Scarcity in ecosystem resources is known to constrain density and structure of vegetation communities. Due to this scarcity, competition between plant individuals often becomes necessary to partition available resources. The presence of competition is typically assessed via selective removals of individuals and measurement of resulting growth responses. In water-limited systems, the medium of competition is the consumption of water. By measuring the movement of water through stems with sapflow methods, a more direct measure of competition may be possible. Using this theoretical foundation, we asked how stand- and site-level structural metrics, e.g., density and basal area, affect water consumption and competition of individual plants at a global scale. The recent development of the SapFluxNet global sapflow database allows for data access across hundreds of sites on six continents. We used the SapFluxNet database and corresponding site metadata to explore the relationship between sapflow and woody plant density with the aim of detecting density-dependent competitive interactions.
Preliminary results of our analyses of the SapFluxNet database suggest a number of key conclusions. Aggregated across biomes and continents, average sapflux increases linearly with increasing mean annual precipitation (MAP). This relationship is highly significant (p < 0.001), but weak (adj. R2 = 0.09), which is expected across the large variation in climates and plant functional types in the database. We modeled the underlying relationship between sapflow and rainfall across all biomes, then examined residuals for correlation with stand density (p < 0.001; adj. R2 = 0.2). When partitioned by biome, density-dependent relationships increase in complexity. For biomes with sufficient data, we detect consistent declines in sapflow with stand density, indicating that competition between the instrumented stem and neighbors reduces resource acquisition in individual plants. Our results provide direct evidence of density-dependent community interactions and competition for resources. Results suggest that, on average, decreasing resource availability limits resource use of individual plants. In water-limited biomes, such as grassland desert, this relationship is especially visible. Future analyses will further investigate differences in biome responses to stand density and precipitation.