Comment on "The global tree restoration potential".

Bastin et al's estimate (Reports, 5 July 2019, p. 76) that tree planting for climate change mitigation could sequester 205 gigatonnes of carbon is approximately five times too large. Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration.

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

Despite growing recognition of the conservation values of grassy biomes, our understanding of how to maintain and restore biodiverse tropical grasslands (including savannas and open-canopy grassy woodlands) remains limited. To incorporate grasslands into large-scale restoration efforts, we synthesised existing ecological knowledge of tropical grassland resilience and approaches to plant community restoration. Tropical grassland plant communities are resilient to, and often dependent on, the endogenous disturbances with which they evolved - frequent fires and native megafaunal herbivory. In stark contrast, tropical grasslands are extremely vulnerable to human-caused exogenous disturbances, particularly those that alter soils and destroy belowground biomass (e.g. tillage agriculture, surface mining); tropical grassland restoration after severe soil disturbances is expensive and rarely achieves management targets. Where grasslands have been degraded by altered disturbance regimes (e.g. fire exclusion), exotic plant invasions, or afforestation, restoration efforts can recreate vegetation structure (i.e. historical tree density and herbaceous ground cover), but species-diverse plant communities, including endemic species, are slow to recover. Complicating plant-community restoration efforts, many tropical grassland species, particularly those that invest in underground storage organs, are difficult to propagate and re-establish. To guide restoration decisions, we draw on the old-growth grassland concept, the novel ecosystem concept, and theory regarding tree cover along resource gradients in savannas to propose a conceptual framework that classifies tropical grasslands into three broad ecosystem states. These states are: (1) old-growth grasslands (i.e. ancient, biodiverse grassy ecosystems), where management should focus on the maintenance of disturbance regimes; (2) hybrid grasslands, where restoration should emphasise a return towards the old-growth state; and (3) novel ecosystems, where the magnitude of environmental change (i.e. a shift to an alternative ecosystem state) or the socioecological context preclude a return to historical conditions.

Reforestation or conservation? The attributes of old growth grasslands in South Africa.

Deforestation as a result of burning and land conversion in the tropics and subtropics has been widely studied and active restoration of forests has been widely promoted. Besides other benefits, reforestation can sequester carbon thereby reducing CO2 emissions to the atmosphere. However, before grasslands are targeted for 'reforestation', it is necessary to distinguish whether they are ancient natural grasslands or secondary vegetation colonizing deforested areas. Here we report the results of a study comparing primary grasslands in South Africa with 4-40 year old secondary grasslands recovering from afforestation with Pinus species. Primary grasslands had significantly higher plant species richness overall, especially of forb species. Ground cover of primary grasslands was more evenly distributed among species than secondary grasslands which tended to mono-dominance. Forbs with underground storage organs (USOs) were common in primary grasslands but conspicuously absent in the recovering systems. Comparison of secondary grasslands of different ages (up to 40 years) showed negligible recovery of the original species composition. Three key features distinguish old growth primary from secondary grasslands: total and forb species numbers, evenness of species contributions to cover and the presence of USOs. Old growth grasslands also differed in their fire response, showing significant post-burn resprouting and fire-stimulated flowering in contrast to secondary grasslands. Though similar contrasting attributes of ancient and secondary grasslands have been reported in South America, more studies are needed to explore their generality in other geographical regions.This article is part of the themed issue 'Tropical grassy biomes: linking ecology, human use and conservation'.

The deforestation story: testing for anthropogenic origins of Africa's flammable grassy biomes.

Africa has the most extensive C4 grassy biomes of any continent. They are highly flammable accounting for greater than 70% of the world's burnt area. Much of Africa's savannas and grasslands occur in climates warm enough and wet enough to support closed forests. The combination of open grassy systems and the frequent fires they support have long been interpreted as anthropogenic artefacts caused by humans igniting frequent fires. True grasslands, it was believed, would be restricted to climates too dry or too cold to support closed woody vegetation. The idea that higher-rainfall savannas are anthropogenic and that fires are of human origin has led to initiatives to 'reforest' Africa's open grassy systems paid for by carbon credits under the assumption that the net effect of converting these system to forests would sequester carbon, reduce greenhouse gases and mitigate global warming. This paper reviews evidence for the antiquity of African grassy ecosystems and for the fires that they sustain. Africa's grassy biomes and the fires that maintain them are ancient and there is no support for the idea that humans caused large-scale deforestation. Indicators of old-growth grasslands are described. These can help distinguish secondary grasslands suitable for reforestation from ancient grasslands that should not be afforested.This article is part of the themed issue 'The interaction of fire and mankind'.