Robert T. Fahey

Quantifying canopy complexity and effects on productivity and resilience in late‐successional hemlock–hardwood forests

The regrowing forests of eastern North America have been an important global C sink over the past 100+ years, but many are now transitioning into late succession. The consequences of this transition are unclear due to uncertainty around the C dynamics of old- growth forests. Canopy structural complexity (CSC) has been shown to be an important source of variability in C dynamics in younger forests (e.g., in productivity and resilience to disturbance), but its role in late-successional forests has not been widely addressed. We investigated patterns of CSC in two old-growth forest landscapes in the Upper Peninsula of Michigan, USA, to assess factors associated with CSC and its influence on productivity and disturbance resilience (to moderate-severity windstorm). CSC was quantified using a portable below-canopy LiDAR (PCL) system in 65 plots that also had long-term (50-70+ years). inventory data, which were used to quantify aboveground net primary productivity (ANPP), disturbance history, and stand characteristics. We found high and variable CSC relative to younger forests across a suite of PCL-derived metrics. Variation in CSC was driven by species composition and size structure, rather than disturbance history or site characteristics. Recent moderate severity wind disturbance decreased plot-scale CSC, but increased stand-scale variation in CSC. The strong positive correlation between CSC and productivity illustrated in younger forests was not present in undisturbed portions of these late-successional ecosystems. Moderate severity disturbance appeared to reestablish the positive link between CSC and productivity, but this relationship was scale and severity dependent. A positive CSC-productivity relationship was evident at the plot scale with low-severity, dispersed disturbance, but only at a patch scale in more severely disturbed areas. CSC does not appear to strongly correlate With variation in productivity in undisturbed old-growth forests, but may play a very important (and scale/severity-dependent) role in their response to disturbance. Understanding potential, drivers and consequences of CSC in late-successional forests will inform management focused on promoting complexity and old-growth conditions, and illustrate potential inipacts of such treatments on regional C dynamics.

Net primary production of a temperate deciduous forest exhibits a threshold response to increasing disturbance severity

The global carbon (C) balance is vulnerable to disturbances that alter terrestrial C storage. Disturbances to forests occur along a continuum of severity, from low-intensity disturbance causing the mortality or defoliation of only a subset of trees to severe stand- replacing disturbance that kills all trees; yet considerable uncertainty remains in how forest production changes across gradients of disturbance intensity. We used a gradient of tree mortality in an upper Great Lakes forest ecosystem to: (1) quantify how aboveground wood net primary production (ANPP,) responds to a range of disturbance severities; and (2) identify mechanisms supporting ANPPw resistance or resilience following moderate disturbance. We found that ANPPw declined nonlinearly with rising disturbance severity, remaining stable until >60% of the total tree basal area senesced. As upper canopy openness increased from disturbance, greater light availability to the subcanopy enhanced the leaf-level photosynthesis and growth of this formerly light-limited canopy stratum, compensating for upper canopy production losses and a reduction in total leaf area index (LAI). As a result, whole-ecosystem production efficiency (ANPPw/LAI) increased with rising disturbance severity, except in plots beyond the disturbance threshold. These findings provide a mechanistic explanation for a nonlinear relationship between ANPPw, and disturbance severity, in which the physiological and growth enhancement of undisturbed vegetation is proportional to the level of disturbance until a threshold is exceeded. Our results have important ecological and management implications, demonstrating that in some ecosystems moderate levels of disturbance minimally alter forest production.

Substantial variation in leaf senescence times among 1360 temperate woody plant species: implications for phenology and ecosystem processes

BACKGROUND AND AIMS Autumn leaf senescence marks the end of the growing season in temperate ecosystems. Its timing influences a number of ecosystem processes, including carbon, water and nutrient cycling. Climate change is altering leaf senescence phenology and, as those changes continue, it will affect individual woody plants, species and ecosystems. In contrast to spring leaf out times, however, leaf senescence times remain relatively understudied. Variation in the phenology of leaf senescence among species and locations is still poorly understood. METHODS Leaf senescence phenology of 1360 deciduous plant species at six temperate botanical gardens in Asia, North America and Europe was recorded in 2012 and 2013. This large data set was used to explore ecological and phylogenetic factors associated with variation in leaf senescence. KEY RESULTS Leaf senescence dates among species varied by 3 months on average across the six locations. Plant species tended to undergo leaf senescence in the same order in the autumns of both years at each location, but the order of senescence was only weakly correlated across sites. Leaf senescence times were not related to spring leaf out times, were not evolutionarily conserved and were only minimally influenced by growth habit, wood anatomy and percentage colour change or leaf drop. These weak patterns of leaf senescence timing contrast with much stronger leaf out patterns from a previous study. CONCLUSIONS The results suggest that, in contrast to the broader temperature effects that determine leaf out times, leaf senescence times are probably determined by a larger or different suite of local environmental effects, including temperature, soil moisture, frost and wind. Determining the importance of these factors for a wide range of species represents the next challenge for understanding how climate change is affecting the end of the growing season and associated ecosystem processes.

Forest Adaptation Resources: climate change tools and approaches for land managers, 2nd edition

Forests across the United States are expected to undergo numerous changes in response to the changing climate. This second edition of the Forest Adaptation Resources provides a collection of resources designed to help forest managers incorporate climate change considerations into management and devise adaptation tactics. It was developed as part of the Climate Change Response Framework and reflects the expertise, creativity, and feedback of dozens of direct contributors and hundreds of users of the first edition over the last several years. Six interrelated chapters include: (1) a description of the overarching Climate Change Response Framework, which generated these resources; (2) a brief guide to help forest managers judge or initiate vulnerability assessments; (3) a “menu” of adaptation strategies and approaches that are directly relevant to forests of the Northeast and upper Midwest; (4) a second menu of adaptation strategies and approaches oriented to urban forests; (5) a workbook process with step-by-step instructions to assist land managers in developing on-theground climate adaptation tactics that address their management objectives; and (6) five real-world examples of how these resources have been used to develop adaptation tactics. The ideas, tools, and resources presented in the different chapters are intended to inform and support existing decisionmaking processes of multiple organizations with diverse management goals. Quality Assurance This publication conforms to the Northern Research Station’s Quality Assurance Implementation Plan which requires technical and policy review for all scientific publications produced or funded by the Station. The process included a blind technical review by at least two reviewers, who were selected by the Assistant Director for Research and unknown to the author. This review policy promotes the Forest Service guiding principles of using the best scientific knowledge, striving for quality and excellence, maintaining high ethical and professional standards, and being responsible and accountable for what we do. Cover Photo A forest containing red pine and northern red oak trees. Photo by Maria Janowiak, U.S. Forest Service and Northern Institute of Applied Climate Science. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. Published by: For additional copies, contact: USDA FOREST SERVICE USDA Forest Service 11 CAMPUS BLVD., SUITE 200 Publications Distribution NEWTOWN SQUARE, PA 19073-3294 359 Main Road Delaware, OH 43015-8640 September 2016 Fax: 740-368-0152 Manuscript received for publication January 2016 Visit our homepage at: http://www.nrs.fs.fed.us/ Forest Adaptation Resources: Climate Change Tools and Approaches for Land Managers, 2nd edition Christopher W. Swanston, Maria K. Janowiak, Leslie A. Brandt, Patricia R. Butler, Stephen D. Handler, P. Danielle Shannon, Abigail Derby Lewis, Kimberly Hall, Robert T. Fahey, Lydia Scott, Angela Kerber, Jason W. Miesbauer, Lindsay Darling, Linda Parker, and Matt St. Pierre

Habitat associations and 150 years of compositional change in white pine-hemlock-hardwood forests based on resurvey of public land survey corners1

Abstract Recent studies have demonstrated substantial changes in the composition and structure of forests in northeastern North America following Euro-American settlement. Especially striking are the “homogenization” of tree species composition across the region and a steep decline in conifer abundance. However, the presettlement distribution and subsequent changes for individual species at a finer scale are not well documented. In this study, we examine the presettlement and current distribution of Pinus strobus L. (eastern white pine) on specific habitats in the northern hardwood forest of Wisconsin by re-surveying 201 nineteenth-century land survey corners. The status of P. strobus regeneration was evaluated in the second-growth forests, as well as in a set of remnant old-growth stands on comparable sites. P. strobus occurred across a broad range of habitats in the presettlement landscape, including highly productive, mesic sites where it rarely occurs in the modern forest. Former white pine-hemlock-hardwood forests most often transitioned to Acer or Populus dominance, matching compositional transitions of the overall landscape. Pinus strobus was retained at ∼28% of presettlement locations, but a large majority of these were dry-mesic or lower-productivity mesic/wet-mesic sites. The modern distribution of P. strobus in hemlock-hardwood forests appears to be strongly influenced by historical factors, rather than site limitations. Pinus strobus regeneration was rare across habitats in the modern landscape; thus, the habitat differences that led to differential retention may not be as influential on its future distribution. Our findings indicate that restoration of P. strobus to a wide variety of habitats, including high-productivity mesic sites, is supported by historical information and that active restoration is probably necessary across a spectrum of habitats.

Chicago Wilderness region urban forest vulnerability assessment and synthesis: a report from the Urban Forestry Climate Change Response Framework Chicago Wilderness pilot project

The urban forest of the Chicago Wilderness region, a 7-million-acre area covering portions of Illinois, Indiana, Michigan, and Wisconsin, will face direct and indirect impacts from a changing climate over the 21st century. This assessment evaluates the vulnerability of urban trees and natural and developed landscapes within the Chicago Wilderness region to a range of future climates. We synthesized and summarized information on the contemporary landscape, provided information on past climate trends, and illustrated a range of projected future climates. We used this information to inform models of habitat suitability for trees native to the area. Projected shifts in plant hardiness and heat zones were used to understand how nonnative species and cultivars may tolerate future conditions. We also assessed the adaptability of planted and naturally occurring trees to stressors that may not be accounted for in habitat suitability models such as drought, flooding, wind damage, and air pollution. The summary of the contemporary landscape identifies major stressors currently threatening the urban forest of the Chicago Wilderness region. Major current threats to the region?s urban forest include invasive species, pests and disease, land-use change, development, and fragmentation. Observed trends in climate over the historical record from 1901 through 2011 show a temperature increase of 1 ?F in the Chicago Wilderness region. Precipitation increased as well, especially during the summer. Mean annual temperature is projected to increase by 2.3 to 8.2 ?F by the end of the century, with temperature increases across all seasons. Projections for precipitation show an increase in winter and spring precipitation, and summer and fall precipitation projections vary by model. Species distribution modeling for native species suggests that suitable habitat may decrease for 11 primarily northern species and increase or become newly suitable for 40 species. An analysis of tree species vulnerability that combines model projections, shifts in hardiness and heat zones, and adaptive capacity showed that 15 percent of the trees currently present in the region have either moderate-high or high vulnerability to climate change, and many of those trees with low vulnerability are invasive species. We developed a process for self-assessment of urban forest vulnerability that was tested by urban forestry professionals from four municipalities, three park districts, and three forest preserve districts in the region. The professionals generally rated the impacts of climate change on the places they managed as moderately negative, mostly driven by the potential effects of extreme storms and heavy precipitation on trees in the area. The capacity of forests to adapt to climate change ranged widely based on economic, social, and organizational factors, as well as on the diversity of species and genotypes of trees in the area. These projected changes in climate and their associated impacts and vulnerabilities will have important implications for urban forest management, including the planting and maintenance of street and park trees, management of natural areas, and long-term planning. will have important implications for urban forest management, including the planting and maintenance of street and park trees, management of natural areas, and long-term planning.

Effects of canopy structure and species diversity on primary production in upper Great Lakes forests

Canopy structure and tree species diversity, shaped by succession, disturbance, and community composition, are linked to numerous ecosystem functions, including net primary production (NPP). Understanding of how ecosystem structural metrics are interrelated and mechanistically link to NPP, however, is incomplete. We characterized leaf area index (LAI), Simpson’s index of Diversity (D′, a measure of species diversity), and canopy rugosity (Rc, a measure of canopy physical complexity) in 11 forest stands comprising two chronosequences varying in establishing disturbance, and in three late successional communities. We related LAI, D′, and Rc to wood NPP (NPPw), and examined whether absorption of photosynthetically active radiation and light use-efficiency (LUE) link NPPw with ecosystem structure. We found that recovery of LAI and D′ was delayed following more severe establishing disturbances, but that the development of Rc was strikingly conserved regardless of disturbance, converging on a common mean value in late-successional stands irrespective of differences in leaf area index and species diversity. LAI was significantly correlated with NPPw in each stage of ecosystem development, but NPPw was only correlated with Rc in early successional stages and with D′ in late successional stages. Across all stands, NPPw was coupled with LAI and Rc, (but not D′) through positive relationships with light absorption and LUE. We conclude by advocating for better integration of ecological disciplines investigating structure–function interactions, suggesting that improved understanding of such relationships will require ecologists to traverse disciplinary boundaries.