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

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Abstract

We simulated loblolly pine (Pinus taeda L.) net canopy assimilation, using BIOMASS version 13.0, for the southeastern United States (1° latitude by 1° longitude grid cells) using a 44-year historical climate record, estimates of available water-holding capacity from a natural resource conservation soils database, and two contrasting leaf area indices (LAI) (low; peak LAI of 1.5 m2 m−2 projected, and high; 3.5 m2 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 also varied considerably (growing season precipitation ranged from 200 to 1600 mm while mean growing season temperature ranged from 13° to 26°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 annual 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 available water-holding capacity. Shallow soils had, naturally, low water-holding capacity (<100 mm) and, subsequently, low productivity. However, median available water-holding capacity (125–150 mm) was sufficient to maintain near maximum production potential in these cells.

Simulations were also conduced to examine the direct affects of soil available water on photosynthesis (PN) and stomatal conductance (gS) on net canopy assimilation. In the absence of water limitations on PN and gS, net canopy assimilation increased by only 10% or less over most of the loblolly pine region (when compared to simulations for median available water-holding capacity with water influences in place). However, the production differences between high and low LAI, at the median soil available water-holding capacity, ranged from 30% to 60% across the loblolly pine range. Vapor pressure deficit was found to dramatically reduce productivity for stands of similar LAI, incident radiation, rainfall, and available water-holding capacity. Thus, these simulations suggest that, regionally, loblolly pine productivity 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 assimilation for regions of otherwise favorable available water and LAI.

Introduction

Available water and soil nutrient availability are two principal factors that limit temperate forest growth (cf. Kozlowski, 1982; Linder and Rook, 1984); however, their relative contribution to growth potential, at least for loblolly pine (Pinus taeda L), remains obscured (e.g. Albaugh et al., 1998). Reduced growth potential associated with sandy soils, or soils low in water-holding capacity in general has, historically, been attributed to low soil water availability (cf. Moehring and Ralston, 1967; Cregg et al., 1988). Premature foliage senescence due to increased water stress diminishes growth potential under these conditions in loblolly pine (Vose and Swank, 1990; Hennessey et al., 1992) and Monterey pine (P. radiata) (Linder et al., 1987) plantations. However, Albaugh et al. (1998)has found evidence contrary to our `historical' expectations. Loblolly pine stands (ages nine through 14) on deep sandy soils with low fertility and low moisture availability (average annual precipitation of 1200 mm) (∼7.5 cm available water) responded significantly more to nutrition (increased leaf area index; LAI), rather than water (irrigation). Similar results were reported for Scots pine (P. sylvestris L.) (Linder, 1987; Linder and Flower-Ellis, 1992; Bergh et al., 1998).

Dynamic LAI enables the potential for a rapid growth response to nutrition in loblolly pine. Loblolly pine LAI often varies two-fold during the coarse of the year with, generally, a maximum of two foliage cohorts that each persists for roughly 18–22 months (Albaugh et al., 1998). As such, loblolly pine LAI has a seasonal pattern with a minimum in February or March and a maximum (peak) in September. The typical range in peak LAI for unfertilized loblolly pine stands across its range is 1.5–3.7 m2 m−2 (projected) (NCSFNC, 1991). However, a doubling of LAI with improved plant nutrient status from fertilization is not uncommon (Vose and Allen, 1988; Colbert et al., 1990; Albaugh et al., 1998). Thus, LAI becomes a corollary for soil nutritional status in loblolly pine.

Growth response to soil available water and nutrient availability, and their interaction, have been extensively studied for spruce and pine stands of Sweden (Aronsson et al., 1977; Linder, 1987), and pine stands in the United States (Albaugh et al., 1998), and Australia (Linder et al., 1987; McMurtrie et al., 1990). In each study, the climate/environment determines the relative importance of water or nutrition on forest growth. For example, irrigation increased productivity in Monterey pine stands by extending the growth period for a droughty site (average annual precipitation of 790 mm) with low water-holding capacity (∼10 cm) (McMurtrie et al., 1990). Growth response to increased LAI associated with fertilization was of lesser importance for this site. Conversely, Albaugh et al. (1998)found no water or nutrient effects on the length of the active growth period for developing loblolly pine stands. And, as mentioned above, they determined that low soil nutrient availability limited growth more than water on deep sandy soils with low fertility and low moisture availability.

Simulations that examine the influence of soil water-holding capacity, LAI, and climate on loblolly pine productivity have produced conflicting results. For instance, McNulty et al. (1994)found no clear correlation between simulated net primary production (NPP) and soil water-holding capacity using a generic model in Georgia. Indeed, areas with the lowest simulated NPP had high soil water-holding capacities. However, Sampson et al. (1996), using the BIOMASS model (McMurtrie and Landsberg, 1992) adapted for loblolly pine (Sampson et al., 1998), simulated loblolly pine NPP across the southeastern US. They suggest that low LAI, and not soil water-holding capacity, limited loblolly pine growth potential. A regional analyses that uses local estimates of soil water-holding capacity, in relation to climate, in a loblolly pine model has heretofore not been attempted.

Our objectives were to use the modified BIOMASS model (Sampson et al., 1998) to examine the effects of soil available water-holding capacity, LAI, and climate on loblolly pine net canopy assimilation (herein defined as gross photosynthesis minus autotrophic respiration). We used regional climate data (Cooter et al., 1993; Cooter et al., in press), and corresponding soils information (NRCS, 1996) for low (peak LAI of 1.5 m2 m−2, projected) and high LAI (3.5 LAI units) simulations. Analyses were evaluated for a 44-year historical climate record for each of 108 1° × 1° latitude by longitude cells covering the loblolly pine range.

Section snippets

Soils data

We used the USDA Natural Resources Conservation Service (NRCS) State Soil Geographic (STATSGO) Data Base (Anonymous, 1994) (ArcView 3.0 format) to determine soil water characteristics for the loblolly pine range: defined here as an area roughly delineated by the region located between 76 and 98 west longitude and 29 to 38 north latitude (excluding those northwestern regions above the 35th parallel and west of 81st meridian). The minimum spatial resolution of these data corresponded to a soil

Regional variation in available water and climate

Not surprisingly, soil available water expressed as a percentage (by weight or by volume) varied from ≈5% to more than 20%, with local and regional variation due to the individual soil series present. Regionally, median (50th percentile) available water-holding capacity, expressed on an absolute basis, ranged from 100 to 250 mm (Fig. 1). Greater water-holding capacity was found along the Mississippi Valley, with decreasing water storage capacity associated with increasing distance west into east

Discussion

Simulations examined here suggest that LAI, rather than soil available water-holding capacity, may be more important in determining the long-term average productivity of loblolly pine on soils of median available water-holding capacity or greater. As a corollary for soil nutritional status, LAI responses in southern pine to nutrient additions are well documented (Vose and Allen, 1988; Colbert et al., 1990; Vose and Swank, 1990; Gholz et al., 1991; Albaugh et al., 1998). Clearly, we have not

Conclusions

These simulations suggest that, regionally, loblolly pine productivity may be more limited by low LAI than by soil available water, at least for soils of median available water-holding capacity or greater. The production response to available water (that simulated for unlimited available water as compared to median available water) was generally 10% or less throughout most of the forest type. The production response to increased LAI, however, was much greater. An increase of 2.0 LAI units

Acknowledgements

We thank Drs. Sune Linder and Mark Ducey for reading an earlier draft of this manuscript and two anonymous reviewers. We also thank Dr. Robert Luxmoore and Oak Ridge National Laboratories for cooperative participation in this project. This study was funded, in part, by the DOE subcontract: SW918C - - and cooperative work from the Global Change in Terrestrial Ecosystems (GCTE) network.

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