Background/Question/Methods Global forest diebacks are the beginnings of change that will be controlled by tree migration, which combines two uncertain processes, tree fecundity and recruitment. Knowledge of how, and how fast, tree migration can proceed is critical for adaptive management of forest resources and conservation efforts. The initial stage of seed production is erratic and poorly observed, with most studies limited to few trees of few species on few sites. At the next stage, tree recruitment is typically too sporadic to characterize at landscape scales. Neither seed production nor seedling recruitment have been quantified or linked to climate and habitat variables at scales needed to evaluate the changes happening now or to anticipate the diversity and structure of 21st century forests. As part of the masting inference and forecasting (MASTIF) project, we synthesized continental-scale data for tree fecundity gathered over the last half century and combined it with forest inventories to connect adult trees (basal area) to i) fecundity (seeds per basal area) and ii) recruitment (recruits per seed). A dynamic model fitted to > 107 tree years of fecundity data provided estimates tree-by-year fecundity. A predictive distribution for the continent combines the fitted model with > 105 trees from Forest Inventory Analysis (FIA), Canadian National Forest Inventory (CNFI) and the National Ecological Observatory Network (NEON).
Results/Conclusions Results show continent-wide tree migration as a balance between regional shifts in fecundity that can diverge from conditions that favour establishment, with clear differences in eastern and western North America. In moist eastern states, the geographic centers for fecundity are most commonly displaced south of tree basal area for the same species. This relationship would be expected if optimal conditions for seed production lie to the south of optimal conditions for growth and survival, despite potential benefits of warming poleward. In the dry west and north-central, fecundity is for most species displaced northwest of tree basal area, as would be expected if the high-rainfall NW is predisposed to lead migration as the continent warms. The east-west contrast diminishes at the transition from fecundity to recruits per seed, which tends to be shifted north in both regions. The net continent-wide migration by contrasting east-west controls highlight interactions, with fecundity primed to lead tree migration in the west, and fecundity slowing progress in the east. The possibility of fecundity limitation offers one explanation for migration lag in species expected to track climate warming by expanding poleward.
Interactions that control the pace of forest change in North America
James S. Clark, Statistical Science, Duke University, Durham, NC, Robert A. Andrus, Department of Geography, University of Colorado, Boulder, CO, Mélaine Aubry-Kientz, school of natural sciences, UC Merced, Merced, CA, Yves Bergeron, Forest Research Institute, University of Quebec in Abitibi-Temiscamingue, Rouyn-Noranda, QC, Canada, Michal Bogdziewicz, Department of Systematic Zoology, Adam Mickiewicz University, Poznan, Poland, Don C. Bragg, USDA Forest Service, Southern Research Station, Monticello, AR, Natalie L. Cleavitt, Natural Resources, Cornell University, Ithaca, NY, Susan Cohen, University of North Carolina, Adrian Das, USGS Western Ecological Research Center, Three Rivers, CA, Michael C. Dietze, Earth and Environment, Boston University, Boston, MA, Timothy J. Fahey, Department of Natural Resources, Cornell University, Ithaca, NY, Istem Fer, Finnish Meteorological Institute, Helsinki, MA, Finland, Jerry F. Franklin, School of Environmental and Forest Sciences, University of Washington, Seattle, WA, Cathrine A. Gehring, Biology, Northern Arizona University, Flagstaff, AZ, Gregory S. Gilbert, Environmental Studies, University of California Santa Cruz, Santa Cruz, CA, Cathryn H. Greenberg, Bent Creek Experimental Forest, USDA Forest Service, Southern Research Station, Asheville, NC, Qinfeng Guo, Eastern Forest Environmental Threat Assessment Center, USDA Forest Service – Southern Research Station, Asheville, NC, Janneke HilleRisLambers, Department of Biology, University of Washington, Seattle, WA, Ines Ibanez, University of Michigan, Jill F. Johnstone, Department of Biology, University of Saskatchewan, Saskatoon, SK, Canada, Christopher L. Kilner, Duke University, Durham, NC, Johannes M. H. Knops, School of Biological Sciences, University of Nebraska, Lincoln, Lincoln, NE, Walter D. Koenig, Lab of Ornithology, Cornell University, Ithaca, NY, Jalene M. LaMontagne, Department of Biological Sciences, DePaul University, Chicago, IL, Jordan Luongo, Duke University, James A. Lutz, Department of Wildland Resources, and the Ecology Center, Utah State University, Logan, UT, Diana S. Macias, Department of Biology, University of New Mexico, Albuquerque, NM, Eliot McEntire, Pacific Forestry Centre, Yassine Messaoud, University of Quebec, Christopher M. Moore, Biology, Case Western Reserve University, Cleveland, OH, Emily V. Moran, School of Natural Sciences, UC Merced, Merced, CA, Jonathan A. Myers, Department of Biology, Washington University in St. Louis, St. Louis, MO, Orrin Myers, University of New Mexico, Albuquerque, NM, Chase Nunez, Universität Leipzig, German Centre for Integrative Biodiversity Research (iDiv), Robert R. Parmenter, Valles Caldera National Preserve, National Park Service, Jemez Springs, NM, Ian S. Pearse, United States Geological Survey, Fort Collins, CO, Miranda D. Redmond, Forest and Rangeland Stewardship, Colorado State University, Fort Collins, CO, Chantal D. Reid, Nicholas School of the Environment and Earth Sciences, Duke University, Durham, NC, Kyle Rodman, University of Colorado, Boulder, CO, C. Lane Scher, University Program in Ecology, Duke University, Durham, NC, William H. Schlesinger, Cary Institute of Ecosystem Studies, Millbrook, NY, Amanda M. Schwantes, Ecology & Evolutionary Biology, University of Toronto, Toronto, ON, Canada, Shubhi Sharma, Nicholas School of Environment, Duke University, Durham, NC, Michael A. Steele, Biology Dept., Wilkes University, Wilkes-Barre, PA, Nathan L. Stephenson, Sequoia and Kings Canyon Field Station, United States Geological Survey, Three Rivers, CA, Samantha Sutton, Biological Sciences, Duke University, Durham, NC, Jennifer J. Swenson, Nicholas School of the Environment, Duke University, Durham, NC, Margaret Swift, Nicholas School for the Environment, Duke University, Durham, NC, Thomas T Veblen, Geography, University of Colorado, CO, Amy V. Whipple, Biological Sciences, Northern Arizona University, Flagstaff, AZ, Thomas G. Whitham, Northern Arizona University, Flagstaff, AZ, Andreas P. Wion, Department of Forest and Rangleland Stewardship, Colorado State University, Fort Collins, CO, Kai Zhu, Environmental Studies, University of California, Santa Cruz, Santa Cruz, CA and Roman I. Zlotin, Geography, Indiana University, Bloomington, IN
Background/Question/Methods: Forests can continue to deliver ecosystem services if tree fecundity keeps pace with 21st century climate change. Not only is fecundity unknown at the continental scale, but so too is its rate of change (ROC). Is seed production responding to climate change and, if so, how? At least two forces could be controlling trends in forest regeneration and the capacity to migrate, i) climate change, and ii) geographic variation in stand growth and structure. Effective management will depend on an understanding of both effects. For example, warming during vulnerable spring months might increase reproductive success. If fecundity is limited by thermal energy that coincides with moisture availability, then warming might benefit reproduction only in regions where moisture remains abundant. At the same time, indirect effects could dominate the response if changing growth moves stands into more or less fecund size classes. We combined an unprecedented synthesis of North American fecundity data from the masting inference and forecasting (MASTIF) collaboration with a novel dynamic bio-physical model to infer continental ROC in fecundity. The biophysical model captures the direct and indirect effects of climate change, including the interactions between them (e.g., size differences matter most in dry climates, or, moisture limitation is most important for small trees).
Results/Conclusions: Fecundity, discussed here as seeds/ha/yr, is highest in the midwest (MW) and west (W) owing to high seeds per basal area in the MW and high basal area of trees in the W. However, fecundity is increasing throughout most of the continent, with highest ROC in the north (N) and northwest (NW). Exceptions include parts of the south (S) and southwest (SW). The full effect of climate change is accelerating ROC in the MW and SW both directly and through effects on growth. Two processes are offsetting a positive effect of warming springs. First, a positive effect on growth moves stands into larger, less fecund size classes in western Canada, the Rockies, and much of the East. However, positive effects on growth do not transfer to increased ROC in much of the W and NE. Despite positive ROC throughout most of the continent, climate change is reducing ROC in the East and NW. Taken together, the full effect of climate change on ROC is primarily positive, despite negative indirect effects. We discuss the implications of species-level differences for migration potential and food web dynamics.
