ALBERT

Description: We review and synthesize information on invasions of nonnative forest insects and diseases in the United States, including their ecological and economic impacts, pathways of arrival, distribution within the United States, and policy options for reducing future invasions. Nonnative insects have accumulated in United States forests at a rate of ~2.5 per yr over the last 150 yr. Currently the two major pathways of introduction are importation of live plants and wood packing material such as pallets and crates. Introduced insects and diseases occur in forests and cities throughout the United States, and the problem is particularly severe in the Northeast and Upper Midwest. Nonnative forest pests are the only disturbance agent that has effectively eliminated entire tree species or genera from United States forests within decades. The resulting shift in forest structure and species composition alters ecosystem functions such as productivity, nutrient cycling, and wildlife habitat. In urban and suburban areas, loss of trees from streets, yards, and parks affects aesthetics, property values, shading, stormwater runoff, and human health. The economic damage from nonnative pests is not yet fully known, but is likely in the billions of dollars per year, with the majority of this economic burden borne by municipalities and residential property owners. Current policies for preventing introductions are having positive effects but are insufficient to reduce the influx of pests in the face of burgeoning global trade. Options are available to strengthen the defenses against pest arrival and establishment, including measures taken in the exporting country prior to shipment, measures to ensure clean shipments of plants and wood products, inspections at ports of entry, and post-entry measures such as quarantines, surveillance, and eradication programs. Improved data collection procedures for inspections, greater data accessibility, and better reporting would support better evaluation of policy effectiveness. Lack of additional action places the nation, local municipalities, and property owners at high risk of further damaging and costly invasions. Adopting stronger policies to reduce establishments of new forest insects and diseases would shift the major costs of control to the source and alleviate the economic burden now borne by homeowners and municipalities.

Description: Question Small mammals are important consumers of tree seeds in forests worldwide. Few studies, however, have addressed the impacts of small mammals as consumers of herbaceous species in the forest understorey. Local neighbourhood-scale variation in canopy tree composition has strong effects on resource availability for understorey plants, but can also influence the distribution, abundance and foraging behaviour of small mammals, primarily through spatial and temporal variation in seed rain. In this study we examine the direct and indirect effects of canopy tree neighbourhoods and herbivory by small mammals on the diversity, abundance and flowering of herbaceous species in a temperate forest. Location and Methods In 1994 we established 36 1 m × 2 m fenced small mammal exclosures, paired with similar-sized control plots, under the major canopy tree species found at Great Mountain Forest, in Norfolk, CT, US. Results When re-sampled in 2005, after 10 yrs of small mammal exclusion, the abundance of herbaceous plants had increased inside the exclosures ( F  = 5.005, df  = 1,38, P  =   0.031), as did the proportion of plants flowering ( t  =   −2.11, df  = 60, P  =   0.039). However, species richness and Shannon–Wiener diversity ( H ′) were similar inside and outside of exclosures. At one site the identity of the canopy tree species did impact species richness ( F  = 4.494, df  = 5,18, P  =   0.008), independent of exclosure. Plots under Fraxinus americana (white ash) had the highest species richness (average number of species = 7.5) while the lowest diversity was found in plots under Tsuga canadensis (Eastern hemlock; average number of species = 2.25) and Quercus rubra (northern red oak; average number of species = 1.5). Conclusions The results demonstrate that small mammal herbivores can directly affect the abundance and reproduction of herbaceous species, and that local conditions (such as the surrounding canopy tree neighbourhood) can also play direct and indirect roles in the maintenance of the herbaceous layer in temperate deciduous forests. We investigated the importance of small mammals as consumers of herbaceous vegetation after 10-yrs of exclusion in temperate deciduous forests. We found that there was no change in species richness or diversity, but that herbaceous vegetation was more abundant and flowered more inside exclosures. Canopy tree neighborhood impacted species richness at one site, independent of exposure to herbivory.

Description: Climate and competition are often presented from two opposing views of the dominant driver of individual tree growth and species distribution in temperate forests, such as those in the eastern United States. Previous studies have provided abundant evidence indicating that both factors influence tree growth, and we argue that these effects are not independent of one another and rather that interactions between climate, competition, and size best describe tree growth. To illustrate this point, we describe the growth responses of five common eastern tree species to interacting effects of temperature, precipitation, competition, and individual size using maximum likelihood estimation. Models that explicitly include interactions among these four factors explained over half of the variance in annual growth for four out of five species using annual climate. Expanding temperature and precipitation analyses to include seasonal interactions resulted in slightly improved models with a mean R 2 of 0.61 (SD 0.10). Growth responses to individual factors as well their interactions varied greatly among species. For example, growth sensitivity to temperature for Quercus rubra increased with maximum annual precipitation, but other species showed no change in sensitivity or slightly reduced annual growth. Our results also indicate that three-way interactions among individual stem size, competition, and temperature may determine which of the five co-occurring species in our study could have the highest growth rate in a given year. Continued consideration and quantification of interactions among climate, competition, and individual-based characteristics are likely to increase understanding of key biological processes such as tree growth. Greater parameterization of interactions between traditionally segregated factors such as climate and competition may also help build a framework to reconcile drivers of individual-based processes such as growth with larger-scale patterns of species distribution. This article is protected by copyright. All rights reserved.

Description: Wild et al. (2014, Journal of Vegetation Science 25: 1327–1340) document persistent effects of the spatial distribution of canopy trees on the distribution of regeneration following stand-replacing disturbance in montane Norway spruce forests. The authors suggest a simple physical process for these legacy effects – the accumulation of winter-dispersed seeds in ‘tree-wells’ with lower snow depth around the trunks of living and recently dead trees. Wild et al. (2014, this issue) document persistent effects of the spatial distribution of canopy trees on the distribution of regeneration following stand-replacing disturbance in montane Norway spruce forests. The authors suggest a simple physical process for these legacy effects – the accumulation of winter-dispersed seeds in ‘tree-wells’ with lower snow depth around the trunks of living and recently dead trees.

Description: Climate and competition are often presented from two opposing views of the dominant driver of individual tree growth and species distribution in temperate forests, such as those in the eastern United States. Previous studies have provided abundant evidence indicating that both factors influence tree growth, and we argue that these effects are not independent of one another and rather that interactions between climate, competition, and size best describe tree growth. To illustrate this point, we describe the growth responses of five common eastern tree species to interacting effects of temperature, precipitation, competition, and individual size using maximum likelihood estimation. Models that explicitly include interactions among these four factors explained over half of the variance in annual growth for four out of five species using annual climate. Expanding temperature and precipitation analyses to include seasonal interactions resulted in slightly improved models with a mean R 2 of 0.61 (SD 0.10). Growth responses to individual factors as well their interactions varied greatly among species. For example, growth sensitivity to temperature for Quercus rubra increased with maximum annual precipitation, but other species showed no change in sensitivity or slightly reduced annual growth. Our results also indicate that three-way interactions among individual stem size, competition, and temperature may determine which of the five co-occurring species in our study could have the highest growth rate in a given year. Continued consideration and quantification of interactions among climate, competition, and individual-based characteristics are likely to increase understanding of key biological processes such as tree growth. Greater parameterization of interactions between traditionally segregated factors such as climate and competition may also help build a framework to reconcile drivers of individual-based processes such as growth with larger-scale patterns of species distribution.

Description: Ecological Applications, Volume 0, Issue 0, Ahead of Print.

Description: Seedling recruitment and survival are critical bottlenecks in tree population dynamics and are likely to play central roles in shifts in species distributions under climate change. We use data from the Forest Inventory and Analysis program to quantify the relationships between two key climate variables—mean annual temperature and growing season water deficit—and rates of seedling recruitment and survival for the 50 most common tree species in the eastern United States. Our statistical models include the positive effects of conspecific adult abundance on recruitment and the potentially negative competitive effects of total canopy abundance on seedling survival. The tradeoff between these two effects creates a range from positive to negative conspecific density dependence, depending on the absolute and relative abundance of conspecific vs. heterospecific adults in a plot. Variation along the climate gradients mirrors patterns found previously in adult distributions. The clearest signal is in variation in the presence/absence of seedlings, while seedling density when present is only weakly related to local climate. The relatively narrow niche breadths for the presence of both seedlings and adults suggest that the frequency of occurrence of species within the landscape, rather than their relative abundance when present, will show the greatest response to climate change. Our analyses predict seedling survival as a function of mean annual temperature independent of the effects of competition and water deficit and thus provide an indication of the fundamental niche for seedling distribution along the temperature gradient. For more than half of the 50 species, their realized seedling niches are displaced to warmer climates. This reflects the prevalence of species in which survival declines with increasing temperature across a significant portion of at least their southern range. Our results show that when the effects of warmer climates are taken into account, seedling survival generally increases with increasing water deficit in the generally humid climates of the eastern United States. This result is consistent with recent global surveys of the relationship between net primary productivity and forest turnover rates, but contrasts with recent studies highlighting the potential impact of drought stress on tree mortality in more arid climates.

Description: Abstract Climate and competition are often presented from two opposing views of the dominant driver of individual tree growth and species distribution in temperate forests, such as those in the eastern United States. Previous studies have provided abundant evidence indicating that both factors influence tree growth, and we argue that these effects are not independent of one another and rather that interactions between climate, competition, and size best describe tree growth. To illustrate this point, we describe the growth responses of five common eastern tree species to interacting effects of temperature, precipitation, competition, and individual size using maximum likelihood estimation. Models that explicitly include interactions among these four factors explained over half of the variance in annual growth for four out of five species using annual climate. Expanding temperature and precipitation analyses to include seasonal interactions resulted in slightly improved models with a mean R2 of 0.61 (SD 0.10). Growth responses to individual factors as well their interactions varied greatly among species. For example, growth sensitivity to temperature for Quercus rubra increased with maximum annual precipitation, but other species showed no change in sensitivity or slightly reduced annual growth. Our results also indicate that three‐way interactions among individual stem size, competition, and temperature may determine which of the five co‐occurring species in our study could have the highest growth rate in a given year. Continued consideration and quantification of interactions among climate, competition, and individual‐based characteristics are likely to increase understanding of key biological processes such as tree growth. Greater parameterization of interactions between traditionally segregated factors such as climate and competition may also help build a framework to reconcile drivers of individual‐based processes such as growth with larger‐scale patterns of species distribution. This article is protected by copyright. All rights reserved.

Description: Traits affecting survival from seedling through adult stages are key elements of tree life histories, and it is widely assumed that variation in survival of adult trees plays an important role in the distribution of species along climate gradients. We use data from plots censused by the U.S. Forest Service Forest Inventory and Analysis program during the years 2000–2011 to quantify relationships between two key aspects of climate—mean annual temperature and growing season water deficit—and rates of sapling and canopy tree survival for the 50 most common tree species in the eastern United States. Our analyses include consideration of the effects of tree size, competition, and nitrogen deposition to avoid confounding effects and to provide context for the importance of variation in climate relative to other factors. Tree size and competitive effects, including the effect of tree size on sensitivity to competition, had the greatest impact on observed variation in survival for all of the species. Survival varied as a function of nitrogen deposition in 20 of the 50 species, and responses were stronger in saplings than in canopy trees. Despite clear sorting of the presence of the tree species along regional gradients of temperature and water deficit, there was only modest evidence that either sapling or canopy tree mortality varied systematically along those gradients. For 24 of the 50 species, the most parsimonious models did not include either temperature or water deficit variables. The exceptions to this were for several species of colder climates in which survival declined significantly in warmer climates. In 40 of the 50 species, there was no significant variation in survival as a function of either average growing season water deficit or the most extreme individual growing season water deficit during the 20 yr preceding the end of the census interval. The frequency of all but the most xeric of our study species declines at some point along a water deficit gradient. But it is seedling survival (reported in earlier work), rather than survival of saplings and canopy trees, that varies systematically along water deficit gradients.