Donald E. Todd

Changes in nutrient distribution in forests and soils of Walker Branch watershed, Tennessee, over an eleven-year period

Changes in vegetation, litter, and soil nutrient content were measured in selected plots on Walker Branch watershed, Tennessee, from 1972–73 to 1982. The watershed has been allowed to revert to forest since 1942, before which it consisted of small subsistence farms and woodland pastures. Changes in Ca status were of particular interest because initial nutrient cycling characterizations indicated that net Ca accumulation in vegetation could have caused large decreases in soil exchangeable Ca2+ within 20 years.Decreases in forest floor and subsoil (45–60 cm) N, exchangeable Ca2+, and Mg2+ content were noted in several plots from 1972 to 1982. Surface soils (0–15 cm) showed either no change or, in some cases (e.g., N and exchangeable K+ in certain plots), increases over the 11-year period. Reductions in forest floor and subsoil exchangeable Ca2+ and exchangeable Mg2+ on cherty, upper slope oak-hickory and chestnut oak forests were most striking. The changes in Ca2+ are thought to be due primarily to high rates of Ca2+ incorporation into woody tissues of oak and hickory species. Reductions in forest floor and subsoil exchangeable Mg2+ could not be accounted for by woody increment; leaching may have played a major role in causing these decreases. Changes in P and exchangeable K+ were variable, with both increases and decreases.There were significant increases in exchangeable Al3+ in both subsoils and surface soils of certain plots, but these were not accompanied by decreases in exchangeable base cations or consistent decreases in pH. Dissolution of interlayer Al from 2:1 clays may be the cause of the exchangeable Al3+ increases.These results suggest a general decline in fertility, especially with regard to Ca and Mg in those forests with low soil Ca and Mg supplies. Monitoring of further changes (if any) in these ecosystems will continue as the currently aggrading forests approach steady state.

Evidence of modern accumulations of adsorbed sulfate in an East Tennessee forested Ultisol

Previous laboratory studies on sulfate content and adsorption in an eastern Tennessee forested Ultisol showed that, although considerable amounts of adsorbed, insoluble sulfate were present in B horizon samples, further accumulations into this form could not be induced with additional inputs. These results posed two questions addressed in this paper: (1) what proportion, if any, of current adsorbed, insoluble sulfate is the result of modern inputs; and (2) are these soils capable of further accumulation of adsorbed, insoluble sulfate under field conditions? Field studies, including sampling beneath and adjacent to an old house, a lysimeter investigation, and sulfate salt application, showed that sulfate has accumulated over the last 80 years, is now accumulating, and can continue to accumulate at higher input levels in this soil. The discrepancies between laboratory and field results pose problems for easy estimation of sulfate adsorption capacity, and further research on the reasons for them is necessary.

Description and field performance of the Walker Branch throughfall displacement experiment: 1993--1996

The authors are conducting a large-scale manipulative field experiment in an upland oak forest on the Walker Branch Watershed in eastern Tennessee to identify important ecosystem responses that might result from future precipitation changes. The manipulation of soil water content is being implemented by a gravity-driven transfer of throughfall from one 6400-m{sup 2} treatment plot to another. Throughfall is intercepted in {approx}1850 subcanopy troughs suspended above the forest floor of the dry plot and transferred by gravity flow across an ambient plot for subsequent distribution onto the wet treatment plot. Soil water content is being monitored at two depths with time domain reflectometers at 310 sampling locations across the site. The experimental system is able to produce statistically significant differences in soil water content in years having both dry and wet conditions. Maximum soil water content differentials between wet and dry plots in the 0- to 0.35-m horizon were 8 to 10% during summers with abundant precipitation and 3 to 5% during drought periods. Treatment impacts on soil water potential were restricted to the surface soil layer. Comparisons of pre- and post-installation soil and litter temperature measurements showed the ability of the experimental design to produce changes in soil water content and water potential without creating large artifacts in the forest understory environment.

Field performance of the Walker Branch throughfall displacement experiment

The authors are conducting a large-scale manipulative field experiments in an upland oak forest on the Walker Branch Watershed in eastern Tennessee USA to identify important ecosystem responses that might result from future precipitation changes. The manipulation of soil moisture is being implemented by a gravity-driven transfer of throughfall precipitation from one treatment plot to another. Throughfall is intercepted in {approx} 2,000 subcanopy troughs (0.3 x 5 m) suspended above the forest floor of the dry plots ({approx} 33% of the ground area is covered) and transferred by gravity flow across an ambient plot for subsequent distribution onto the wet treatment plot. Percent soil water is being monitored with time domain reflectometers at 310 sampling locations across the site. The experimental system is able to produce statistically significant differences in soil water content in years having both extremely dry and extremely wet conditions. Furthermore, comparisons of pre- and post-installation soil temperature measurements have documented the ability of the experimental design to produce these changes without changing the microclimate of the forest understory.

Walker Branch Throughfall Displacement Experiment

The Walker Branch Watershed (∼100 ha), located at 35°58′ N latitude and 84°17′ W longitude, is a part of the U.S. Department of Energy’s (DOE’s) National Environmental Research Park near Oak Ridge, Tennessee (Johnson and Van Hook 1989). Long-term (50-year) mean annual precipitation is 1352 mm, and mean annual temperature is 14.2°C. The soils are primarily Typic Paleudults derived from dolomitic bedrock. The soils are highly weathered and very deep (> 10 m) on ridge tops and therefore retain little evidence of their carbonate parent material. Plant-extractable water (water held between 0 and − 2.5 MPa) for the upper meter of soil is approximately 183 mm. A large fraction of this water (44%) is held in the upper 0.35 m of the soil profile, where 74% of all fine roots in the upper 0.90 m of the profile are located (Joslin and Wolfe 1998; Chapter 16, this volume). Depth to bedrock at this location is approximately 30 m (McMaster 1967), and deep rooting may be a source of some water.

Precipitation Change and Soil Leaching: Field Results and Simulations from Walker Branch Watershed, Tennessee

To investigate the potential effects of changing precipitation on a deciduous forest ecosystem, an experiment was established on Walker Branch Watershed, Tennessee that modified the amount of throughfall at −33%, ambient (no change), and +33% using a system of rain gutters and sprinklers. We hypothesized that the drier treatments would cause: 1) disproportionate changes in soil water flux, 2) increased total ionic concentrations in soil solution that would in tum cause 3) decreased SO 4 2− /Cl− ratios, 4) decreased HCO3 − concentrations, and 5) increased ratios of Al to (Ca2+ + Mg2+) and of (Ca2+ + Mg2+) to K+. Hypothesis 1 was supported by simulation results. Hypotheses 2 and 3 were supported in part by field results, although interpretation of these was complicated by pre-treatment biases. Hypotheses 4 and 5 were not supported by the field results. Comparisons of field data and Nutrient Cycling Model (NuCM) simulations were favorable for most ions except Cl− and K+. The disparities may be due to underestimation of soil buffering in the case of Cl− and overestimation of soil buffering in the case of K+ in the model. Long-term simulations with NuCM suggest that reducing water inputs will slow the rate of soil acidification and P loss, but will not materially affect growth or ecosystem N status.

The combined effects of thinning and prescribed fire on carbon and nutrient budgets in a Jeffrey pine forest

Abstract• Both burning and harvesting cause carbon and nutrient removals from forest ecosystems, but few studies have addressed the combination of these effects. For a Pinus jeffreyii forest in the Sierra Nevada Mountains of California, we posed the question: what are the relative impacts of thinning and subsequent burning on carbon and nutrient removals?• The thinning methods included whole-tree thinning (WT, where all aboveground biomass was removed) cut to length (CTL, where branches and foliage were left on site in a slash mat on top of skid trails) and no harvest (CONT). Total C and nutrient exports with thinning and burning were greater in the WT and CTL than in the CONT treatments. Total C and N removals were approximately equal for the WT and CTL treatments, although harvesting dominated exports in the WT treatment and burning dominated exports in the CTL treatment. Total removals of P, K, Ca, Mg and S were greatest in the WT treatments, where harvesting dominated removals.• Comparisons of nutrient removals with ecosystem capital and calculations of potential replenishment by atmospheric deposition suggested that N is the nutrient likely to be most depleted by harvesting and burning treatments.Résumé• L’un et l’autre, le brûlage et les récoltes, causent un enlèvement important de carbone et de nutriments dans les écosystèmes forestiers, mais peu de travaux se sont attelés à l’étude de la combinaison de ces effets. Pour une forêt de Pinus jeffreyii nous posons la question suivante : quels sont les impacts relatifs d’une éclaircie et d’un brûlage ultérieur sur l’enlèvement du carbone et des nutriments, dans les Montagnes de la Sierra Nevada en Californie?• Les méthodes d’éclaircie comprennent : éclaircie totale des arbres (WT, toute la biomasse au-dessus du sol est enlevée), coupe à la dimension désirée (CTL, branches et feuillage ont été laissés sur site en andains) et pas de récolte (CONT). L’exportation totale de carbone et de nutriments avec éclaircie et brûlage dans les traitements WT et CTL a été plus importante que dans le traitement CONT. Les sommes totales de carbone et d’azote enlevées ont été approximativement les mêmes pour les traitements WT et CTL, bien que la récolte est été prédominante dans les exportations pour le traitement WT et que le brûlage soit prédominant dans les exportations pour le traitement CTL. Le total des enlèvements de P, K, Ca, Mg et S dans le traitement WT a été le plus grand dans les traitements WT où la récolte était plus importante que l’enlèvement.• La comparaison des nutriments enlevés par rapport au capital de l’écosystème et le calculs de réapprovisionnement par des apports atmosphériques suggère que l’azote (N) est le nutriment probablement le plus épuisé par les traitements de récolte et de brûlage.

Tree and Sapling Growth and Mortality

Plant-dry-matter accumulation ultimately depends on the yield of carbon building blocks (i.e., nonstructural carbohydrates) from the difference between carbon assimilation (Chapter 3, this volume) and autotrophic respiration (Chapter 4, this volume). Accumulated sugar and starch reserves in roots and shoots (Chapter 5, this volume) represent the primary compounds, along with stored elements, that must be present to support mass accumulation in the stems of saplings and trees. Waring and Pitman (1985) proposed a hierarchy of photosynthate allocation priorities for trees that considers stem growth to be a relatively low allocation priority, suggesting that changes in stem-growth rates would be a sensitive indicator of water stress response. Sustained low stem-growth rates have also been linked to mortality (Kohyama and Hara 1989; Pedersen 1998; Swaine et al. 1987; Tainter et al. 1984). Because growth and mortality are the integrated result of of physiological responses to environmental stress, they represent key end points for investigations of forest response to changing precipitation regimes. This chapter describes the annual growth and mortality of saplings and large trees during 7 full years of throughfall manipulation (1994–2000) and provides a quantitative description of the response of tree growth to soil-water deficits. The Throughfall Displacement Experiment (TDE) observations are also contrasted with published data for other deciduous hardwood forests, and their application for predicting growth responses to soil-water deficits throughout the eastern deciduous hardwood forest is discussed.

Influences of Thinning and Prescribed Fire on Water Relations of Jeffrey Pine I. Xylem and Soil Water Potentials

Abstract Forest thinning accomplished with cut-to-length and whole-tree harvesting systems, and prescribed underburning were assessed for their impacts on water relations in eastern Sierra Nevada Jeffrey pine (Pinus Jeffreyi Grev. & Balf.) during a period of extended drought. Predawn and midday measurements of xylem water potential in dominant and codominant crown class trees more than a century old were made on six days spread over three growing seasons, accompanied by measurements of soil water potential completed between the predawn and midday sessions of each day. With the exception of a single predawn session, the only one of a total of 12 in which xylem water potentials did not differ among treatments, the potentials in trees of thinned stand subunits were 0.67 MPa higher on average during predawn sessions and 0.71 MPa higher during midday sessions than those in trees of the unthinned treatment. Differences between the cut-to-length and whole-tree treatments were marginal and uncommon, but when they occurred, potentials were higher in the former. Prescribed fire effects on xylem water potential were also uncommon, but when occurring generally indicated lower stress levels in the burned than in the unburned treatment. Soil water potentials largely coincided with those of xylem water, with higher potentials in either the cut-to-length or whole-tree treatments, and usually both, than in the unthinned treatment on each of the six days of measurement. Underburning influences on soil water were rare, but when evident, potentials were higher in the burned than in the unburned treatment by substantial margins. For a majority of the measurement sessions, xylem water potential was found to be negatively correlated with residual basal area but positively correlated with soil water potential. In turn, coarse fragments and organic matter in the soil profile intermittently influenced soil water potential, with the former a negative factor while the latter was positive. Overall, results of this study suggest that substantial ecophysiological advantages can be derived from density management in older, dry site forests, which at minimum are not compromised by subsequent implementation of controlled underburning.

AIR FLOW AND HEAT TRANSFER IN A TEMPERATURE-CONTROLLED OPEN TOP ENCLOSURE

A large 12-meter-diameter open top enclosure (OTE) equipped with two unique belowground and above ground heating systems was built and intensively tested in Oak Ridge, TN, USA. The OTE is a prototype for use within an environmental change experiment, in which replica units will be built in Minnesota to assess the response of northern peatland ecosystems to increases in temperature and elevated atmospheric CO2. For several months, temperatures, energy, wind speed and relative humidity were monitored throughout the enclosure space to assess the enclosure performance and efficiency. In parallel, Computational Fluid Dynamics (CFD) simulations were performed with ANSYS-CFX to investigate the impacts of external wind, buoyancy, and OTE design on the temperatures achieved within the enclosure. The addition of a frustum that partially reduced the top opening was also investigated experimentally and numerically. The OTE is capable of achieving a temperature differential of at least +6°C for air using a combination of 8 electrical heaters. Differential temperatures were sustained for several months. The experimental data and the numerical results showed that the addition of a frustum dramatically decreases the operating cost of the OTE and leads to better control over the differential air temperature in the enclosure. Buoyancy forces and winds heavily impacted enclosure performance. It was also found that the heating efficiency of the OTE depends mainly on the wind speed, and that there exists a critical wind speed at which the heating efficiency is the highest.Copyright