Savanna research has emphasized tree effects on soils, microclimate and grasses. Little is known of tree-tree interactions and how these might influence sacanna function and dynamics. Furthermore, savanna research has largely been conducted where individual trees are scattered throughout a grassy matrix. However, the formation of tree and shrub islands is a common, but little-studied phenomenon in many arid land ecosystems.
This collaborative project will quantify mechanisms of facilitative and competitive interactions between an overstory tree, honey mesquite, and associated understory shrubs in a subtropical savanna ecosystem in south-central Texas. The goal of the project is to elucidate mechanisms of patch dynamics associated with the physiognomic transformation of grassland and open savanna to closed-canopy thorn woodland. The specific objectives are to elucidate the mechanisms associated with the initiation and development of tree-shrub islands, and to predict how species interactions in these islands change across topoedaphically diverse landscapes over time. The approach to this research will involve a combination of selective removal and reciprocal transplant experiments that will be coupled with manipulations of resources (light, nutrients and soil moisture) and measurements of plant growth, foliar chemistry, leaf gas exchange, plant/soil water relations and isotopic abundances. Results will increase our ability to generalize the roles of facilitation and competition in succession and mechanisms of woody plant increase in grasslands and savannas, enhance our ability to represent and parameterize spatially explicit simulation models for tree-shrub-grass ecosystems, and contribute directly to the development of a sound basis for monitoring, managing and manipulating grassland and savanna ecosystems.
