Robert F. Wittwer

Water use in thinned loblolly pine plantations

Abstract Soil moisture and throughfall were monitored for two growing seasons under three thinning intensities: basal areas of 7.8 m 2 ha −1 , 12.6 m 2 ha −1 and 26.6 m 2 ha −1 (unthinned control) in an 11- to 12-year-old loblolly pine ( Pinus taeda L.) plantation. The first year after thinning soil water decreased at a rate of 0.8 mm day −1 , 1.0 mm day −1 and 1.4 mm day −1 , respectively, in the 7.8 m 2 ha −1 , 12.6 m 2 ha −1 , and 26.6 m 2 ha −1 basal area plots between May and late August. The more rapid development of soil moisture deficits in the unthinned stand was owing to both greater soil moisture use and reduced throughfall. For the same time period in the second year, with below-normal rainfall, soil moisture deficits developed at a rate of 1.5, 1.5 and 1.7 mm day −1 for the two heavily thinned and the unthinned treatments. Estimated growing season soil water use rates were 4.4 mm day −1 , 4.2 mm day −1 , and 4.0 mm day −1 , respectively for the 7.8 m 2 ha −1 , 12.6 m 2 ha −1 and 26.6 m 2 ha −1 basal area levels in Year 1. The second year after thinning water use rates for the same period were 2.7 mm day −1 , 2.6 mm day −1 , and 2.5 mm day −1 , respectively. The results of this study suggest that the capacity to manage available water in loblolly pine stands by thinning is more a function of reduced interception loss and increased throughfall than reduced water use and is also weather dependent. Growing seasons with low rainfall or only one or two large rainfall events will not permit much opportunity to manage soil moisture.

Relationship between throughfall and stand density in a Pinus taeda plantation

Abstract Throughfall was sampled from mid-May to November 1 during 1984 and 1985 in an 11-year-old Pinus taeda L. plantation subjected to various intensities of precommercial thinning. A prediction equation relating throughfall to basal area and gross rainfall for individual storms was developed. Throughfall data collected periodically during 1986 were used to test the prediction equation. Throughfall during the growing-season increased by approximately 3% of total rainfall for every 4m2/ha reduction in basal area. Depending upon the role of stemflow, additional input to the forest floor from throughfall, when reducing the basal area from 26.6 m2/ha to 12.62/ha, would range from 22 to 60 mm. This additional water would increase the amount of soil moisture available to the stand during the growing-season.

Effects of stand development and weather on monthly leaf biomass dynamics of a loblolly pine (Pinus taeda L. ) stand

Abstract 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 years 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.

Needle litterfall prediction models for even-aged natural shortleaf pine (Pinus echinata Mill.) stands

Abstract Needle litterfall was collected over seven growing seasons in two even-aged natural shortleaf pine (Pinus echinata Mill.) stands in southeast Oklahoma. The data were used to develop a model quantifying the relationship of stand and site characteristics and weather variables to annual needlefall. This model is of use to forest managers interested in forecasting future fuel loads, or producing alternative forest products. Because of their proximity to the western edge of the natural range of shortleaf pine, long-term monitoring of these stands may yield information about the effects of climate change on litterfall. Spring temperatures during the growing season in which the needles were produced, the amount of needlefall that occurred two seasons previous, and site index were statistically significant explanatory variables in the model. The model explained 63 percent of the variation in the data. A comprehensive set of statistical misspecification tests employed to verify the assumptions of linear regression models indicated the hypotheses of normality, linear functional form, homoskedasticity, parameter stability, and independence could not be rejected. Finally, because historical needlefall data may not exist for many stands, a three-stage least-squares model was developed to forecast needlefall quantities.

Estimating seed quantity and quality in shortleaf pine (Pinus echinata Mill.) cones from natural stands

The quantity and quality of seeds were studied in 886 shortleaf pine cones collected from 48 trees in each of two 15-ha natural stands in western Arkansas, USA. The number of seeds per cone ranged from 0 to 102, and averaged 34.6 and 38.0 in the two stands. Between 30 and 50 percent of the seeds were sound. Overall mean values for total number and total sound seeds compare with previous work. However, the wide variation between individual trees in a stand needs to be considered when evaluating cone abundance and estimating potential seed yields. For the stands observed in this study, a sample of 65 to 85 cones would be needed to estimate the number of sound seeds per cone (within ±3 seeds) with 95 percent probability. Sampling 1 or 2 cones from each of about 3 trees per ha would provide a reliable estimate of sound seed contents.

Comparison of branch biomass relationships for North Carolina and Oklahoma/Arkansas loblolly pine seed sources growing in southeastern Oklahoma

Abstract Comparison of branch-level foliage and branch (wood+bark) biomass relationships for North Carolina Coastal (NCC) and Oklahoma/Arkansas (O/A) loblolly pine ( Pinus taeda L.) seed sources provided an indication of biomass partitioning differences between these two seed sources. The objective of this study was to quantify and compare the branch-level foliage and branch biomass of NCC and O/A provenances on an excessively drained site in southeastern Oklahoma to assess their branch-level biomass partitioning patterns; a modeling process was developed to accomplish this objective. It was found that seed source significantly influenced the amount of foliage per branch. If tree and branch dimensions were held constant, NCC branches would carry approximately 30% more foliage per branch than O/A branches. The relationship between tree and branch dimensions and branch production did not differ for the two seed sources. Vertical distributions of branch and foliage biomass were found to be similar for the two seed sources as well. Thus, on the droughty site observed in this study: (1) the NCC seed source tended to partition more biomass into the foliage component at the individual branch level than did the O/A seed source; (2) the two seed sources were similar in their propensity to partition biomass into the branch component at the branch level, and (3) the two seed sources were similar in the vertical distribution of branch and foliage biomass within the crown.

Segregation and linkage relationships of isoenzymes in shortleaf pine (Pinus echinata Mill.)

Abstract Segregation and linkage relationships were analyzed between 28 isoenzyme loci in ten natural stands representing much of the natural range of Pinus echinata Mill. (shortleaf pine). A total of 203 possible two-locus combinations were tested. Three linkage groups were revealed in this study at a linkLOD of 4.0. The first linkage group (A) consisted of Pgi and Adh-1; Gdh, Idh, Skdh-2, G6pd-2 and Aco were mapped to the second linkage group (B); the third group (C) had 2 loci: Mdh-2 and Mdh-3. A moderate linkage between Mnr-2 and Dia-2 and weak linkages between Mnr-1 and Dia-1, and Got-2 and 6pgd-2 were also detected. The significance of these results in shortleaf pine is discussed and compared to linkage maps previously reported in other conifers, including pines.