Tsuga Canadensis Research Articles

Impact of Hemlock Woolly Adelgid (Hemiptera: Adelgidae) Infestation on the Jasmonic Acid-Elicited Defenses of Tsuga canadensis (Pinales: Pinaceae).

Hemlock woolly adelgid is an invasive piercing-sucking insect in eastern North America, which upon infestation of its main host, eastern hemlock ('hemlock'), improves attraction and performance of folivorous insects on hemlock. This increased performance may be mediated by hemlock woolly adelgid feeding causing antagonism between the the jasmonic acid and other hormone pathways. In a common garden experiments using hemlock woolly adelgid infestation and induction with methyl jasmonate (MeJA) and measures of secondary metabolite contents and defense-associated enzyme activities, we explored the impact of hemlock woolly adelgid feeding on the local and systemic induction of jasmonic acid (JA)-elicited defenses. We found that in local tissue hemlock woolly adelgid or MeJA exposure resulted in unique induced phenotypes, whereas the combined treatment resulted in an induced phenotype that was a mixture of the two individual treatments. We also found that if the plant was infested with hemlock woolly adelgid, the systemic response of the plant was dominated by hemlock woolly adelgid, regardless of whether MeJA was applied. Interestingly, in the absence of hemlock woolly adelgid, hemlock plants had a very weak systemic response to MeJA. We conclude that hemlock woolly adelgid infestation prevents systemic induction of JA-elicited defenses. Taken together, compromised local JA-elicited defenses combined with weak systemic induction could be major contributors to increased folivore performance on hemlock woolly adelgid-infested hemlock.

Using a Trait-Based Approach to Compare Tree Species Sensitivity to Climate Change Stressors in Eastern Canada and Inform Adaptation Practices

Despite recent advances in understanding tree species sensitivities to climate change, ecological knowledge on different species remains scattered across disparate sources, precluding their inclusion in vulnerability assessments. Information on potential sensitivities is needed to identify tree species that require consideration, inform changes to current silvicultural practices and prioritize management actions. A trait-based approach was used to overcome some of the challenges involved in assessing sensitivity, providing a common framework to facilitate data integration and species comparisons. Focusing on 26 abundant tree species from eastern Canada, we developed a series of trait-based indices that capture a species’ ability to cope with three key climate change stressors—increased drought events, shifts in climatically suitable habitat, increased fire intensity and frequency. Ten indices were developed by breaking down species’ response to a stressor into its strategies, mechanisms and traits. Species-specific sensitivities varied across climate stressors but also among the various ways a species can cope with a given stressor. Of the 26 species assessed, Tsuga canadensis (L.) Carrière and Abies balsamea (L.) Mill are classified as the most sensitive species across all indices while Acer rubrum L. and Populus spp. are the least sensitive. Information was found for 95% of the trait-species combinations but the quality of available data varies between indices and species. Notably, some traits related to individual-level sensitivity to drought were poorly documented as well as deciduous species found within the temperate biome. We also discuss how our indices compare with other published indices, using drought sensitivity as an example. Finally, we discuss how the information captured by these indices can be used to inform vulnerability assessments and the development of adaptation measures for species with different management requirements under climate change.

Hammond Hill Research Catchment: Supporting hydrologic investigations of rooting zone and vegetation water dynamics under climate change

AbstractThe Hammond Hill Research Catchment (HH) is a small (120 ha), temperate, second order tributary to Six Mile Creek, Cayuga Lake, and the Great Lakes (42.42°, −76.32°). The HH has been monitored since January 2017 for the purpose of understanding how recent infiltration mixes with antecedent soil water on hillslope forest floors and the spatial and temporal patterns of Root Water Uptake (RWU) by temperate northeastern US tree species (eastern hemlock [Tsuga canadensis], American beech [Fagus grandifolia], and sugar maple [Acer saccharum]). These data are informing us about the hydrologic consequences of anticipated tree species composition change and supporting the development of more refined ecohydrological models. The glaciated catchment is underlain by a shallow confining siltstone layer (1–1.5 m depth) and densely covered with an approximately 60 year old regrowth mixed species forest of hemlock, beech, and other deciduous tree species common to the northeastern US. Current datasets from the HH include precipitation snow water equivalent, discharge, and associated isotopic water compositions, δ2H & δ18O. Measurements of (top 10 cm) soil water content, as well as bulk soil water and hemlock and beech xylem isotopic compositions are made at several locations across a topographic wetness gradient. The near‐term role of the HH is to support an understanding of the environmental and ecological drivers of plant RWU competition. All data from the HH are publicly available.

North American temperate conifer (Tsuga canadensis) reveals a complex physiological response to climatic and anthropogenic stressors.

Rising atmospheric CO2 (ca) is expected to promote tree growth and lower water loss via changes in leaf gas exchange. However, uncertainties remain if gas-exchange regulation strategies are homeostatic or dynamical in response to increasing ca, as well as evolving climate and pollution inputs. Using a suite of tree ring-based δ13C-derived physiological parameters (Δ13C, ci, iWUE) and tree growth from a mesic, low elevation stand of canopy-dominant Tsuga canadensis in north-eastern USA, we investigated the influence of rising ca, climate and pollution on, and characterised the dynamical regulation strategy of, leaf gas exchange at multidecadal scales. Isotopic and growth time series revealed an evolving physiological response in which the species shifted its leaf gas-exchange strategy dynamically (constant ci; constant ci/ca; constant ca-ci) in response to rising ca, moisture availability and site conditions over 111yr. Tree iWUE plateaued after 1975, driven by greater moisture availability and a changing soil biogeochemistry that may have impaired a stomatal response. Results suggested that trees may exhibit more complex physiological responses to the changing environmental conditions over multidecadal periods, and complicating the parameterisation of Earth system models and the estimation of future carbon sink capacity and water balance in midlatitude forests and elsewhere.

Monitoring Forest Infestation and Fire Disturbance in the Southern Appalachian Using a Time Series Analysis of Landsat Imagery

The southern Appalachian forests have been threatened by several large-scale disturbances, such as wildfire and infestation, which alter the forest ecosystem structures and functions. Hemlock Woolly Adelgid (Adelges tsugae Annand, HWA) is a non-native pest that causes widespread foliar damage and eventual mortality, resulting in irreversible tree decline in eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlocks throughout the eastern United States. It is important to monitor the extent and severity of these disturbances over space and time to better understand their implications in the biogeochemical cycles of forest landscapes. Using all available Landsat images, we investigate and compare the performance of Tasseled Cap Transformation (TCT)-based indices, Normalized Difference Vegetation Index (NDVI), Enhanced Vegetation Index (EVI), and Disturbance Index (DI) in capturing the spectral-temporal trajectory of both abrupt and gradual forest disturbances (e.g., fire and hemlock decline). For each Landsat pixel, the temporal trajectories of these indices were fitted into a time series model, separating the inter-annual disturbance patterns (low frequency) and seasonal phenology (high frequency) signals. We estimated the temporal dynamics of disturbances based on the residuals between the observed and predicted values of the model, investigated the performance of all the indices in capturing the hemlock decline intensity, and further validated the results with the number of individual dead hemlocks identified from high-resolution aerial images. Our results suggested that the overall performance of NDVI, followed by TCT wetness, was most accurate in detecting both the disturbance timing and hemlock decline intensity, explaining over 90% of the variability in the number of dead hemlocks. Despite the overall good performance of TCT wetness in characterizing the disturbance regime, our analysis showed that this index has some limitations in characterizing disturbances due to its recovery patterns following infestation.

Baseline to novel ecosystems in Michigan, USA, with a quantitative and qualitative assessment

ABSTRACT Pre-Euro-American settlement vegetation provides information about historical ecology. I evaluated baseline conditions and novel status of current forests in Michigan using historical (1836 to 1858) and current (2010–2015) surveys and assessed quantitative and qualitative measures of novel status. Aspen (increased from 2% to 11% of all trees) and red maple (<2% to 12.5%) replaced eastern hemlock (15% to 2%) and American beech (8% to <1%) as most abundant species. Density was similar between surveys but mean diameter (trees ≥12.7 cm) decreased from 39 to 22 cm. The emerging forest type is a mix of early- to mid-successional species, particularly red maple, from eastern broadleaf forests of the central-eastern US.Openlands in southern Michigan have been replaced by agriculture and closed forests. Historical forests dominated by few tree species have transitioned to diverse eastern broadleaf forests throughout the eastern US, conforming to quantitative and qualitative measures of novel ecosystem status. Besides exceeding a quantitative threshold (e.g., squared chord distance), current forests meet novel status because they are ubiquitous, constitute a new normal, arise predictably, and unavoidably in response to disturbance or land-use change, auto-organize, and retain novelty after crossing thresholds challenging to reverse.

Microbiome Variation Across Two Hemlock Species With Hemlock Woolly Adelgid Infestation.

The hemlock woolly adelgid (Adelges tsugae, HWA), an invasive insect, is devastating native hemlock populations in eastern North America, and management outcomes have so far had limited success. While many plant microbiomes influence and even support plant immune responses to insect herbivory, relatively little is known about the hemlock microbiome and its interactions with pathogens or herbivores such as HWA. Using 16S rRNA and ITS gene amplicon sequencing, we characterized the needle, branch, root, and rhizosphere microbiome of two hemlock species, Tsuga canadensis and T. sieboldii, that displayed low and high levels of HWA populations. We found that both archaeal/bacterial and fungal needle communities, as well as the archaeal/bacterial branch and root communities, varied in composition in both hemlock species relative to HWA population levels. While host species and plant-associated habitats explained a greater proportion of the variance in the microbiome than did HWA population level, high HWA populations were associated with enrichment of 100 likely fungal pathogen sequence variants across the four plant-associated habitats (e.g., needle, branch, root, rhizosphere) compared to trees with lower HWA populations. This work contributes to a growing body of literature linking plant pathogens and pests with the changes in the associated plant microbiome and host health. Furthermore, this work demonstrates the need to further investigate plant microbiome effects across multiple plant tissues to understand their influences on host health.

An invasive insect, hemlock woolly adelgid, indirectly impacts Louisiana Waterthrush nest site selection and nest survival in the southern Appalachians

Abstract Eastern hemlock (Tsuga canadensis) is declining throughout the eastern United States due to hemlock woolly adelgid (Adelges tsugae Annand), an invasive insect from Asia. In the southern Appalachians, hemlock is concentrated in moist ravines and its decline threatens riparian ecosystems. Previous research on this invasion has focused on adelgid control and how hemlock decline affects community composition or forest processes; few studies have evaluated the consequences for demography of obligate riparian species. The Louisiana Waterthrush (Parkesia motacilla) is an obligate riparian species that could be sensitive to hemlock condition in this region, but how individuals respond to decline is currently unknown. To address this knowledge gap, we leveraged recent adelgid treatment efforts in Great Smoky Mountains National Park to evaluate the relationship(s) between hemlock decline and waterthrush habitat selection (foraging and nest site) and vital rates (nest and adult survival). We found that hemlock decline was unrelated to foraging habitat selection and apparent adult survival, but was related to nest site selection through an interaction with percent ground cover of exposed live tree roots: birds selected for nest sites in areas with more exposed live roots but only when hemlock was in poor condition. Nest survival was lower in areas where deciduous species (vs. evergreen species) dominated the understory, suggesting that adelgid invasion could indirectly impact waterthrush fitness depending on how vegetative succession proceeds following hemlock decline. Our results suggest that the short-term consequences of adelgid invasion on this riparian avian species are minimal in this area, but these relationships are likely dynamic and dependent on local habitat features and the predator community response to hemlock decline.

Do southern Appalachian Mountain summer stream temperatures respond to removal of understory rhododendron thickets?

AbstractDense understory thickets of the native evergreen shrub Rhododendron maximum expanded initially following elimination of American chestnut by the chestnut blight, and later in response to loss of the eastern hemlock due to hemlock woolly adelgid invasion. Rhododendron thickets often blanket streams and their riparian zones, creating cool, low‐light microclimates. To determine the effect of such understory thickets on summer stream temperatures, we removed riparian rhododendron understory on 300 m reaches of two southern Appalachian Mountain headwater streams, while leaving two 300 m reference reaches undisturbed. Overhead canopy was left intact in all four streams, but all streams were selected to have a significant component of dead or dying eastern hemlock in the overstory, creating time‐varying canopy gaps throughout the reach. We continuously monitored temperatures upstream, within and downstream of treatment and reference reaches. Temperatures were monitored in all four streams in the summer before treatments were imposed (2014), and for two summers following treatment (2015, 2016). Temperatures varied significantly across and within streams prior to treatment and across years for the reference streams. After rhododendron removal, increases in summer stream temperatures were observed at some locations within the treatment reaches, but these increases did not persist downstream and varied by watershed, sensor, and year. Significant increases in daily maxima in treatment reaches ranged from 0.9 to 2.6°C. Overhead canopy provided enough shade to prevent rhododendron removal from increasing summer temperatures to levels deleterious to native cold‐water fauna (average summer temperatures remained below 16°C), and local temperature effects were not persistent.

Differential sunlight exposure affects settling behaviour of hemlock woolly adelgid crawlers

Abstract Previous research demonstrated that elevated sunlight improves carbon balance and growth of eastern hemlock (Tsuga canadensis (L.) Carrière) seedlings infested with the invasive hemlock woolly adelgid (Adelges tsugae Annand). This study examined the hypothesis that elevated visible and ultraviolet sunlight directly affects the settling behaviour of A. tsugae crawlers. Hemlock seedlings were manually infested with equal densities of A. tsugae and placed into artificial shade treatments (0%, 40% and 80% shade). The density of settled nymphs of the ensuing generation on shaded seedlings was three times higher than the density on unshaded seedlings. Adelges tsugae ovisacs were also attached to hydrated hemlock branches placed beneath acrylic filter treatments that selectively allowed transmission of various sunlight wavelengths. The proportion of adelgid crawlers that settled on the branch (vs. dropping from it) was higher beneath filters that blocked the widest spectrum of solar radiation (opaque and amber filters) than beneath full sun. In the March‐April trial, exposure to full sun and the full visible spectrum only (clear‐UV filter) significantly increased the proportion of crawlers that settled on the bottom (vs. top) side of the branch. Our results suggest that A. tsugae crawlers are negatively phototactic and/or thermotactic, with behaviour influenced more by visible light than ultraviolet light.

Water permeability of monolithic wood biocarbon

Abstract Monolithic wood biocarbon is an emerging functional material for energy and environmental applications. Its water permeability directly affects its performance but hasn't been quantified. Via slow pyrolysis to 1000 °C, biocarbon monoliths were prepared from five wood precursors -two hardwoods: sugar maple (acer saccharum) and balsa (ochroma pyramidale), and three softwoods: white pine (pinus strobus), hemlock (tsuga canadensis), and red wood (sequoia sempervirens). Their water permeability was evaluated experimentally along both axial and radial directions. The highest permeability (1.3 × 10−11 m2) was obtained with balsa along the axial direction. The rank in axial permeability followed balsa > sugar maple > red wood > pine > hemlock (1.0 × 10−13 m2). Anatomic characteristics of biocarbons were quantified by analyzing Scanning Electron Microscope (SEM) images. Vessels in hardwoods were about one order of magnitude large in diameter than tracheids. These large vessels were responsible for over 90% of water transported through monoliths, making hardwoods more permeable than softwoods. Porosity is not a reliable predictor of permeability which is proportional to the square of diameter and the cross sectional area of vessels and/or tracheids. The rank in permeability of softwoods was attributed to the difference in tracheids diameter. Biocarbon monoliths in radial direction was less permeable than axial direction. The axial/radial permeability ratio of sugar maple was over 400 while that of hemlock was about two, highlighting the importance of anatomic characteristics in determining water transport in biocarbon monoliths.

Shear Properties of Eastern Hemlock with Respect to Fiber Orientation for Use in Cross Laminated Timber

AbstractShear properties of the cross layer in cross laminated timber (CLT) panels are critical for design purposes, especially for applications with high concentrated loads and low span-to-depth r...

Detecting Change in Forest Structure with Simulated GEDI Lidar Waveforms: A Case Study of the Hemlock Woolly Adelgid (HWA; Adelges tsugae) Infestation

The hemlock woolly adelgid (HWA; Adelges tsugae) is an invasive insect infestation that is spreading into the forests of the northeastern United States, driven by the warmer winter temperatures associated with climate change. The initial stages of this disturbance are difficult to detect with passive optical remote sensing, since the insect often causes its host species, eastern hemlock trees (Tsuga canadensis), to defoliate in the midstory and understory before showing impacts in the overstory. New active remote sensing technologies—such as the recently launched NASA Global Ecosystem Dynamics Investigation (GEDI) spaceborne lidar—can address this limitation by penetrating canopy gaps and recording lower canopy structural changes. This study explores new opportunities for monitoring the HWA infestation with airborne lidar scanning (ALS) and GEDI spaceborne lidar data. GEDI waveforms were simulated using airborne lidar datasets from an HWA-infested forest plot at the Harvard Forest ForestGEO site in central Massachusetts. Two airborne lidar instruments, the NASA G-LiHT and the NEON AOP, overflew the site in 2012 and 2016. GEDI waveforms were simulated from each airborne lidar dataset, and the change in waveform metrics from 2012 to 2016 was compared to field-derived hemlock mortality at the ForestGEO site. Hemlock plots were shown to be undergoing dynamic changes as a result of the HWA infestation, losing substantial plant area in the middle canopy, while still growing in the upper canopy. Changes in midstory plant area (PAI 11–12 m above ground) and overall canopy permeability (indicated by RH10) accounted for 60% of the variation in hemlock mortality in a logistic regression model. The robustness of these structure-condition relationships held even when simulated waveforms were treated as real GEDI data with added noise and sparse spatial coverage. These results show promise for future disturbance monitoring studies with ALS and GEDI lidar data.

Rebound of Adelges tsugae spring generation following predation on overwintering generation ovisacs by the introduced predator Laricobius nigrinus in the eastern United States

Hemlock woolly adelgid (HWA), Adelges tsugae Annand (Hemiptera: Adelgidae), has devastated eastern hemlock (Tsuga canadensis [L.] Carriere) in a major portion of its native range in eastern North America. Population dynamics of HWA in the absence of predators have been studied for decades. After many years and much effort directed towards rearing and releasing biological control agents to manage HWA, one of these agents, Laricobius nigrinus Fender (Coleoptera: Derodontidae), is now successfully established at significant densities at sites from the southern to the mid-Atlantic states of the eastern U.S. However, high densities of HWA still persist at many locations throughout the region and spread of HWA and associated damage to hemlock continues. Population models for HWA have suggested that even upwards of 90% predation on eggs laid by the overwintering sistens generation will have minimal effect in reducing the population densities of HWA, if HWA are at high density. In this study, we tested the ability of L. nigrinus to reduce HWA densities, and experimentally tested these model predictions to better understand what impact, if any, L. nigrinus has on HWA densities. By using predator exclusion cages at field sites with well-established populations of L. nigrinus, we were able to record HWA densities, fecundity, overwintering mortality, and predation by L. nigrinus, as well as the proportion of branch tips producing new growth on study trees. Using our field-collected data, we refitted the model in ways that allowed us to predict what population densities we could expect for the following summertime progrediens generation given previous HWA density and levels of L. nigrinus. In both years, we found that despite high rates (greater than 80% ovisac predation) of predation by L. nigrinus on uncaged branches compared to caged branches, there were no significant differences in subsequent densities of the HWA spring generation between caged and uncaged treatments, as predicted by our model. In 2018, our field-collected densities of the summer progrediens generation were lower than what was predicted by the model in both predator exclusion treatments, possibly due to the model not incorporating tree health and climatic factors. Simulation models of pest insect populations based on field-collected data such as fecundity, density, overwintering mortality, and predation, could prove to be important in informing researchers and managers about the role of the biological control agent in the population dynamics of the target host.

Influence of forest-to-silvopasture conversion and drought on components of evapotranspiration

The northeastern U.S. is projected to experience more frequent short-term (1−2 month) droughts interspersed among larger precipitation events. Agroforestry practices such as silvopasture may mitigate these impacts of climate change while maintaining economic benefits of both agricultural and forestry practices. This study evaluated the effects of forest-to-silvopasture (i.e., 50 % thinning) conversion on the components of evapotranspiration (transpiration, rainfall interception, and soil evaporation) during the growing season of 2016. The study coincided with a late-summer drought throughout the northeastern U.S., which allowed us to also evaluate the effects of forest-to-silvopasture conversion on drought responses of multiple tree species, including Pinus strobus, Tsuga canadensis, and Quercus rubra. In the reference forest and silvopasture, we observed declining soil moisture and tree water use during the drought for all three tree species. However, the decline in P. strobus water use in response to declining soil moisture in the silvopasture was not as steep as compared with the reference forest, resulting in greater water use in the silvopasture for this species. In contrast, we did not detect different water-use responses between forest and silvopasture in T. canadensis or Q. rubra. This suggests that forest-to-silvopasture conversion via thinning can alleviate drought stress for P. strobus and that this species may be more sensitive to moisture stress when competition for water is high in denser stands. Evapotranspiration was 35 % lower in the silvopasture compared with the reference forest, primarily a result of lower transpiration and rainfall interception. While soil evaporation was greater in the silvopasture, this was not enough to offset the considerably lower transpiration and interception. We observed greater radial tree growth 1–3 years following conversion in the silvopasture as compared with the reference forest for T. canadensis and Q. rubra, but not for P. strobus. Overall, our results suggest that forest conversion to silvopasture (in lieu of clearcutting for new pasture) may mitigate the impacts of agricultural land use intensification and climate change on ecosystem services, especially in terms of sustaining hydrologic regulation functions. Further study is required to determine the generality of these results and whether these benefits extend beyond the first few years post-conversion.

Seasonal changes in eastern hemlock (Tsuga canadensis) foliar chemistry

Eastern hemlock (Tsuga canadensis (L.) Carrière) is an eastern North American conifer threatened by the invasive hemlock woolly adelgid (Adelges tsugae Annand). Changes in foliar terpenes and phenolics were evaluated in new (current-year growth) and mature (1-year-old growth) hemlock needles during the growing season and into plant dormancy. From April through September, foliar concentrations of nonvolatile soluble phenolics, condensed tannins, lignin, mono- and sesquiterpenes, α-pinene, camphene, isobornyl acetate, and diterpene resin were quantified. After September, additional analyses of metabolites that continued to differ significantly between new and mature foliage were carried out. Total soluble phenolic and condensed tannin concentrations in new foliage remained low relative to those of mature foliage throughout the growing season and converged in December. Lignin concentration in new foliage converged with that of mature foliage by July. Concentrations of α-pinene, camphene, isobornyl acetate, and diterpene resin in new foliage converged with those of mature foliage within 1 month of budbreak. The convergence of terpene concentrations in new and mature foliage suggests that these metabolites may play a role in herbivore defense during the peak growing season. Conversely, soluble phenolics, including condensed tannins, may defend foliage from herbivory outside of the spring growth period.

A Four-Year, Seven-State Reforestation Trial with Eastern Hemlocks (Tsuga canadensis) Resistant to Hemlock Woolly Adelgid (Adelges tsugae)

We conducted over a decade of research into individual eastern hemlock (Tsuga canadensis; hemlock) trees that are potentially resistant to hemlock woolly adelgid (Adelges tsugae; HWA), an invasive xylem-feeding insect that is capable of rapidly killing even mature trees. Following clonal propagation of these individuals, in 2015 we planted size- and age-matched HWA-resistant and HWA-susceptible hemlocks in HWA-infested forest plots in seven states. In 2019, we re-surveyed the plots; 96% of HWA-resistant hemlocks survived compared to 48% of susceptible trees. The surviving HWA-resistant trees were also taller, produced more lateral growth, retained more foliage, and supported lower densities of the elongate hemlock scale Fiorinia externa, another invasive hemlock pest, than the surviving HWA-susceptible trees. Our results suggest that HWA management may benefit from additional research exploring the identification, characterization, and use of HWA-resistant eastern hemlocks in future reforestation efforts.

Using isotopes to incorporate tree water storage and mixing dynamics into a distributed ecohydrologic modelling framework

AbstractRoot water uptake (RWU) by vegetation influences the partitioning of water between transpiration, evaporation, percolation, and surface runoff. Measurements of stable isotopes in water have facilitated estimates of the depth distribution of RWU for various tree species through methodologies based on end member mixing analysis (EMMA). EMMA often assumes that the isotopic composition of tree‐stored xylem water (δXYLEM) is representative of the isotopic composition of RWU (δRWU). We tested this assumption within the framework of EcH2O‐iso, a process‐based distributed tracer‐aided ecohydrologic model, applied to a small temperate catchment with a vegetation cover of coniferous eastern hemlock (Tsuga canadensis) and deciduous American beech (Fagus grandifolia). We simulated three scenarios for tree water storage and mixing: (a) zero storage (ZS), (b) storage with a well‐mixed reservoir (WM), and (c) storage with piston flow (PF). Simulating tree storage (WM and PF) improved the fit to δXYLEMobservations over ZS in the summer and fall seasons and substantially altered calibrated RWU depths and stomatal conductance. Our results suggest that there are likely to be advantages to considering tree storage and internal mixing when attempting to interpret δXYLEMin the estimation of RWU depths and critical zone water residence times, particularly during periods of low transpiration. Improved representations of tree water dynamics could yield more accurate ecohydrologic and earth system model representations of the critical zone.

Skewed distributions of lifetime reproductive success: beyond mean and variance

Lifetime reproductive performance is quantified here by the LRS (lifetime reproductive success), the random number of offspring an individual produces over its lifetime. Many field studies find that distributions of LRS among individuals are non-normal, zero-inflated and highly skewed. These results beg the question, what is the distribution of LRS predicted by demographic models when the only source of randomness is demographic stochasticity? Here we present the first exact analysis of the probability distribution of LRS for species described by age+stage models; our analysis starts with estimated vital rates. We illustrate with three examples: the Hadza, human hunter-foragers (age-only), the evergreen tree Tsuga canadensis (stage-only) and Roe deer, Capreolus capreolus (age+stage). For each we obtain the exact distribution of LRS, but also calculate and discuss the first three moments. Our results point to important questions about how such LRS distributions affect natural selection, and life history evolution.

Physiological responses of eastern hemlock (Tsuga canadensis) to light, adelgid infestation, and biological control: Implications for hemlock restoration

The rapid loss of eastern hemlock (Tsuga canadensis) in the southern Appalachian Mountains due to hemlock woolly adelgid (Adelges tsugae, HWA) infestation has resulted in substantial changes to ecosystem structure and function. Several restoration strategies have been proposed, including silvicultural treatments that increase incident light in forest understories. We conducted a four-year manipulative field experiment on surviving mid-story hemlock trees to investigate the effects of release from light limitation on HWA infestation and physiological parameters, expecting that higher light levels would improve tree carbon balance. Mixed hardwood forest sites were either previously uninfested with HWA (UN), infested with HWA (I), or infested with HWA and had a history of predatory beetle releases for biological control (I+P). At each site, we identified ten eastern hemlock trees in the mid-story and cut ~15 m radius canopy gaps around half of them while leaving the canopy intact over the other half. We compared short- and long-term indices of carbon gain and stress: leaf net photosynthesis (Anet); leaf fluorescence (Fv/Fm); leaf total non-structural carbohydrate concentration ([TNC]); new shoot growth; HWA density; and basal area growth. We found that trees experienced greater leaf-level stress in gaps and when HWA was actively feeding. Despite being more stressed, trees in gaps fixed 2x more carbon than those in reference conditions. High Anet in the spring translated into high leaf [TNC] in the spring, coinciding with when HWA was actively feeding. Although infested and uninfested trees had similar leaf [TNC] maxima, infestation prevented trees from allocating this carbon to shoot and basal area growth; this was particularly true for reference trees. Greater shoot growth in gap trees translated to greater annual basal area growth—by the end of the study, trees in gaps were growing 9x more than trees in reference conditions, and this was generally regardless of infestation status. In terms of growth and carbon balance, eastern hemlock consistently benefited from the increased light and soil moisture found in gaps; there was inconsistent and rather weak evidence that predator beetles conferred an additional advantage. Our results indicate that silvicultural treatments may improve long-term health and survival of infested trees and that integration of such treatments with existing strategies is worthy of continued exploration.