Unveiling above‐ and below‐ground ecological strategies that underlie woody plant encroachment in grasslands

Abstract Forests and grasslands often occur side by side in the landscape, forming a complex mosaic system with contrasting environmental conditions, maintained by different fire‐vegetation stabilising feedbacks. Woody species that occur along this sharp gradient must adopt viable ecological strategies to deal with the contrasting environments of these ecosystems. For this, plants are challenged to efficiently coordinate the functioning of ecological strategy dimensions above‐ and below‐ground. We tested hypotheses related to structural changes in vegetation and associated shifts in community‐level trait patterns and ecological strategies during woody plant encroachment. We surveyed 60 permanent plots in forest‐grassland mosaics at two different times (2012–2022) to obtain data on changes in vegetation structure, species composition, abundance and ecological strategies after 10 years without disturbance, capturing a gradient from open and woody plant‐encroached grasslands to closed forests. An integrated functional approach was used to assess the different dimensions of plant trait variation, including 10 above‐ and below‐ground traits, representing whole‐plant, leaf, stem and root strategies. Woody plant encroachment led to a substantial increase in woody plant density in former grasslands, transforming their structure to resemble that of young forests. Interestingly, we found clear trade‐offs between above‐ and below‐ground traits among woody species. On the one hand, the species occurring in grassland had conservative leaves, a strategy for protection against high solar incidence, physical damage and drought, and had roots with a ‘do‐it‐yourself’ strategy, which ensures efficiency in the acquisition of nutrients and water in nutrient‐limited soils, and had thick bark related to fire resistance. On the other hand, forest species were usually taller and had acquisitive leaves, indicating highly competitive ability in light‐limited forests, whereas their roots had an ‘outsourcing’ strategy of resource uptake to mycorrhizal fungi in the nutrient‐rich soils of forests. Synthesis: We advanced the current understanding of woody plant encroachment in grasslands by showing the underlying trait‐based trade‐offs that enable woody species to occur along the transition between forest and grassland through space and time. Importantly, we have shown how below‐ground traits are important in explaining the species strategies, with a negative covariance between above‐ and below‐ground. Our integrative trait‐based approach will be helpful in better understanding and managing forest‐grassland mosaics in southern Brazil and analogous patchy ecosystems around the world.