James M. Dyer

The Hocking Hills of southeastern Ohio are situated at the western range boundary of eastern hemlock, where this foundation species occurs in isolated pockets in ravines and on steep slopes. The goal of this study is to characterize these hemlock stands prior to infestation by hemlock woolly adelgid (HWA), which recently entered the state and is likely to cause high mortality if untreated. Individuals in 30 plots established from 2008 to 2011 were resurveyed in 2020, allowing determination of growth rates and mortality rates. Each plot was paired with an upslope transect in the resurvey to record non-hemlock species likely to seed in from above. Tree species diversity in the plots is low (Shannon H’ < 1), as eastern hemlock remained the dominant species in all plots; storm damage and competitive thinning appeared to account for most mortality across all species. Hemlock growth rates were comparable to deciduous species. Common co-occurring species are statistically associated with particular topographic and soil properties on the plots. Following future hemlock mortality, tulip-poplar, chestnut oak, white oak, sweet birch, and red maple may be among the first species to dominate the canopy; Japanese stiltgrass currently appears to be the invasive species of greatest concern. An understanding of pre-HWA composition, structure, and forest dynamics can inform restoration efforts, if in the future long-term, sustainable HWA control or host-tree resistance is developed; this understanding can also apprise management and conservation efforts in the unique hemlock stands of southeast Ohio.

The Hocking Hills of southeastern Ohio are situated at the western range boundary of eastern hemlock, where this foundation species occurs in isolated pockets in ravines and on steep slopes. The goal of this study is to characterize these hemlock stands prior to infestation by hemlock woolly adelgid (HWA), which recently entered the state and is likely to cause high mortality if untreated. Individuals in 30 plots established from 2008 to 2011 were resurveyed in 2020, allowing determination of growth rates and mortality rates. Each plot was paired with an upslope transect in the resurvey to record non-hemlock species likely to seed in from above. Tree species diversity in the plots is low (Shannon H’ < 1), as eastern hemlock remained the dominant species in all plots; storm damage and competitive thinning appeared to account for most mortality across all species. Hemlock growth rates were comparable to deciduous species. Common co-occurring species are statistically associated with particular topographic and soil properties on the plots. Following future hemlock mortality, tulip-poplar, chestnut oak, white oak, sweet birch, and red maple may be among the first species to dominate the canopy; Japanese stiltgrass currently appears to be the invasive species of greatest concern. An understanding of pre-HWA composition, structure, and forest dynamics can inform restoration efforts, if in the future long-term, sustainable HWA control or host-tree resistance is developed; this understanding can also apprise management and conservation efforts in the unique hemlock stands of southeast Ohio.

Climate models project warmer summer temperatures will increase the frequency and heat severity of droughts in temperate forests of Eastern North America. Hotter droughts are increasingly documented to affect tree growth and forest dynamics, with critical impacts on tree mortality, carbon sequestration and timber provision. The growing acknowledgement of the dominant role of drought timing on tree vulnerability to water deficit raises the issue of our limited understanding of radial growth phenology for most temperate tree species. Here, we use well-replicated dendrometer band data sampled frequently during the growing season to assess the growth phenology of 610 trees from 15 temperate species over 6 years. Patterns of diameter growth follow a typical logistic shape, with growth rates reaching a maximum in June, and then decreasing until process termination. On average, we find that diffuse-porous species take 16–18 days less than other wood-structure types to put on 50% of their annual diameter growth. However, their peak growth rate occurs almost a full month later than ring-porous and conifer species (ca. 24 ± 4 days; mean ± 95% credible interval). Unlike other species, the growth phenology of diffuse-porous species in our dataset is highly correlated with their spring foliar phenology. We also find that the later window of growth in diffuse-porous species, coinciding with peak evapotranspiration and lower water availability, exposes them to a higher water deficit of 88 ± 19 mm (mean ± SE) during their peak growth than ring-porous and coniferous species (15 ± 35 mm and 30 ± 30 mm, respectively). Given the high climatic sensitivity of wood formation, our findings highlight the importance of wood porosity as one predictor of species climatic sensitivity to the projected intensification of the drought regime in the coming decades.

Topography exerts strong control on microclimate, resulting in distinctive vegetation patterns in areas of moderate to high relief. Using the Thornthwaite approach to account for hydrologic cycle components, a GIS-based Water Balance Toolset is presented as a means to address fine-scale species–site relationships. For each pixel within a study area, the toolset assesses inter-annual variations in moisture demand (governed by temperature and radiation) and availability (precipitation, soil storage). These in turn enable computation of climatic water deficit, the amount by which available moisture fails to meet demand. Summer deficit computed by the model correlates highly with the Standardized Precipitation–Evapotranspiration Index (SPEI) for drought at several sites across the eastern U.S. Yet the strength of the approach is its ability to model fine-scale patterns. For a 25-ha study site in central Indiana, individual tree locations were linked to summer deficit under different historical conditions: using average monthly climatic variables for 1998–2017, and for the drought year of 2012. In addition, future baseline and drought-year projections were modeled based on downscaled GCM data for 2071–2100. Although small deficits are observed under average conditions (historical or future), strong patterns linked to topography emerge during drought years. The modeled moisture patterns capture vegetation distributions described for the region, with beech and maple preferentially occurring in low-deficit settings, and oak and hickory dominating more xeric positions. End-of-century projections suggest severe deficit, which should favor oak and hickory over more mesic species. Pockets of smaller deficit persist on the landscape, but only when a fine-resolution Light Detection and Ranging (LiDAR)-derived Digital Elevation Model (DEM) is used; a coarse-resolution DEM masks fine-scale variability and compresses the range of observed values. Identification of mesic habitat microrefugia has important implications for retreating species under altered climate. Using readily available data to evaluate fine-scale patterns of moisture demand and availability, the Water Balance Toolset provides a useful approach to explore species–environment linkages.