Evergreen Broadleaf Ecophysiology Under Rapid Light Fluctuations And Transitional Winter Conditions In The Southern Appalachian Mountains

The southern Appalachian Mountains experience harsh winter conditions, including extremely cold air and soil temperatures, stochastic precipitation, often in the form of snow, high light, and strong winds. All of these contribute to the ecophysiological challenges that an evergreen plant faces during this season. Most wintertime research on evergreens has been done on conifers; we know less about how broadleaved angiosperms respond. Differences in leaf morphology between the two groups may influence mechanisms for coping with winter conditions. In addition, climate change is resulting in winters with stochastically dispersed warm days, and we do not know how broadleaved evergreens will respond to this. We are investigating the wintertime ecophysiology of the holly, Ilex x ‘Nellie R. Stevens,’ a broadleaved evergreen angiosperm commonly planted in this region. We are measuring diurnal gas exchange, water potential, maximum potential quantum efficiency (Fv/Fm), chlorophyll content, and leaf, soil, and air temperatures before, during, and after winter. We have found positive photosynthetic rates coupled with low stomatal conductances even at air temperatures slightly below or at 0oC, in part due to heating of the leaf above freezing when in full sun. Chlorophyll fluorescence measurements, however, indicate substantial photoinhibition for leaves exposed to high light at low temperatures, yet, they do not appear permanently injured and can achieve high rates on subsequent warmer days. Although rates are higher on warmer days in winter, cold soils may constrain the degree to which this species can take advantage of those days, due to the inability to transport water. Under sunny, winter conditions, low stomatal conductance may reduce latent heat loss, thus warming the broad leaves so they better cope with photoinhibition. This research can improve our understanding of how other broadleaf evergreen angiosperms may respond to natural as well as anthropogenically-influenced seasonal trends.