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We review and synthesize information on invasions of nonnative forest insects and diseases in the United States, including their ecological and economic impacts, pathways of arrival, distribution within the United States, and policy options for reducing future invasions. Nonnative insects have accumulated in United States forests at a rate of

In humid, broadleaf-dominated forests where gap dynamics and partial canopy mortality appears to dominate the disturbance regime at local scales, paleoecological evidence shows alteration at regional-scales associated with climatic change. Yet, little evidence of these broad-scale events exists in extant forests. To evaluate the potential for the occurrence of large-scale disturbance, we used 76 tree-ring collections spanning ∼840 000 km 2 and 5327 tree recruitment dates spanning ∼1.4 million km 2 across the humid eastern United States. Rotated principal component analysis indicated a common growth pattern of a simultaneous reduction in competition in 22 populations across 61 000 km 2 . Growth-release analysis of these populations reveals an intense and coherent canopy disturbance from 1775 to 1780, peaking in 1776. The resulting time series of canopy disturbance is so poorly described by a Gaussian distribution that it can be described as \"heavy tailed,\" with most of the years from 1775 to 1780 comprising the heavy-tail portion of the distribution. Historical documents provide no evidence that hurricanes or ice storms triggered the 1775-1780 event. Instead, we identify a significant relationship between prior drought and years with elevated rates of disturbance with an intense drought occurring from 1772 to 1775. We further find that years with high rates of canopy disturbance have a propensity to create larger canopy gaps indicating repeated opportunities for rapid change in species composition beyond the landscape scale. Evidence of elevated, regional-scale disturbance reveals how rare events can potentially alter system trajectory: a substantial portion of old-growth forests examined here originated or were substantially altered more than two centuries ago following events lasting just a few years. Our recruitment data, comprised of at least 21 species and several shade-intolerant species, document a pulse of tree recruitment at the subcontinental scale during the late-1600s suggesting that this event was severe enough to open large canopy gaps. These disturbances and their climatic drivers support the hypothesis that punctuated, episodic, climatic events impart a legacy in broadleaf-dominated forests centuries after their occurrence. Given projections of future drought, these results also reveal the potential for abrupt, meso- to large-scale forest change in broadleaf-dominated forests over future decades.

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.

The ability to automatically delineate individual tree crowns using remote sensing data opens the possibility to collect detailed tree information over large geographic regions. While individual tree crown delineation (ITCD) methods have proven successful in conifer-dominated forests using Light Detection and Ranging (LiDAR) data, it remains unclear how well these methods can be applied in deciduous broadleaf-dominated forests. We applied five automated LiDAR-based ITCD methods across fifteen plots ranging from conifer- to broadleaf-dominated forest stands at Harvard Forest in Petersham, MA, USA, and assessed accuracy against manual delineation of crowns from unmanned aerial vehicle (UAV) imagery. We then identified tree- and plot-level factors influencing the success of automated delineation techniques. There was relatively little difference in accuracy between automated crown delineation methods (51–59% aggregated plot accuracy) and, despite parameter tuning, none of the methods produced high accuracy across all plots (27—90% range in plot-level accuracy). The accuracy of all methods was significantly higher with increased plot conifer fraction, and individual conifer trees were identified with higher accuracy (mean 64%) than broadleaf trees (42%) across methods. Further, while tree-level factors (e.g., diameter at breast height, height and crown area) strongly influenced the success of crown delineations, the influence of plot-level factors varied. The most important plot-level factor was species evenness, a metric of relative species abundance that is related to both conifer fraction and the degree to which trees can fill canopy space. As species evenness decreased (e.g., high conifer fraction and less efficient filling of canopy space), the probability of successful delineation increased. Overall, our work suggests that the tested LiDAR-based ITCD methods perform equally well in a mixed temperate forest, but that delineation success is driven by forest characteristics like functional group, tree size, diversity, and crown architecture. While LiDAR-based ITCD methods are well suited for stands with distinct canopy structure, we suggest that future work explore the integration of phenology and spectral characteristics with existing LiDAR as an approach to improve crown delineation in broadleaf-dominated stands.

The nonnative hemlock woolly adelgid is steadily killing eastern hemlock trees in many parts of eastern North America. We summarize impacts of the adelgid on these forest foundation species; review previous models and analyses of adelgid spread dynamics; and examine how previous forecasts of adelgid spread and ecosystem dynamics compare with current conditions. The adelgid has reset successional sequences, homogenized biological diversity at landscape scales, altered hydrological dynamics, and changed forest stands from carbon sinks into carbon sources. A new model better predicts spread of the adelgid in the south and west of the range of hemlock, but still under-predicts its spread in the north and east. Whether these underpredictions result from inadequately modeling accelerating climate change or accounting for people inadvertently moving the adelgid into new locales needs further study. Ecosystem models of adelgid-driven hemlock dynamics have consistently forecast that forest carbon stocks will be little affected by the shift from hemlock to early-successional mixed hardwood stands, but these forecasts have assumed that the intermediate stages will remain carbon sinks. New forecasting models of adelgid-driven hemlock decline should account for observed abrupt changes in carbon flux and ongoing and accelerating human-driven land-use and climatic changes.

The study of vegetation community and structural change has been central to ecology for over a century, yet the ways in which disturbances reshape the physical structure of forest canopies remain relatively unknown. Moderate severity disturbances affect different canopy strata and plant species, resulting in variable structural outcomes and ecological consequences. Terrestrial lidar (light detection and ranging) offers an unprecedented view of the interior arrangement and distribution of canopy elements, permitting the derivation of multidimensional measures of canopy structure that describe several canopy structural traits (CSTs) with known links to ecosystem function. We used lidar‐derived CSTs within a machine learning framework to detect and describe the structural changes that result from various disturbance agents, including moderate severity fire, ice storm damage, age‐related senescence, hemlock woolly adelgid, beech bark disease, and chronic acidification. We found that fire and ice storms primarily affected the amount and position of vegetation within canopies, while acidification, senescence, pathogen, and insect infestation altered canopy arrangement and complexity. Only two of the six disturbance agents significantly reduced leaf area, counter to common assumptions regarding many moderate severity disturbances. While findings are limited in their generalizability due to lack of replication among disturbances, they do suggest that the current limitations of standard disturbance detection methods—such as optical‐based remote sensing platforms, which are often above‐canopy perspectives—limit our ability to understand the full ecological and structural impacts of disturbance, and to evaluate the consistency of structural patterns within and among disturbance agents. A more broadly inclusive definition of ecological disturbance that incorporates multiple aspects of canopy structural change may potentially improve the modeling, detection, and prediction of functional implications of moderate severity disturbance as well as broaden our understanding of the ecological impacts of disturbance.

The development of old-growth forests in northeastern North America has largely been within the context of gap-scale disturbances given the rarity of stand-replacing disturbances. Using the 10-ha old-growth Harvard Tract and its associated 90-year history of measurements, including detailed surveys in 1989 and 2009, we document the long-term structural and biomass development of an old-growth Tsuga canadensis–Pinus strobus forest in southern New Hampshire, USA following a stand-replacing hurricane in 1938. Measurements of aboveground biomass pools were integrated with data from second- and old-growth T. canadensis forests to evaluate long-term patterns in biomass development following this disturbance. Ecosystem structure across the Tract prior to the hurricane exhibited a high degree of spatial heterogeneity with the greatest levels of live tree basal area (70–129 m2/ha) on upper west-facing slopes where P. strobus was dominant and intermixed with T. canadensis. Live-tree biomass estimates for these stratified mixtures ranged from 159 to 503 Mg/ha at the localized, plot scale (100 m2) and averaged 367 Mg/ha across these portions of the landscape approaching the upper bounds for eastern forests. Live-tree biomass 71 years after the hurricane is more uniform and lower in magnitude, with T. canadensis currently the dominant overstory tree species throughout much of the landscape. Despite only one living P. strobus stem in the 2009 plots (and fewer than five stems known across the entire 10-ha area), the detrital legacy of this species is pronounced with localized accumulations of coarse woody debris exceeding 237.7–404.2 m3/ha where this species once dominated the canopy. These patterns underscore the great sizes P. strobus attained in pre-European landscapes and its great decay resistance relative to its forest associates. Total aboveground biomass pools in this 71-year-old forest (255 Mg/ha) are comparable to those in modern old-growth ecosystems in the region that also lack abundant white pine. Results highlight the importance of disturbance legacies in affecting forest structural conditions over extended periods following stand-replacing events and underscore that post-disturbance salvage logging can alter ecosystem development for decades. Moreover, the dominant role of old-growth P. strobus in live and detrital biomass pools before and after the hurricane, respectively, demonstrate the disproportionate influence this species likely had on carbon storage at localized scales prior to the widespread, selective harvesting of large P. strobus across the region in the 18th and 19th centuries.

The duration of tree seedling persistence in the understory varies greatly between forests and across environmental conditions within a forest ecosystem. To examine species‐level variation in understory persistence and passage to the sapling life stage, we followed 5236 seedlings in single‐tree canopy gaps and closed canopy conditions over three years and simulated seedling passage times and the number of seedlings required to produce one 1.5‐m tall sapling of five common tree species in a hemlock–hardwood forest of Massachusetts, USA. Averaged across species, it took 26 years in gaps and 31 years under closed canopies to go from a first‐year seedling to a 1.5‐m sapling. Across species, the average number of seedlings needed for one sapling was 294 in gaps and 2674 in closed canopy environments. We observed high interspecific variation in passage times and number required for one sapling. Betula congeners and Pinus strobus took less time and significantly fewer individuals than Acer rubrum and Tsuga canadensis, which are generally regarded as more tolerant of understory conditions. The largest intraspecific difference in gaps versus closed canopy environments was for Quercus rubra, where we estimated the number of seedlings required to produce one sapling in closed canopies to be 172 times higher than in gaps. Stem breakage also increased the number of seedlings needed per sapling, especially in closed canopy environments. We evaluated our estimates in the lab by aging cross‐sections obtained from seedlings in gap and closed canopy conditions. Compared to our empirical age‐to‐height relationships, most simulations tended to underpredict seedling age for a given height, suggesting that passage times may be even longer than our simulations indicated. Our study shows that trees can persist for decades in the seedling life stage, highlighting a need for better‐parameterized recruitment processes in demographic forecasting.

Land-use history is the template upon which contemporary plant and tree populations establish and interact with one another and exerts a legacy on the structure and dynamics of species assemblages and ecosystems. We use the first census (2010–2014) of a 35-ha forest-dynamics plot at the Harvard Forest in central Massachusetts to describe the composition and structure of the woody plants in this plot, assess their spatial associations within and among the dominant species using univariate and bivariate spatial point-pattern analysis, and examine the interactions between land-use history and ecological processes. The plot includes 108,632 live stems ≥ 1 cm in diameter (2,215 individuals/ha) and 7,595 standing dead stems ≥ 5 cm in diameter. Live tree basal area averaged 42.25 m 2 /ha, of which 84% was represented by Tsuga canadensis (14.0 m 2 / ha), Quercus rubra (northern red oak; 9.6 m2/ ha), Acer rubrum (7.2 m 2 / ha) and Pinus strobus (eastern white pine; 4.4 m 2 / ha). These same four species also comprised 78% of the live aboveground biomass, which averaged 245.2 Mg/ ha. Across all species and size classes, the forest contains a preponderance (> 80,000) of small stems (<10-cm diameter) that exhibit a reverse-J size distribution. Significant spatial clustering of abundant overstory species was observed at all spatial scales examined. Spatial distributions of A. rubrum and Q. rubra showed negative intraspecific correlations in diameters up to at least a 150-m spatial lag, likely indicative of crowding effects in dense forest patches following intensive past land use. Bivariate marked point-pattern analysis, showed that T. canadensis and Q. rubra diameters were negatively associated with one another, indicating resource competition for light. Distribution and abundance of the common overstory species are predicted best by soil type, tree neighborhood effects, and two aspects of land-use history: when fields were abandoned in the late 19th century and the succeeding forest types recorded in 1908. In contrast, a history of intensive logging prior to 1950 and a damaging hurricane in 1938 appear to have had little effect on the distribution and abundance of present-day tree species. Our findings suggest that current day composition and structure are still being influenced by anthropogenic disturbances that occurred over a century ago.

In the forests of northeastern North America, invasive insects and pathogens are causing major declines in some tree species and a subsequent reorganization of associated forest communities. Using observations and experiments to investigate the consequences of such declines are hampered because trees are long‐lived. Simulation models can provide a means to forecast possible futures based on different scenarios of tree species decline, death, and removal. Such modeling is particularly urgent for species such as eastern hemlock (Tsuga canadensis), a foundation species in many northeastern forest regions that is declining due to the hemlock woolly adelgid (Adelges tsugae). Here, we used an individual‐based forest simulator, SORTIE‐ND, to forecast changes in forest communities in Central Massachusetts over the next 200 yr under a range of scenarios: a no‐adelgid, status quo scenario; partial resistance of hemlock to the adelgid; adelgid irruption and total hemlock decline over 25 yr, adelgid irruption and salvage logging of hemlock trees; and two scenarios of preemptive logging of hemlock and hemlock/white pine. We applied the model to six study plots comprising a range of initial species mixtures, abundances, and levels of hemlock dominance. Simulations indicated that eastern white pine, and to a lesser extent black birch and American beech, would gain most in relative abundance and basal area following hemlock decline. The relative dominance of these species depended on initial conditions and the amount of hemlock mortality, and their combined effect on neighborhood‐scale community dynamics. Simulated outcomes were little different whether hemlock died out gradually due to the adelgid or disappeared rapidly following logging. However, if eastern hemlock were to become partially resistant to the adelgid, hemlock would be able to retain its dominance despite substantial losses of basal area. Our modeling highlights the complexities associated with secondary forest succession due to ongoing hemlock decline and loss. We emphasize the need both for a precautionary approach in deciding between management intervention or simply doing nothing in these declining hemlock forests, and for clear aims and understanding regarding desired community‐ and ecosystem‐level outcomes.