Charles D. Canham

CANOPY TREE–SOIL INTERACTIONS WITHIN TEMPERATE FORESTS: SPECIES EFFECTS ON SOIL CARBON AND NITROGEN

In a northwestern Connecticut forest, we quantified the carbon (C) and ni- trogen (N) content of the forest floor and the top 15 cm of mineral soil and the rate of midsummer net N mineralization beneath six different tree species. There were large in- terspecific differences in forest floor depth and mass, in the size and distribution of C and N pools at varying soil depths, and in rates of midsummer net N mineralization and nitri- fication. Forest floor mass ranged from 3.2 kg/m 2 to 11.0 kg/m 2 and was smallest beneath sugar maple and largest beneath hemlock. The pool size of C in the forest floor ranged from 1.1 kg/m 2 to 4.4 kg/m 2 while the N content of the forest floor ranged from 83 g/m 2 to 229 g/m 2 . Forest floor C and N pools were smallest beneath sugar maple and highest beneath hemlock. Soil C:N ratios (range: 14.8-19.5) were lower beneath sugar maple, red maple, and white ash than beneath beech, red oak, and hemlock, whereas the opposite was true of the midsummer rate of net N mineralization (range: 0.91-2.02 g·m 22 ·28 d 21 ). The rate of net nitrification was positively correlated with the rate of net N mineralization. Interspecific differences in litter production and quality explain the large differences among species in the size of the forest floor C and N pools and in net N mineralization rates. The differences in the size and distribution of C and N pools beneath the different species suggest that the mechanisms regulating the process of species replacement in these forests will mediate the effects of anthropogenic, environmental changes in soil C and N dynamics.

Growth and Canopy Architecture of Shade‐Tolerant Trees: Response to Canopy Gaps

Patterns of aboveground growth, branching, and leaf display were examined in saplings of Acer saccharum Marsh. (sugar maple) and Fagus grandifolia Ehrh. (beech) to determine (1) the responses of aboveground growth rates to variation in forest light regimes and (2) the importance of branching and leaf display patterns to the ability of saplings of these two species to respond to changes in forest light regimes produced by canopy gaps. For both species, rates of height growth, lateral growth, and the production of new shoots in even the low gap light levels created by small canopy gaps (15-75 M2) were as much as an order of magnitude greater than growth rates of saplings beneath closed canopies. However, saplings of both species showed little response to further increases in gap light levels. The strong response of maple saplings to low gap light levels was correlated with an increase in sapling leaf area index and the efficiency of leaf display (measured as leaf area per unit length or surface area of branches). In beech, the more modest response to low light levels in small gaps and the higher growth rates than maple beneath a closed canopy were paralleled by a lack of significant increase in beech leaf area indices in small gaps and a higher efficiency of leaf display beneath a closed canopy than in small gaps. Thus, the magnitude of the response of these two species to small canopy gaps is correlated with their degree of plasticity in patterns of branching and leaf display. Both species can be considered small-gap specialists in the sense that their combinations of shade tolerance, growth responses, and canopy architecture make them particularly successful at exploiting small canopy gaps. However, the two species differ in their placement on a gradient in the degree to which woody plants respond to canopy disturbances.

Different Respones to Gaps Among Shade‐Tollerant Tree Species

Most species of trees in the Eastern deciduous forests of North America can be considered shade tolerant (sensu Whitmore 1975, 1982, Swaine and Whitmore 1988) in that germination and seedling establishment can occur beneath a closed canopy. I propose that the ability to tolerate shade allows a range of responses to disturbances that are qualitatively different from those of intolerant species. Furthermore, quantitative differences exist among shade-tolerant species in their responses to gaps. Differentiation of the responses of shade-tolerant species to gaps has significant implications for general models of forest dynamics.

Forest Ecosystem Responses to Exotic Pests and Pathogens in Eastern North America

Abstract The forests of eastern North America have been subjected to repeated introductions of exotic insect pests and pathogens over the last century, and several new pests are currently invading, or threatening to invade, the region. These pests and pathogens can have major short- and long-term impacts on forest ecosystem processes such as productivity, nutrient cycling, and support of consumer food webs. We identify six key features of the biology of exotic animal pests and the ecology of their hosts that are critical to predicting the general nature and severity of those impacts. Using three examples of introduced pests and pathogens in eastern forest ecosystems, we provide a conceptual framework for assessing potential ecosystem-scale effects.

CANOPY TREE-SOIL INTERACTIONS WITHIN TEMPERATE FORESTS: SPECIES EFFECTS ON pH AND CATIONS

We quantified soil acidity and exchangeable cations in the forest floor and upper 7.5 cm of mineral soil beneath the canopies of individual trees of six different species in a mixed-species forest in northwestern Connecticut. Soil pH decreased in a sequence starting with sugar maple (Acer saccharum) . white ash (Fraxinus americana) . red maple (Acer rubrum) . beech (Fagus grandifolia) . red oak (Quercus rubra) . eastern hemlock (Tsuga canadensis). The differences among species were largest in the forest floor and the top 7.5 cm of mineral soil. Exchangeable Ca and Mg in the 0-7.5 cm mineral soil layer were significantly higher beneath sugar maple than all other species, with the exception of white ash. There were negligible differences among species in the quantity of exchangeable Ca and Mg in the forest floor. In the 0-7.5 cm mineral soil layer, exchangeable Ca was positively correlated with the content of unweathered Ca in the parent material, but the relationship differed among species. There was a large increase in exchangeable Ca in the soils beneath sugar maple but a negligible increase in the soils beneath hemlock and red maple. Exchangeable Al and Fe were highest beneath hemlock and lowest beneath sugar maple. The differences in pH and exchangeable cations between sugar maple and hemlock are likely due to interspecific differences in the introduction of acidity (e.g., organic acids) and Ca uptake and allocation. Observing an association between tree species and specific soil chemical properties within mixed-species stands implies that changes in the distribution and abundance of tree species alters the spatial and temporal pattern of soil acidity and cation cycling in this forest.

Catastrophic Windthrow in the Presettlement Forests of Wisconsin

Presettlement survey records for the state of Wisconsin (circa 1834-1873) reveal a widespread pattern of catastrophic windthrow. From these records, we have calculated that there were 51.8 separate patches (> 1.0 ha) of complete canopy windthrow covering a total of 4828 ha annually within the region of the presettlement hemlock-northern hardwood forests, in the northeastern part of the state. The estimated return time for catastrophic windthrow at a site in this region is 1210 yr. Catastrophic windthrow was much less common in the forests of southern Wisconsin. Comparisons of the presettlement disturbance regime with contemporary climatological records suggest that cata- strophic thunderstorms were the principal mechanism for large-scale windthrow in northern Wisconsin.

Climate, Deer, Rodents, and Acorns as Determinants of Variation in Lyme-Disease Risk

Risk of human exposure to vector-borne zoonotic pathogens is a function of the abundance and infection prevalence of vectors. We assessed the determinants of Lyme-disease risk (density and Borrelia burgdorferi-infection prevalence of nymphal Ixodes scapularis ticks) over 13 y on several field plots within eastern deciduous forests in the epicenter of US Lyme disease (Dutchess County, New York). We used a model comparison approach to simultaneously test the importance of ambient growing-season temperature, precipitation, two indices of deer (Odocoileus virginianus) abundance, and densities of white-footed mice (Peromyscus leucopus), eastern chipmunks (Tamias striatus), and acorns ( Quercus spp.), in both simple and multiple regression models, in predicting entomological risk. Indices of deer abundance had no predictive power, and precipitation in the current year and temperature in the prior year had only weak effects on entomological risk. The strongest predictors of a current years risk were the prior years abundance of mice and chipmunks and abundance of acorns 2 y previously. In no case did inclusion of deer or climate variables improve the predictive power of models based on rodents, acorns, or both. We conclude that interannual variation in entomological risk of exposure to Lyme disease is correlated positively with prior abundance of key hosts for the immature stages of the tick vector and with critical food resources for those hosts.

Why forests appear resistant to exotic plant invasions: intentional introductions, stand dynamics, and the role of shade tolerance

Invasion ecology has traditionally focused on exotic plant species with early successional life-history traits, adapted to colonize areas following disturbance. However, the ecological importance of these traits may be overstated, in part because most invasive plants originate from intentional introductions. Furthermore, this focus neglects the types of plants most likely to invade established communities, particularly forests – namely shade-tolerant, late-successional species. In invasion ecology, it is generally assumed that undisturbed forests are highly resistant to plant invasions. Our review reveals that this assumption is not justified: in temperate and tropical regions around the world, at least 139 exotic plant species are known to have invaded deeply shaded forest understories that have not undergone substantial disturbance. These exotics present a particular management challenge, as they often increase in abundance during succession. While forest invasions may develop comparatively slowly under natural disturbance regimes, anthropogenic processes, including the spread of exotic pests and pathogens, can be expected to accelerate the rate of invasion.

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves

Abstract We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species –Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina– with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots.

Neighborhood Analyses Of Canopy Tree Competition Along Environmental Gradients In New England Forests

We use permanent-plot data from the USDA Forest Services Forest Inventory and Analysis (FIA) program for an analysis of the effects of competition on tree growth along environmental gradients for the 14 most abundant tree species in forests of northern New England, USA. Our analysis estimates actual growth for each individual tree of a given species as a function of average potential diameter growth modified by three sets of scalars that quantify the effects on growth of (1) initial target tree size (dbh), (2) local environmental conditions, and (3) crowding by neighboring trees. Potential growth of seven of the 14 species varied along at least one of the two environmental axes identified by an ordination of relative abundance of species in plots. The relative abundances of a number of species were significantly displaced from sites where they showed maximum potential growth. In all of these cases, abundance was displaced to the more resource-poor end of the environmental gradient (either low fertility or low moisture). The pattern was most pronounced among early successional species, whereas late-successional species reached their greatest abundance on sites where they also showed the highest growth in the absence of competition. The analysis also provides empirical estimates of the strength of intraspecific and interspecific competitive effects of neighbors. For all but one of the species, our results led us to reject the hypothesis that all species of competitors have equivalent effects on a target species. Most of the individual pairwise interactions were strongly asymmetric. There was a clear competitive hierarchy among the four most shade-tolerant species, and a separate competitive hierarchy among the shade-intolerant species. Our results suggest that timber yield following selective logging will vary dramatically depending on the configuration of the residual canopy, because of interspecific variation in the magnitude of both the competitive effects of different species of neighbors and the competitive responses of different species of target trees to neighbors. The matrix of competition coefficients suggests that there may be clear benefits in managing for specific mixtures of species within local neighborhoods within stands.