J. For. 115(3):190-201 RESEARCH ARTICLE https://doi.org/10.5849/jof.2016-024 silviculture Managing Hardwood-Softwood Mixtures for Future Forests in Eastern North America: Assessing Suitability to Projected Climate Change John M. Kabrick, Kenneth L. Clark, Anthony W. D’Amato, Daniel C. Dey, Laura S. Kenefic, Christel C. Kern, Benjamin O. Knapp, David A. MacLean, Patricia Raymond, and Justin D. Waskiewicz Despite growing interest in management strategies for climate change adaptation, there are few methods for assessing 1998). In temperate forests of eastern North the ability of stands to endure or adapt to projected future climates. We developed a means for assigning climate America, naturally occurring mixedwoods “Compatibility” and “Adaptability” scores to stands for assessing the suitability of tree species for projected climate are found in the pine-oak (Pinus-Quercus), scenarios. We used these scores to determine whether mixed hardwood-softwood stands or “mixedwoods” were better hemlock-hardwood (Tsuga-hardwood), and suited to projected future climates than pure hardwood or pure softwood stands. We also examined the quantity of spruce-fir-hardwood (Picea-Abies-hardwood) aboveground carbon (C) sequestered in the overstory of these mixtures. In the four different mixedwood types that types (Table 1; Figure 1 ). Mixedwoods can we examined, we found that Pinus echinata-Quercus mixtures in the Ozark Highlands had greater Compatibility scores occur as isolated stands within hardwood- or than hardwood stands and greater Adaptability scores than pure Pinus echinata stands; however, these mixtures did softwood-dominated landscapes or they can not store more aboveground overstory C than pure stands. For Pinus strobus-Quercus rubra, Picea-Abies-hardwood, and comprise a large proportion of a forest land- Tsuga canadensis-hardwood mixtures, scores indicated that there were no advantages or disadvantages related to scape. climate compatibility. Those mixtures generally had greater Adaptability scores than their pure softwood analogs but Mixedwood stands are often structur- stored less aboveground overstory C. Despite the many benefits of maintaining mixedwoods, regenerating and/or ally complex and vertically stratified because recruiting the softwood component of these mixtures remains a persistent silvicultural challenge. individual species of hardwoods and soft- woods have differing shade tolerances, Keywords: hardwood-softwood mixtures, climate change adaptation, forest management, aboveground growth rates, longevities, phenology, and overstory carbon crown and root structure (Kelty et al. 1992, Pre´vost 2008, Pretzsch 2014). Moreover, species within mixedwoods often employ “ ixedwoods” are stands contain- rouche et al. 2013, Leak et al. 2014), we use differing regeneration and growth strategies. ing hardwoods and softwoods. the term to describe stands in which neither Because of this structural and compositional M Although variations in the defi- component comprises more than approxi- complexity (Kelty et al. 1992), there has nition occur (e.g., Sauvageau 1995, La- mately 75–80% of the composition (Helms long been interest in the benefits of mixed- Received July 27, 2016; accepted December 12, 2016; published online January 19, 2017. Affiliations: John Kabrick ([email protected]), USDA Forest Service, Northern Research Station, Columbia, MO. Kenneth L. Clark ([email protected]), USDA Forest Service, Northern Research Station. Anthony W. D’Amato ([email protected]), University of Vermont. Daniel C. Dey ([email protected]), USDA Forest Service, Northern Research Station. Laura S. Kenefic (lkenefi[email protected]), USDA Forest Service, Northern Research Station. Christel C. Kern ([email protected]), USDA Forest Service, Northern Research Station. Benjamin O. Knapp ([email protected]), University of Missouri. David A. MacLean ([email protected]), University of New Brunswick. Patricia Raymond ([email protected]), Gouvernement du Que´bec, Ministe`re des Foreˆts, de la Faune et des Parcs, Direction de la Recherche Forestie`re. Justin D. Waskiewicz ([email protected]), University of Vermont. Acknowledgments: We express our appreciation to Maria Janowiak and Louis Iverson for their comments and feedback about our approach and methods for assessing climate compatibility and adaptability described in this article. We thank Rachel Knapp for preparing the spruce-fir-hardwood data from the Penobscot Experimental Forest and Kenneth Laustsen (Maine Forest Service) for his assistance with spruce-fir-hardwood assessment. We also thank two anonymous reviewers and the associate editor for helpful suggestions. We are appreciative of Thomas L. Schmidt and John C. Brissette, USDA Forest Service, Northern Research Station, for supporting this research. 190 Journal of Forestry • May 2017 Table 1. Summary of selected mixedwood types in eastern North America. Common Mixedwood type Hardwood component Softwood component Approximate Extent disturbances Stand structure Shortleaf pine-oak Quercus alba, Quercus coccinea, Pinus echinata Ͼ1 million ha in Ozark and Frequent surface fire; Range from complex Quercus falcata, Quercus southern Appalachian historical and vertical structure rubra, Quercus stellata, regions on well-drained current logging; to open Quercus velutina acidic soils historical grazing; woodlands with drought; minor increasing fire wind and ice frequency Pitch pine-oak Quercus alba, Quercus coccinea, Pinus rigida, Pinus echinata, Ͼ1 million ha in New Frequent surface fire; Range from complex Quercus falcata, Quercus Pinus pungens, Pinus England, Mid-Atlantic, and historical logging; vertical structure ilicifolia, Quercus rubra, virginiana Appalachian regions and historical grazing; to open Quercus stellata, Quercus excessively well drained drought; minor woodlands with velutina sandy soils wind and ice increasing fire frequency White pine-red oak Quercus rubra, Quercus alba, Pinus strobus, Tsuga 0.5 million ha in New England Historical Two-storied, closed Quercus velutina, Betula canadensis on coarse-textured soils agricultural canopy stands alleghaniensis, Betula lenta, derived from granite and abandonment, with white pine Acer rubrum, Fraxinus gneiss or residuum from logging, wind, and occurring as americana sandstone and shale occasional surface emergent stratum fire over hardwood canopy Spruce-fir-hardwoods Betula papyrifera, Betula Picea rubens, Abies Ͼ10 million ha in the Historical and Range from complex alleghaniensis, Acer rubrum, balsamea, Picea glauca, Northeastern United States, current logging, vertical and Acer saccharum, Populus Tsuga canadensis, Thuja Quebec, and New periodic spruce horizontal spp., Fagus grandifolia occidentalis, Pinus strobus Brunswick on shallow, budworm structure to rocky, or acid soils derived outbreaks, single- or two- from glacial till hurricanes, minor storied stands wind and ice, with few to many rarely fire residuals over a dense canopy, depending on type and scale of disturbance Hemlock-hardwoods Acer saccharum, Betula Tsuga canadensis, Abies 0.5 million ha in western Great Infrequent, Complex vertical alleghaniensis, Betula balsamea, Picea glauca, Lakes, southeastern Canada, moderate-severity and horizontal papyrifera, Acer rubrum, Pinus strobus, Thuja New England, and north wind disturbance; structure Fraxinus americana, occidentalis central Appalachian regions historical logging; Fraxinus nigra, Ostrya on mesic sites with well- minor wind and virginiana, Prunus drained medium or coarse- ice pensylvanica, Prunus textured soils serotina, Quercus rubra, Tilia americana wood stands because of their potential to management practices affect the rate of car- D’Amato et al. 2011). By integrating species produce a greater timber volume or biomass bon accumulation (carbon sequestration) or with differing morphology and growth pat- (Waldrop 1989, Waskiewicz et al. 2013), to the amount of carbon stored in live trees and terns, mixed-species stands can theoretically provide more diverse or unique habitats deadwood (e.g., Hoover and Stout 2007, use growing space more completely than sin- (Comeau 1996, Jung et al. 1999, Girard et al. 2004), and to be more resistant or re- silient to contemporary insect outbreaks and Management and Policy Implications diseases (Su et al. 1996, Campbell et al. Forest management agencies are increasingly interested in establishing desired future conditions that are 2008) than pure stands. Correspondingly, mixedwood stands may be well suited for compatible with projected changes in climate. Maintaining, conserving, or restoring tree species diversity and achieving emerging management objectives enhancing carbon stocks are often identified as important climate mitigation strategies. Mixed hardwood- related to climate change mitigation and/or softwood stands or “mixedwoods” are often structurally and compositionally diverse because of the differing adaptation (D’Amato et al. 2011, Gauthier shade tolerances, growth rates, longevities, phenology, and crown and root structures of the constituent species. et al. 2014). There has long been interest in the benefits of mixedwoods because of their potential to produce a greater timber Forests remove carbon dioxide from the volume or biomass, to provide more diverse habitats, and to be more resistant or resilient to contemporary pests atmosphere and convert it to biomass, and pathogens than pure stands. They also may be better suited for projected climates although assessing this thereby providing opportunities for mitigat- has remained a challenge.