Ecology, 94(5), 2013, pp. 1165–1175 Ó 2013 by the Ecological Society of America Thirty-two years of change in an old-growth Ohio beech–maple forest 1 JAMES R. RUNKLE Department of Biological Sciences, Wright State University, Dayton, Ohio 45435 USA Abstract. Old-growth forests dominated by understory-tolerant tree species are among forest types most likely to be in equilibrium. However, documentation of the degree to which they are in equilibrium over decades-long time periods is lacking. Changes in climate, pathogens, and land use all are likely to impact stand characteristics and species composition, even in these forests. Here, 32 years of vegetation changes in an old-growth beech (Fagus grandifolia)–sugar maple (Acer saccharum) forest in Hueston Woods, southwest Ohio, USA, are summarized. These changes involve canopy composition and structure, turnover in snags, and development of vegetation in treefall gaps. Stand basal area and canopy density have changed little in 32 years. However, beech has decreased in canopy importance (49% to 32%) while sugar maple has increased (32% to 47%). Annual mortality was about 1.3% throughout the study period. Mortality rates increased with stem size, but the fraction of larger stems increased due to ingrowth from smaller size classes. Beech was represented by more very large stems than small canopy stems: over time, death of those larger stems with inadequate replacement has caused the decrease in beech importance. Sugar maple was represented by more small canopy stems whose growth has increased its importance. The changes in beech and sugar maple relative importance are hypothesized to be due to forest fragmentation mostly from the early 1800s with some possible additional effects associated with the formation of the state park. Snag densities (12–16 snags/ha) and formation rates (1–3 snagsÁhaÀ1ÁyrÀ1) remained consistent. The treefall gaps previously studied are closing, with a few, large stems remaining. Death of gap border trees occurs consistently enough to favor species able to combine growth in gaps and survival in the understory. Key words: Acer; disturbance; Fagus; fragmentation; Hueston Woods, Ohio; long-term study; old growth forest; snag; tree demography; treefall gaps. INTRODUCTION Oliver and Larson (1996) state that true old-growth Models of forest dynamics, often framed in terms of forests have probably never been common because ecological succession, predict a series of changes in the major disturbances occur quite frequently in most areas forest as it ages, including increases in biomass and compared to the life spans of trees. Pickett et al. (2009) shifts in species composition from fast-growing light- state that stabilization of vegetation composition or demanding species to those better able to survive in the structure is recognized to be a rare outcome of understory until their opportunity to reach the canopy vegetation dynamics at many spatial and temporal occurs (e.g., Whittaker 1975, Perry et al. 2008). If scales and in many habitats. However, in parts of the disturbances are infrequent enough, this process results Lake States and in parts of the central eastern deciduous in a steady state or equilibrium community in which the forest, such forests may have been frequent (Runkle dominant canopy species also reproduce well. At that 1996, Frelich 2002, Perry et al. 2008). stage, the community will change relatively little in It is not clear which aspect of forests is the best species composition or ecosystem properties until some indicator of old-growth conditions. Bormann and new disturbance occurs. This simple model, based on Likens (1979) used stand biomass as an indicator: in ideas from Clements (1936) and their subsequent old-growth forests, biomass fluctuations should be development (Pickett et al. 2009), provides a useful minor compared to the mean value. They proposed a framework. However, it leaves several questions unan- shifting-mosaic steady state model in which the ecosys- swered. tem as a whole is in equilibrium although at any one Are there locations and forests in which disturbances location it changes with the establishment, growth, and are rare enough that a steady state community exists? death of individual canopy trees. Oliver and Larson (1996) consider true old growth to occur when all trees that invaded immediately after the last major distur- Manuscript received 8 December 2011; revised 25 October bance have died. They do not assume that species 2012; accepted 13 November 2012; final version received 6 December 2012. Corresponding Editor: J. J. Battles. composition will remain constant at that time but are 1 E-mail: [email protected] unclear under what conditions it might change. Frelich 1165 1166 JAMES R. RUNKLE Ecology, Vol. 94, No. 5 (2002) uses the definition of Clements (1936) that the The opportunity for measuring these features of old- climax forest will be able to reproduce its own growth mesic forests in some forest types is very limited. composition in the absence of severe disturbance. Perry Few old-growth remnants exist in some areas of fertile et al. (2008) also characterize the steady state forest as agricultural soils (Davis 1996). Available sites are one in which species composition becomes relatively further restricted by several recently introduced and stable if species reproduce under their own canopies or spreading pathogens and pests affecting such important in small gaps. trees as American beech (Fagus grandifolia), ash It is uncertain how much change occurs in the final (Fraxinus spp.), eastern hemlock (Tsuga canadensis), stage. At that stage, most tree death occurs as single and, to a lesser extent, sugar maple (Acer saccharum) treefalls (referred to here as gaps) or, at most, small (Griffin et al. 2003, Small et al. 2005, Watmough 2010, clusters of treefalls. The frequency of such deaths Garnas et al. 2011, Kashian and Witter 2011). Such undoubtedly varies from year to year. It is not known increased recent mortality obscures the disturbance how variable over short (a few years) to medium (a few regimes under which current tree species have evolved. decades) time spans are mortality rates, stand charac- Hueston Woods State Park in southwestern Ohio, teristics such as total basal area and total density, and USA, is an ideal place to examine the nature of an old- species composition. growth mesic forest. It is dominated by two long-lived In many parts of eastern North America, the mesic species, beech and sugar maple. There is no indication of a major overstory disturbance for several disturbance regime is dominated by individual treefalls tree generations. It is one of the very few old-growth and replacement in the resultant canopy gaps (Runkle forest stands in this forest type and has not yet been 1990, 1998, Lorimer et al. 2001, Busing 2005, D’Amato impacted by invasive tree pathogens. My 32-yr study and Orwig 2008, Gravel et al. 2010). Several generaliza- there allows me to distinguish between trends and tions about this disturbance regime are possible. fluctuations and allows me to quantify variations Average annual mortality is about 1% for trees around average conditions. I will examine (1) the corresponding to average canopy residence times of dynamics of canopy trees, including their rates of about a century for many tree species (Runkle 1996, growth, mortality, and replacement overall, by species Tyrrell et al. 1998). Less clear is the degree to which and by size class; (2) the dynamics of snags (dead stems mortality varies over short to moderate time periods. It .1.5 m high); and (3) changes in the vegetation of also is not clear what deviations around that 1% figure treefall gaps originally sampled in 1977. entail and why mortality in some forest types is consistently greater than 1% while in other forest types METHODS it is consistently less. Study area Related to the stand structure of living trees is the The study site is a 67-ha old-growth stand in Hueston stand structure of dead trees and other coarse woody Woods State Park, Ohio, USA (398340 N, 848450 W). debris (see Plate 1). Those components of the forest are That stand has remained relatively undisturbed since its important for many species, including birds and other purchase in 1797. Its overstory is dominated by beech vertebrates, fungi and other decomposers, and plants and sugar maple with white ash (Fraxinus americana), (Harmon et al. 1986, Hura and Crow 2004, Garber et al. tulip tree (Liriodendron tulipifera), and black cherry 2005, Siccama et al. 2007). Some information on these (Prunus serotina) as codominants in some areas. Soils resources is accumulating (Tyrrell et al. 1998) but some are Russell silt loams (a Typic Hapludalf) and are deep forest types, including the beech–maple forest type, have and well drained, having formed in silt-mantled glacial been the focus of relatively few studies. Standing dead till overlying limestone (Lerch et al. 1969). Summers are trees (snags) are of particular interest with regard to hot (mean July temperature 23.78C), winters are cold forest change because they are ephemeral. Some trees die (mean January temperature À2.08C), and precipitation and immediately become fallen logs. Others die standing occurs year-round (mean annual precipitation 102.5 cm or broken off and gradually deteriorate. The time during [averages 1950–1988 for Hamilton, Ohio; National which they remain standing is an important component Climatic Data Center]). Braun (1950) included it in her of the forest habitat for many species. Long-term studies characterization of the beech–maple forest region and are necessary to determine the rates at which snags form, recognized three topographic areas: upland, slope, and the rates at which they deteriorate and the variation in lowland; elevation ranges from 268–296 m above sea the availability of this resource through time. level. More recently it has been the subject of several A third component of forest change is the filling of studies on forest dynamics (Vankat et al.