John J. Battles

Assessing the relative importance of disturbance, herbivory, diversity, and propagule pressure in exotic plant invasion

The current rate of invasive species introductions is unprecedented, and the dramatic impacts of exotic invasive plants on community and ecosystem properties have been well documented. Despite the pressing management implications, the mechanisms that control exotic plant invasion remain poorly understood. Several factors, such as disturbance, propagule pressure, species diversity, and herbivory, are widely believed to play a critical role in exotic plant invasions. However, few studies have examined the relative importance of these factors, and little is known about how propagule pressure interacts with various mechanisms of ecological resistance to determine invasion success. We quantified the relative importance of canopy disturbance, propagule pressure, species diversity, and herbivory in determining exotic plant invasion in 10 eastern hemlock forests in Pennsylvania and New Jersey (USA). Use of a maximum-likelihood estimation framework and information theoretics allowed us to quantify the strength of evidence for alternative models of the influence of these factors on changes in exotic plant abundance. In addition, we developed models to determine the importance of interactions between ecosystem properties and propagule pressure. These analyses were conducted for three abundant, aggressive exotic species that represent a range of life histories: Alliaria petiolata, Berberis thunbergii, and Microstegium vimineum. Of the four hypothesized determinants of exotic plant invasion considered in this study, canopy disturbance and propagule pressure appear to be the most important predictors of A. petiolata, B. thunbergii, and M. vimineum invasion. Herbivory was also found to be important in contributing to the invasion of some species. In addition, we found compelling evidence of an important interaction between propagule pressure and canopy disturbance. This is the first study to demonstrate the dominant role of the interaction between canopy disturbance and propagule pressure in determining forest invasibility relative to other potential controlling factors. The importance of the disturbance-propagule supply interaction, and its nonlinear functional form, has profound implications for the management of exotic plant species populations. Improving our ability to predict exotic plant invasions will require enhanced understanding of the interaction between propagule pressure and ecological resistance mechanisms.

Forest carbon storage: ecology, management, and policy

The objective of this review is to give ecologists and policy makers a better understanding of forest carbon dynamics and recent policy and management activities in this arena. The ecology of forest carbon is well understood, but measurement and projection of carbon sequestration at small scales can be costly. Some forest management activities qualify as offsets in various carbon markets. To promote wider use, a system is needed that will provide inexpensive and standardized approaches to forest carbon accounting that are not prone to dishonest handling. The prospects are fairly promising for development of such a system, but first, technical and organizational constraints must be overcome. In contrast, the benefits – in terms of greenhouse-gas reduction – of substituting wood for other building materials, and in displacing fossil fuel energy, could be realized immediately, if standards for calculations can be developed.

RESPONSES OF EARLY SUCCESSIONAL NORTHERN HARDWOOD FORESTS TO CHANGES IN NUTRIENT AVAILABILITY

In many mesic forests the dominant trees are limited concurrently by light and soil resources, and understanding the mechanisms of competition and predicting outcomes of competition are especially difficult when co-limitation exists. We altered soil resource availability during the early stages of stand development after clearcutting of northern hardwood forests to examine the mechanism of competition. Specifically, we sought empirical evidence about the role of various physiological, morphological, allocational, and architectural responses in regulating plant competition. We expected the competitive ability of the extreme pioneer species, Prunus pensylvanica (pin cherry), to be enhanced by increased nutrient supply, with consequent effects at the community and ecosystem levels of organization. Nutrient availability was increased by about three-fold by monthly fertilization for 6 yr in nine even-aged northern hardwood stands dominated by pin cherry, three each of three ages (6, 12, and 18 yr at initiation...

Synchrony of Seed Dispersal, Hydrology and Local Climate in a Semi-arid River Reach in California

The temporal availability of propagules is a critical factor in sustaining pioneer riparian tree populations along snowmelt-driven rivers because seedling establishment is strongly linked to seasonal hydrology. River regulation in semi-arid regions threatens to decouple seed development and dispersal from the discharge regime to which they evolved. Using the lower Tuolumne River as a model system, we quantified and modeled propagule availability for Populus fremontii (POFR), Salix gooddingii (SAGO), and Salix exigua (SAEX), the tree and shrub species that dominate near-channel riparian stands in the San Joaquin Basin, CA. A degree-day model was fit to field data of seasonal seed density and local temperature from three sites in 2002–2004 to predict the onset of the peak dispersal period. To evaluate historical synchrony of seed dispersal and seasonal river hydrology, we compared peak spring runoff timing to modeled peak seed release periods for the last 75 years. The peak seed release period began on May 15 for POFR (range April 23–June 10), May 30 for SAGO (range May 19–June 11) and May 31 for SAEX (range May 8–June 30). Degree-day models for the onset of seed release reduced prediction error by 40–67% over day-of-year means; the models predicted best the interannual, versus site-to-site, variation in timing. The historical analysis suggests that POFR seed release coincided with peak runoff in almost all years, whereas SAGO and SAEX dispersal occurred during the spring flood recession. The degree-day modeling approach reduce uncertainty in dispersal timing and shows potential for guiding flow releases on regulated rivers to increase riparian tree recruitment at the lowest water cost.

Acceleration of Exotic Plant Invasion in a Forested Ecosystem by a Generalist Herbivore

The successful invasion of exotic plants is often attributed to the absence of coevolved enemies in the introduced range (i.e., the enemy release hypothesis). Nevertheless, several components of this hypothesis, including the role of generalist herbivores, remain relatively unexplored. We used repeated censuses of exclosures and paired controls to investigate the role of a generalist herbivore, white-tailed deer (Odocoileus virginianus), in the invasion of 3 exotic plant species (Microstegium vimineum, Alliaria petiolata, and Berberis thunbergii) in eastern hemlock (Tsuga canadensis) forests in New Jersey and Pennsylvania (U.S.A.). This work was conducted in 10 eastern hemlock (T. canadensis) forests that spanned gradients in deer density and in the severity of canopy disturbance caused by an introduced insect pest, the hemlock woolly adelgid (Adelges tsugae). We used maximum likelihood estimation and information theoretics to quantify the strength of evidence for alternative models of the influence of deer density and its interaction with the severity of canopy disturbance on exotic plant abundance. Our results were consistent with the enemy release hypothesis in that exotic plants gained a competitive advantage in the presence of generalist herbivores in the introduced range. The abundance of all 3 exotic plants increased significantly more in the control plots than in the paired exclosures. For all species, the inclusion of canopy disturbance parameters resulted in models with substantially greater support than the deer density only models. Our results suggest that white-tailed deer herbivory can accelerate the invasion of exotic plants and that canopy disturbance can interact with herbivory to magnify the impact. In addition, our results provide compelling evidence of nonlinear relationships between deer density and the impact of herbivory on exotic species abundance. These findings highlight the important role of herbivore density in determining impacts on plant abundance and provide evidence of the operation of multiple mechanisms in exotic plant invasion.

SPATIAL ELEMENTS OF MORTALITY RISK IN OLD-GROWTH FORESTS

For many species of long-lived organisms, such as trees, survival appears to be the most critical vital rate affecting population persistence. However, methods commonly used to quantify tree death, such as relating tree mortality risk solely to diameter growth, almost certainly do not account for important spatial processes. Our goal in this study was to detect and, if present, to quantify the relevance of such processes. For this purpose, we examined purely spatial aspects of mortality for four species, Abies concolor, Abies magnifica, Calocedrus decurrens, and Pinus lambertiana, in an old-growth conifer forest in the Sierra Nevada of California, USA. The analysis was performed using data from nine fully mapped long-term monitoring plots. In three cases, the results unequivocally supported the inclusion of spatial information in models used to predict mortality. For Abies concolor, our results suggested that growth rate may not always adequately capture increased mortality risk due to competition. We also found evidence of a facilitative effect for this species, with mortality risk decreasing with proximity to conspecific neighbors. For Pinus lambertiana, mortality risk increased with density of conspecific neighbors, in keeping with a mechanism of increased pathogen or insect pressure (i.e., a Janzen-Connell type effect). Finally, we found that models estimating risk of being crushed were strongly improved by the inclusion of a simple index of spatial proximity. Not only did spatial indices improve models, those improvements were relevant for mortality prediction. For P. lambertiana, spatial factors were important for estimation of mortality risk regardless of growth rate. For A. concolor, although most of the population fell within spatial conditions in which mortality risk was well described by growth, trees that died occurred outside those conditions in a disproportionate fashion. Furthermore, as stands of A. concolor become increasingly dense, such spatial factors are likely to become increasingly important. In general, models that fail to account for spatial pattern are at risk of failure as conditions change.

Edge effects in mixed conifer group selection openings: tree height response to resource gradients

Replicated circular openings ranging in size from 0.1 to 1 ha were cleared on a Sierran mixed conifer forest in 1996 at the Blodgett Forest Research Station, California and planted with seedlings of six native species. After 3 years of growth, heights of all trees were measured and analyzed according to species, opening size, and location within the opening. To determine the cause of the edge influence on height, we measured differences along north‐south transects in extension growth, pre-dawn water potential, and light availability for three species of trees: giant sequoia (Sequoiadendron giganteum), ponderosa pine (Pinus ponderosa), and Douglas-fir (Ptseudotsuga menziesii var. menziesii). The sequence of mean height from tallest to shortest based on species was: giant sequoia > incense cedar > Douglas-fir > ponderosa pine > white fir > sugar pine. For all species combined, a ten-fold increase in the area of the opening corresponded to a 34% increase in mean height. Trees were tallest on average in the north rows and shortest in the south rows. There was no difference in height between trees in the east and west rows. As expected, resource availability was greatest near the center and least near the edges with north edges receiving significantly more light than southern edges. In general, observed edge effects on sapling height growth were correlated with light and water supply. However there were important differences between species in the nature of the co-limitation. Giant sequoia growth was most sensitive to light and water availability. Together they explained more than 47% of the observed variation in giant sequoia height. In contrast, only light was a significant predictor of ponderosa pine performance. Douglas-fir heights were significantly related to both light and water but there was more unexplained variability in the Douglas-fir model compared to the other species. These highly controlled experimental group openings provide a standard reference for silviculturalists using the group selection method of regeneration. # 2002 Elsevier Science B.V. All rights reserved.

Forest Composition, Structure, and Change in an Old-Growth Mixed Conifer Forest in the Northern Sierra Nevada

density was 721 stems/ha, and total basal area was 75.3 m2/ha. Pseudotsuga menziesii (Mirbel) Franco var. menziesii and Abies concolor (Gordon & Glend.) Lindley codominated the plot. A. concolor was the most abundant tree species, 420 stems/ha, but P. menziesii had the highest basal area, 31.3 m2/ha. A retrospective analysis of trees > 24 cm in dbh showed that in the last 39 years, stand density increased 39% from 157 to 219 stems/ha, and stand basal area increased 15% from 57.9 to 66.7 m2/ha. These increases were due largely to the canopy recruitment of A. concolor (60% of new recruits) and the continued growth and low mortality (0.3%/ yr) of P. menziesii. Overall stand mortality of trees originally measured by Baker was 0.6%/yr. Regeneration size classes were dominated (in order of frequency) by A. concolor, P. menziesii, and Calocedrus decurrens (Torrey) Florin. There was little regeneration of the less shade-tolerant pine species, Pinus lambertiana Douglas and Pinus ponderosa Laws.

GAP DYNAMICS FOLLOWING FOREST DECLINE: A CASE STUDY OF RED SPRUCE FORESTS

Forest decline is perceived as a threat to forest health in many regions of the world. The decline of red spruce in the mountains of the northeastern United States is one well-known example. We evaluated the impact of spruce decline by considering its effect on the prevailing gap dynamics. We compared plant composition, height growth, and indices of resource availability in gaps to the forest as a whole in four old-growth stands. In our study, there were no appreciable compositional differences between gaps and the forest as a whole. However, the relative height growth rate of saplings and seedlings was greater in gaps and was significantly different in three of the four old-growth communities studied. There were also significant differences in the response of species to gaps; birches grew faster than balsam fir which grew faster than spruce. The initial stage of vegetation recovery in the spruce–fir forest was dominated by advance regeneration of balsam fir. Much of the recovery in the hardwood–conifer t...

The importance of quantifying propagule pressure to understand invasion: an examination of riparian forest invasibility

The widely held belief that riparian communities are highly invasible to exotic plants is based primarily on comparisons of the extent of invasion in riparian and upland communities. However, because differences in the extent of invasion may simply result from variation in propagule supply among recipient environments, true comparisons of invasibility require that both invasion success and propagule pressure are quantified. In this study, we quantified propagule pressure in order to compare the invasibility of riparian and upland forests and assess the accuracy of using a communitys level of invasion as a surrogate for its invasibility. We found the extent of invasion to be a poor proxy for invasibility. The higher level of invasion in the studied riparian forests resulted from greater propagule availability rather than higher invasibility. Furthermore, failure to account for propagule pressure may confound our understanding of general invasion theories. Ecological theory suggests that species-rich communities should be less invasible. However, we found significant relationships between species diversity and invasion extent, but no diversity-invasibility relationship was detected for any species. Our results demonstrate that using a communitys level of invasion as a surrogate for its invasibility can confound our understanding of invasibility and its determinants.