Rosebay Rhododendron

Rosebay rhododendron (Rhododendron maximum) is an important component of the understory in Appalachian forest ecosystems. Growing up to 8 m in height, it occurs in a range of habitats, but especially cool mesic riparian corridors. In the summer you might encounter this species exhibiting its showy white, pink or light purple flowers. It maintains its deep green foliage throughout the year and individuals retain their waxy leaves up to eight years. Retention of the evergreen foliage allows for photosynthesis and continuous growth throughout the year, which provides the rosebay rhododendron a competitive advantage over deciduous trees and shrubs that are in a state of dormancy in the winter. Winters in the Appalachians can be very harsh, but the rosebay rhododendron has several adaptations that allow it to withstand very cold temperatures. One very noticeable adaptation to cold is the curling of its leaves. The curling of the leaves occurs in direct response to leaf temperatures and is believed to be a mechanism to prevent additional water loss, and to avoid injury to the leaves' photosynthetic machinery from freezing and thawing. The curling of the rhododendron leaves may change dramatically over the course of the day as temperatures change. If you happen to be hiking on one of our Western Pennsylvania trails on a cold winter day, you might notice that the leaves of the rosebay rhododendron are tightly curled in the shade. However, just around the corner in a patch of sunlight where temperatures are much higher, the leaves are unfurled and appear much the same as they would during the warmer months.

One of the places to view this species is along Bear Run at WPC's Bear Run Nature Reserve in Fayette County, but there are many others throughout the Pennsylvania mountains.

Photo by WPC's Ephraim Zimmerman.

References: Nilsen, E.T. 1985. Seasonal and diurnal leaf movements of Rhododendron maximum L . in contrasting irradiance environments. Oecologia 65: 296-302.

Nilsen, E.T. 1992. Thermostatic leaf movements: a synthesis of research with Rhododendron. Bot. J.. Linnean Society 110:205-233.

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