgently needed to guide restoration decision making. We analysed the dynamics of secondary savannas in the Brazilian Cerrado by sampling 29 abandoned pastures (time since abandonment ranging from 3 to 25 years) and applying the space‐for‐time substitution method. We modelled the temporal changes in plant community attributes and estimated the time (years) required for these attributes to match those of two reference ecosystems (three replicates each), old‐growth savanna and a forest‐type savanna, which had encroached following fire suppression (encroached savanna). We also analysed the plant community composition of the study sites. Our models showed that tree canopy cover, richness and density rapidly increased with time since pasture abandonment, easily surpassing the values of the old‐growth savanna (28 years) and reaching the values of encroached savanna 49 years after abandonment. The cover and richness of the ground layer increased at a much slower pace. Since the species in this layer, including the exotic grasses, are shade intolerant, they will be eliminated by canopy closure over time. Up to 25 years after abandonment, secondary savannas continued to lack many (37%) old‐growth savanna species, mostly from the ground layer (82% of grasses absent). This period was also not sufficient for the secondary savannas to become floristically similar to the encroached savannas, which are dominated by shade‐tolerant tree species. Synthesis and applications. Despite the reported high natural regeneration of Neotropical savanna vegetation, abandoned pastures will not spontaneously return to an old‐growth savanna state. Protected from fire and lacking the native ground layer, the end state of secondary savannas will be a low‐diversity forest. If restoration goals include the recovery of old‐growth savanna biodiversity and structure, interventions are required to prevent woody encroachment and reintroduce native grasses, forbs and shrubs. However, if the desirable endpoint is a low‐diversity forest, passive restoration (non‐intervention) and fire protection are appropriate.

Resilience and restoration of tropical and subtropical grasslands, savannas, and grassy woodlands

“…Overgrazing, fire exclusion, and woody encroachment can replace tropical old-growth grasslands with low-diversity swards of exotic grasses or dense tree cover in a matter of years to a few decades (e.g. Cava et al, 2018). Grassland conversion for agriculture, plantation forestry, or mining not only destroys plant and animal communities, but also profoundly changes chemical and physical soil features Le Stradic et al, 2018a).…”

“…Yet in the absence of fire, exotic-dominated tropical grasslands will become low-diversity forests. Indeed, across a chronosequence of abandoned pastures invaded by African grasses in Brazil, Cava et al (2018) found that herbaceous savanna plants did not spontaneously regenerate and that fire exclusion promoted the formation of low-diversity forests, which lacked the historical old-growth savanna structure and species composition. Further complicating matters, exotic grass invasions can be promoted by cattle, which disperse seeds and influence apparent competition between native and invasive species (e.g.…”

“…One aspect of fire management in restoration is clear: without fire, grasslands do not recover on abandoned agricultural lands. Instead, they develop into low-diversity forests Abreu et al, 2017;Cava et al, 2018).…”

Tailoring restoration interventions to the grassland‐savanna‐forest complex in central Brazil

“…Fertilizers and agricultural limestone are commonly added to enrich soil and neutralize acidity, frequently favoring invasions by exotic grasses (Bustamante et al ). Pastures are typically seeded with highly competitive exotic grass species, which rapidly spread into agricultural fields and degraded areas, increasing fire frequency (Pivello et al ), outcompeting native species, and decreasing the resilience of the herbaceous layer (Cava et al ; Silva & Vieira ; Coutinho et al ).…”

Energy and Environmental Aspects of Using Eucalyptus from Brazil for Energy and Transportation Services in Europe

“…Land use changes are intrinsically uncertain to model, and difficult to reverse since, for example, pasture abandonment does not lead to the spontaneous restoration of original old-growth ecosystems [79]. Despite the fact that conversion of pasture to annual crops is the most frequent conversion in Brazil [80], its conversion to forest or perennial bioenergy crops is likely to promote many environmental benefits, including the increase in soil carbon stocks [81,82].…”