Anthony W. D'Amato

Multi-year ecosystem response to hemlock woolly adelgid infestation in southern New England forests

The introduced hemlock woolly adelgid (HWA) (Adelges tsugae Annand) has generated widespread tree decline and substantial mortality of eastern hemlock (Tsuga canadensis (L.) Carriere) throughout th...

Effects of thinning on drought vulnerability and climate response in north temperate forest ecosystems

Reducing tree densities through silvicultural thinning has been widely advocated as a strategy for enhancing resistance and resilience to drought, yet few empirical evaluations of this approach exist. We examined detailed dendrochronological data from a long-term (> 50 years) replicated thinning experiment to determine if density reductions conferred greater resistance and/or resilience to droughts, assessed by the magnitude of stand-level growth reductions. Our results suggest that thinning generally enhanced drought resistance and resilience; however, this relationship showed a pronounced reversal over time in stands maintained at lower tree densities. Specifically, lower-density stands exhibited greater resistance and resilience at younger ages (49 years), yet exhibited lower resistance and resilience at older ages (76 years), relative to higher-density stands. We attribute this reversal to significantly greater tree sizes attained within the lower-density stands through stand development, which in turn increased tree-level water demand during the later droughts. Results from response-function analyses indicate that thinning altered growth-climate relationships, such that higher-density stands were more sensitive to growing-season precipitation relative to lower-density stands. These results confirm the potential of density management to moderate drought impacts on growth, and they highlight the importance of accounting for stand structure when predicting climate-change impacts to forests.

The impacts of increasing drought on forest dynamics, structure, and biodiversity in the United States

We synthesize insights from current understanding of drought impacts at stand-to-biogeographic scales, including management options, and we identify challenges to be addressed with new research. Large stand-level shifts underway in western forests already are showing the importance of interactions involving drought, insects, and fire. Diebacks, changes in composition and structure, and shifting range limits are widely observed. In the eastern US, the effects of increasing drought are becoming better understood at the level of individual trees, but this knowledge cannot yet be confidently translated to predictions of changing structure and diversity of forest stands. While eastern forests have not experienced the types of changes seen in western forests in recent decades, they too are vulnerable to drought and could experience significant changes with increased severity, frequency, or duration in drought. Throughout the continental United States, the combination of projected large climate-induced shifts in suitable habitat from modeling studies and limited potential for the rapid migration of tree populations suggests that changing tree and forest biogeography could substantially lag habitat shifts already underway. Forest management practices can partially ameliorate drought impacts through reductions in stand density, selection of drought-tolerant species and genotypes, artificial regeneration, and the development of multistructured stands. However, silvicultural treatments also could exacerbate drought impacts unless implemented with careful attention to site and stand characteristics. Gaps in our understanding should motivate new research on the effects of interactions involving climate and other species at the stand scale and how interactions and multiple responses are represented in models. This assessment indicates that, without a stronger empirical basis for drought impacts at the stand scale, more complex models may provide limited guidance.

The influence of successional processes and disturbance on the structure of Tsuga canadensis forests.

Old-growth forests are valuable sources of ecological, conservation, and management information, yet these ecosystems have received little study in New England, due in large part to their regional scarcity. To increase our understanding of the structures and processes common in these rare forests, we studied the abundance of downed coarse woody debris (CWD) and snags and live-tree size-class distributions in 16 old-growth hemlock forests in western Massachusetts. Old-growth stands were compared with eight adjacent second-growth hemlock forests to gain a better understanding of the structural differences between these two classes of forests resulting from contrasting histories. In addition, we used stand-level dendroecological reconstructions to investigate the linkages between disturbance history and old-growth forest structure using an information-theoretic model selection framework. Old-growth stands exhibit a much higher degree of structural complexity than second-growth forests. In particular, old-growth stands had larger overstory trees and greater volumes of downed coarse woody debris (135.2 vs. 33.2 m3/ha) and snags (21.2 vs. 10.7 m3/ha). Second-growth stands were characterized by either skewed unimodal or reverse-J shaped diameter distributions, while old-growth forests contained bell-shaped, skewed unimodal, rotated sigmoid, and reverse J-shaped distributions. The variation in structural attributes among old-growth stands, particularly the abundance of downed CWD, was closely related to disturbance history. In particular, old-growth stands experiencing moderate levels of canopy disturbance during the last century (1930s and 1980s) had greater accumulations of CWD, highlighting the importance of gap-scale disturbances in shaping the long-term development and structural characteristics of old-growth forests. These findings are important for the development of natural disturbance-based silvicultural systems that may be used to restore important forest characteristics lacking in New England second-growth stands by integrating structural legacies of disturbance (e.g., downed CWD) and resultant tree-size distribution patterns. This silvicultural approach would emulate the often episodic nature of CWD recruitment within old-growth forests.

Recognizing trade-offs in multi-objective land management

As natural resource management and conservation goals expand and evolve, practitioners and policy makers are increasingly seeking options that optimize benefits among multiple, often contradictory objectives. Here, we describe a simple approach for quantifying the consequences of alternative management options in terms of benefits and trade-offs among multiple objectives. We examine two long-term forest management experiments that span several decades of stand (forest tree community) development and identify substantial trade-offs among carbon cycling and ecological complexity objectives. In addition to providing improved understanding of the long-term consequences of various management options, the results of these experiments show that positive benefits resulting from some management options are often associated with large trade-offs among individual objectives. The approach to understanding benefits and trade-offs presented here provides a simple yet flexible framework for quantitatively assessing the ...

STAND AND LANDSCAPE‐LEVEL DISTURBANCE DYNAMICS IN OLD‐GROWTH FORESTS IN WESTERN MASSACHUSETTS

Natural disturbances strongly influence the dynamics and developmental patterns of forest ecosystems; however, relatively little is known about the historic patterns of natural disturbance for many portions of eastern North America, such as southern New England, where human disturbance has predominated for centuries. For these regions, much of our understanding of natural disturbance dynamics comes from studies of younger second- growth forests or isolated old-growth stands, thus limiting the temporal and spatial resolution of our knowledge of dynamics in these regions. To address these limitations, we analyzed dendroecological data from the 18 largest remaining old-growth stands in western Massachusetts, ranging in proximity from 1-60 km apart, in an effort to characterize the historic stand and landscape-level patterns of natural disturbance. Our results indicate that disturbance regimes for these systems were dominated by relatively frequent, low-intensity disturbances (average 5.0% 6 0.2% canopy area disturbed per decade) operating somewhat randomly on the landscape. Across the study areas, most decadal disturbances (86.2%) involved ,10% canopy loss. There was no evidence of stand-replacing disturbances during the period examined (1700-1989), and the maximum canopy area disturbed in any given decade was 26.3%. Nonmetric multidimensional scaling demonstrated that several forests shared similar disturbance histories despite being separated by .50 km. Comparisons of these patterns with model simulations of past hurricane events and historical documents suggest that broadscale disturbances, such as hurricanes and ice storms, resulted in common disturbance peaks and subsequent recruitment peaks at spatially disparate areas in the 1790s, 1870s, 1900s, and 1920s. Conversely, the lack of synchrony in proximate areas during these events highlights the patchy nature of these disturbances on the landscape. Compositional and physiographic factors influenced disturbance patterns, as stands located on northwest-facing slopes or containing significant Picea rubens components in the forest overstory experienced the highest levels of disturbance. Our results highlight the utility of incorporating dendroecological reconstructions across numerous old-growth stands to interpret the historic stand and landscape-level disturbance dynamics in areas devoid of large, contiguous old- growth landscapes.

Climate remains an important driver of post‐European vegetation change in the eastern United States

Department of Geography, University of Wisconsin-Madison, 550 North ParkStreet, Madison, WI 53706, USAThe influence of climate on forest change during thepast century in the eastern United States was evalu-ated in a recent paper (Nowacki & Abrams, 2014)that centers on an increase in ‘highly competitivemesophytic hardwoods’ (Nowacki & Abrams, 2008)and a concomitant decrease in the more xerophyticQuercus species. Nowacki & Abrams (2014) con-cluded that climate change has not contributed sig-nificantly to observed changes in forest composition.However, the authors restrict their focus to a singleelement of climate: increasing temperature since theend of the Little Ice Age ca. 150 years ago. In theirstudy, species were binned into four classifications(e.g., Acer saccharum – ‘cool-adapted’, Acer rubrum –‘warm-adapted’) based on average annual tempera-ture within each species range in the United States,reducing the multifaceted character of climate into asingle, categorical measure. The broad temperatureclasses not only veil the many biologically relevantaspects of temperature (e.g., seasonal and extremetemperatures) but they may also mask other influ-ences, both climatic (e.g., moisture sensitivity) andnonclimatic (e.g., competition).Understanding the primary drivers of forest changeis critically important. However, using annual tem-perature reduces the broad spectrum of climaticinfluence on forests (e.g., Jackson & Overpeck, 2000;Jackson et al., 2009) to a single variable. Tsuga canad-ensis illustrates one example of the complex interac-tion between trees and temperature. In the southernpart of its range, Tsuga canadensis growth is weakly,but positively correlated with early growing-seasontemperature. However, this relationship becomesstronger and shifts to later in the season toward thenorthern part of its range (Cook & Cole, 1991). More-over, Tsuga canadensis growth is significantly andnegatively correlated with just May temperaturesduring the current growing season in the northeast-ern United States (Cook, 1991; Cook & Cole, 1991;Vaganov et al., 2011), while in the southeastern Uni-ted States it is strongly and negatively correlatedwith summer (June–August) temperatures (Hart et al.,2010). Trees can also be sensitive to diverse and ofteninteracting climate variables at various stages of theirlife cycles (Jackson et al., 2009). Interactions betweenprecipitation and temperature are clearly important(Harsch & Hille Ris Lambers, 2014; Martin-Benito &Pederson, accepted), and often lead to counterintui-tive responses. For example, some plant species thatwould have been expected to move north and ups-

The efficacy of salvage logging in reducing subsequent fire severity in conifer-dominated forests of Minnesota, USA

Although primarily used to mitigate economic losses following disturbance, salvage logging has also been justified on the basis of reducing fire risk and fire severity; however, its ability to achieve these secondary objectives remains unclear. The patchiness resulting from a sequence of recent disturbances-blowdown, salvage logging, and wildfire-provided an excellent opportunity to assess the impacts of blowdown and salvage logging on wildfire severity. We used two fire-severity assessments (tree-crown and forest-floor characteristics) to compare post-wildfire conditions among three treatment combinations (Blowdown-Salvage-Fire, Blowdown-Fire, and Fire only). Our results suggest that salvage logging reduced the intensity (heat released) of the subsequent fire. However, its effect on severity (impact to the system) differed between the tree crowns and forest floor: tree-crown indices suggest that salvage logging decreased fire severity (albeit with modest statistical support), while forest-floor indices suggest that salvage logging increased fire severity. We attribute the latter finding to the greater exposure of mineral soil caused by logging operations; once exposed, soils are more likely to register the damaging effects of fire, even if fire intensity is not extreme. These results highlight the important distinction between fire intensity and severity when formulating post-disturbance management prescriptions.

Density‐dependent vulnerability of forest ecosystems to drought

Climate models predict increasing drought intensity and frequency for many regions, which may have negative consequences for tree recruitment, growth and mortality, as well as forest ecosystem services. Furthermore, practical strategies for minimizing vulnerability to drought are limited. Tree population density, a metric of tree abundance in a given area, is a primary driver of competitive intensity among trees, which influences tree growth and mortality. Manipulating tree population density may be a mechanism for moderating drought-induced stress and growth reductions, although the relationship between tree population density and tree drought vulnerability remains poorly quantified, especially across climatic gradients. In this study, we examined three long-term forest ecosystem experiments in two widely distributed North American pine species, ponderosa pine Pinus ponderosa (Lawson & C. Lawson) and red pine Pinus resinosa (Aiton), to better elucidate the relationship between tree population density, growth and drought. These experiments span a broad latitude and aridity range and include tree population density treatments that have been purposefully maintained for several decades. We investigated how tree population density influenced resistance (growth during drought) and resilience (growth after drought compared to pre-drought growth) of stand-level growth during and after documented drought events. Our results show that relative tree population density was negatively related to drought resistance and resilience, indicating that trees growing at lower densities were less vulnerable to drought. This result was apparent in all three forest ecosystems, and was consistent across species, stand age and drought intensity. Synthesis and applications. Our results highlighted that managing pine forest ecosystems at low tree population density represents a promising adaptive strategy for reducing the adverse impacts of drought on forest growth in coming decades. Nonetheless, the broader applicability of our findings to other types of forest ecosystems merits additional investigation.

Growth, yield, and structure of extended rotation Pinus resinosa stands in Minnesota, USA.

Extended rotations are increasingly used to meet ecological objectives on forestland; however, information about long-term growth and yield of these systems is lacking for most forests in North Ame...