Sapflow Database Reveals Density-Dependent Competition Among Woody Plants at Global Scale

Though limiting resources differ among systems, water is limiting in most arid and many mesic systems, potentially allowing for direct measurement of competition by measurement of water uptake. Sapflow measurements provide a direct measure of water movement through plant stems, but, to our knowledge, sapflow has never been used to study density dependence and competition at large (regional or global) scales. Here, we examine a global database of sapflow measurements, the SapFluxNet database, for signs of density-dependent competition for water. We find that plant-level water uptake decreases with increasing competition from neighbours (specifically, neighbourhood basal area). Further analysis demonstrates that global-scale variability in annual sapflow can be largely explained (R² = 0.522) by the combination of average vapour pressure and neighbourhood summed basal area. This analysis provides a rare quantification of plant competition for a limiting resource inferred directly via measurements of resource acquisition (i.e., sapflow).