Water use and carbon exchange of red oak- and eastern hemlock-dominated forests in the northeastern USA: implications for ecosystem-level effects of hemlock woolly adelgid

Water use and carbon exchange of a red oak-dominated (Quercus rubra L.) forest and an eastern hemlock-dominated (Tsuga canadensis L.) forest, each located within the Harvard Forest in north-central Massachusetts, were measured for 2 years by the eddy flux method. Water use by the red oak forest reached 4 mm day(-1), compared to a maximum of 2 mm day(-1) by the eastern hemlock forest. Maximal carbon (C) uptake rate was also higher in the red oak forest than in the eastern hemlock forest (about 25 versus 15 micromol m(-2) s(-1)). Sap flux measurements indicated that transpiration of red oak, and also of black birch (Betula lenta L.), which frequently replaces eastern hemlock killed by hemlock woolly adelgid (Adelges tsugae Annand.), were almost twice that of eastern hemlock. Despite the difference between species in maximum summertime C assimilation rate, annual C storage of the eastern hemlock forest almost equaled that of the red oak forest because of net C uptake by eastern hemlock during unusually warm fall and spring weather, and a near-zero C balance during the winter. Thus, the effect on C storage of replacing eastern hemlock forest with a forest dominated by deciduous species is unclear. Carbon storage by eastern hemlock forests during fall, winter and spring is likely to increase in the event of climate warming, although this may be offset by C loss during hotter summers. Our results indicate that, although forest water use will decrease immediately following eastern hemlock mortality due to the hemlock woolly adelgid, the replacement of eastern hemlock by deciduous species such as red oak will likely increase summertime water use over current rates in areas where hemlock is a major forest species.     

Growth and Infestation by Hemlock Woolly Adelgid of Two Exotic Hemlock Species in a New England Forest

ABSTRACT

The hemlock woolly adelgid (HWA, Adelges tsugae Annand) an invasive exotic insect, may extirpate eastern hemlock (Tsuga canadensis (L.) Carrière) trees from native forests, but other hemlock species could be planted to occupy their ecological niche. This study tests two of the most likely replacement species candidates: western hemlock (T. heterophylla (Raf.) Sargent) and Chinese hemlock (T. chinensis (Franchet) Pritzel). Low survival rates, slow growth, and infestation by HWA of western hemlock in eastern hemlock forests shows that the western hemlock is not a likely candidate for planting in the northern portion of eastern hemlock's range. In contrast, Chinese hemlock grew at rates similar to eastern hemlock and did not show any signs of HWA infestation. In this study, damage from deer was a much bigger problem than growth reductions from HWA.     

Response of arthropod biodiversity to foundation species declines: The case of the eastern hemlock

Widespread declines of foundation species, such as many corals, kelps, and overstory trees, are of grave concern because, by definition, these species create and maintain habitat that supports other species. Nevertheless, past responses to their declines, many of which were caused by invasive species, have been late and ineffective, underscoring the need to predict changes in biodiversity and ecosystem function associated with species invasions and foundation species losses. One predictive, but under-used, approach is to compare the species and functions associated with the afflicted foundation species to its projected replacement communities. The taxa associated with the foundation species and subsequent successional stages would be expected to decline and increase, respectively. We used this approach to generate hypotheses for how arthropod diversity might change in response to extensive losses of eastern hemlock trees caused by the invasive, hemlock woolly adelgid (insect: Hemiptera, Adelgidae). Our all-strata survey of the arthropods in an eastern hemlock forest and its expected replacement climax community in the mid-Atlantic region of the United States, mixed hardwood forest, suggests that eastern hemlock losses might initiate increases in arthropod abundance, alpha diversity, and 23 arthropod taxa, might produce no change in evenness or composition of arthropod functional groups, but might trigger decreases in beta diversity and seven hemlock indicator taxa. These predictions are consistent with observed trends in arthropod responses to hemlock losses in other studies, and thus might be useful for targeting early monitoring, management, and conservation efforts. This research is exploratory, however, and tests of these predictions across larger spatial scales will be necessary to determine the generality of the findings.     

Oak seedling growth and ectomycorrhizal colonization are less in eastern hemlock stands infested with hemlock woolly adelgid than in adjacent oak stands

Invasive, non-indigenous, phytophagous insects have caused widespread declines in several dominant tree species. The decline in dominant tree species may lead to cascading effects on other tree and microbial species and their interactions, affecting forest recovery following the decline. In the eastern USA, eastern hemlock (Tsuga canadensis (L.) Carr) is declining because of infestation by the hemlock woolly adelgid (HWA; Adelges tsugae Annand). Northern red oak (Quercus rubra L.) is a common replacement species in declining hemlock stands, but reduced mycorrhizal inoculum potential in infested hemlock stands may cause oak to grow more slowly compared with oak in oak stands. We grew red oak seedlings for one growing season in declining hemlock-dominated stands infested with HWA and in adjacent oak-dominated stands. Ectomycorrhizal root tip density and morphotype richness in soil cores were 63 and 27% less, respectively, in declining hemlock stands than in oak stands. Similarly, ectomycorrhizal percent colonization and morphotype richness on oak seedlings were 33 and 30% less, respectively, in declining hemlock stands than in oak stands. In addition, oak seedlings in declining hemlock stands had 29% less dry mass than oak seedlings in oak stands. Analysis of covariance indicated that morphotype richness could account for differences in oak seedling dry mass between declining hemlock stands and oak stands. Additionally, oak seedling dry mass in declining hemlock stands significantly decreased with decreasing ectomycorrhizal percent colonization and morphotype richness. These results suggest that oak seedling growth in declining hemlock stands is affected by reduced ectomycorrhizal inoculum potential. Further, the rate of forest recovery following hemlock decline associated with HWA infestation may be slowed by indirect effects of HWA on the growth of replacement species, through effects on ectomycorrhizal colonization and morphotype richness.     

Oak loss increases foliar nitrogen, δ15N and growth rates of Betula lenta in a northern temperate deciduous forest

Oak forests dominate much of the eastern USA, but their future is uncertain due to a number of threats and widespread failure of oak regeneration. A sudden loss of oaks (Quercus spp.) could be accompanied by major changes in forest nitrogen (N) cycles with important implications for plant nutrient uptake and tree species composition. In this study, we measured the changes in N use and growth rates of black birch trees (Betula lenta L.) following oak girdling at the Black Rock Forest in southeastern New York, USA. Data were collected from nine experimental plots composed of three treatments: 100% oaks girdled (OG), 50% oaks girdled (O50) and control (C). Foliar N concentration and foliar (15)N abundance increased significantly in the oak-girdled plots relative to the control, indicating that the loss of oaks significantly altered N cycling dynamics. As mineralization and nitrification rates increase following oak loss, black birch trees increase N absorption as indicated by higher foliar N content and increased growth rates. Foliar N concentration increased by 15.5% in the O50 and 30.6% in the OG plots relative to the control, while O50 and OG plots were enriched in (15)N by 1.08‰ and 3.33‰, respectively (P?相似文献   

Effects of winter temperatures on two birch (Betula) species

In Massachusetts, low winter temperatures delay the onset of flowering in black birch (Betula lenta L.), but not in gray birch (B. populifolia Marsh.). During the winter of 2006, male inflorescences and twigs of black birch had higher water contents than those of gray birch, and the inflorescences of black birch experienced greater frost kill than those of gray birch. Vessels diameters were greater in black than in gray birch, a difference associated with a higher incidence of winter xylem embolism, as indicated by reduced xylem hydraulic conductance. In both species, recovery of hydraulic conductance in twigs that survived the winter coincided with the development of root pressure. Frost kill to male inflorescences or associated damage to plant tissues may account for the difference between species in the effect of winter temperature on the time of first flowering. In a comparison of 24 birch species, sensitivity of the first flowering date to temperature was also correlated with water content in male inflorescences.     

Interspecific variation in nighttime transpiration and stomatal conductance in a mixed New England deciduous forest

Transpiration is generally assumed to be insignificant at night when stomata close in response to the lack of photosynthetically active radiation. However, there is increasing evidence that the stomata of some species remain open at night, which would allow for nighttime transpiration if there were a sufficient environmental driving force. We examined nighttime water use in co-occurring species in a mixed deciduous stand at Harvard Forest, MA, using whole-tree and leaf-level measurements. Diurnal whole-tree water use was monitored continuously with Granier-style sap flux sensors in paper birch (Betula papyrifera Marsh.), red oak (Quercus rubra L.) and red maple (Acer rubrum L.). An analysis was conducted in which nighttime water flux could be partitioned between refilling of internal water stores and transpiration. Substantial nighttime sap flux was observed in all species and much of this flux was attributed to the refilling of depleted water stores. However, in paper birch, nighttime sap flux frequently exceeded recharge estimates. Over 10% of the total daily sap flux during the growing season was due to transpiration at night in paper birch. Nighttime sap flux was over 8% of the total daily flux in red oak and 2% in red maple; however, this flux was mainly associated with recharge. On nights with elevated vapor pressure deficit, sap flux continued through the night in paper birch, whereas it reached zero during the night in red oak and red maple. Measurements of leaf-level gas exchange on a night with elevated vapor pressure deficit showed stomatal conductance dropping by only 25% in paper birch, while approaching zero in red oak and red maple. The study highlighted differences in ecophysiological controls on sap flux exerted by co-occurring species. Paper birch is a fast-growing, shade-intolerant species with an earlier successional status than red oak and red maple. Risking water loss through nighttime transpiration may provide paper birch with an ecological advantage by enabling the species to maximize photosynthesis and support rapid growth. Nighttime transpiration may also be a mechanism for delivering oxygen to respiring cells in the deep sapwood of paper birch.     

Restocking with broadleaved species during the conversion of Tsuga heterophylla plantations to native woodland using natural regeneration

The removal of conifers planted during the twentieth century on sites that had been woodland for many centuries, with the intention of restoring native broadleaved species, is an important aim of forestry policy in Great Britain. Current guidance generally advocates gradual removal of plantation trees using continuous cover silviculture and restocking by natural regeneration, but methods are largely untested. This study investigated natural regeneration of trees and shrubs at sites where western hemlock had been established either pure or in mixture with native broadleaved species. Western hemlock had been cleared from 70% of the sites. There were c. 10?C300 stems ha^?1 providing canopy cover of 10?C90%; the predominant broadleaved canopy trees were ash, beech, birch and oak. The ground flora of all sites was species poor; bramble was the predominant vegetation type overall, but grasses, rushes and shrubs were relatively more abundant on open sites. A total of 15 native tree species were regenerating, birch was most common and at most sites there were few valuable broadleaved timber species. Although combined numbers of seedlings and saplings varied from 200 to 20,000 ha^?1, large areas of most sites had few regenerating trees and <10% of their area was becoming restocked with timber species. Relationships between seedling numbers and site characteristics were complex and varied with species, but there was a consistent positive relationship between the presence of nearby parent trees and the frequency and abundance of seedlings. The results indicate that the current reliance on natural regeneration may be unwise especially for those broadleaved species such as oak and beech which are valued for their timber.     

Physiological, morphological and allocational plasticity in understory deciduous trees: importance of plant size and light availability

In a 4-year study, we investigated changes in leaf physiology, crown morphology and whole-tree biomass allocation in seedlings and saplings of shade-tolerant sugar maple (Acer saccharum Marsh.) and intermediate shade-tolerant yellow birch (Betula alleghaniensis Britt.) growing in natural understory light (0.5 to 35% of full sunlight) or in understory light reduced by 50% with shade nets to simulate the effect of gap closure. Leaf physiological parameters were mainly influenced by the light gradient, whereas crown morphological and whole-tree allocational parameters were mainly influenced by tree size. No single physiological, morphological or allocational trait was identified that could explain the difference in shade tolerance between the species. Yellow birch had higher growth rates, biomass allocation to branches and leaf physiological plasticity and lower crown morphological plasticity in unmodified understory light than sugar maple. Sugar maple did not display significant physiological plasticity, but showed variation with tree size in both crown morphology and whole-tree biomass allocation. When sugar maple was small, a greater proportion of whole-tree biomass was allocated to roots. However, physiological differences between the species decreased with decreasing light and most morphological and allocational differences tended to disappear with increasing tree size, suggesting that many species differences in shade-tolerance are expressed mainly during the seedling stage. Understory trees of both species survived for 4 years under shade nets, possibly because of higher plasticity when small and the use of stored reserves when taller.     

Dynamics of transpiration, sap flow and use of stored water in tropical forest canopy trees

In large trees, the daily onset of transpiration causes water to be withdrawn from internal storage compartments, resulting in lags between changes in transpiration and sap flow at the base of the tree. We measured time courses of sap flow, hydraulic resistance, plant water potential and stomatal resistance in co-occurring tropical forest canopy trees with trunk diameters ranging from 0.34-0.98 m, to determine how total daily water use and daily reliance on stored water scaled with size. We also examined the effects of scale and tree hydraulic properties on apparent time constants for changes in transpiration and water flow in response to fluctuating environmental variables. Time constants for water movement were estimated from whole-tree hydraulic resistance (R) and capacitance (C) using an electric circuit analogy, and from rates of change in water movement through intact trees. Total daily water use and reliance on stored water were strongly correlated with trunk diameter, independent of species. Although total daily withdrawal of water from internal storage increased with tree size, its relative contribution to the daily water budget (approximately 10%) remained constant. Net withdrawal of water from storage ceased when upper branch water potential corresponded to the sapwood water potential (Psi(sw)) at which further withdrawal of water from sapwood would have caused Psi(sw) to decline precipitously. Stomatal coordination of vapor and liquid phase resistances played a key role in limiting stored water use to a nearly constant fraction of total daily water use. Time constants for changes in transpiration, estimated as the product of whole- tree R and C, were similar among individuals (~0.53 h), indicating that R and C co-varied with tree size in an inverse manner. Similarly, time constants estimated from rates of change in crown and basal sap flux were nearly identical among individuals and therefore independent of tree size and species.     

Hydraulic architecture and photosynthetic capacity as constraints on release from suppression in Douglas-fir and western hemlock

We compared hydraulic architecture, photosynthesis and growth in Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), a shade-intolerant species, and western hemlock (Tsuga heterophylla (Raf.) Sarg.), a shade-tolerant species, to study the temporal pattern of release from suppressive shade. In particular, we sought to determine whether hydraulic architecture or photosynthetic capacity is most important in constraining release. The study was conducted at two sites with mixed stands of 10- to 20-year-old Douglas-fir and western hemlock. At one site, the stand had been thinned allowing release of the understory trees, whereas at the other site, the stand remained unthinned. Douglas-fir had lower height growth (from 1998-2003) and lower relative height growth (height growth from 1998 to 2003/height in 1998) than western hemlock. However, relative height growth of released versus suppressed trees was higher in Douglas-fir (130%) than in western hemlock (65%), indicating that, although absolute height growth was less, Douglas-fir did release from suppression. Release seemed to be constrained initially by a limited photosynthetic capacity in both species. Five years after release, Douglas-fir trees had 14 times the leaf area and 1.5 times the leaf nitrogen concentration (N (area)) of suppressed trees. Needles of released western hemlock trees had about twice the maximum assimilation rate (A (max)) at ambient [CO(2)] as needles of suppressed trees and exhibited no photoinhibition at the highest irradiances. After release, trees increased in leaf area, leaf N concentration and overall photosynthetic capacity. Subsequently, hydraulic architecture appeared to constrain release in Douglas-fir and, to a lesser extent, in western hemlock. Released trees had significantly less negative foliar delta(13)C values than suppressed trees and showed a positive relationship between leaf area:sapwood area ratio (A (L)/A (S)) and delta(13)C, suggesting that trees with more leaf area for a given sapwood area experienced a stomatal limitation on carbon gain. Nonetheless, these changes had no significant effects on leaf specific conductivities of suppressed versus released trees of either species, but leaf specific root conductance was significantly lower in released Douglas-fir.     

Radial growth changes following hemlock woolly adelgid infestation of eastern hemlock

? Context

Hemlock woolly adelgid (Adelges tsugae) is an invasive insect that is defoliating and killing eastern hemlock (Tsuga canadensis) in the USA.

? Aims

We quantified changes in tree-ring growth rates and wood anatomy for living trees infested with hemlock woolly adelgid across six sites from Massachusetts (42°41′N) to Georgia (34°53′N) to identify growth responses of eastern hemlock that had survived infestation.

? Methods

Annual ring widths from infested eastern hemlocks were cross-dated and measured. Growth rates before and after infestation were compared. Two infested trees from Virginia were cut, and thin sections were prepared to identify changes in cell properties.

? Results

At three sites, trees experienced a significant decrease in radial growth after hemlock woolly adelgid arrival; however, the other three sites showed no change or increase in growth. Latewood produced after hemlock woolly adelgid infestation had significantly smaller cells with reduced cell wall thickness compared to latewood prior to infestation.

? Conclusion

At half the sites where hemlock woolly adelgid infested eastern hemlock trees were sampled, radial growth increased or remained unchanged. This unexpected response may be due to reduced competition due to mortality of other eastern hemlocks or physiological compensatory responses of increased photosynthetic rate and increased water use efficiency experienced by eastern hemlock infested with hemlock woolly adelgid.     

Species differences in total and vertical distribution of branch- and tree-level leaf area for the five primary conifer species in Maine, USA

Vertical distribution of leaf area largely governs both tree structure and function. Models of this important tree attribute have been constructed for several commercially important conifers. However, a limited number of studies have compared alternative modeling techniques and inherent species differences. This study used several existing datasets for the five primary conifer species in Maine, namely balsam fir [Abies balsamea (L.) Mill.], northern white-cedar [Thuja occidentalis (L.)], eastern hemlock [Tsuga canadensis (L.) Carr.], eastern white pine [Pinus strobus (L.)], and red spruce [Picea rubens (Sarg.)] to examine species variation in total and vertical distribution of projected leaf area at the individual branch- and tree-levels. In addition, multiple methods for modeling the vertical distribution of leaf area were examined across the species. For a given branch diameter and location within the crown, eastern hemlock branches held the greatest amount of leaf area, followed by balsam fir, northern white-cedar, white pine, and red spruce. At the tree-level, eastern white pine held the greatest amount of leaf area followed by eastern hemlock, balsam fir, red spruce, and northern white-cedar for a given tree size. Across species, the two-parameter, right-truncated Weibull distribution performed the best for predicting vertical distribution of leaf area when compared to the four-parameter beta and Johnson's S_B distributions (reduction of root mean square error of 1.7–21.1%). Northern white-cedar had a relative distribution of leaf area distinctly different than other species in this study with a mode shifted towards the upper crown. In contrast to red spruce and white pine, the mode of the relative distribution of leaf area for balsam fir and eastern hemlock occurred lower in the crown. Results of this study suggest that differences in total and vertical distribution of leaf area exist between species, but significant amounts of their variation are largely accounted for by bole and crown size.     

Afforestation of low-productivity peatlands in Sweden — the potential of natural seeding

In 1971, a series of five afforestation trials were established along a north-south gradient on open peatlands in Sweden. All areas were drained, fertilized and planted with Scots pine (Pinus sylvestris). Survival and height of the planted trees as well as the amount of natural establishment, i.e. downy birch (Betula pubescens), silver birch (Betula pendula), Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), and other deciduous species: (e.g. Salix sp., Sorbus aucuparia), by seeding from nearby forests were measured 18–22 years later. The amount of naturally established trees exceeded the number of living planted trees in all five areas. Next to downy birch, Scots pine was the most common naturally established species. Large variations in both amount and height of natural establishment were found along the climatic gradient. Planted trees were taller than naturally established trees, except for the northernmost area. Number and height of naturally established trees correlated positively to closeness to nearest forest and to nearest ditch. These trees also responded positively to PK fertilization in the three southernmost areas. In the two northernmost areas no such response was found. It was concluded that natural seeding from neighbouring forests can be counted on and recommended in afforestation of low-productive peatlands in Sweden, provided that downy birch is accepted as a dominant tree species.     

Fertilization effects on mean stomatal conductance are mediated through changes in the hydraulic attributes of mature Norway spruce trees

Stomatal conductance was quantified with sap flux sensors and whole-tree chambers in mature Norway spruce (Picea abies (L.) Karst.) trees after 3 years of exposure to elevated CO(2) concentration ([CO(2)]) in a 13-year nutrient optimization experiment. The long-term nutrient optimization treatment increased tree height by 3.7 m (67%) and basal diameter by 8 cm (68%); the short-term elevated [CO(2)] exposure had no effect on tree size or allometry. Nighttime transpiration was estimated as approximately 7% of daily transpiration in unchambered trees; accounting for the effect of nighttime flux on the processing of sap flux signals increased estimated daily water uptake by approximately 30%. Crown averaged stomatal conductance (g(s)) was described by a Jarvis-type model. The addition of a stomatal response time constant (tau) and total capacitance of stored water (C(tot)) improved the fit of the model. Model estimates for C(tot) scaled with sapwood volume of the bole in fertilized trees. Hydraulic support-defined as a lumped variable of leaf-specific hydraulic conductivity and water potential gradient (K(l)DeltaPsi) -was estimated from height, sapwood-to-leaf area ratio (A(s):A(l)) and changes in tracheid dimensions. Hydraulic support explained 55% of the variation in g(s) at reference conditions for trees across nutrient and [CO(2)] treatments. Removal of approximately 50% of A(l) from three trees yielded results suggesting that stomatal compensation (i.e., an increase in g(s)) after pruning scales inversely with K(l)DeltaPsi, indicating that the higher the potential hydraulic support after pruning, the less complete the stomatal compensation for the increase in A(s):A(l).     

Allozyme variation and recent evolutionary history of eastern hemlock (<Emphasis Type="Italic">Tsuga canadensis</Emphasis>) in the southeastern United States

Eastern hemlock (Tsuga canadensis [L.] Carr.) is a widespread and ecologically important conifer species of eastern North America that is threatened by the hemlock woolly adelgid (Adelges tsugae Annand), a pest introduced into the United States from Asia in the 1920s. Information about the genetic composition of eastern hemlock is necessary to guide ex situ conservation efforts in the southeastern United States, where the species is expected to harbor relatively high amounts of genetic variation in areas of Pleistocene glacial refuge. Nineteen allozyme markers were used to quantify the genetic variation present in 20 eastern hemlock populations in the southeastern United States. Results indicate that the species has low levels of genetic diversity in the region compared to most other conifers, but greater population differentiation (F _ST = 0.126). Populations along the eastern periphery and in the Appalachian interior exhibited higher levels of diversity than those along the western periphery of its geographic range. The results suggest that the glacial refuge area for eastern hemlock was likely located east of the southern Appalachian Mountains, and indicate that ex situ conservation seed collections should be concentrated in these areas of higher diversity.     

Changes in the water relations of balsam fir and white birch saplings after thinning

In the autumn of 1987, young balsam fir (Abies balsamea (L.) Mill.) and white birch (Betula papyrifera Marsh.) trees were thinned and their water relations followed during the next two growing seasons. At the beginning of the first summer following treatment, thinned trees of both species had lower osmotic potentials at full saturation (Psi(pi,sat)) and at turgor loss point (Psi(pi,tlp)) compared with controls. At this time, Psi(pi,sat) was linearly related to the percentage of full sunlight reaching the trees. A higher sugar concentration in leaves was an important component of the lower Psi(pi,sat) of thinned trees. For the other two sampling dates during the first growing season after treatment and all three sampling dates during the second growing season after treatment, little osmotic adjustment of the thinned trees relative to the control tress was observed in either species. The absence of osmotic adjustment during the second growing season following thinning suggests that other mechanisms were responsible for the acclimation of the treated trees to the higher atmospheric evaporative demand. Sapwood permeability (k) of white birch was higher than that of balsam fir, but no differences in k or in sapwood area were found between treated and control trees of either species. Predawn water potentials (Psi(pred)) of treated trees were less negative than those of controls.     

Impact and recovery of western hemlock following disturbances by forestry and insect defoliation

Impact and recovery of juvenile western hemlock stands were measured 5 years after the end of an outbreak of western blackheaded budworm Acleris gloverana (Walsingham) (Lepidoptera: Tortricidae) and hemlock sawfly Neodiprion tsugae Middleton (Hymenoptera: Diprionidae) on Haida Gwaii, British Columbia, Canada. Overall, whole-tree mortality was infrequent (3.6–6.8%) but aggregated in a few, severely defoliated stands. Top-kill of the apical leader of surviving trees was more frequent (7.2–22.1%) and many of these trees had not recovered a growing dominant leader 5 years after the original damage. These severe impacts were most common in the largest trees in the youngest stands which had been spaced just before the outbreak. Radial growth was also reduced for 2 years following defoliation. However, mean annual ring width and basal area increment in surviving trees recovered quickly and was even enhanced in post-defoliation stands, particularly in spaced stands that had been moderately defoliated. Management recommendations include planning for likely outbreaks by avoiding creation of expanses of young regenerating stands of western hemlock during harvest and modifying spacing treatments when an insect outbreak is imminent to reflect the greater risk to trees <20 years of age.     

Changes in composition, structure and aboveground biomass over seventy-six years (1930-2006) in the Black Rock Forest, Hudson Highlands, southeastern New York State

We sought to quantify changes in tree species composition, forest structure and aboveground forest biomass (AGB) over 76 years (1930-2006) in the deciduous Black Rock Forest in southeastern New York, USA. We used data from periodic forest inventories, published floras and a set of eight long-term plots, along with species-specific allometric equations to estimate AGB and carbon content. Between the early 1930s and 2000, three species were extirpated from the forest (American elm (Ulmus americana L.), paper birch (Betula papyrifera Marsh.) and black spruce (Picea mariana (nigra) (Mill.) BSP)) and seven species invaded the forest (non-natives tree-of-heaven (Ailanthus altissima (Mill.) Swingle) and white poplar (Populus alba L.) and native, generally southerly distributed, southern catalpa (Catalpa bignonioides Walt.), cockspur hawthorn (Crataegus crus-galli L.), red mulberry (Morus rubra L.), eastern cottonwood (Populus deltoides Bartr.) and slippery elm (Ulmus rubra Muhl.)). Forest canopy was dominated by red oak and chestnut oak, but the understory tree community changed substantially from mixed oak-maple to red maple-black birch. Density decreased from an average of 1500 to 735 trees ha(-1), whereas basal area doubled from less than 15 m(2) ha(-1) to almost 30 m(2) ha(-1) by 2000. Forest-wide mean AGB from inventory data increased from about 71 Mg ha(-1) in 1930 to about 145 Mg ha(-1) in 1985, and mean AGB on the long-term plots increased from 75 Mg ha(-1) in 1936 to 218 Mg ha(-1) in 1998. Over 76 years, red oak (Quercus rubra L.) canopy trees stored carbon at about twice the rate of similar-sized canopy trees of other species. However, there has been a significant loss of live tree biomass as a result of canopy tree mortality since 1999. Important constraints on long-term biomass increment have included insect outbreaks and droughts.     

Sampling strategies and density–defoliation relationships for the pale-winged gray moth, Iridopsis ephyraria, on mature eastern hemlock

The pale-winged gray moth, Iridopsis ephyraria (Walker), a native geometrid moth species found across eastern North America, has recently caused severe defoliation of eastern hemlock trees, Tsuga canadensis (L.) Carrière, in south-central Nova Scotia, Canada. Field studies were carried out to develop sampling techniques that could be used to predict subsequent defoliation of mature hemlocks, which are too large to sample by conventional methods. Densities of newly emerged, first-instar larvae, estimated by sticky tape traps, explained 77% of the subsequent current-year shoot defoliation by I. ephyraria. Egg densities from foam oviposition traps and mid-instar larval densities from beating sheets were weakly and usually non-significantly related to defoliation. Therefore, foam oviposition traps could be used to qualitatively estimate egg densities of I. ephyraria, but first-instar larvae must be sampled with sticky tape if accurate predictions of hemlock defoliation are desired.