Individual Tree and Stand Level Influences on the Growth, Vigor, and Decline of Red Oaks in the Ozarks Steven L. Voelker, Rose-Marie Muzika, and Richard P. Guyette Abstract: Repeated oak decline and mortality events have occurred in the Ozark region for decades and probably longer. We sampled an age sequence of 1,259 black and scarlet oaks (Quercus velutina Lam. and Quercus coccinea Muench.) to better describe the process of oak decline and mortality in the red oak group (subgenera Erythrobalanus). Trend in basal area increment (BAI) over the most recent 40 years was used to establish three vigor classes for trees with decreasing, stable, or increasing growth (Declining, Stable, or Healthy). We compared crown condition measures with absolute BAI and boundary line BAI, a measure of radial growth adjusted for tree size. A pulse of mortality was found to occur just subsequent to the most recent drought, although decline often started decades previously. Time series of individual tree BAI suggests that half of all oak decline events were incited by one or two drought-related step-changes in growth and variance. Predisposing factors to decline generally showed significant but weak relationships with crown conditions. Surviving oaks growing in high-mortality stands had poorer crown conditions and grew more slowly than trees in low-mortality stands. When recently dead trees were accounted for, the same high-mortality stands had significantly greater predecline basal area and stocking than low-mortality stands. Thus, a less competitive growth environment may afford some buffer to drought stress before oak decline but does not appear to help afflicted stands improve their growth and vigor. FOR.SCI. 54(1):8 –20. Keywords: oak decline, drought, shoot dieback, forest health, boundary line basal area increment. N UMEROUS DECLINE AND MORTALITY EVENTS have been recorded in red oak forests (Hursh and Hassis 1931, Millers et al. 1989, Biocca et al. 1993, Clin- ton et al. 1993, Jenkins and Pallardy 1995, LeBlanc 1998, Oak et al. 2004, Starkey et al. 2004). The classic etiology of oak decline suggests that an inciting event causes physical injury (often shoot dieback or defoliation), which reduces whole tree photosynthetic potential and depletes energy reserves in the form of nonstructural carbohydrates. These afflicted trees are then more susceptible to opportunistic pathogens (Manion 1991, Wargo 1996, Marc ¸ais and Bre ´da 2006). Consequently, crown conditions, or a tree’s pattern of branching and hydraulic architecture providing water from the soil to the leaves are closely tied to past and present capacity for photosynthesis and growth (Dwyer et al. 1995, Rust and Roloff 2002). Indeed stem growth can be interpreted as a fixed record of tree vigor because of the priorities for carbon allocation during the growing season (Waring 1987, Barbaroux et al. 2003). Therefore, the lower basal area growth of declining oaks before an inciting event is evidence that some combination of predisposing factors play an important role in the decline process (Pederson 1998). Despite continued oak decline events and their study, forest managers and silviculturists have been frustrated by a lack of fundamental knowledge that could prioritize treat- able versus unavoidable influences to decline and mortality of mature red oaks. Inciting events such as drought are unavoidable, but an improved knowledge of how trees re- spond to predisposing factors to oak decline may allow forest scientists to better focus and experimentally test pre- ventative silvicultural approaches in maturing forests. To monitor forest health and vigor, crown condition measure- ments provide an immediate assessment (Zarnoch et al. 2004) whereas stem growth patterns determined from incre- ment cores have often been viewed as too labor-intensive despite the detailed information they provide (Biondi 1999). The need to assess forest declines with respect to poten- tial predisposing factors such as stand composition, stand age, stand density, and site productivity has been acknowl- edged for some time (Hyink and Zedaker 1987), but few studies are designed within the context of evaluating these factors. Some research has suggested that certain site char- acteristics interacting with stand age predispose oaks to decline (Starkey and Oak 1988, Tainter et al. 1990, Biocca et al. 1993, Dwyer et al. 1995, Oak et al. 1986, 1996) or that declining oaks were more often located on xeric landforms or other site-specific conditions (Oak et al. 1986, 1996, Jenkins and Pallardy 1993, 1995). In the Missouri Ozarks, drought and Armillaria root disease are the most important inciting and contributing factors in the mortality of red oaks (Jenkins and Pallardy 1995, Bruhn et al. 2000). Our study builds on this knowledge by more thoroughly considering how predisposing factors at the stand level can affect tree Steven L. Voelker, Department of Forestry, University of Missouri, Columbia, MO 65211; current address: Oregon State University, Richardson Hall, Department of Wood Science and Engineering, Corvallis, OR 97339 —Fax: (541) 737-3385; steve.voelker@oregonstate.edu. Rose-Marie Muzika, Depart- ment of Forestry, University of Missouri, Columbia, MO 65211. Richard P. Guyette, Department of Forestry, University of Missouri, Columbia, MO 65211. Acknowledgments: The authors acknowledge Jessie Bebb, Jered Hayes, Molly Martin, and Mark Yates for their help during this project. We also sincerely thank Randy Jensen and MOFEP for granting us access to the MOFEP trees database. Johann Bruhn, Peter Becker, John Kabrick, an anonymous associate editor, and two anonymous reviewers provided comments that greatly improved this work. This research was supported by the US Forest Service, Northeastern Area Forest Health Protection, and Southern Research Station. Manuscript received May 18, 2006, accepted June 20, 2007 Copyright © 2008 by the Society of American Foresters 8 Forest Science 54(1) 2008