Water use, aboveground net primary production, and water use efficiency in a longleaf pine wiregrass savanna

Savannas are prevalent throughout the world in temperate and tropical climates. While savanna structure and diversity are well studied, few studies concurrently investigate water use and productivity patterns of the dominant life-forms, trees and grasses, and how they vary with N fertility. We investigated annual aboveground net primary productivity (ANPP) and transpiration (E) of the dominant plant types—longleaf pine and oak trees and wiregrass— in fire-maintained longleaf pine-wiregrass community. Experimental plots spanned a natural hydrologic (xeric and mesic) gradient and were randomly assigned to an N fertilization treatment (0 or 100 kg N ha^-1 yr^-1 added).

ANPP (1.3-2.2 Mg ha^-1) and annual E (108-380 mm) were low in these communities; both varied across the landscape with soil moisture and water table depth. Water use efficiency (WUE) was twice as high in xeric compared to mesic sites, and increased as water table depth increased. N increased ANPP in all plant groups, but only increased E on shallow water table sites. The ANPP response to fertilization was proportionally greater on xeric sites. In both site types, evidence that trees were using groundwater existed—WUE increased, and E and canopy stomatal conductance decreased as water table depth increased.

While some of the patterns in our data are consistent with Walter’s two-layer hypothesis regarding functional rooting zone separation between savanna plant life-forms, many are not. Our results are best explained by a conceptual model in which trees have a bimodal rooting distribution that overlaps with, and extends deeper than that of grasses.