Brian D. Kloeppel

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.

Hillslope nutrient dynamics following upland riparian vegetation disturbance

AbstractWe investigated the effects of removing near-stream Rhododendron and of the natural blowdown of canopy trees on nutrient export to streams in the southern Appalachians. Transects were instrumented on adjacent hillslopes in a first-order watershed at the Coweeta Hydrologic Laboratory (35°03′N, 83°25′W). Dissolved organic carbon (DOC), K+, Na+, Ca2+, Mg2+, NO3−-N, NH4+-N, PO43−-P, and SO42− were measured for 2 years prior to disturbance. In August 1995, riparian Rhododendron on one hillslope was cut, removing 30% of total woody biomass. In October 1995, Hurricane Opal uprooted nine canopy trees on the other hillslope, downing 81% of the total woody biomass. Over the 3 years following the disturbance, soilwater concentrations of NO3−-N tripled on the cut hillslope. There were also small changes in soilwater DOC, SO42−, Ca2+, and Mg2+. However, no significant changes occurred in groundwater nutrient concentrations following Rhododendron removal. In contrast, soilwater NO3−-N on the storm-affected hillslope showed persistent 500-fold increases, groundwater NO3−-N increased four fold, and streamwater NO3−-N doubled. Significant changes also occurred in soilwater pH, DOC, SO42−, Ca2+, and Mg2+. There were no significant changes in microbial immobilization of soil nutrients or water outflow on the storm-affected hillslope. Our results suggest that Rhododendron thickets play a relatively minor role in controlling nutrient export to headwater streams. They further suggest that nutrient uptake by canopy trees is a key control on NO3−-N export in upland riparian zones, and that disruption of the root–soil connection in canopy trees via uprooting promotes significant nutrient loss to streams.

BAAD: a biomass and allometry database for woody plants

Understanding how plants are constructed—i.e., how key size dimensions and the amount of mass invested in different tissues varies among individuals—is essential for modeling plant growth, carbon stocks, and energy fluxes in the terrestrial biosphere. Allocation patterns can differ through ontogeny, but also among coexisting species and among species adapted to different environments. While a variety of models dealing with biomass allocation exist, we lack a synthetic understanding of the underlying processes. This is partly due to the lack of suitable data sets for validating and parameterizing models. To that end, we present the Biomass And Allometry Database (BAAD) for woody plants. The BAAD contains 259 634 measurements collected in 176 different studies, from 21 084 individuals across 678 species. Most of these data come from existing publications. However, raw data were rarely made public at the time of publication. Thus, the BAAD contains data from different studies, transformed into standard units and variable names. The transformations were achieved using a common workflow for all raw data files. Other features that distinguish the BAAD are: (i) measurements were for individual plants rather than stand averages; (ii) individuals spanning a range of sizes were measured; (iii) plants from 0.01–100 m in height were included; and (iv) biomass was estimated directly, i.e., not indirectly via allometric equations (except in very large trees where biomass was estimated from detailed sub-sampling). We included both wild and artificially grown plants. The data set contains the following size metrics: total leaf area; area of stem cross-section including sapwood, heartwood, and bark; height of plant and crown base, crown area, and surface area; and the dry mass of leaf, stem, branches, sapwood, heartwood, bark, coarse roots, and fine root tissues. We also report other properties of individuals (age, leaf size, leaf mass per area, wood density, nitrogen content of leaves and wood), as well as information about the growing environment (location, light, experimental treatment, vegetation type) where available. It is our hope that making these data available will improve our ability to understand plant growth, ecosystem dynamics, and carbon cycling in the worlds vegetation.

Differential effects of understory and overstory gaps on tree regeneration

Abstract Gaps in the forest canopy can increase the diversity of tree regeneration. Understory shrubs also compete with tree seedlings for limited resources and may depress tree recruitment. We compared effects of shrub removal and canopy windthrow gaps on seedling recruitment and understory resource levels. Shrub removal, with the canopy left intact, was associated with increased levels of understory light and soil moisture and coincided with increased species richness and diversity of tree regeneration compared to both control plots and canopy gaps. Canopy windthrow gaps, however, resulted in a more than 500 fold increase in soil nitrate concentrations, and seedling growth rates that were twice as high as that observed with shrub removal. Our results suggest that gaps in the understory shrub layer and the overstory canopy may have complementary effects on resource availability with corresponding benefits to seedling establishment and growth.

Seasonal Tissue Water Relations of Four Successional Pennsylvania Barrens Species in Open and Understory Environments

Seasonal tissue water relations were measured in co-occurring saplings of Quercus velutina Lam., Quercus prinus L., Sassafras albidum (Nutt.) Nees, and Acer rubrum L. from adjacent open and understory sites in the central Pennsylvania barrens. Open-growing plants exhibited greater and earlier phenological shifts in osmotic potentials under moist conditions, whereas understory plants had greater osmotic adjustment during a mild, late-season drought. Sassafras albidum was an exception, exhibiting steadily declining osmotic potentials at full and zero turgor over the course of the season on both sites. Elastic modulus (ε) steadily increased for all species on the open site, while A. rubrum showed a decrease and S. albidum and Q. velutina showed an increase in ε in the understory. Relative water content at zero turgor (RWC0) was similar in the understory and open sites except during the drought period when understory plants had lower values. Quercus prinus generally exhibited the lowest RWC0 values, although A. rubrum, a later successional species, had a seasonal decrease in RWC0 at both sites. Thus, each species exhibited somewhat unique combinations of seasonal osmotic and elastic adjustment, which acted in concert to balance tissue water loss with turgor maintenance under changing environmental conditions. These results indicate that a variety of ecophysiological mechanisms operate to allow species of different successional rank to tolerate open and understory barrens environments.