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1. Landowner functional types to characterize response to invasive forest insects

   https://doi.org/10.1002/pan3.10065  

   Holt, Jonathan R. ; Borsuk, Mark E. ; Butler, Brett J. ; Kittredge, David B. ; Laflower, Danelle ; MacLean, Meghan G. ; Markowski‐Lindsay, Marla ; Orwig, David ; Thompson, Jonathan R. ; Chen, ed., Xiaodong ( December 2019 , People and Nature)

   Invasive forest insects can induce tree mortality in two ways: (a) by directly harming trees; or (b) by influencing forest owners to pre‐emptively harvest threatened trees. This study investigates forest owners’ intentions to harvest trees threatened by invasive insects.

   Our first objective is to identify and characterize agent functional types (AFTs) of family forest owners in the northeastern United States using a set of contingent behaviour questions contained in a mail survey. We establish AFTs as a form of dimension reduction, effectively casting landowners into a typology in which each type (AFT) has distinct probabilities of tree harvesting in response to forest insects. Our analysis identifies three functional types of landowners: ‘Cutters’ (46% of respondents; high intent to harvest trees impacted by invasive forest insects), ‘Responsive Cutters’ (42% of respondents; intent sensitive to insect impact severity), and ‘Non‐cutters’ (12% of respondents; low intent to cut).

   Our second objective is to model AFT membership to predict the distribution of AFTs across the landscape. Predictors are chosen from a set of survey, geographic and demographic features. Our best AFT‐prediction model has three predictor variables: parcel size (hectares of forest), geographical region, and town‐level forested fraction. Application of the model provides a high‐resolution probability distribution of AFTs across the landscape.

   By coupling human and insect behaviour, our results allow for holistic assessments of how invasive forest insects disturb forests, inclusive of the management response to these pests.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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2. Quantifying within‐species trait variation in space and time reveals limits to trait‐mediated drought response

   https://doi.org/10.1111/1365-2435.14112  

   Kerr, Kelly L. ; Anderegg, Leander D. L. ; Zenes, Nicole ; Anderegg, William R. L. ( June 2022 , Functional Ecology)

   Climate change is stressing many forests around the globe, yet some tree species may be able to persist through acclimation and adaptation to new environmental conditions. The ability of a tree to acclimate during its lifetime through changes in physiology and functional traits, defined here as its acclimation potential, is not well known.

   We investigated the acclimation potential of trembling aspenPopulus tremuloidesand ponderosa pinePinus ponderosatrees by examining within‐species variation in drought response functional traits across both space and time, and how trait variation influences drought‐induced tree mortality. We measured xylem tension, morphological traits and physiological traits on mature trees in southwestern Colorado, USA across a climate gradient that spanned the distribution limits of each species and 3 years with large differences in climate.

   Trembling aspen functional traits showed high within‐species variation, and osmotic adjustment and carbon isotope discrimination were key determinants for increased drought tolerance in dry sites and in dry years. However, trembling aspen trees at low elevation were pushed past their drought tolerance limit during the severe 2018 drought year, as elevated mortality occurred. Higher specific leaf area during drought was correlated with higher percentages of canopy dieback the following year. Ponderosa pine functional traits showed less within‐species variation, though osmotic adjustment was also a key mechanism for increased drought tolerance. Remarkably, almost all traits varied more year‐to‐year than across elevation in both species.

   Our results shed light on the scope and limits of intraspecific trait variation for mediating drought responses in key southwestern US tree species and will help improve our ability to model and predict forest responses to climate change.

   Read the freePlain Language Summaryfor this article on the Journal blog.

    

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3. Chemical defense over decadal scales: Ontogenetic allocation trajectories and consequences for fitness in a foundation tree species

   https://doi.org/10.1111/1365-2435.13425  

   Cope, Olivia L. ; Kruger, Eric L. ; Rubert‐Nason, Kennedy F. ; Lindroth, Richard L. ; Barton, ed., Kasey ( August 2019 , Functional Ecology)

   Expression of herbivore defense traits can change dramatically during the course of plant development. Little is known, however, about the degree of genetic or sexual variation in these ontogenetic defense trajectories or whether the trajectories themselves are adaptive, especially in long‐lived species.

   We used a 13‐year dataset of chemical defense traits, growth and survivorship from a common garden of trembling aspen (Populus tremuloides) genotypes to document long‐term defense trajectories and their relationship to tree fitness during juvenile and early mature stages.

   Overall, concentrations of the two principal classes of aspen defense compounds (salicinoid phenolic glycosides [SPGs] and condensed tannins [CTs]) decreased to differing degrees in foliage of juvenile trees and then remained relatively constant in maturity. Initial values, juvenile rates of change and average mature values all exhibited significant genetic variation for both SPGs and CTs.

   Relationships between defense trajectory parameters and metrics of tree fitness (growth and survivorship) depended on compound type and tree sex. Females with higher‐allocation SPG trajectories (high initial juvenile concentrations, slow juvenile declines, high mature concentrations) grew more slowly relative to females with lower‐allocation trajectories. In males, higher‐allocation SPG trajectories had a lesser effect on growth but were associated with reduced mortality. Juvenile CT trajectories were not correlated with tree fitness, but average CT concentration in maturity was positively related to growth in females.

   These results suggest that ontogenetic defense trajectories are adaptive and subject to natural selection. Genotypic variation and ontogeny shape tree defensive chemistry, both independently and interactively. These patterns of defense expression have the potential to structure trophic interactions and the genetic composition of forests in both space and time.

   A freePlain Language Summarycan be found within the Supporting Information of this article.

    

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4. Independent and interactive effects of plant genotype and environment on plant traits and insect herbivore performance: A meta‐analysis with Salicaceae

   https://doi.org/10.1111/1365-2435.13249  

   Barker, Hilary L. ; Holeski, Liza M. ; Lindroth, Richard L. ; Koricheva, ed., Julia ( December 2018 , Functional Ecology)

   Ecological research has increasingly highlighted the importance of intraspecific variation in shaping the structure and function of communities and ecosystems. Indeed, the effects of intraspecific variation can match or exceed those of interspecific variation. Previous reviews of intraspecific variation in plant traits across heterogeneous environments have focused primarily onmeanphenotypic effects. We propose that a richer and fuller understanding of the ecological causes and consequences of intraspecific variation would be provided by partitioning traitvarianceinto its subcomponents (genetic, environment, genotype by environment interaction).

   We used a meta‐analysis of 352 sets of genetic, environment and genotype by environment (G×E) variation estimates from 72 studies of Salicaceae to compare these sources of variation across plant traits (growth, foliar nitrogen, defence compounds), insect herbivore performance metrics (e.g., survival, growth, fecundity) and environmental conditions (e.g., soil nutrients, water, defoliation).

   Our findings revealed that variation in levels of defence compounds (both condensed tannins and salicinoids) and insect herbivore performance were primarily genetically determined, while variation in plant growth and foliar nitrogen was more environmentally determined.

   Plasticity in plant growth, foliar nitrogen levels and insect herbivore performance varied substantially across different sites (year × location), and nutrient, water and carbon dioxide environments. Plasticity was lowest for chemical defence traits and all traits in contrasting ozone and defoliation environments.

   Our quantitative review also revealed several gaps in the literature, including a need for surveying more mature plants, a wider variety of insect herbivore species (e.g., leaf‐galling insects, specialist insects) and underrepresented environmental treatments (e.g., competition, defoliation, disease, light and water availability).

   Findings from this analysis highlight the importance of, and patterns within, intraspecific variation with respect to shaping the evolvability and plasticity of traits and governing the interactions of plants and insects.

   Aplain language summaryis available for this article.

    

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5. Defoliation severity is positively related to soil solution nitrogen availability and negatively related to soil nitrogen concentrations following a multi-year invasive insect irruption

   https://doi.org/10.1093/aobpla/plaa059  

   Conrad-Rooney, Emma ; Barker Plotkin, Audrey ; Pasquarella, Valerie J ; Elkinton, Joseph ; Chandler, Jennifer L ; Matthes, Jaclyn Hatala ( December 2020 , AoB PLANTS)

   Atkins, Jeff (Ed.)

   Abstract Understanding connections between ecosystem nitrogen (N) cycling and invasive insect defoliation could facilitate the prediction of disturbance impacts across a range of spatial scales. In this study we investigated relationships between ecosystem N cycling and tree defoliation during a recent 2015–18 irruption of invasive gypsy moth caterpillars (Lymantria dispar), which can cause tree stress and sometimes mortality following multiple years of defoliation. Nitrogen is a critical nutrient that limits the growth of caterpillars and plants in temperate forests. In this study, we assessed the associations among N concentrations, soil solution N availability and defoliation intensity by L. dispar at the scale of individual trees and forest plots. We measured leaf and soil N concentrations and soil solution inorganic N availability among individual red oak trees (Quercus rubra) in Amherst, MA and across a network of forest plots in Central Massachusetts. We combined these field data with estimated defoliation severity derived from Landsat imagery to assess relationships between plot-scale defoliation and ecosystem N cycling. We found that trees in soil with lower N concentrations experienced more herbivory than trees in soil with higher N concentrations. Additionally, forest plots with lower N soil were correlated with more severe L. dispar defoliation, which matched the tree-level relationship. The amount of inorganic N in soil solution was strongly positively correlated with defoliation intensity and the number of sequential years of defoliation. These results suggested that higher ecosystem N pools might promote the resistance of oak trees to L. dispar defoliation and that defoliation severity across multiple years is associated with a linear increase in soil solution inorganic N. 

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