Regional influences of soil available water-holding capacity and climate, and leaf area index on simulated loblolly pine productivity

We simulated loblolly pine (Pinus taeda L.) net canopy assimilation, using BIOMASS version 13.0, for the southeastern United States (1 degrees latitud e by 1 degrees longitude grid cells) using a 44-year historical climate rec ord, estimates of available water-holding capacity from a natural resource conservation soils database, and two contrasting leaf area indices (LAI) (l ow; peak LAI of 1.5 m(2) m(-2) projected, and high; 3.5 m(2) m(-2)). Median (50th percentile) available water-holding capacity varied from 100 to 250 mm across the forest type for a normalized 1.25 m soil profile. Climate als o varied considerably (growing season precipitation ranged from 200 to 1600 mm while mean growing season temperature ranged from 13 to 26 degrees C). Net canopy assimilation ranged from 9.3 to 19.2 Mg C ha(-1) a(-1) for high LAI and the 95th percentile of available water holding capacity simulations . We examined the influence of soil available water-holding capacity, and ann ual variation in temperature and precipitation, on net canopy assimilation for three cells of similar latitude. An asymptotic, hyperbolic relationship was found between the 44-year average net canopy assimilation and soil ava ilable water-holding capacity. Shallow soils had, naturally, low water-hold ing capacity (<100 mm) and, subsequently, low productivity. However, median available water-holding capacity (125-150 mm) was sufficient to maintain n ear maximum production potential in these cells. Simulations were also conduced to examine the direct affects of soil availa ble water on photosynthesis (P-N) and stomatal conductance (g(s)) on net ca nopy assimilation. In the absence of water limitations on P-N and g(s), net canopy assimilation increased by only 10% or less over most of the lobloll y pine region (when compared to simulations for median available water-hold ing capacity with water influences in place). However, the production diffe rences between high and low LAT, at the median soil available water-holding capacity, ranged from 30% to 60% across the loblolly pine range. Vapor pre ssure deficit was found to dramatically reduce productivity for stands of s imilar LAI, incident radiation, rainfall, and available water-holding capac ity. Thus, these simulations suggest that, regionally, loblolly pine produc tivity may be more limited by low LAI than by soil available water-holding capacity (for soils of median available water-holding capacity or greater). In addition, high atmospheric forcing for water vapor will reduce net assi milation for regions of otherwise favorable available water and LAI. (C) 19 99 Elsevier Science B.V. All rights reserved.