Intensive forest management (fertilization, weed control) and the planting of fast-growing families of loblolly pine (Pinus taeda L.) can dramatically increase the rate of tree biomass accumulation, but it is unclear how tree genetics and management intensity interact to affect belowground processes. For 2.5 years in a 10–12-year-old plantation in north central Florida, we examined ecosystem carbon (C) accumulation, soil respiration (SR), total belowground C flux (TBCF), and litterfall in forests receiving different levels and types of fertilizer and weed control treatments that effectively reflected a contrast in ‘high’ vs. ‘operational’ management intensity. A fast-growing family was compared with a slower-growing family using single-family block plots. Applying high intensity silviculture treatments significantly (p < 0.05) increased C accumulation in aboveground biomass on average by 55% (20.9 Mg C ha−1) relative to less intensive silviculture, and the fast growing family accumulated 14% (6.3 Mg C ha−1) more C than the slower growing family at the end of 12 years. For the organic layer C, the high intensity silvicultural treatments significantly (p = 0.02) increased C accumulation (9.0 Mg C ha−1) and biomass increment (p = 0.04, 0.7 Mg C ha−1 y−1); however, the family treatment was not significant (p > 0.05) for either annual increment or organic horizon C. In contrast, the response of belowground C dynamics to silvicultural intensity were family specific, with the fast-growing family having significantly (p < 0.001) greater SR and TBCF under the operational treatment, while the slow-growing family showed no change in allocation with silvicultural intensity. The faster growing family also concentrated SR on its mounded planting bed position under the low silvicultural intensity, potentially making it better adapted to receiving silvicultural treatments concentrated near the tree base. These results suggest that loblolly pine’s C allocation belowground could be a characteristic to use for selecting pine families for greater growth potential, compatibility with silvicultural practices, or as a means to affect ecosystem C accumulation.

Because of scaling problems point measurements of soil CO2 efflux on a small volume of soil may not necessarily reflect an overall community response. The aim of this study was to test this hypothesis in the Biosphere 2 facility and achieve the following broad goals: (1) investigate soil net CO2 exchange–temperature relationship at the community level; (2) compare soil net CO2 exchange at the community level to the traditional sample point estimates of CO2 efflux scaled up to the community level; (3) evaluate the usefulness of a facility such as Biosphere 2 for conducting community level experiments for studying response to a climatic perturbation under controlled environmental conditions. A 550 m3 volume of soil with 282, 15 cm tree stumps was enclosed at the Biosphere 2 Center and warmed from 10 to 25 °C over a period of 34 days. Net CO2 exchange from this community was measured at various points on the soil surface with 78.5 cm2 chambers and for the whole community using each of the three bays at Biosphere 2 Center as a closed system. Soil CO2 efflux rates obtained by point measurements showed tremendous variability from location to location. At the community level and with point measurements, net CO2 exchange increased exponentially with increasing soil temperatures. Q10 values from both the point and community level measurements ranged from 1.7 to 2.5. Scaling of point measurements by soil surface area and time overestimated community rates by 36% revealing some of the limitations of point measurements. This experiment demonstrates how Biosphere 2 facility could be used to study behavior of individual components and measure responses at the community level and test our capacity to scale point in time and space measures of community processes to the community level.

We quantified the effects of nutrient and water availability on monthly whole-tree carbohydrate budgets and determined allocation patterns of storage carbohydrates in loblolly pine (Pinus taeda) to test site resource impacts on internal carbon (C) storage. A factorial combination of two nutrient and two irrigation treatments were imposed on a 7-year-old loblolly pine stand in the Sandhills of North Carolina. Monthly collections of foliage, branch, stem, bark, and root tissues were made and total non-structural carbohydrate analyses were performed on samples collected in years 3 and 4 after treatment initiation. Seasonal fluxes of carbohydrates reflected the hypothesized use and storage patterns. Starch concentrations peaked in the spring in all tissues measured; however, minimum concentrations in aboveground tissue occurred in late winter while minimum concentrations in below ground tissue occurred in late fall. Increased nutrient availability generally decreased starch concentrations in current year tissue, while increasing starch in 1-year-old woody tissue. Irrigation treatments did not significantly impact carbohydrate flux. The greatest capacity for starch storage was in below ground tissue, accounting for as much as 400kgC/ha per year, and more than 65% of the total stored starch C pool. The absolute amount of C stored as starch was significantly increased with increased nutrient availability, however, its relative contribution to the total annual C budget was not changed.

In managed forests, the amount of carbon further sequestered will be determined by (1) the increased amount of carbon in standing biomass (resulting from land-use changes and increased productivity); (2) the amount of recalcitrant carbon remaining below ground at the end of rotations; and (3) the amount of carbon sequestered in products created from harvested wood. Because of the region's high productivity and industrial infrastructure, carbon sequestration via southern pine forests could be increased, and this may benefit the nation in terms of global policy commitments.

In managed forests, the amount of carbon further sequestered will be determined by (1) the increased amount of carbon in standing biomass (resulting from land-use changes and increased productivity); (2) the amount of recalcitrant carbon remaining below ground at the end of rotations; and (3) the amount of carbon sequestered in products created from harvested wood. Because of the region's high productivity and industrial infrastructure, carbon sequestration via southern pine forests could be increased, and this may benefit the nation in terms of global policy commitments.

Loblolly pine (Pinus taeda L.) forests represent the major forest type in the southern United States. The loblolly pine region extends from Delaware and centraI Maryland south to central Florida and west to eastern Oklahoma and Texas (Fowells, 1965). The wide range of loblolly pine largely results from its rapid growth and its successful adaptation to many varieties of soil types and environmental conditions. These and other factors have made loblolly pine an important commercial species in the region. However, although loblolly pine occurs on a many types of sites, its commercial value, as measured by net primary productivity (NPP), varies tremendously and is strongly determined by variability in the local climate and stand and site conditions (McNulty et al., 1997). Uncertainty regarding potential changes in climate as a result of increasing atmospheric carbon dioxide (CO2) concentration has caused concern for the future commercial viability of loblolly pine forests.

A 2 × 2 nutrient and water factorial experiment with four replications was installed in an 8-yr-old stand of loblolly pine (Pinus taeda L.) growing on an infertile, excessively drained sandy site in Scotland County, North Carolina. After the fourth year of treatment, estimated stem volume increment, total biomass production, and peak leaf area index (LAI) increased 152

Fertilizer and irrigation treatments were applied in a 7‐ to 10‐year‐old loblolly pine ( Pinus taeda L.) plantation on a sandy soil near Laurinburg, North Carolina. Rainfall, throughfall, stemflow, and soil water content were measured throughout the study period. Monthly interception losses ranged from 4 to 15% of rainfall. Stemflow ranged from 0.2 to 6.5% of rainfall. Rainfall, leaf area index (LAI), basal area (BA), and the interactions of rainfall with LAI or BA influenced prediction models of throughfall, but not stemflow, on a stand level. We found significant differences due to the effects of treatments in the soil water of the top 0.5‐ and 1‐m soil layers by the beginning of the second growing season and throughout the remainder of the study period. Average daily water use and loss from a 1‐m soil layer reflected the low water‐holding capacity of the sand. Soil water in a 1‐m layer was rapidly depleted to within 10% of available water during periods of little or no rainfall. Irrigation did not significantly affect productivity and created a greater potential for loss of water to drainage below 1 m. On the basis of Zahner 's [1966] method of soil water depletion in a sandy soil under forest cover, total drainage to below l m was 55% of evapotranspiration in unirrigated plots and 150% of evapotranspiration in irrigated plots.

Effects of N fertilization on shoot phenology and foliage production were determined in loblolly pine (Pinus taeda L.) trees growing in the Georgia Piedmont. Two rates of N fertilizer were applied in February 1991. Shoot and foliage growth were determined at upper, middle, and lower crown positions at 1-month intervals throughout 1991. Shoot elongation of individual flushes overlapped, although there was about a I-month delay between the initiation of each successive flush. Needle elongation was closely associated with shoot elongation but lasted 2 months longer. On average, the first flush contributed about 69% to total leaf area. Shoot and foliage growth increased with crown height. Shoot elongation, needle length, number of fascicles per flush, and leaf area per flush were 9.7, 1.4, 4.7, and 6.9 times higher, respectively, in the upper crown than in the lower crown. The effects of N fertilization on shoot and foliage growth were most pronounced in the middle crown position. Leaf area production per shoot was increased by 12.4, 66.4, and 41.1% by fertilization for shoots in the upper, middle, and lower crowns, respectively. N fertilization did not alter the seasonal patterns of shoot and foliage development Both fertilized and control trees began, peaked, and ended flush and fascicle elongation concurrently.

We used the process model BIOMASS version 13.0 to simulate contemporary net primary production (NPP) and NPP response to climate projections for a doubling of atmospheric CO2 concentration from 2 general circulation models (GCMs) that vary in their CO2 sensitivity: the less sensitive GFDL and the more sensitive UKMO. Increased GCM sensitivity to CO2 is reflected in increased predictions in the magnitude, variation, and range of the climate variables. Simulations used a 40 yr historical climate record, and 2 stand and site conditions to standardize the total NPP response estimates for eighteen 1x1 degrees grid cells across the southern United States, Contemporary NPP and NPP response estimates from the 18 cells were smoothed using a cell search algorithm to obtain an NPP response index matrix for the entire loblolly pine (Pinus taeda) forest-type. We conducted a sensitivity analysis of the environmental variables projected to change in a 2xCO(2) environment to help interpret simulation output. Contemporary NPP varied from 2.5 to 8.5 Mg C ha(-1) yr(-1) over the range of loblolly pine, High leaf area index (LAI) simulations had 1.5 to 2 times the productivity of low LAI simulations, but the regional patterns were similar; NPP was correlated with regional differences in precipitation and temperature. The NPP response to future climate and atmospheric changes depended on the GCM used, and on the stand and site condition assumed. Inter-annual estimates for the 18 cell simulations resulted in a +22 to +84% NPP response for the GFDL climate projections and a -30 to +94% NPP response for the UKMO climate projections. The 40-year average NPP response for the smoothed data ranged from +43 to +65% and -1 to +94% for the GFDL and the UKMO climate projections, respectively. Consequently, the magnitude and range of the 40-year average NPP response to the climate projections was directly correlated with the GCM CO2 sensitivity. Although increased CO2 sensitivity resulted in broader extremes in the predicted temperature response, precipitation response for the 2 models was similar. The NPP response was also correlated with the patterns in predicted climate change, with regional differences coupled to local climatic conditions. Climate projections from both models produced similar NPP responses when predicted temperatures and precipitation regimes were similar. Elevated ambient CO2 had a greater effect on NPP response than temperature or precipitation in the sensitivity comparisons. Simulations indicate that a CO2 fertilizer effect, assuming no CO2 acclimation, more than compensates for declines in productivity over most of the loblolly pine forest-type associated with projected decreased precipitation and/or projected low to moderate increases in temperature and, therefore, increased maintenance respiration costs.

Annual leaf biomass production, monthly needle accretion and monthly needlefall were measured in an 11-to 17-year-old thinned stand of loblolly pine. Initial thinning levels were 7.8 m2 ha−1, 12.6 m2 ha−1, and 25.5 m2 ha−1 (unthinned). A light thinning was done again at Age 14. Annual variations in annual leaf biomass production and monthly variations in monthly needle accretion and needlefall were related to measured stand and weather variables. Age variations in annual leaf biomass production occurred over the 6 year study period. The variation in annual leaf biomass production was best quantified as a quadratic function of stand basal area and average weighted temperature for the months of June, July, August and September. Although stand basal area was the major determinant of annual leaf biomass production, an increase in average temperature from 24.5 to 26.5°C resulted in a 27% reduction in annual leaf biomass production. This was translated to an approximate reduction of 7.3 m2 ha−1 year−1 of stemwood. Monthly needle accretion varied little between years or with stand density. Thus, a single normalized logistic function was suitable for describing monthly needle accretion for all 6 years. Monthly needlefall was variable from year to year. Variation in needlefall was low for a period of 7 months (January 16–August 15). During this period monthly needlefall averaged from 3 to 8% of the previous year's annual leaf biomass production at the beginning of the phenological year. Variation in this 7 month period was not consistently related to stand density or any of the weather variables considered in this study. Monthly needlefall from August 16 to January 15 was extremely variable. This variability was not related to stand density. The weather variable that explained most of the monthly variation in needlefall during this period was the average rain-potential evapotranspiration determined for the 2 months preceding a monthly needlefall event. Peak needlefall was found to occur 2 months earlier in a drought year than in a year when rain-potential evapotranspiration was high.

Temperature-independent fluctuations in stem CO(2) efflux were measured in Pinus taeda L. seedlings. Stem CO(2) efflux was measured during high and low transpiration rates, high and low net photosynthesis rates, and normal and interrupted substrate supply conditions. Stem CO(2) efflux rates were an average of 6.7% lower during periods of high transpiration compared to periods of low transpiration. This difference in stem CO(2) efflux rates was not due to water stress. The most likely cause was movement of respiratory CO(2) in the transpiration stream. Interruption of substrate supply to the stem by phloem girdling reduced stem CO(2) efflux rates. Increasing net photosynthesis rates from low to high had no effect on stem CO(2) efflux, but decreasing net photosynthesis from high to low caused relatively small reductions in stem CO(2) efflux. These results indicate that diurnal changes in net photosynthesis rate may play a small role in temperature-independent afternoon depressions of stem CO(2) efflux. The transport of respiratory CO(2) by the transpiration stream compromises measurements of woody tissue respiration obtained by commonly accepted gas exchange techniques. This phenomenon could also affect measurement of leaf net photosynthesis and branch woody tissue respiration.

Objectives, approach, and results of the modeling required to adapt the management oriented growth and yield model (PTAEDA2) to function as a driver for the biologically based process model (MAESTRO), the first step towards a complete linkage of the two, are presented. MAESTRO is a model that estimates the carbon gain of an array of trees in a stand by calculating and integrating the effects of numerous biological and physical variables on photosynthesis and respiration processes. Model outputs are the radiation flux densities at any point within the stand, hourly and daily totals of radiation absorbed, and photosynthesis and transpiration by leaves within the crown of one or more target trees. The tree factors are then summed to the stand level. PTAEDA2 is a distant-dependent, individual-tree model that simulates the growth and yield of a plantation of loblolly pine (Pinus taeda L.). Each individual tree's survival and maximum growth potential at any given age is adjusted by the influences of competition and change in photosynthetic capacity. Tree and stand growth are predicted on an annual basis. For the initial linkage, PTA-EDA2 outputs tree and stand descriptive information of the type and format required for input into MAESTRO, with only accompanying minor changes necessary to the receptor model. New relationships and modifications of prediction equations for individual tree mean crown radius, crown shape, crown length, and the vertical and horizontal distributions of foliage biomass necessary for this linkage are presented.

Xylem pressure potential, leaf conductance, transpiration, and soil moisture were measured during three summers following precommercial thinning of a 10-year-old stand of loblolly pine (Pinustaeda L.) in southeastern Oklahoma. The stand was thinned to three target basal-area levels: 5.8, 11.5, and 23 m2•ha−1 (control). Soil water potential increased significantly in response to thinning during the summer of each year studied. However, plant water relations were relatively unaffected by the treatments. Significant thinning effects on diurnal xylem pressure potential were observed on only 7 of 55 measurement periods. Treatment differences in conductance and transpiration observed during the first year of the study appeared to be related to differences in light interception and crown exposure. Regression analysis indicated response of leaf conductance and transpiration to predawn xylem pressure potential and vapor pressure deficit was not affected by the thinning treatments. Overall, the results of this study are consistent with a hypothesis in which transpiration, leaf area, and water potential interact to form a homeostatic relationship.

A 10-year-old stand of loblolly pine (Pinustaeda L.) in southeastern Oklahoma was thinned to three target basal-area levels: 5.8, 11.5, and 23 m2•ha−1 (control). Specific gravity, latewood percentage, date of transition from earlywood to latewood, growth, and climate variables were measured for 2 years after thinning. Variation in the measured wood properties was more influenced by climatic variation than by the thinning treatments. Diameter growth and per-tree basal-area growth were significantly greater on the thinned treatments both years after thinning. However, stand basal-area growth was greatest on the unthinned treatment. Basal-area growth rates were significantly related to stand basal area, tree size, soil water potential, and air temperature. Early in the summer, growth was positively related to mean daily temperature, while later in the summer, growth was negatively related to mean daily temperature, reflecting the influence of high-temperature stress on growth. A year with high summer rainfall (1984) resulted in wood with a higher percentage of latewood and higher specific gravity than wood produced in a year with low summer rainfall (1985). The date of latewood initiation was significantly related to tree size, soil moisture, and evaporative demand. The date of transition from earlywood to latewood occurred 10–14 days sooner on the unthinned plots in both years. However, annual ring latewood percentage and specific gravity were not significantly affected by thinning. Increased late-season growth rates compensated for the later transition date on the thinned treatments, resulting in no net change in ring latewood percentage due to thinning. The results indicate that individual tree basal-area growth can be increased by thinning without reducing wood density.