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.
Contacts:
Photosynthesis -- Paul Hanson (hansonpj@ornl.gov)
Leaf conductance -- Robert Auge -- (auge@utk.edu)
Water potential and osmotic adjustment -- Tim Tschaplinski (tschaplinstj@ornl.gov)
Storage carbohydrates -- Tim Tschaplinski (tschaplinstj@ornl.gov)
Contact: Nelson Edwards -- (edwardsnt@ornl.gov)
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 -- (wullschlegsd@ornl.gov)
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.
Contacts
Tree and sapling growth: Paul Hanson -- (hansonpj@ornl.gov)
Seedling growth and survival -- Michael Huston (hustonma@ornl.gov)
Recruitment from seed -- Jake Weltzin (jweltzin@utk.edu)
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 -- (jdjoslin@esper.com )
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 -- (hansonpj@ornl.gov)
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 -- (hansonpj@ornl.gov)
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 -- (dwj@unr.edu)
Contact: Johann Bruhn for additional information (Bruhn@PSU.Missouri.edu)
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 (dshure@biology.emory.edu)
Contact: Ken Cramer -- (kenc@monm.edu)
![See figure]](TNC.gif){kind=link}
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