TDE Research Findings (Updated April 2002)

Individual investigators listed below can be contacted for reprints of publications in their respective areas of expertise.  See also the TDE publications list.

Foliar Photosynthesis, Conductance and Biochemistry

Foliar photosynthesis and conductance of mature trees and saplings were confirmed to be responsive to soil water deficits, but significant water stress was not present in all years reducing the overall impact of reductions in carbon gain on growth. For example, at the depth of the 1995 drought, light-saturated photosynthesis of overstory tree foliage was 55% greater and 28% lower for the wet and dry treatment plots when compared to foliage on the control plot ([See figure]). A similar pattern was also apparent for the understory plants throughout the 1995 growing season.

A considerable degree of variation in drought tolerance among species has been demonstrated, with American beech, dogwood, black gum, and sugar maple least tolerant, red maple intermediate, and white oak and chestnut oak most tolerant, as determined by osmotic potential at full turgor (OFT ; a component of drought tolerance). During the wet 1994 growing season, dogwood and chestnut oak were the only two species that displayed osmotic adjustment (lowering of OFT) to drought, having lower OFT in the dry treatment. The capacity for osmotic adjustment was species-specific and not restricted to only species having low OFT. There was no effect of canopy position on OFT in red maple, but OFT of understory chestnut oak seedlings were higher (less drought tolerant) than that of the overstory trees, because understory trees did not display osmotic adjustment in response to drought, whereas overstory trees did. A large fraction of the osmotic adjustment to drought in dogwood and chestnut oak trees could be accounted for by the major classes of solutes, including soluble carbohydrates, organic acids, and cations, but the relative contribution of any given class of solutes varied with time of season and species.

Total dormant-season nonstructural carbohydrate levels showed substantial interannual variation with large reductions following the drought of 1998 [See figure]. Treatment-induced changes in dormant-season total nonstructural carbohydrates (TNC) can be detected in some, but not all species. Poor correlations between TNC and subsequent canopy leaf production or tree growth suggest substantial TNC redundancy in stored reserves, and indicate that mature tree components of the upland oak forest on Walker Branch are seldom carbohydrate limited.

Photosynthesis -- Paul Hanson (
Leaf conductance -- Robert Auge -- (
Water potential and osmotic adjustment -- Tim Tschaplinski (
Storage carbohydrates -- Tim Tschaplinski (

Stem Respiration

Seasonal patterns of stem respiration showed only minimal effects of soil water deficits, but stem respiration is related to basal area growth rates [See figure].  The lack of correlation between stem growth and soil water deficits is discussed further below.  Typical dormant season bole respiration rates at 0°C for mature canopy oaks, maples and yellow-poplar are approximately 7 to 8, 8, and 3 µmol m-3 s-1, respectively with Q10 values between 2 and 2.6.  Growing season bole respiration rates are much higher with values at a reference temperature of 10°C ranging from 30 to 160 µmol m-3 s-1.   Annual stem respiration for the upland oak forest on Walker branch averaged 389 gC m-2 y-1 from 1993 to 2000.

Contact: Nelson Edwards -- (

Plant Water Use

Red maple saplings growing in the understory showed strong seasonal patterns of whole-plant water use with differences due to treatment (wet vs. dry) being evident during a mid-summer drought of 1995 [See figure]. These effects were observed across a range of sapling stem diameters suggesting that position within the understory was not a factor in regulating whole-plant water use.  Soil water potentials less than -1.5 MPa are required to completely stop water use by saplings [See figure].  Significant reductions in mature tree water use by TDE treatment during late-season droughts have also been documented.

Stand-level water use, scaled from individual tree sapflow observations, was 25% lower on the dry than ambient plot in 2000.  Diffuse porous trees contribute a disproportionate amount to stand water use representing 69, 71, and 71 percent of total stand water use on the wet, ambient, and dry treatment plots in 2000.  While sapflow based estimates of tree water use are an excellent measure of relative water use among species and treatments, scaling these data to the stand level results in an underestimate of forest water use when compared with other data [See figure].

Contact: Stan Wullschleger -- (

Plant Growth and Survival

Seedlings -- Mortality of bare root seedlings planted on the TDE (Acer, Liriodendron, Quercus) showed significant effects of species, treatments and topography, plus significant interactions between the main factors. Treatment effects were particularly dramatic, with no mortality in the wet treatment and the highest mortality in the dry treatment.  Results of this and other research indicate that plant species representative of typical upland deciduous forests in East Tennessee exhibit a wide range of sensitivity to experimentally imposed drought, ranging from little, if any, impact to significant increases in mortality. In particular, C. florida may be particularly sensitive to changes in precipitation regimes. Wetter climates in the future may contribute to enhanced establishment of seedlings and survival of saplings, whereas drier climates may reduce the likelihood of recruitment and survival of seedlings and saplings. Alternatively, wetter climates may promote pests and disease to which C. florida is prone, although this has yet to be tested empirically.

Mature trees and saplings -- Overstory trees showed minimal growth response to experimentally altered precipitation levels from 1993 to 2000 whereas understory saplings showed reduced and increased growth on the dry and wet plots, respectively [See Tree figure or Sapling figure]. Although mortality of mature trees was minimal from 1993 to 2000 (1 to 3 % per year), the mortality of Cornus florida and Acer rubrum saplings reached nearly 70 and 35% for C. florida and A. rubrum dry plot results, respectively [Mortality figure].  After only 3 years (1995) the predominant treatment response was increased mortality of only C. florida on the dry plot, but after 8 full years of manipulation both sapling species showed reduced mortality on the wet treatment when compared to both the ambient and the dry plots.

Insensitivity of mature tree growth responses to TDE treatments and periodic drought can be explained by a disconnect between annual growth phenology and the late-season occurrence of droughts [See figure].  Future models need to incorporate such data if long term assessments of climatic change are to provide correct outputs.

Multiyear growth responses and the suite of physiological observations to date, support our hypotheses that the small stature vegetation of an upland oak stand will be more sensitive to changing precipitation patterns than established canopy dominant trees. If future climate change were to lead to reduced growing season soil water contents (resulting from either lower precipitation or elevated evapotranspiration) early forest ecosystem responses are likely to be exhibited by regenerating vegetation instead of the existing dominant overstory trees.

Tree and sapling growth: Paul Hanson -- (
Seedling growth and survival -- Michael Huston (
Recruitment from seed -- Jake Weltzin (

Root Growth

We examined minirhizotron and repeated root biomass sampling data for treatment effects on (1) fine root biomass, (2) fine root to foliage ratio, (3) altered rates of fine root turnover; and (4) depth of rooting during the period 1994 to 2000.  Differences across treatments in net fine root production (using minirhizotron root elongation observations as indices of biomass production) were small and non-significant [See figure].  Periods of lower root production in the dry treatment were compensated for by higher growth during favorable periods. While not statistically significant, both the highest production and mortality rates were found in the wet treatment, resulting in the highest index of fine root turnover.

The combination of slightly higher production and higher mortality in the wet treatment, based on minirhizotron data, resulted in estimates of no net changes in the standing-pool of fine root biomass. Direct measurements of fine root biomass between 1993 and 2000 indicate slight net increases in biomass may have occurred in the wet plot, relative to the other two treatments, but only in the surface 30 cm of the lower slope. On the other hand, slight net decreases in fine root biomass in the wet plot may have occurred at 60 to 90 cm on the upper slope,  relative to the other two treatments. In the context of the belowground carbon budget, these changes are small indeed.

After seven years, a clear picture of stand fine root system response to drought exposure has yet to emerge in this forest ecosystem. Our results provide little support for either an increase in net fine root production, nor a shift towards an increasing root to shoot ratio with long-term drought exposure. This finding, corroborated by the absence of treatment effects on FRB changes after 7 years, appears to be the result of the resilience of this forest ecosystem in maintaining a relatively constant fine-root mass over the long term.

Contact: Dev Joslin -- ( )

Soil Respiration and Decomposition

The fixed change in precipitation inputs implemented by the TDE (i.e., ±33%) has not impacted soil respiration in a consistent manner, but severe, short-term soil water deficits lead to clear reductions in soil respiration [See figure].  Dynamic wetting and drying of the forest organic horizons, originally ignored, was found to be a major factor controlling the dynamics of  soil respiration [See figure].

Although short-term litter decomposition is highly dependent on litter water content,  long term patterns of litter decomposition show only minimal impacts of the TDE throughfall manipulations [See figure].  Rapid decomposition during periods of optimum moisture content tend to make up for dry periods of limited heterotrophic activity.

Simulations of soil respiration for the upland oak forest on Walker Branch Watershed show mean annual fluxes of 938, 929,  and 907 gC m-2 y-1 for the wet, ambient, and dry treatments, respectively.  To adequately capture daily and seasonal patterns of soil respiration ecosystem models of soil respiration in deciduous forests should include separate functions for litter layer and mineral soil activity, and they can be improved by independent  accounting of the carbon costs associated with root growth activity.

Contact: Paul Hanson -- (

Stand Carbon and Net Ecosystem Production

The upland oak forest on Walker Branch Watershed is estimated to contain 172 MgC ha-1 to a soil depth of 1 meter. Of this total 58% is located in the live vegetation biomass.  Including soil carbon data to a depth of 9 meters shifts the majority of the carbon to the soils [See figure].

Interannual estimates of NEP based on biometric data and the extrapolation of site-specific processes suggest a mean annual net primary production (NPP) of 729 gC m-2 y-1 and net ecosystem production (NEP) of 187 gC m-2 y-1.  Year-to-year differences in environmental variables (e.g., growing season length and drought) can produce shifts in NEP of ±60%.  These estimates of NPP and NEP are in good agreement with historical observations, and, while they are considerably lower than the Walker Branch eddy covariance estimates of NEP, they are in a range similar to other deciduous hardwood forest NEP estimates in Indiana and Massachusetts.

Contact: Paul Hanson -- (

Forest Nutrient Cycling

The effects of changing precipitation on soil leaching in a deciduous forest were examined by experimentally manipulating throughfall fluxes in the field. Soil leaching was measured with resin lysimeters in the O horizons and with ceramic cup lysimeters in the E (25 cm) and Bt (70 cm) horizons. Treatment effects differed by time and by soil horizon.

In the O horizons, large and statistically significant treatment effects on N fluxes were found (lower N fluxes in the DRY and higher N fluxes in the WET treatment [See figure]). The DRY treatment also had significantly greater forest floor N content than the AMB or WET treatment after five years, more N retention in the litter of the DRY treatment than in the others. In the E horizons, there were no statistically significant treatment effects on soil solution concentrations of any measured ion. In the Bt horizons, statistically significant treatment effects on electrical conductivity (EC), pH, Ca2+, Mg2+, K+, Na+, SO42-, and Cl- were found [See figure].  These were due exclusively to differences between the DRY and other treatments; differences between  WET and AMB treatments were rarely significant. The greater treatment effects in the DRY than in the WET treatments in the Bt horizon were probably due to the disproportionate effects of the DRY treatment on soil water flux. Sharp increase in Na+ concentrations (and, to a lesser extent, Cl- and EC) were noted in soil solutions in all treatments during the spring of 1999, an apparent reflection of the extreme drought in 1998.

Throughfall manipulations caused disproportionate changes in soil water flux which in turn caused changes in estimated ion leaching rates. Soil solution concentrations in the dry treatment were more concentrated than in the ambient or wet treatments, but this did not outweigh the reduction in estimated soil water flux and thus estimated ion leaching rates were therefore reduced. Soil solutions in the wet treatment were unaffected by treatment, and thus estimated ion leaching rates were considerably higher than in the ambient or wet treatments. Because soil water fluxes cannot be directly measured, ion leaching rates cannot be known with certainty, however.

Simulations of the nutrient cycling process mimicked the patterns observed in the field in some cases (soil solution concentration increases in the dry treatment), but failed to do so in other cases (i.e., the observed increase in forest floor mass and N content in the field was not indicated). Predictions of long-term changes in base saturation in soils with the potential to impact forest productivity seem reasonable but remain to be tested over time.

Contact: Dale W. Johnson -- (

Spatial Patterns of Armillaria Fungi Populations

Species in the white-rot fungal genus Armillaria vary in parasitic aggressiveness as root pathogens of trees. Armillaria genets were mapped in the TDE plots using basidiomata and rhizomorphs collected in 1994 and 1995. Isolates from 36 rhizomorph and 48 basidioma collections identified as A. gallica, and from 21 basidioma collections identified as A. Mellea, were further classified as 5 A. gallica genets and 18 A. Mellea genets. Only 4 of the 105 collections were obtained from the "dry" plot.

Contact: Johann Bruhn for additional information (

Plant-Herbivore Processes

Donald Shure of Emory University is conducting a NIGEC funded project to determine the possible changes in plant resource allocation to foliar N and phenolic defenses (i.e., tannins) following rainfall manipulations and quantifying the type and extent of insect herbivory from tree species experience wet, ambient or drought conditions.

Early treatment results in 1993 and 1994 showed no change in plant-heribivore processes between dry, ambient and wet-treatment plots. The absence of treatment differences in 1994 reflects the impact of an especially wet summer which prevented soil moisture reductions on the dry plot from affecting plant foliar chemistry. Nevertheless, rainfall effects were evident when comparing our results over a three-year period (1992-1994). Leaf N and phenolic concentrations were lower in all tree species during the much drier summer in 1993 and 1992. Foliar N levels in oaks returned to pre-drought levels during the wet 1994 summer, although N concentrations in maple leaves remained low. Total phenolic and hydrolyzable tannin levels were also low in 1994, whereas condensed tannin production was much higher for all species during the wet summer. These results show consistent reductions in plant allocation to foliar nutrients and phenolic defenses during drought. In contrast, the degree of recovery following drought varied between species and for different categories of phenolics. Insect herbivore damage was directly correlated with rainfall totals over the three year period. Herbivore damage on oaks was significantly reduced during the 1993 drought whereas insect feeding on oaks and maples in 1994 was elevated during the wet summer. These findings suggest that herbivore damage at Walker Branch appears largely dependent on the response of specific insect feeding guilds to moisture-related changes in foliar N and plant phenolics.

Contact: Donald J. Shure for additional information (

Spider Populations in the Forest Litter Layer

Spiders are a top predator in the detrital system of the deciduous forest and may influence decomposition through top down cascades on populations of Collembola that function as fungivores and detritovores [Food web figure].  Ken Cramer of Monmouth College, Monmouth, Illinois surveyed the TDE experimental area for spider populations during August and November of 2000.  Spider density and species richness were significantly impacted by changes in the litter layer water content driven by treatment and by slope position across the experimental area.

Contact: Ken Cramer -- (

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