Brian C. McCarthy

EFFECT OF ENVIRONMENTAL VARIATION ON THE INVASIVE SUCCESS OF A NONINDIGENOUS FOREST HERB

With the ever-burgeoning spread of nonindigenous plants, often facilitated by human activities, it is imperative to conduct case studies of particular invasive plants and the sites they invade in order to develop effective and efficient habitat management strategies. Alliaria petiolata is a native European biennial mustard that has become a serious invasive pest in many North American forests. In order to better characterize habitat vul- nerability to invasion by A. petiolata, we conducted an experiment to test the effects of environmental heterogeneity in the form of habitat, microenvironment, and small-scale litter disturbance on A. petiolata germination, survival, growth, and reproduction. Treatments consisted of two habitats (upland and lowland), two microenvironments (forest edge and forest interior), and three small-scale litter disturbance treatments (control, litter completely removed, and litter partially removed). Seeds were sown into plots in November, and plots were monitored for two years. Lowland plots had greater soil moisture and less litter per unit area than upland plots. In general, forest edge plots had greater understory cover and light availability and lower overstory cover than forest interior plots. Rosette survival, mature plant survival, plant biomass, height, and fruit and seed production were significantly greater for plants in the lowland compared with plants in the upland. Germination, rosette survival, mature plant survival, and reproduction were significantly greater in the forest edge plots when compared with interior plots. Litter disturbance had no significant effect on germination, growth, or reproduction. It is apparent that site colonization by A. petiolata was not dependent on the creation of patches of bare soil by disturbance, since plants were capable of invading sites with an intact litter layer. Alliaria petiolata growth and repro- duction was greatest in plots with adequate soil moisture and increased light availability. Therefore, mesic forests with a more open canopy structure and forest trails or edges may be the most vulnerable to invasion and the most difficult sites to manage. Drier upland forest interiors, which were more resistant to invasion, may be more responsive to man- agement techniques.

Competitive Ability of Alliaria petiolata (Garlic Mustard, Brassicaceae), an Invasive, Nonindigenous Forest Herb

Alliaria petiolata (Bieb.) Cavara & Grande (garlic mustard, Brassicaceae) is a Eurasian herb that is currently invading forested areas throughout portions of the northern United States and southern Canada. Alliaria petiolata enters plant communities in an aggressive fashion, often displacing resident understory species. One possible mechanism that may enable garlic mustard to successfully invade these areas is hypothesized superiority as a competitor. In order to determine the competitive potential of A. petiolata, a multiple deWit replacement series was conducted in the greenhouse between A. petiolata and three native species: Impatiens capensis, an herbaceous annual, and Acer negundo and Quercus prinus, two woody perennials. Each target species was grown in mixture with A. petiolata, and all species were grown in monoculture. After 21 wk, plants were harvested and dried. Aboveground dry‐weight biomass (yield) was then used to calculate the relative yield per plant, relative yield total, and mean aggressivity index. Impatiens capensis and A. negundo experienced greater intraspecific competition than interspecific competition with A. petiolata, while A. petiolata grown with A. negundo experienced more interspecific competition. Impatiens capensis and A. petiolata were approximately equal in aggressivity, while A. negundo was more aggressive than A. petiolata. In contrast, Q. prinus experienced more interspecific competition when grown with A. petiolata and had a lower aggressivity value. Alliaria petiolata rosettes had a greater competitive ability and aggressivity than Q. prinus, indicating that oak forest understories may be more vulnerable to A. petiolata invasion and that A. petiolata may negatively affect oak regeneration.

Spatial and temporal variability of herbaceous vegetation in an eastern deciduous forest

We examined spatial and temporal variability of understory herbaceousvegetation on opposing north- and south-facing slopes in an eastern deciduousold-growth forest in southeastern Ohio, USA. Secondly, we explored theinfluenceof sampling scale and analytical technique on our assessment of diversitypatterns. The influence of aspect and seasonality were examined at varyingsampling scales using observed richness, evenness, andH′ diversity measures, non-parametric richnessestimators, species-area curves, and SHE analysis. Herb layer composition,abundance, and diversity were strongly influenced by location (north slope vs.south slope), seasonal sampling period (April, June, August), and plot size(micro (2 m2)- vs. meso (70m2)-scale samples). Although north and south plots werecompositionally distinct, they followed similar courses of change through thegrowing season. Richness, evenness, and H′ diversitywere generally greater on the south plot whereas herbaceous abundance wasgreater on the north plot. Species composition and diversity showed markedphenological (temporal) changes, and comparison of diversity measures at micro-and meso-scales produced markedly different results. Minimum sample areas of150–200 m2 were needed to evaluate micro-scalerichness in these species rich communities, suggesting that forest understoriesmay be frequently undersampled in ecological studies. Comparison of observedandestimated meso-scale richness also suggested underestimation of richness in thenorth plot, particularly earlier in the growing season. Thus, sample size,area,and time of sampling appear critical to assessment of diversity in spatiallyandtemporally variable communities such as herbaceous forest understories.

Evaluation of coarse woody debris and forest vegetation across topographic gradients in a southern Ohio forest

In highly dissected landscapes, topographic gradients and their associated microhabitat conditions have been shown to greatly influence the distribution of woody vegetation. However, little is known about the influence of these gradients on the spatial distribution of coarse woody debris (CWD), especially in unglaciated eastern North America. The main goal of this study was to examine the influence of topography (slope aspect, percent slope, and slope position as calculated from a landform index (LI)) and plot characteristics (tree height, age, and canopy cover) on the distribution of trees (diameter at breast height, dbh≥10 cm), saplings (dbh<10 cm), and CWD in a mixed-oak forest of unglaciated southern Ohio (USA). CWD was defined in this study as any downed bole or limb with a diameter≥10 cm over a length≥0.5 m. Redundancy analysis (RDA) of the three structural layers revealed that the measured parameters explained a significant (P<0.05) percentage of the variation in individual species distributions. In all three structural layers, slope aspect and LI were consistently important in explaining individual taxa distributions. Total CWD density (pieces ha−1) was significantly (P<0.05) correlated with LI (r=0.299) and percent slope (r=−0.433). Total tree density was significantly negatively correlated with slope position (r=−0.469), slope aspect (r=−0.328), and canopy cover (r=−0.391), and total sapling density was significantly negatively correlated with height of the dominant tree in each plot (r=−0.283). According to an analysis of similarities between the three structural layers, species assemblages were significantly (P<0.05) different between all three layers. The tree and CWD strata showed the highest similarity (rho, R=0.216) while the CWD and sapling layers showed the lowest (R=0.762). Oak (Quercus spp.) dominated the tree and CWD layers while mesophytic species (maple (Acer spp.) and beech (Fagus grandifolia)) dominated the sapling layer. Analyses of CWD distributions in relation to both living vegetation and topographic gradients suggest that a complex interplay of factors dictate the distribution of CWD loads across the landscape. Differences in distributions were observed for total and individual CWD taxa. Given its importance in forested ecosystems, a better understanding of CWD spatial distribution is necessary if CWD dynamics and function (habitat and nutrient cycling) are to be fully understood.

A new index of interspecific competition for replacement and additive designs

The literature is reviewed to summarize the major indices of interspecific competition used in De Wit replacement experiments. Of the many indices that have been defined, some are less than clear as to their meaning, so interpretations are often difficult to make. In an effort to explore the performance of individual indices and to permit cross-correlation among indices, a series of hypothetical results in different competition scenarios is created. A standardized notation for all indices is also provided, along with equations and proofs. Nine indices are reviewed and analyzed for their behavior under the hypothetical scenarios and a new index that provides increased clarification and interpretability over other indices is proposed. Relative Yield Total, Aggressivity, and Relative Replacement Rate were shown to be poor measures of competition. Relative Crowding Coefficient has many restrictions to its use. The clearest index that includes two or more species is Relative Yield of Mixture. Indices that describe single species accurately were found to be Relative Yield (RY) and Relative Competition Intensity (RCI), both of which are mathematically convertible. An index introduced here, Change in Contribution (CC), is a single species index that differs from RY and RCI because it takes into account the overall biomass each species contributes.

Changes in forest structure and species composition following invasion by a non-indigenous shrub, Amur honeysuckle (Lonicera maackii)

Abstract Heavy infestations of a non-indigenous shrub, Amur honeysuckle (Lonicera maackii) are frequently linked to poor individual performance and reduced species diversity. We used chronosequence methods and analysis of forest strata to investigate potential impacts of L. maackii on 1) species richness, 2) density of sub-canopy recruitment layers, 3) indicator species affinity, and 4) within- and among-strata compositional similarities. We compared sites ranging from 12 to 26 years since initial invasion as well as non-invaded, control sites. When assessing non-invaded versus long-invaded sites, we found significantly (P ≤ 0.05) reduced densities in the herb (−57%), seedling (−58%), and sapling layers (−90%), and also reduced species richness in the seedling (−34%), and sapling (−58%), and herbaceous layers (−4%). Interestingly, the germinable seed bank density showed significant increases among non-invaded and long-invaded sites (+78%), while the species richness of the seed bank demonstrated a marked decrease (−41%). Indicator species analysis found that most species had greater frequencies and abundances in non-invaded sites, and nearly all species had greater affinities for the non-invaded sites, especially in the seedling, sapling, and seed bank layers. Responses were species-specific in the herbaceous layer. Within-layer species similarity decreased with increased time of L. maackii occupancy in all sub-canopy strata. This pattern was also found with reductions in between-layer compositional similarity for the long-invaded (r = 0.16) versus the recently-invaded (r = 0.37) and non-invaded (r = 0.51) sites. Our data suggest that in many invaded sites, habitats are experiencing wholesale alterations in species composition, structure, and successional trajectory, and invasion may also change competitive interactions in forest understories as well as alter between-layer species linkages. These results validate the candidacy of L. maackii sites for increased removal and restoration efforts.

Spatial and temporal variation in the response of understory vegetation to disturbance in a central Appalachian oak forest

SMALL, C.J. AND B.C. MCCARTHY (Department of Environmental and Plant Biology, Ohio University, Athens, Ohio 45701). Spatial and Temporal Variation in the Response of Understory Vegetation to Disturbance in a Central Appalachian Oak Forest. J. Torrey Bot. 129: 136-153. 2002.-Understory vegetation in stand-initiating (7 year clearcuts) and understory-reinitiating (> 125 year mature second-growth) oak-dominated forest stands in southeastern Ohio was examined relative to topographic aspect, micro-environmental conditions, and sampling season to explore patterns of understory dynamics and stand development following management disturbance. Plots were established on upper and lower slopes across a range of aspect positions and sampled in spring and summer to capture topographic and seasonal changes in herb layer dynamics. Micro-site light, temperature, and physical soil properties differed significantly with stand age whereas fertility and moisture varied with aspect. Species richness was greater in clearcut than mature forest stands, associated with greater resource availability and spatial heterogeneity. Herb richness and abundance were greater on NWand SE-facing slopes and lower slope positions, corresponding to greater soil moisture and fertility. Greater frequencies of non-native species were also found in resource rich, clearcut plots. Canonical correspondence analysis showed stand age, aspect, and fertility to strongly influence herb layer composition. Non-metric multidimensional scaling showed compositional divergence of spring vs. summer and clearcut vs. mature forest herb samples. Composition of clearcuts and mature forests differed markedly in June, suggesting that management influences may be more pronounced in summer herb communities. Most studies of understory dynamics in eastern forests fail to account for spatial and temporal variations in site and vegetation characteristics, however, understory disturbance response appears strongly linked to these ecosystem properties. A better understanding of these factors is needed to fully understand the effects of forest management on herb layer communities in our eastern forests.

Effect of Population Density on the Demography of an Invasive Plant (Alliaria petiolata, Brassicaceae) Population in a Southeastern Ohio Forest

Abstract As interest in invasive species management increases, new information with respect to invasive species abundance and distribution in invaded habitats is imperative. One essential type of information is demographic data. When invasive plants colonize a new habitat, their numbers may be low at first, but the population may undergo rapid expansion. We were interested in the effect of intraspecific density on the population dynamics and life history attributes of Alliaria petiolata a Eurasian biennial herb that has become an invasive pest in portions of North America. Thirty plots were established in a mesic second-growth deciduous forest in high, medium and low density patches of A. petiolata rosettes. Demographic data were collected for all A. petiolata cohorts present in the plots from 1996–1998. In June 1998 all first year rosette and second year mature individuals were harvested, dried and weighed. Stage-based population projection matrices were constructed in order to compare demography among plots and years, and models were used to predict trends in future population growth. There were significant differences among demographic parameters as a function of density and year. Survival to flowering in 1998 was greatest for plants in low density plots. These plants were also larger and produced more fruits than plants in either medium or high density plots. Initial differences among plots in plant density diminished and by 1998 there was no significant difference among density treatment plots in number of flowering plants or number of seeds produced. Seed bank formation ensures that, even under less favorable circumstances, A. petiolata can remain at a site for a number of years. Lambda values indicated that the number of plants in plots of each density is increasing, with the greatest increase in low density plots (λ = 1.45). As this study shows, due to abundant seed production, patches of low A. petiolata density in a newly colonized mesic forest can grow rapidly and in a few years form a dense stand.

An Ethnobotanical analysis of the tree species common to the subtropical moist forests of the petén, Guatemala

The purpose of this study was to analyze the utilization of tree species within and around the borders of the Maya Biosphere Reserve in Guatemala and to compare two sampling methodologies for the acquisition of such data. Residents in the communities of Caoba (border of reserve) and Uaxactún (within reserve) demonstrated significantly different utilization of tree species. Differences were due to the unique ecological and socio-economic conditions in each of the towns. Residents of both communities relied most heavily onSwietenia macrophylla, Manilkara achras. Cedrela odorata, Pouteria mammosa and Caesalpinia spp. Residents of Caoba considered 39% of all useful tree species to be marketable compared with only 18% in Uaxactún. Overall, more than 80 tree species were identified as useful in each of the communities. Changes in forest composition along with dynamic economic conditions were found to be altering the commercial utilization of tree species throughout the region. Some suggestions are offered for community development projects.

Climate remains an important driver of post‐European vegetation change in the eastern United States

Department of Geography, University of Wisconsin-Madison, 550 North ParkStreet, Madison, WI 53706, USAThe influence of climate on forest change during thepast century in the eastern United States was evalu-ated in a recent paper (Nowacki & Abrams, 2014)that centers on an increase in ‘highly competitivemesophytic hardwoods’ (Nowacki & Abrams, 2008)and a concomitant decrease in the more xerophyticQuercus species. Nowacki & Abrams (2014) con-cluded that climate change has not contributed sig-nificantly to observed changes in forest composition.However, the authors restrict their focus to a singleelement of climate: increasing temperature since theend of the Little Ice Age ca. 150 years ago. In theirstudy, species were binned into four classifications(e.g., Acer saccharum – ‘cool-adapted’, Acer rubrum –‘warm-adapted’) based on average annual tempera-ture within each species range in the United States,reducing the multifaceted character of climate into asingle, categorical measure. The broad temperatureclasses not only veil the many biologically relevantaspects of temperature (e.g., seasonal and extremetemperatures) but they may also mask other influ-ences, both climatic (e.g., moisture sensitivity) andnonclimatic (e.g., competition).Understanding the primary drivers of forest changeis critically important. However, using annual tem-perature reduces the broad spectrum of climaticinfluence on forests (e.g., Jackson & Overpeck, 2000;Jackson et al., 2009) to a single variable. Tsuga canad-ensis illustrates one example of the complex interac-tion between trees and temperature. In the southernpart of its range, Tsuga canadensis growth is weakly,but positively correlated with early growing-seasontemperature. However, this relationship becomesstronger and shifts to later in the season toward thenorthern part of its range (Cook & Cole, 1991). More-over, Tsuga canadensis growth is significantly andnegatively correlated with just May temperaturesduring the current growing season in the northeast-ern United States (Cook, 1991; Cook & Cole, 1991;Vaganov et al., 2011), while in the southeastern Uni-ted States it is strongly and negatively correlatedwith summer (June–August) temperatures (Hart et al.,2010). Trees can also be sensitive to diverse and ofteninteracting climate variables at various stages of theirlife cycles (Jackson et al., 2009). Interactions betweenprecipitation and temperature are clearly important(Harsch & Hille Ris Lambers, 2014; Martin-Benito &Pederson, accepted), and often lead to counterintui-tive responses. For example, some plant species thatwould have been expected to move north and ups-