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.

Emergence and establishment of native and non-native species in soils of remnant and converted highland grasslands - southern Brazil / Emergência e estabelecimento de espécies nativas e exóticas em solos de campos convertidos e nativos remanescentes do Planalto - Sul do Brasil

Abstract Native grasslands in the Campos de Cima da Serra, Brazil, are being converted at speed for exotic tree plantations and cropland. The impact of modified and novel soil conditions on the establishment of native grassland species is unknown; establishment of non-native species, deliberately or accidentally introduced, could be favoured. In a common garden composed of fully randomized replicate samples of soils collected from remnant grassland, former cropland and pine plantations, we tested emergence and establishment of five cold-season species: Native low-tussock grass Piptochaetium montevidense (Spreng.) Parodi; native legume Trifolium riograndense Burkart; naturalized low-tussock grass Vulpia bromoides (L.) Gray; low-tussock grass Holcus lanatus L., cultivated and naturalized in Brazil; and a cultivar of non-native Trifolium repens. Other than expected, soil type and species*soil type interactions had no significant effect on seedling emergence after 132 days in the field. Species effect on seedling emergence, however, was highly significant. Vulpia bromoides emergence was significantly highest in all soil types. Holcus lanatus and Trifolium riograndense both achieved second highest emergence rates and did not differ significantly from each other. Lowest overall emergence rates were found in the non-native clover cultivar. Lab germination tests failed for Piptochaetium, although it showed reasonable emergence in the field. Good performance of the native clover is encouraging for future grassland restoration, but the value of highly germinable Vulpia as a forage remains to be tested. Holcus tolerates a wide range of soil conditions and its life history traits may promote naturalization, or even invasiveness. Native grasslands of the region should be monitored for this species. Studies like these, but set up on a larger geographical scale and with a wider array of native species, will be essential in developing ecological restoration methods for southern Brazilian grasslands.

Resumo Uma acelerada conversão do campo nativo em plantações de espécies florestais exóticas e lavouras tem ocorrido nos Campos de Cima da Serra, Sul do Brasil. Ainda é desconhecido o impacto que as condições edáficas, em solos alterados ou preservados, exercem no estabelecimento de espécies nativas do campo; as espécies exóticas, introduzidas deliberada ou acidentalmente, talvez sejam favorecidas. Em um common garden composto por repetições aleatórias de amostras de solo, coletadas em áreas de antigas plantações de pinus, de antigas lavouras ou em áreas com campo nativo preservado, foram testados a emergência e o estabelecimento de cinco espécies hibernais: Piptochaetium montevidense (Spreng.) Parodi, gramínea cespitosa nativa; Trifolium riograndense Burkart, leguminosa nativa; Vulpia bromoides (L.) Gray, gramínea cespitosa naturalizada no sul do Brasil; Holcus lanatus L., gramínea cespitosa exótica, cultivada e disseminada no Brasil; e um cultivar de Trifolium repens, leguminosa exótica largamente utilizada. Diferentemente do esperado, o tipo de solo e interações espécie*tipo de solo não tiveram efeito significativo na emergência de plântulas após 132 dias de teste a campo. O efeito da espécie na emergência das plântulas, entretanto, foi altamente significativo. A emergência de Vulpia bromoides foi significativamente superior em qualquer tipo de solo. Ambos Holcus lanatus e Trifolium riograndense apresentaram as segundas maiores taxas de emergência, não diferindo significativamente entre si; as taxas mais baixas foram apresentadas pelo cultivar de trevo não nativo. O Teste de Germinação em laboratório falhou para o Piptochaetium, apesar de este ter demonstrado razoável emergência a campo. O bom desempenho do trevo nativo é encorajador para futuras restaurações de pastagens nativas; o valor forrageiro de Vulpia, que apresentou alta germinação, ainda precisa ser testado. Holcus tolera uma ampla faixa de condições de solo, e suas características adaptativas podem vir a torná-lo naturalizado ou mesmo invasivo. Os campos da região devem ser monitorados em função dessa espécie. Estudos como esse, mas configurados em escala geográfica maior e com maior variedade de espécies nativas, serão essenciais no desenvolvimento de métodos de restauração para os Campos Sulinos Brasileiros.

Plants adapted to warmer climate do not outperform regional plants during a natural heat wave.

With ongoing climate change, many plant species may not be able to adapt rapidly enough, and some conservation experts are therefore considering to translocate warm-adapted ecotypes to mitigate effects of climate warming. Although this strategy, called assisted migration, is intuitively plausible, most of the support comes from models, whereas experimental evidence is so far scarce. Here we present data on multiple ecotypes of six grassland species, which we grew in four common gardens in Germany during a natural heat wave, with temperatures 1.4-2.0°C higher than the long-term means. In each garden we compared the performance of regional ecotypes with plants from a locality with long-term summer temperatures similar to what the plants experienced during the summer heat wave. We found no difference in performance between regional and warm-adapted plants in four of the six species. In two species, regional ecotypes even outperformed warm-adapted plants, despite elevated temperatures, which suggests that translocating warm-adapted ecotypes may not only lack the desired effect of increased performance but may even have negative consequences. Even if adaptation to climate plays a role, other factors involved in local adaptation, such as biotic interactions, may override it. Based on our results, we cannot advocate assisted migration as a universal tool to enhance the performance of local plant populations and communities during climate change.