Browsing by Author "Smith, David W."
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- Effects of understory vegetation manipulation on hardwood regeneration recruitment and growth in southern Appalachian forestsThompson, Jesse Warren (Virginia Tech, 1996)The successful regeneration of mature oak (Quercus spp.) forests is thought to be dependent on the presence of oak advance regeneration. However, the advance regeneration must be of sufficient size and density for oak to be competitive and become a dominant species after harvest. The presence of a dense midstory canopy of shade tolerant species has been implicated with the poor development of oak advance regeneration. Understory Vegetation Control was conducted in 1994 in average quality (SI₅₀ 17.7 - 21.9 m for upland oak) southern Appalachian forest stands to determine the effects on oak (Quercus spp.) and maple (Acer spp.) advance seedling abundance, growth, and development. Three study sites were located in southwest Virginia, and the following two treatments were implemented at each site: Understory Vegetation Control (UVC) and Control. Permanent sampling plots and individual seedlings were located to quantify the density, recruitment, and growth of advance regeneration. Competing vegetation was significantly reduced after one year by the UVC treatment, where the mean relative change in the sum of the heights of competing stems between 1 and 5 m in height was -15.9 percent for the UVC plots vs. 22.8 percent for the Control plots. Neither oak seedling recruitment nor height growth was enhanced by the UVC treatment after one year. Insufficient time has elapsed to allow for a growth response, or to determine if seedling recruitment will be enhanced by UVC. Several years may be required to determine if the UVC treatment can enhance the growth and competitive status of oak regeneration.
- Environmental determinism and forest structure and composition: a naturally replicated experimentSchiffman, Paula M. (Virginia Tech, 1990)Ecological theory states that forest succession is largely environmentally determined. Many investigators have suggested, however, that stochastic processes can frequently alter successional pathways. In particular, the colonization phase is thought to be very stochastic. This study utilizes a unique series of 6 naturally replicated sites on Brush Mountain (Montgomery Co., VA) to determine whether forest structure and species composition is primarily deterministic or stochastic in nature. Although the canopy stratum of the mature forest at these environmentally similar sites was very Similar in structure and species composition (p>0.05), the composition of the subcanopy strata vegetation differed Significantly among sites (p<0.05). This indicated that stochastic events (e.g. low intensity ground fires, deer browsing, cattle grazing) were were important factors during the development of understory and ground layer vegetation. Site-to-site differences in subcanopy species composition remained apparent during the first 2 years following disturbance (removal of canopy cover) of 4 sites. Brush Mountain lacked a significant seed bank (< 1 seed/m²), and the dominant sub canopy species reproduced largely by vegetative sprouting. Post-disturbance colonization of the sites was very stochastic, but had relatively little immediate effect on vegetation structure and composition. In the future, however, as colonist species reproduce and increase in density, stochastic colonization events may become an important aspect of forest development at some of the sites.
- Laboratory and field ecophysiological studies on the impact of air pollution on red spruce and Fraser firTyszko, Piotr (Virginia Tech, 1991)Three studies were performed to investigate the impact of air pollution on high-elevation red spruce-Fraser fir forests in the Southern Appalachians. In the first study, red spruce (Picea rubens Sarg.) and Fraser fir (Abies fraseri (Pursh.) Poir.) seedlings were submitted to long-term (2.5 yrs), multiple growing cycle (4 and 5, respectively), intermittent ozone fumigations (0.025, 0.070, and 0.150 ppm). No effect of ozone exposure on growth and gas exchange of the seedlings was found. Net photosynthesis at saturating light intensity was reduced in both species and the light compensation point was shifted upwards in spruce when exposed to ozone. Fraser fir seedlings showed inconsistent responses of CO₂ curve parameters to ozone exposure. There were indications that ozone exposure modified cell wall modulus of elasticity in both species. In the second study, the impact of summer exposure to ambient pollutants on winter hardiness in red spruce seedlings was examined. The seedlings were subjected to the following summertime treatments while kept in exclusion chambers on the top of Whitetop Mountain (Virginia): ambient air and clouds, ambient air with clouds excluded, charcoal filtered air, and chamberless control treatment. During the following winter the seedlings were placed in Blacksburg (Virginia), in two locations: in the open and in a shadehouse. A number of conducted tests indicated that there were significant differences in winter damage between the chamber treatments and chamberless control, as well as between the winter exposure locations. Among the summer chamber exposure regimes, the treatment excluding clouds seemed to perform the best (although not all the evidence supports the latter statement). In the third study, the physiology of red spruce trees of various sizes (seedlings, saplings, and overstory trees), growing on two sites on the top of Whitetop Mtn., was compared and related to ambient ozone concentration. Some seedlings were treated with an antioxidant EDU, to help evaluate the impact of ozone on their physiology. The trees of various sizes showed clear differences in gas exchange, with overstory trees photosynthesizing at the lowest rates, and seedlings - at the highest. Overstory trees also showed more negative shoot water potential and higher night respiration than smaller tree sizes. No deleterious effects of ambient ozone on red spruce physiology were detected.
- Modeling thinning effects on ring width distribution and wood specific gravity of loblolly pine (Pinus taeda L.)Tasissa, Gudaye (Virginia Tech, 1996)An appropriate accounting for thinning effects on growth rate and wood quality requires a clear understanding and quantification of these effects. In this regard, four basic interrelated issues were addressed in this study: 1) thinning effects on ring specific gravity 2) thinning effects on ring width distribution 3) thinning effects on stem form, and 4) prediction models for these quantities. The study showed that thinning does not significantly affect ring specific gravity, whereas its effects on ring width distribution and stem form were significant. Thinning increases ring width significantly over most of the tree bole and increases the earlywood and latewood components proportionally maintaining an approximately constant latewood proportion. As a consequence, thinning effects on latewood proportion is not significant; confirming the results obtained in the specific gravity study and further dispelling the concern that thinning may substantially reduce wood specific gravity. Thinning affects stem form by increasing the form exponent especially near the tree base accentuating the neiloid form expected in that area. High up in the stem, the form exponent changes little within a tree and among thinning treatments, with a general tendency towards a paraboloid shape. Differences due to thinning intensities, in general, were not significant indicating the applicability of results within a wide range of densities. Prediction models for ring specific gravity, ring width, latewood proportion and stem profile based on ring, tree, stand and site factors were developed Influences of stand level factors, density measures in particular, in prediction models are minor probably because tree level factors such as, stem diameter at breast height, crown ratio, etc. themselves manifest stand conditions. The mixed-effects analysis technique was used in data analysis to account for correlation among observations from the same subject. Direct covariance modeling yielded better fits than accounting for correlation indirectly using random effects covariates in many cases; however, both could not be accommodated simultaneously. Structures which assume decreasing correlation with increasing distance between observations, such as the first-order autoregressive structure, performed better than alternative specifications. Results consistently showed that accounting for correlation among observations substantially improves the fits over ignoring correlation; effectively addressing the issue of bias in the standard errors of estimates.
- The molecular ecology of Vaccinium macrocarpon Aiton, the American cranberryStewart, C. Neal (Virginia Tech, 1993)Cranberry (Vaccinium macrocarpon), a commercially grown evergreen dwarf shrub, is a dominant taxon in temperate bogs in North America. It spreads clonally by runners, and reproduces sexually predominantly by self-fertilization on upright stems. The objective of this project was to investigate genetic and clonal variation and phenotypic plasticity of V. macrocarpon. Specifically, I wanted to test whether there exists an inverse relationship between population genetic variation and the amount of overall phenotypic plasticity of vegetative characteristics. As background information I assessed the vegetation and edaphic factors of marginal cranberry bogs found in the mid-to-southern Appalachians. A gradient of nutrient availabilities was found among bogs that was positively associated strongly with the dominance of the more generalist Rubus hispidus and negatively with ericaeous bog shrubs such as V. macrocarpon. Eutrophication may lead to the replacement of endemic bog species with generalist plastic species. Theoretically, it would be plausible for environmental heterogeneity or stress to allow selection for more phenotypically plastic clones within a species. A single adaptively plastic clone for growth strategy could sweep a site, excluding intraspecific competitors. That is, selection could favor clones with high plasticity that could subsequently lead to a loss of genetic variation within a population. The environmental and genetic conditions favoring this would more likely exist in distributionally marginal sites because of spatial and temporal heterogeneity and island-like biogeography. Field and common garden experiments in which nutrients were manipulated were performed to test for an inverse relationship between phenotypic plasticity and genetic heterogeneity. Random amplified polymorphic DNA (RAPD) profiling was coupled with ecological measurements of plant growth on the experimental clones and other clones from the experimental populations to estimate genetic heterogeneity. Genetic heterogeneity was found to be significantly lower in marginal populations than in central populations. Phenotypic plasticity was somewhat higher in a more marginal population in the field sites, but direct statistical comparisons could not be made. The common garden study was inconclusive, possibly due to transplant shock, but a trend among natural populations was towards higher plasticity among marginal clones. Additional research on other species is needed to clarify the possible inverse relationship between phenotypic plasticity and genetic variation within populations.
- Stand structure, growth, and mortality in southern Appalachian spruce-firNicholas, N. S. (Virginia Tech, 1992)Current stand structure and composition, biomass levels and distributions, stand level foliage surface area (LAI) estimations, and mortality and growth patterns were determined using consistent sampling methodology for a network of 142 (20 m x 20 m) permanent plots at three southern Appalachian spruce-fir sites (Mt. Rogers National Recreation Area (NRA) of Virginia, the Black Mountains of North Carolina, and the Great Smoky Mountains of Tennessee and North Carolina). Baseline conditions were documented to accommodate future efforts to determine actual phenomena of forest decline in a highly disturbed ecosystem. Information on structure, composition, and growth and mortality trends were combined to develop a model to predict forest change for the next two decades. Past studies indicated that undisturbed spruce-fir species distribution tended to follow an elevation gradient: red spruce (Picea rubens Sargent) dominance changing to Fraser fir (Abies fraseri (Pursh) Poiret) dominance as elevation increased. Current stand composition at the Black Mountains and the Great Smokies also showed a shift from spruce to fir; however, Mt. Rogers NRA was an exception to that trend. As fir abundance increased with elevation there were increasing levels of balsam woolly adelgid-caused (Adelgid piceae Ratz.) mortality at the Black and Smoky Mountains where there was a greater proportion of standing dead fir than live fir. Unlike these two sites, Fraser fir on Mount Rogers still had escaped major damage from the adelgid. Projected leaf area index (LAI)(m²/m²) was developed for spruce, fir and yellow birch (Betula lutea Michaux f.), based on predicted foliage weight from overstory biomass equations, as a quantifiable measure of forest productivity. Primarily old-growth spruce-fir stands at the Great Smoky Mountains had an average LAI (11.9) significantly greater than stands at Mt. Rogers NRA (9.1) or the Black Mountains (8.3) which both have a patchwork of disturbance histories. Some conversion to increased hardwoods may have occurred in second growth stands at lower elevations with a resulting lower leaf area capability. At higher elevations (1830-1980 m), LAI was predicted to decrease if the remaining adelgid-infested fir die for both virgin and logged sites. Past studies have inferred information on mortality patterns from assessment of standing dead stems density. Overstory annual mortality was directly measured each year from 1985 and 1989 and found to vary among the four dominant overstory species; mountain-ash (Sorbus americana Marshall) had the highest rate (6.4 %), followed by fir (5.8 %), birch (2.7 %), and spruce had the lowest (2.1 %). Results suggested that enumerations of standing dead trees should not be used to assess mortality patterns since a substantial proportion (20-30 %) of all trees that died, fell to the ground in the same year, and were never part of the pool of standing dead stems. Comparisons of fir diameter distribution indicated that at sites where the balsam woolly adelgid was causing significant fir mortality, stand structure was shifting because of the elimination of larger (> 35 cm DBH) live fir stems. Prediction of individual tree mortality using logistic regression was unsuccessful for birch and mountain-ash, while equations to predict spruce and fir mortality depended on crown condition (amount of intact needles), as a predictor variable. Since an accelerated rate of change in stand structure has been predicted to occur with increased mortality and reduced growth rates, a short-term (twenty year) projection model of forest composition and structure was developed. Individual tree basal area increment equations for red spruce, Fraser fir, and yellow birch, along with ingrowth and mortality information were combined to provide predictions starting from the year 1989 and ending in 2009. Where the adelgid has been dominating fir mortality patterns for several decades, such as in the Black Mountains, little overall change is expected. For most elevations basal area is projected to be stable while stem densities decrease. In the Smokies, where little fir is found at or below 1675 m elevation, stand structure is predicted to change little during the 20 year period. However, the highest elevations of the Smokies are predicted to eventually be similar to the current stand structure of high elevations of the Black Mountains. The adelgid infestation of the peaks of the central Smokies lagged by twenty-some years behind the Black Mountains and the model predicts a deterioration of fir as well as spruce in that area.