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Eastern hemlock (Tsuga canadensis (L.) Carr.) is one of the principal riparian and cove canopy species in the southern Appalachian Mountains. Throughout its range, eastern hemlock is facing potential widespread mortality from the hemlock woolly adelgid (HWA). If HWA-induced eastern hemlock mortality alters hydrologic function, land managers will be challenged to develop management strategies that restore function or mitigate impacts. To estimate the impact that the loss of this forest species will have on the hydrologic budget, we quantified and modeled transpiration over a range of tree sizes and environmental conditions. We used heat dissipation probes, leaf-level gas-exchange measurements, allometric scaling, and time series modeling techniques to quantify whole-tree and leaf-level transpiration (EL) of eastern hemlock. We monitored trees ranging from 9.5 to 67.5 cm in diameter along a riparian corridor in western North Carolina, USA during 2004 and 2005. Maximum rates of daily tree water use varied by diameter and height, with large trees transpiring a maximum of 178-186 kg H2O·tree-1·d-1. Values of EL could be predicted from current and lagged environmental variables. We forecasted eastern hemlock EL for inventoried stands and estimated a mean annual transpiration rate of 63.3 mm/yr for the hemlock component, with 50% being transpired in the winter and spring. In typical southern Appalachian stands, eastern hemlock mortality would thus reduce annual stand-level transpiration by 10% and reduce winter and spring stand-level transpiration by 30%. Eastern hemlock in the southern Appalachians has two distinct ecohydrological roles: an evergreen tree that maintains year-round transpiration rates and a riparian tree that has high transpiration rates in the spring. No other native evergreen in the southern Appalachians will likely fill the ecohydrological role of eastern hemlock if widespread mortality occurs. With the loss of this species, we predict persistent increases in discharge, decreases in the diurnal amplitude of streamflow, and increases in the width of the variable source area.

The primary objective of this study was to determine whether the exotic, invasive shrub, glossy buckthorn ( Frangula alnus ), is more abundant in canopy gaps created by logging than in uncut forests. Secondary objectives were to determine whether buckthorn abundance in gaps is related to gap size, and whether or not buckthorn exhibits advanced regeneration. The abundance of glossy buckthorn was estimated in five patch cuts and three single-tree cuts in a 90 year old eastern hemlock–eastern white pine–sweet birch forest at the Woodman Horticultural Farm in Durham, NH, USA. Glossy buckthorn was 96 times more abundant in logged areas than in uncut control plots. The three largest but youngest gaps (>0.08 ha; 5 years old) had the greatest proportion of tall (>2 m), reproductively mature glossy buckthorn individuals, with 18.4% fruiting. The older, medium-sized gaps (ca. 0.03 ha; 10 years old) contained the highest overall densities of glossy buckthorn, but few stems were flowering (~2%) and none were fruiting at the time of sampling. Small gaps (<0.01 ha; 10 years old) appeared to be sinks for glossy buckthorn, as all individuals were <0.5 m tall and none were >4 years old. As age and size of gaps were correlated, it was difficult to determine which factor played a larger role in the establishment and persistence of glossy buckthorn. However, the greater proportion of individuals >2 m tall and greater reproductive vigor of glossy buckthorn in large gaps relative to small gaps—despite fewer years available for growth—suggest that larger disturbances lead to more resources available for buckthorn growth, survival, and reproduction. Individuals <0.5 m tall were observed in uncut control plots at low density (<30 stems/ha) and 5% of stems in large gaps were older than the gaps themselves, suggesting that gap formation released previously established glossy buckthorn individuals (i.e., advanced regeneration).

During a 33-year sampling period, we observed species richness and calculated species evenness and Shannon Diversity for understory woody seedlings and herbaceous species on three small islands in Lake Winnipesaukee, New Hampshire, and noted consistency of dominant plant species over time. Seedlings and herbaceous species were recorded and measured in 25 permanent plots that were created on the three islands in 1978. The understory species data were compiled by frequency and dominance of woody seedlings and herbaceous species. Data from 250 individual quadrats show that species richness more than doubled from 41 in 1978 to 83 species on all three islands in 2011. Species evenness on all the islands remained relatively constant in each of the four samplings. The combined Shannon’s Diversity for the three islands rose from 2.76 in 1978 to 3.37 in 2011. Dominant species in the study were Aralia nudicaulis, Gaultheria procumbens, Gaylussacia baccata, Maianthemum canadense, and Tsuga canadensis seedlings.

The abundance of eastern hemlock ( Tsuga canadensis ) in eastern US forests has declined since the 1950s owing to the introduction of the non-native insect, hemlock woolly adelgid (HWA, Adelges tsugae ). In southern New England, eastern hemlock is being replaced by the deciduous tree species, black birch ( Betula lenta ). To date there is little understanding of whether hemlock loss will fundamentally alter ecosystem C balance and component fluxes. In this study, we use a comparative approach to study potential changes in C fluxes and N cycling associated with HWA-induced hemlock decline and replacement. The stands include primary- and secondary-growth hemlock forests (>230 and 132 years old, respectively), recently disturbed stands (5 and 18 years old) that now have rapidly growing black birch saplings, and a mature black birch stand of age similar to the second-growth hemlock stand. We found that aboveground net primary production was higher in the aggrading black birch stand and significantly so at 18-years post-HWA compared to the secondary-hemlock stand it would likely replace. Rapid forest regrowth was accompanied by significantly higher rates of N uptake from the soil but also higher N-use efficiency because most of the N taken up from the soil was allocated to the production of wood with a high C-to-N ratio. In contrast to patterns of aboveground production, the rate of soil respiration was lowest in the young stands and not significantly different from the second-growth hemlock stand, suggesting little net effect of stand replacement on soil C efflux. The leaf litter decomposition study showed that black birch litter decomposed more rapidly than hemlock litter but that there was no effect of stand type on the rate of decomposition. Analyses of extracellular microbial exoenzyme activity painted a more nuanced pattern of variation among stands, with fine root biomass the only weakly explanatory variable. In combination with our prior work on C stocks, these results suggest that forests affected by HWA in southern New England will remain a sink for atmospheric CO 2 despite reorganization of stand structure and species composition.

Eastern hemlock (Tsuga canadensis) is projected to experience shifts in suitable habitat distribution in response to climate change. Models predict the southern boundary of eastern hemlock will remain geographically stable, but that eastern hemlock will decline in dominance and abundance along the boundary. In 2003 and 2013, 10 disjunct eastern hemlock stands on the southernmost portion of the Cumberland Plateau in Alabama were surveyed to characterize the vertical structure and diameter distributions of eastern hemlock. This work represents the third survey over 20 years and provides more complete insight into the stand dynamics of these disjunct stands. No trends were common among all stands, but there was a general decrease in the number of canopy eastern hemlock from 2013 to 2023. Despite the loss of canopy dominance, the majority of stands experienced an increase in the number of stems in the larger diameter classes. We expect these stands to remain viable and predict no shift in the southern range limit of the species. If, however, climate change effects begin to manifest in the stand structure or a severe disturbance occurs, eastern hemlock may lose dominance and be unable to regenerate. In 2021, an EF1 tornado obliterated one stand, and removed all canopy stems from another. Although the hemlock woolly adelgid (Adelges tsugae) is perhaps the most well-known threat to eastern hemlock, non-species specific disturbances have the potential to be equally as devastating to these disjunct populations.

We present fossil pollen data from a sediment core at Lake Grinnell in northern New Jersey of the northeastern United States. The 12 500-yr chronology of the sediment record was controlled by seven calibrated AMS radiocarbon dates on terrestrial plant macrofossils. Similar to many pollen profiles in the region, our data show that vegetation changed from Picea- and Pinus-dominated woodland/forest at 12.5—11.4 ka (1 ka = 1000 cal. yr BP), through Pinus-dominated mixed forest at 11.4—9.3 ka, to Quercus-dominated forest after 9.3 ka. Some main tree taxa, including Tsuga, Ulmus and Acer, arrived and expanded during the Quercus expansion phase at 11.4—9.3 ka in the early Holocene, while other trees, including Fagus and Carya, established and expanded much later around 9 ka. Pollen data show that major forest shifts were in response to climatic change as independently inferred from oxygen-isotope records in the same core. These responses include major turnovers of tree species at the onset of the Holocene after the Younger Dryas, including the disappearance of Picea, declines of boreal taxa Betula and Alnus, and expansions of most other temperate trees. Our new records show two hemlock (Tsuga canadensis) decline events during the Holocene that were both likely caused by climatic change. The early-Holocene hemlock decline centering around 9.5 ka was marked by >10-fold decrease in hemlock populations, which was probably caused by summer high temperature as inferred from high oxygen isotope values. The mid-Holocene hemlock decline from 5.3 ka to 3.0 ka corresponds with a pronounced shift in oxygen isotopes at Grinnell and with a dry climate interval documented from other records in northeastern North America. Our results support the notion that a dry climate caused or triggered the mid-Holocene hemlock decline and that the Appalachian Forest has shown sensitive responses to climatic change in the Holocene.

Flavanols and flavonols of mitotic and post-mitotic nuclei in needles of Taxus baccata L., Tsuga canadensis L., and slow growing dwarf genotypes of both genera are investigated histochemically. The flavanols of nuclear chromatins and in the vacuoles stain blue with the p-dimethylamino-cinnamaldehyde (DMACA) reagent. Flavonols do not react with the reagent but owing to their UV absorbance they can be seen as bright yellow pigments. The nuclei in the photomicrographs obtained by microscopy were measured for flavanols at 640 nm. The vigorously sprouting Taxus baccata L. displays the most rapid cell cycling of the needles and the nuclei reveal clear blue and white mosaic structures. The flavanol component of Taxus baccata nuclei remains relatively stable most of the growing season. The dwarf genotypes also display fairly blue stained meristematic nuclei during the intense spring flush. However, after the spring flush and towards mid-summer the nuclear flavanols slowly decrease in parallel with a gradual increase in yellow staining nuclear flavonols. A mixture of blue stained flavanols and yellow flavonols results in greenish coloration of the nuclei. The greenish tint becomes more pronounced when the parenchyma cells mature and age. At the same time, the cytoplasm of the dwarf genotypes also begins to attain a more yellow tint. This trend continues towards mid-summer and autumn, particularly in the nana genotypes. It would appear that the yellow staining flavonols are linked to restricted growth conditions. In the present study, it becomes evident that the species-typical endogenous growth potential is related to both flavanol and flavonol allocation into the nuclei. The vigorously growing species of Taxus and Tsuga have a higher capacity for recruitment of flavanols into the nuclei than the very slow growing dwarf species.

How, where, and why carbon (C) moves into and out of an ecosystem through time are long-standing questions in biogeochemistry. Here, we bring together hundreds of thousands of C-cycle observations at the Harvard Forest in central Massachusetts, USA, a mid-latitude landscape dominated by 80–120-yr-old closed-canopy forests. These data answered four questions: (1) where and how much C is presently stored in dominant forest types; (2) what are current rates of C accrual and loss; (3) what biotic and abiotic factors contribute to variability in these rates; and (4) how has climate change affected the forest's C cycle? Harvard Forest is an active C sink resulting from forest regrowth following land abandonment. Soil and tree biomass comprise nearly equal portions of existing C stocks. Net primary production (NPP) averaged 680–750 g C.m⁻².yr⁻¹; belowground NPP contributed 38–47% of the total, but with large uncertainty. Mineral soil C measured in the same inventory plots in 1992 and 2013 was too heterogeneous to detect change in soil-C pools; however, radiocarbon data suggest a small but persistent sink of 10–30 g C.m⁻².yr⁻¹. Net ecosystem production (NEP) in hardwood stands averaged ~300 g C.m⁻².yr⁻¹. NEP in hemlock-dominated forests averaged ~450 g C.m⁻².yr⁻¹ until infestation by the hemlock woolly adelgid turned these stands into a net C source. Since 2000, NPP has increased by 26%. For the period 1992–2015, NEP increased 93%. The increase in mean annual temperature and growing season length alone accounted for ~30% of the increase in productivity. Interannual variations in GPP and NEP were also correlated with increases in red oak biomass, forest leaf area, and canopy-scale light-use efficiency. Compared to long-term global change experiments at the Harvard Forest, the C sink in regrowing biomass equaled or exceeded C cycle modifications imposed by soil warming, N saturation, and hemlock removal. Results of this synthesis and comparison to simulation models suggest that forests across the region are likely to accrue C for decades to come but may be disrupted if the frequency or severity of biotic and abiotic disturbances increases.

The response of understory trees to climate variability is key to understanding current and future forest dynamics. However, analyses of climatic effects on tree growth have primarily focused on the upper canopy, leaving understory dynamics unresolved. We analyzed differences in climate sensitivity based on canopy position of four common tree species (Acer rubrum, Fagus grandifolia, Quercus rubra, and Tsuga canadensis) using growth information from 1,084 trees across eight sites in the northeastern United States. Effects of canopy position on climate response varied, but were significant and often nonlinear, for all four species. Compared to overstory trees, understory trees showed stronger reductions in growth at high temperatures and varied shifts in precipitation response. This contradicts the prevailing assumption that climate responses, particularly to temperature, of understory trees are buffered by the overstory. Forest growth trajectories are uncertain in compositionally and structurally complex forests, and future demography and regeneration dynamics may be misinferred if not all canopy levels are represented in future forecasts.

Species distribution modelling has revealed shifts in the spatial distribution of the range of eastern hemlock (Tsuga canadensis (L.) Carrière) in eastern North America. Models project a decline in eastern hemlock at the southern portion of its range, but not contraction of the southern boundary. In 2003, the vertical, horizontal, and diameter structure and diameter-age relationships of eastern hemlock were quantified in 10 stands thought to represent the species' 10 southernmost stands on the Cumberland Plateau in Alabama. In 2013, we resurveyed these stands to document changes in stand characteristics over the past decade. In addition, we explored additional reaches of stream corridors known to support eastern hemlock to document additional stands previously undescribed in the literature. Results from our resampling revealed that stands had similar stem frequencies over the 10-yr period, but, generally, the number of canopy stems and the number of seedlings declined. The decline in seedlings may have been a result of mortality or recruitment to larger size classes. The decline in canopy trees may have been caused by regional drought in 2007 or localized severe weather events. Our additional sampling yielded one stand not previously described. Although we cannot rule out additional disjunct stands in the area, we speculate that no eastern hemlock stands occur farther south than those documented here. Based on our results, we suggest that these stands are reproductively viable with episodic regeneration, and there has been no evidence of range contraction at the southern range limit over the past decade.