Research Projects

Diversity of soil microbes and the function of beech-maple forests

The natural areas of The Holden Arboretum contain some of the finest examples of mature, intact beech-maple forest in Ohio. Holden is conducting research into the effects of soil physical and chemical properties on the diversity and function of soil microbes in these forests. In particular, we are interested in soil fungi that colonize plant roots (mycorrhizae) and how this symbiotic association controls the structure and function of these forests and how they respond to environmental variability and change. Holden scientists are using molecular techniques that extract and amplify DNA from environmental samples to determine the diversity and relative abundance of these important soil fungi, as well as measuring microbial processes using biogeochemical techniques.

Tree roots in soil

Mycorrhizal fungi on roots

Fungal fruiting body in the forest

Winter ecology and altered precipitation dynamics

Most studies of forest ecosystems and soil microbial communities at northern latitudes ignore functional processes that occur during cold months or under snow. Recent work by Holden scientists and others has suggested that these processes may be more important that previously thought and can represent a significant portion of annual carbon and nutrient cycling in many ecosystems. Moreover, altered precipitation and temperature patterns related to climate change might influence the magnitude and importance of winter processes in the Great Lakes region. Holden scientists are working to understand how forests and soils in Northeast Ohio function during our extended periods of cold temperatures and snow cover.

Manipulating soil temperature and soil freezing

Estimating leaf decomposition under snow

Acidic precipitation and forest health

Acidic precipitation resulting from burning fossil fuel is a chronic problem in areas near population centers or concentrations of industrial activity. The northeastern United States is one area that receives particularly high levels of acid precipitation. The resulting ecosystem acidification can impact plants, soils and water. In hardwood forests, such as those common in Northeast Ohio, there is concern that long-term acidification of forests and can decrease the availably of the essential nutrients phosphorus and calcium in soil, which could result in poor forest health and forest decline. Holden scientists received a grant from the National Science Foundation ( to look at how forests and soils respond to changes in acidity and soil phosphorus availability. They are manipulating soil pH and fertility in a long-term study of Ohio forests using crushed limestone (lime) and phosphorus fertilizer and watching how plants and soil microbes respond

Bags of lime that were spread on forest plots

Transporting lime into the forest

Applying lime by hand

The evolutionary ecology of whole-plant physiological integration

Leaf, xylem and root anatomical traits are critical components of plant growth and survival, because they determine physiological functions like growth rate and provide safety against stresses like freezing and drought. For example, the images below show cross-sections of Rhododendron leaves, with different cell types appearing as different colors. The relative sizes and proportions of the different tissue types influence the photosynthetic rate. Researchers are using the extensive Rhododendron collections at The Holden Arboretum to expand our scientific understanding of which specific suites of traits are associated with particular environments, how trait integration changes across growth environments, and how anatomical traits determine plant physiological functions. In addition, Rhododendron researchers at Holden are investigating the relative influences of species ancestry versus climate in determining anatomical and physiological traits. These two influences have very different implications for the future of Rhododendron lineages in the face of changing climate. If closely related species within a lineage all have very similar traits, then the lineage may have little capacity to respond to rapidly changing climate conditions.  On the other hand, if traits are very diverse within a lineage, with different traits found in species from warmer habitats compared to those from cold habitats for example, that lineage may have a greater capacity to respond to changing climate conditions.

Horticultural Research Projects

Under the direction of plant breeder and geneticist Stephen Krebs, PhD, the Arboretum’s David G. Leach Research Station maintains its commitment to developing superior rhododendrons for continental climates (i.e. cold winters and hot summers). The traditional breeding program is now complemented by a vigorous research component focused on adaptations of rhododendrons to biotic and abiotic stresses, such as winter freezing injury to leaves and buds, summertime ‘bleaching’ of leaves (photoinhibition), and diseases caused by fungal pathogens (Phytophthora root rot and powdery mildew).

Please visit the Leach Research Station webpage to learn more.