Effects of environmental variation and stand development on water use and production of five structurally-diverse Pinus species

Abstract

For five economically and ecologically important southern U.S. Pinus species, I explore: (1) the impact that canopy architecture has on needle-scale and plant-scale hydraulic traits; (2) how these differences may manifest in patterns of water use at the daily scale in response to environmental variation; and (3) how canopy and stand characteristics, as well as stand development, control leaf-scale and stand-scale transpiration on interannual timescales. For two common-garden, multi-year sap-flux monitoring studies, I quantified sensitivity to vapor pressure deficit and soil moisture among species or genotypes within the same environment, compared their sap-flux density and leaf transpiration under standardized reference conditions, and explained long-term variation in water use as a result of increasing sapwood area, leaf area, and tree height. Daily-scale sap-flux density and leaf-level transpiration were broadly conservative within a study for a given planting density, primarily reflecting local environmental conditions even in the presence of established hydraulic trait differences. Stand-scale variation in sap-flux and transpiration at longer timescales broadly reflected the impacts of stand development and intraspecific competition, causing water use per unit sapwood or leaf area to decline over time. This led to a decoupling of stand transpiration from sapwood area per unit of ground area, occurring within the first 8-10 years of rapid stand development for some species. The interacting, often convoluted effects of within-canopy shading and increasing pathlength resistance were each explicitly considered as drivers of long-term change in transpiration. Tree height (pathlength resistance) explained interannual decline in sap-flux and leaf-scale transpiration as well as or better than variation in leaf area index. These findings suggest that Pinus transpiration may be able to be modelled assuming a single physiology, accounting for environmental variability and stage of development. Further, the addition of hydraulic architecture – especially height – to transpiration models could serve to support more accurate estimates of water use in developing stands than those that consider leaf area in isolation. Finally, due to the effects of both competition and development on leaf-scale and stand-transpiration, lower-density plantings may be associated with higher water yield to surrounding ecosystems, as well as broader-crown genotypes.