Earth’s ancient grasslands and savannas—hereafter old-growth grasslands—have long been viewed by scientists and environmental policymakers as early successional plant communities of low conservation value. Challenging this view, emerging research suggests that old-growth grasslands support substantial biodiversity and are slow to recover if destroyed by human land uses (e.g., tillage agriculture, plantation forestry). But despite growing interest in grassland conservation, there has been no global test of whether old-growth grasslands support greater plant species diversity than secondary grasslands (i.e., herbaceous communities that assemble after destruction of old-growth grasslands). Our synthesis of 31 studies, including 92 timepoints on six continents, found that secondary grasslands supported 37% fewer plant species than old-growth grasslands (log response ratio = −0.46) and that secondary grasslands typically require at least a century, and more often millennia (projected mean 1,400 y), to recover their former richness. Young (<29 y) secondary grasslands were composed of weedy species, and even as their richness increased over decades to centuries, secondary grasslands were still missing characteristic old-growth grassland species (e.g., long-lived perennials). In light of these results, the view that all grasslands are weedy communities, trapped by fire and large herbivores in a state of arrested succession, is untenable. Moving forward, we suggest that ecologists should explicitly consider grassland assembly time and endogenous disturbance regimes in studies of plant community structure and function. We encourage environmental policymakers to prioritize old-growth grassland conservation and work to elevate the status of old-growth grasslands, alongside old-growth forests, in the public consciousness.

长期以来，科学家和环境政策制定者一直将地球上的古老草原和热带稀树草原（以下简称旧草原）视为具有低保护价值的早期演替植物群落。具有挑战性的观点表明，新兴研究表明，旧草地支持大量的生物多样性，如果被人类土地用途（例如耕作农业，人工林）破坏，则恢复缓慢。但是，尽管人们越来越关注草地保护，但尚没有关于次生草地（次生草地被破坏后聚集的草本群落）是否支持更大的植物物种多样性的全球检验。我们对31项研究进行了综合，其中包括六大洲的92个时间点，发现次生草原比旧生草原支持的植物种类少37％（对数响应比= −0.46），并且次生草原通常至少需要一个世纪，更通常是几千年（预计平均1400年）才能恢复其先前的丰富度。年轻的（<29岁）次生草地由杂草组成，即使其丰富程度在数十年至数百年间不断增加，次生草地仍缺少典型的老龄草地物种（例如长寿多年生植物）。根据这些结果，关于所有草地都是杂草丛生，被火和大型食草动物困住，处于被捕接续状态的观点是站不住脚的。向前进，我们建议生态学家在研究植物群落结构和功能时应明确考虑草地的聚集时间和内源性干扰机制。我们鼓励环境政策制定者优先考虑旧草场的保护，并在公众意识中努力提高旧草场以及旧林的地位。