Jeffrey S. Ward

Long-term spatial dynamics in an old-growth deciduous forest

Abstract In 1926, 1976, and 1986 stems (≥ 10 cm diameter at 1.37 m (dbh)) were measured and mapped in a 4 ha interior study area on the Davis-Purdue Research Forest in east-central Indiana. Spatial pattern type was determined using the Clark and Evanss index, and Ripleys L function. The G function was used to examine spatial dispersion at intertree distances ( 25 dbh) trees was uniform between 1926 and 1986. Density-dependent mortality and ingrowth processes are maintaining uniform spatial distributions. Initial neighborhood tree distribution was higher around trees which died in the periods 1926–1976 and 1976–1986 than for contemporary survivors, i.e. trees which survived this period had fewer neighboring trees within 6 m at the beginning of the period than did trees which died. The higher initial neighborhood densities around mortality trees than survivors supports density-dependent mortality. Ingrowth was inhibited in a 6 m zone proximite to established trees for both the 1926–1976 and 1976–1986 periods.

Spatial dispersion of woody regeneration in an old-growth forest

Spatial pattern and density for woody regeneration were examined in relation to canopy structure for an old-growth forest in east-central Indiana. All woody stems -2 cm diameter at 1.37 m were recorded by species and diameter on a 4-ha area gridded into 10 x 10 m quadrats. Canopy structure was inventoried using a 5 x 5 m grid. Canopy structure was divided into three classes: canopy gaps, secondary canopy, and primary canopy. The nonparametric dispersion index was used to examine spatial patterns of woody species and the Clark-Evans nearest neighbor index was used to examine spatial pattern of canopy gaps. Canopy gaps covered 9.0% of the 4 ha area, averaged 52.4 m2 in area, and were randomly dispersed. Most species were aggregated at scales >400 m2. While overall regeneration density did not vary significantly with canopy structure, densities for some individual species were significantly different under different canopy structures. The current disturbance regime of this forest, primarily single-tree gaps, has created a canopy structure which favors the regeneration of shade-tolerant species.

Exotic Seed Dispersal by White-tailed Deer in Southern Connecticut

Abstract We examined the role of suburban white-tailed deer (Odocoileus virginianus Zimmerman) in dispersal of exotic plants in forests bordered by medium-density housing in southern Connecticut. Estimated deer density on the research site was 23 deer/km2 with higher local densities along the suburban/woodland interface. In 2002, 90 pellet groups were gathered periodically from September through December. In 2003, eight pellet groups were collected weekly from early June through late December for a total of 236. All pellet groups were vernalized at 5°C for 60 days. Pellet groups were placed in a growing medium in trays in a temperature controlled greenhouse for six months. Seeds germinated from 47% of pellet groups, which included 656 seedlings of 57 species. Seeds (n = 326) of 32 species not native to Connecticut germinated in 23% of pellet groups. We estimated that the deer herd on site had the potential to disperse 586-1046 viable exotic seeds/day/km2 during the 2002 sampling period and 390-696 viable exotic seeds/day/km2 during the 2003 sampling period. Birds, small mammals, and abiotic factors are known dispersal agents for exotic plants, some of which are invasive. Our results indicate that white-tailed deer are another important dispersal agent of exotic species. Thus, white-tailed deer may not only alter vegetation structure through direct browse damage of established plants, but also indirectly by lowering reproductive output of native plants and simultaneously distributing seeds of exotic species.

Managing Japanese Barberry (Ranunculales: Berberidaceae) Infestations Reduces Blacklegged Tick (Acari: Ixodidae) Abundance and Infection Prevalence with Borrelia burgdorferi (Spirochaetales: Spirochaetaceae)

ABSTRACT In many Connecticut forests with an overabundance of white-tailed deer (Odocoileus virginianus Zimmermann), Japanese barberry (Berberis thunbergii DC) has become the dominant understory shrub, which may provide a habitat favorable to blacklegged tick (Ixodes scapularis Say) and white-footed mouse (Peromyscus leucopus Rafinesque) survival. To determine mouse and larval tick abundances at three replicate sites over 2 yr, mice were trapped in unmanipulated dense barberry infestations, areas where barberry was controlled, and areas where barberry was absent. The number of feeding larval ticks/mouse was recorded. Adult and nymphal ticks were sampled along 200-m draglines in each treatment, retained, and were tested for Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt, and Brenner) presence. Total first-captured mouse counts did not differ between treatments. Mean number of feeding larval ticks per mouse was highest on mice captured in dense barberry. Adult tick densities in dense barberry were higher than in both controlled barberry and no barberry areas. Ticks sampled from full barberry infestations and controlled barberry areas had similar infection prevalence with B. burgdorferi the first year. In areas where barberry was controlled, infection prevalence was reduced to equal that of no barberry areas the second year of the study. Results indicate that managing Japanese barberry will have a positive effect on public health by reducing the number of B. burgdorferi-infected blacklegged ticks that can develop into motile life stages that commonly feed on humans.

Effects of Japanese Barberry (Ranunculales: Berberidaceae) Removal and Resulting Microclimatic Changes on Ixodes scapularis (Acari: Ixodidae) Abundances in Connecticut, USA

ABSTRACT Japanese barberry (Berberis thunbergii de Candolle) is a thorny, perennial, exotic, invasive shrub that is well established throughout much of the eastern United States. It can form dense thickets that limit native herbaceous and woody regeneration, alter soil structure and function, and harbor increased blacklegged tick (Ixodes scapularis Say) populations. This study examined a potential causal mechanism for the link between Japanese barberry and blacklegged ticks to determine if eliminating Japanese barberry could reduce tick abundance and associated prevalence of Borrelia burgdorferi (Johnson, Schmid, Hyde, Steigerwalt, and Brenner). Japanese barberry was controlled at five study areas throughout Connecticut; adult ticks were sampled over three years. Each area had three habitat plots: areas where barberry was controlled, areas where barberry remained intact, and areas where barberry was minimal or absent. Sampled ticks were retained and tested for B. burgdorferi presence. At two study areas, temperature and relative humidity data loggers were deployed in each of the three habitat plots over two growing seasons. Intact barberry stands had 280 ± 51 B. burgdorferi-infected adult ticks/ha, which was significantly higher than for controlled (121 ± 17/ha) and no barberry (30 ± 10/ha) areas. Microclimatic conditions where Japanese barberry was controlled were similar to areas without barberry. Japanese barberry infestations are favorable habitat for ticks, as they provide a buffered microclimate that limits desiccation-induced tick mortality. Control of Japanese barberry reduced the number of ticks infected with B. burgdorferi by nearly 60% by reverting microclimatic conditions to those more typical of native northeastern forests.

Response of woody regeneration to thinning mature upland oak stands in Connecticut, USA.

Abstract In 1989 all woody regeneration (less than 10 cm diameter at breast height (dbh)) was inventoried on 27 paired plots in thinned and unthinned mature oak stands throughout Connecticut using four 13.5 m 2 circular subplots per plot. Stocking levels were reduced by 32% on thinned plots. Total regeneration density was not increased by thinning. Thinned plots had higher oak regeneration density than unthinned plots (6761 stems ha −1 and 5018 stems ha −1 , respectively). A greater proportion of oak regeneration was taller than 1.5 m on thinned (3.3%) than on unthinned plots (0.3%). Thinning increased the proportion of taller red maple and minor species. Total non-oak regeneration density was not increased by thinning, but thinning increased non-oak sapling density. Reducing subcanopy density by low thinning should be considered as a method of enhancing oak regeneration prior to initiating traditional regeneration cuts such as shelter-woods.

New derivation reduces bias and increases power of Ripley’s L index

The development of distance-dependent growth and survival models depends on an understanding of the spatial distribution of the population in question. Ripley’s L index (LR) has found wide application for examining the spatial dispersion of plants. LR is calculated as the square root of a weighted sum of the number of observed plant pairs that are less than a certain distance apart. The weighting used by LR inflates the pair count sum to compensate for reduced pair counts for plants near the plot boundary. Using Monte Carlo simulations, we show that the variance in the observed number of tree pairs is not stabilized by the square root transformation at low expected counts. The non-linearity of the square root transformation introduces a consistent bias in both the first and second moments of the tree pair distribution. We present a derived estimator for Ripley’s analytical L index (LA) that provides a more accurate estimate of variance and mean. This new approach, based on a true Poisson variate, includes a modification of the previous edge correction method that incorporates a global estimate of mean pair density, rather than local values. This reduces variance caused by stochastic placement of point pairs near the boundary. Monte Carlo simulations verified the predictions of this model over a wide range of population sizes (25–1400). Simulation results showed that the LR numerical estimate of the confidence limit was overly conservative by nearly a factor of two. The improved power and accuracy provided by LA suggest that it would be fruitful to reexamine population spatial dispersion data in the literature using the analytical estimator (LA). As an illustration, the power and accuracy of LR and LA to detect non-random spatial dispersions is compared using generated populations and six stands of mapped trees in Connecticut.

Influence of crown class and shade tolerance on individual tree development during deciduous forest succession in Connecticut, USA

Abstract Sixty years of individual tree crown class records were used to elucidate the influence of crown class (dominant, codominant, intermediate, or suppressed), shade tolerance (intolerant, midtolerant, or tolerant), and their interactions on the probability of individual tree movement among crown classes. Trees were measured at 10 year intervals between 1927 and 1987, excluding 1947, on 364 nominal 0.01 ha plots. A total of 14 154 individual tree records were used in this analysis. The transition rates among crown classes for 30 year intervals (1927–1957 and 1957–1987) were examined for all combinations of antecedent crown classes and tolerance rankings. The distribution of ingrowth among tolerance rankings was also examined. Mortality rates increased with decreasing crown class for all tolerance rankings, and mortality rates increased with decreasing tolerance. Compared with tolerant trees, midtolerant and intolerant trees had higher rates of ascension into dominant and codominant crown classes and exhibited higher persistence rates in the dominant crown class. These factors suggest that midtolerant and intolerant trees have an advantage over tolerant trees in the higher crown classes, In contrast, tolerant trees had the advantage in suppressed and intermediate crown classes, with lower mortality rates, higher persistence, and higher rates of crown class ascension than for midtolerant or intolerant trees. Crown class stratification was driven by the change in relative advantage of each tolerance ranking among the crown class. Results of this study suggest that the canopy stratum (upper canopy, lower canopy, or mixed) affected by disturbance is as important as disturbance scale, intensity, and frequency in influencing the composition of the suppressed crown class. Because the suppressed crown class is in a constant state of high flux, with fewer than 30% of the suppressed trees remaining in the suppressed crown class for any 30 year period, a small change in the relative persistence or ascension rates among tolerance rankings, whether by a different disturbance or climate regime, could alter the proportion of tolerance rankings in the suppressed crown class and ultimately result in an alternative succession.

Comparing effectiveness and impacts of Japanese barberry (Berberis thunbergii) control treatments and herbivory on plant communities.

Abstract Two factors that can degrade native plant community composition and structure, and hinder restoration efforts, are invasive species and chronic overbrowsing by ungulates such as white-tailed deer. Beginning in 2007, the effectiveness, costs, and impacts of Japanese barberry control treatments and herbivory on nonnative and native plant communities was examined at eight study areas over 4 to 5 yr. Prescribed burning and mechanical mowing by wood shredder or brush saw were utilized as initial treatments to reduce the aboveground portion of established barberry and were equally effective. Without a follow-up treatment, barberry had recovered to 56 to 81% of pretreatment levels 50 to 62 mo after initial treatment. Follow-up treatments in mid-summer to kill new sprouts included directed heating and foliar herbicide applications. Relative to untreated controls, follow-up treatments lowered barberry cover 50 to 62 mo after initial treatment by at least 72%. Although all follow-up treatments were equally effective, the labor cost of directed heating was four times higher than for herbicide applications. Follow-up treatment type (directed heating vs. herbicide) had minimal impact on species other than barberry. White-tailed deer herbivory had a larger impact on other species than did barberry control treatments. Native grass and fern cover was higher outside of exclosures. Areas inside exclosures had higher cover of Oriental bittersweet and multiflora rose, but not Japanese barberry. Thus, recovery of native communities will require more than simply removing the dominant invasive species where deer densities are high. Excellent reduction of Japanese barberry cover can be achieved using either directed heating or herbicides as follow-up treatments in a two-step process, but other invasive plants may become a problem when barberry is removed if deer populations are low. Nomenclature: Glyphosate, triclopyr, 2,4-D, Japanese barberry, Berberis thunbergii DC. BEBTH, Oriental bittersweet Celastrus orbiculatus Thunb. CELOR, multiflora rose, Rosa multiflora Thunb. ex Murr. ROSMU. Management Implications: Japanese barberry is an invasive shrub in the eastern United States and Canada that forms dense thickets that can inhibit forest regeneration and native herbaceous plant populations, and is associated with elevated populations of blacklegged ticks, which can be infected with the causal agent of Lyme disease, Borrelia burgdorferi. A reasonable objective of controlling an invasive species such as Japanese barberry, is not eradication of the invasive species per se, but the restoration of a community dominated by native herbaceous species and woody regeneration. This study found that a single treatment (e.g., mowing) is ineffective without a follow-up treatment. A variety of techniques using a two-step strategy can successfully control Japanese barberry for a period of at least 4 to 5 yr if there is an immediate follow-up treatment, albeit at a wide range of labor costs. Labor costs for follow-up treatments with a propane torch were at least four times higher than for herbicide applications, although both were equally effective at controlling barberry. However, invasive control by itself was insufficient to restore native communities in these areas with high white-tailed deer densities because deer herbivory had a larger impact on plant communities than control of Japanese barberry. Where deer densities are high, a successful restoration program must incorporate a program to reduce deer density to a level sufficient for native forbs to flower and set fruit and for tree seedlings to grow tall enough to be above browse heights—typically 2 m (6 ft). Successful restoration might also require introduction of locally extirpated species. A successful program should also include strategies to control those invasive species, such as Oriental bittersweet and multiflora rose, that might have been suppressed by dense barberry or deer browse. Although applying control techniques to barberry is justifiable where infestations are severe and there is concern about tick abundance, the chance that one invasive species problem might be traded for another must be anticipated.

Nonchemical and Herbicide Treatments for Management of Japanese Stiltgrass (Microstegium vimineum)

Abstract Japanese stiltgrass, an annual grass species native to eastern Asia, has become a serious invasive-plant problem in the eastern United States. We compared the efficacy of herbicides and nonchemical options found effective for controlling stiltgrass in earlier studies, with organic herbicides and herbicides used at reduced rates in a wooded floodplain along the lower Connecticut River. We compared the effect of 2 yr of conventional and alternative treatments on cover of other nonnative and native species. Four blocks of 18 plots (3 by 4 m [9.8 by 13.1 ft]) were established in May 2008. Treatments included directed heating with a propane torch (June, July), hand-pulling (July), mowing with a string trimmer (July, August), foliar applications of household vinegar [5% acetic acid] (June, July) and the herbicides imazapic (June), pelargonic acid (June, July), and pelargonic acid plus pendimethalin (June). The following herbicides were applied at labeled doses and at one-fourth labeled doses: fenoxaprop-p-ethyl (July), glufosinate (August), and glyphosate (August). Stiltgrass cover and height were evaluated periodically, and plant samples were collected in autumn of 2008 and 2009 to determine the number of viable seeds produced. Final evaluations were conducted in June 2010 after 2 yr of treatment. Stiltgrass cover averaged 88% on untreated plots in fall. All treatments reduced stiltgrass cover and seed production. The least-effective treatments were hand-pulling, pelargonic acid, and vinegar in July. Direct heating, mowing, and vinegar in June reduced seed production by more than 90%. All treatments containing imazapic, pelargonic acid plus pendimethalin, fenoxaprop-p-ethyl, glufosinate, and glyphosate completely prevented stiltgrass seed production in the second year of treatment. Effective control of stiltgrass can be achieved during a 2-yr period with a variety of herbicides, including herbicides at one-fourth of the labeled dose, and through nonchemical treatments. Nomenclature: Fenoxaprop-p-ethyl; glufosinate; glyphosate; imazapic; pelargonic acid; pendimethalin; Japanese stiltgrass, Microstegium vimineum (Trin.) A. Camus var. imberbe (Nees) Honda. Management Implications: Japanese stiltgrass, an increasingly common, invasive grass species along riparian and transportation corridors, including hiking trails, can depress native, herbaceous plant populations and alter soil processes. Although earlier studies have shown that several herbicides provide effective control of stiltgrass infestations, many infestations occur in forests where regulations, deeds, or active public opposition can limit herbicide application as a management option. During a 2-yr period, we compared the relative effectiveness of several synthetic herbicides with an “organic” herbicide, acetic acid (household vinegar), and the nonchemical options of hand-pulling, cutting, and directed heating with a propane torch. We also examined the effectiveness of the herbicides fenoxaprop-p-ethyl, glufosinate, and glyphosate at labeled and one-fourth labeled doses. Hand-pulling, pelargonic acid, and vinegar in July were not sufficiently effective to be recommended. Vinegar in June and mowing provided better, but not complete, control. Directed heating and the other herbicides, even at one-fourth doses, provided nearly complete control of stiltgrass. Use of herbicides at low doses might minimize public opposition, provide effective control, and reduce environmental effects. Control of stiltgrass can be achieved without herbicides, but nonchemical methods will require additional time and labor costs.