International audienceWe expand the concept of “old growth” to encompass the distinct ecologies and conservation values of the world's ancient grass-dominated biomes. Biologically rich grasslands, savannas, and open-canopy woodlands suffer from an image problem among scientists, policy makers, land managers, and the general public, that fosters alarming rates of ecosystem destruction and degradation. These biomes have for too long been misrepresented as the result of deforestation followed by arrested succession. We now know that grassy biomes originated millions of years ago, long before humans began deforesting. We present a consensus view from diverse geographic regions on the ecological characteristics needed to identify old-growth grasslands and to distinguish them from recently formed anthropogenic vegetation. If widely adopted, the old-growth grassland concept has the potential to improve scientific understanding, conservation policies, and ecosystem management

Old-growth tropical forests harbor an immense diversity of tree species but are rapidly being cleared, while secondary forests that regrow on abandoned agricultural lands increase in extent. We assess how tree species richness and composition recover during secondary succession across gradients in environmental conditions and anthropogenic disturbance in an unprecedented multisite analysis for the Neotropics. Secondary forests recover remarkably fast in species richness but slowly in species composition. Secondary forests take a median time of five decades to recover the species richness of old-growth forest (80% recovery after 20 years) based on rarefaction analysis. Full recovery of species composition takes centuries (only 34% recovery after 20 years). A dual strategy that maintains both old-growth forests and species-rich secondary forests is therefore crucial for biodiversity conservation in human-modified tropical landscapes.

Seasonally dry tropical forests (SDTFs) provide a habitat for a diverse number of species and cover significant land areas. Yet, the amount of scientific research they have attracted is minimal. Recently, an international debate on the generality, consequences and forces keeping SDTFs in a position of relative negligence has begun. This paper assesses and compares biodiversity-related knowledge across the major Brazilian ecosystems to verify in which extent the Caatinga vegetation (a mosaic of scrubs and patches of seasonally dry forest) has been scientifically unappreciated. In fact, Caatinga attracted the lowest scores from semi-quantitative indexes describing biodiversity-survey efforts and knowledge status, with a negligible number of papers published internationally. Caatinga also supports fewer research teams and senior researchers as compared to humid tropical forests (Amazonia and Atlantic Forest). Both the knowledge index and number of published papers were explained by a model combining ecosystem area (km2), inventory index plus the current number of research teams or senior researchers assigned to Brazilian ecosystems. These findings reinforce the notion that SDTFs have experienced lower scientific efforts, but also reveal an apparent connection between institutional capacity, production/accumulation of scientific knowledge and conservation efforts. Although the research agenda for reconciling land-use and biodiversity conservation has been launched, current institutional anemia across SDTFs is likely to limit its impacts and achievements.

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.