Barton D. Clinton

Loss of foundation species: consequences for the structure and dynamics of forested ecosystems

In many forested ecosystems, the architecture and functional ecology of certain tree species define forest structure and their species-specific traits control ecosystem dynamics. Such foundation tree species are declining throughout the world due to introductions and outbreaks of pests and pathogens, selective removal of individual taxa, and over-harvesting. Through a series of case studies, we show that the loss of foundation tree species changes the local environment on which a variety of other species depend; how this disrupts fundamental ecosystem processes, including rates of decomposition, nutrient fluxes, carbon sequestration, and energy flow; and dramatically alters the dynamics of associated aquatic ecosystems. Forests in which dynamics are controlled by one or a few foundation species appear to be dominated by a small number of strong interactions and may be highly susceptible to alternating between stable states following even small perturbations. The ongoing decline of many foundation species provides a set of important, albeit unfortunate, opportunities to develop the research tools, models, and metrics needed to identify foundation species, anticipate the cascade of immediate, short- and long-term changes in ecosystem structure and function that will follow from their loss, and provide options for remedial conservation and management.

Regeneration Patterns in Canopy Gaps of Mixed-Oak Forests of the Southern Appalachians: Influences of Topographic Position and Evergreen Understory

-Canopy gaps in southern Appalachian mixed-oak forests were assessed for the effects of topographic, gap and stand variables on density of wood seedlings. Seedling density was significantly correlated with percent slope and positively with gap age (1-5 yr). Density varied substantially among topographic positions and increased with gap size. Species richness decreased over time and increased with gap size. Regeneration was dominated by Acer rubrum L. Other important species included Quercus coccinea Muench., Q. rubra L., Q. velutina Lamarck, Liriodendron tulipifera L. and Cornus florida L. Some known gap species increased in density with increasing gap size. Competitive inhibition effects of the evergreen understory (Rhododendron maximum L. and Kalmia latifolia L.) were also examined. Gaps containing over 50% cover of R. maximum had significantly lower densities than all other gaps, including gaps with >50% K latifolia cover. Height distributions of major regenerating species were skewed away from small (<15 cm) height classes. Species establishment was a function of gap area, gap age, topographic position and cover of R. maximum. In addition, species of varying degrees of tolerance of understory conditions are capable of establishment in small to medium size canopy openings in the absence of an evergreen shrub understory.

Catastrophic windthrow in the southern Appalachians: characteristics of pits and mounds and initial vegetation responses.

We characterized pit and mound (PM) topography resulting from catastrophic wind in the Coweeta Basin, and located 48 PMs across a variety of forest types. Our measurements included pit length, width, and depth; and mound height, thickness, and width. Species of fallen trees were identified, and DBH (diameter at breast height, 1.37m) was measured for biomass determination. We identified five distinct microsites at each PM: mound face, mound top, pit bottom, pit-wall, and intact forest floor. On each microsite, we measured photosynthetically active radiation (PAR), soil temperature, and soil moisture, and took soil samples from four microsites (intact forest floor, pit wall, pit bottom, mound top) to determine carbon and nitrogen concentrations. Treefall direction was marginally non-random. Three PM dimensions were significantly related to fallen tree biomass: mound width; mound height; and pit width. Other relationships failed because (1)

Does Rhododendron maximum L. ( Ericaceae ) Reduce the Availibility of Resources Above and Belowground for Canopy Tree Seedlings

Abstract Subcanopy shrubs and perennial herbs inhibit recruitment of canopy trees in forests around the world. Although this phenomenon is widespread, and can have significant effects on community dynamics, the mechanisms of inhibition are not well understood. In the southern Appalachian region, Rhododendron maximum inhibits the recruitment of canopy trees in forests of northern red oak (Quercus rubra). We have shown, in previous research, that processes occurring before canopy tree seed germination are not responsible for this inhibition. Therefore, post-germination processes, such as competition for resources are most important. In this study we show that the presence of a thicket of R. maximum in the understory reduced the availability of light by 80%, the frequency and duration of sunflecks by 96%, the availability of water by 20% and the availability of several soil nutrients (particularly cations) by variable amounts. Moreover, the survival of Q. rubra seedlings in the understory over 3 y was significantly reduced (by about 40%) in the presence of a R. maximum thicket compared with forest without a thicket. Seedling survival was positively associated with light availability, but the slope and intercept of that relationship was different in forest with or without R. maximum. Therefore, belowground processes are involved in reduced seedling survival under the R. maximum thicket. The resources most associated with survival of Q. rubra seedlings were water and light. Although many soil nutrients were significantly lower in forest with R. maximum than in forest without R. maximum, no individual nutrient was a significant covariate with Q. rubra survivorship. Our data indicate that competition for resources both above- and belowground is an important mechanism for inhibition of canopy tree recruitment by R. maximum. Light is important to seedling survival, but is not the only important factor. Water availability and the ability to accumulate soil nutrients are equally or more important than light to survival of canopy tree seedlings in the presence of a subcanopy thicket of R. maximum.

INHIBITION OF SEEDLING SURVIVAL UNDER RHODODENDRON MAXIMUM (ERICACEAE) : COULD ALLELOPATHY BE A CAUSE?

In the southern Appalachian mountains a subcanopy species, Rhododendron maximum, inhibits the establishment and survival of canopy tree seedlings. One of the mechanisms by which seedlings could be inhibited is an allelopathic effect of decomposing litter or leachate from the canopy of R. maximum (R.m.) on seed germination, root elongation, or mycorrhizal colonization. The potential for allelopathy by R.m. was tested with two bioassay species (lettuce and cress), with seeds from four native tree species, and with three ectomycorrhizal fungi. Inhibitory influences of throughfall, fresh litter, and decomposed litter (organic layer) from forest with R.m. (+R.m. sites) were compared to similar extractions made from forest without R.m. (-R.m. sites). Throughfall and leachates of the organic layer from both +R.m. and -R.m. sites stimulated germination of the bioassay species above that of the distilled water control, to a similar extent. There was an inhibitory effect of leachates of litter from +R.m. sites on seed germination and root elongation rate of both bioassay species compared with that of litter from -R.m. sites. Native tree seed stratified in forest floor material from both forest types had a slightly higher seed germination rate compared with the control. A 2-yr study of seed germination and seedling mortality of two tree species, Quercus rubra and Prunus serotina, in field plots showed no significant influence of litter or organic layer from either forest type. Incorporating R.m. leaf material into the growth medium in vitro depressed growth of one ectomycorrhizal species but did not affect two other species. Leaf material from other deciduous tree species depressed ectomycorrhizal growth to a similar or greater extent as leaf material from R.m. In conclusion, R.m. litter can have an allelopathic effect on seed germination and root elongation of bioassay species as well as some ectomycorrhizal species. However, this allelopathic affect is not manifest in field sites and is not likely to be an important cause for the inhibition of seedling survival within thickets of R.m.

Effects of Rhododendron maximum Thickets on Tree Seed Dispersal, Seedling Morphology, and Survivorship

In the southern Appalachian forests, the regeneration of canopy trees is severely inhibited by Rhododendron maximum L., an evergreen understory shrub producing dense thickets. While light availability is a major cause, other factors may also contribute to the absence of tree seedlings under R. maximum. We examined the effects of R. maximum on several life history stages of tree species, including seed dispersal, seed bank germination, seedling growth, and survivorship. We found no significant effect of R. maximum on seed reaching the forest floor for Acer rubrum, Liriodendron tulipifera, Quercus rubra, Quercus prinus, Carya spp., and Nyssa sylvatica. This indicates that either seed output of maternal trees rooted within the thicket were unaffected by R. maximum or seed dispersal from surrounding areas into thickets compensated for a lower seed production of canopy trees rooted in the thickets. Germination of tree seeds (A. rubrum, L. tulipifera, Q. rubra, and Betula lenta) from the seed bank also was not reduced by leaves and substrates within the thickets. Seedling mortality of all species (Q. rubra, Prunus serotina, and Tsuga canadensis) planted in our experimental plots was up to fivefold higher in thickets of R. maximum compared with those outside the thickets. The order of mortality under the R. maximum thickets, \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Leaf Water Relations and Sapflow in Eastern Cottonwood (Populus deltoides Bartr.) Trees Planted for Phytoremediation of a Groundwater Pollutant

Prunus> Quercus> Tsuga

Relationships between stem CO2 efflux, substrate supply, and growth in young loblolly pine trees

\end{document} , was consistent with the shade tolerance ranking of these species. Loss of Tsuga seedlings was attributed to burial by litter rather than shade. Surviving seedlings of Quercus and Prunus in R. maximum thickets were taller than those outside the thickets, but the seedlings in R. maximum thickets produced significantly fewer leaves, smaller total leaf area, leaf mass, and stem mass. Leaf N (%) was significantly higher in Quercus seedlings in R. maximum thickets compared with seedlings outside the thickets. Moreover, no difference was found in leaf N (%) between forest types for Prunus and Tsuga, indicating that seedlings in R. maximum thickets were not N limited. Rather, light limitation, herbivory, and litter fall contributed to the lack of tree regeneration under R. maximum thickets.