The mechanisms affecting seedling establishment in restored savanna understories are seasonally dependent.
Abstract
Agricultural land is being increasingly abandoned on a global scale, with over 200 million hectares recovering from agricultural use. Plant community regeneration differs greatly in structure and composition after agricultural impacts, yet the mechanisms underpinning these dramatic changes are poorly understood. It is critical to determine the relative importance of abiotic and biotic factors that limit plant establishment and success during the recovery process. In particular, below-ground competition for resources in soils affected by former agricultural uses may play an important role in limiting plant establishment. Yet, below-ground competition is generally studied less than above-ground, especially in the context of land-use history. We compare plant establishment with and without below-ground competition in the context of a large-scale experiment manipulating land-use histories (i.e. with and without a history of agriculture) and restoration of historical vegetation structure (i.e. thinned and unthinned canopy trees) and determine how life stage and the local environment (e.g. soil water holding capacity, vegetation cover) influence this relationship. For three of our four target species, below-ground competition strongly limited establishment success, but did not interact with land-use history and canopy thinning directly. Instead, land-use history and canopy thinning interacted to affect establishment during germination and survival in spring, while below-ground competition limited growth during the summer. The strength of below-ground competition was affected by local resources, but the directionality of this relationship depended on agricultural history and canopy thinning. Synthesis and applications. Because adding seeds increased establishment in all cases, we recommend confronting land-use legacies by overcoming dispersal limitation with seed additions (even in degraded sites) and ensuring that below-ground structures are managed during restoration, especially in summer. In addition, managers should consider how the relationship between local resources and below-ground competition at individual sites might depend on land-use history or canopy thinning.