Seasonal drivers of Rhododendron maximum transpiration in forest understories in the southern Appalachians

Background/Question/Methods

Rhododendron maximum, a large, evergreen, ericaceous shrub common to cove forest and riparian areas in the southern Appalachians has increased in growth recently in areas where hemlock woolly adelgid has reduced forest canopy cover. Even in forests with intact canopies, a dense R. maximum shrub canopy increases total community leaf area index and potentially increases forest evapotranspiration. Our objective was to determine the potential impact of R. maximum thickets on forest evapotranspiration in a deciduous forest, with an emphasis on the seasonal drivers of water use. We used thermal dissipation probes to quantify sap flux density for six R. maximum shrubs every 15 minutes for one year. We also measured incident light on the shrub layer, temperature and vapor pressure deficit and related these factors to leaf-level transpiration (E_L).  Leaf-level gas exchange measurements were also used to quantify the response of foliage to light, temperature and vapor pressure deficit. We then used community data to scale up leaf and plant-level measurements and estimate the total contribution of R. maximum to total stand transpiration.

Results/Conclusions

Rhododendron transpiration was strongly influenced by seasonal variations in microclimate. Peak E_L occurred in spring (late April) and a second, smaller peak in E_L was observed in early autumn (late September). Approximately 33% of annual E_L occurred during 8 weeks in late spring and early autumn. Understory light and sap-flux were not well correlated. Incident light on Rhododendron peaked in mid-April, prior to the spring peak in transpiration, and then declined rapidly with canopy leaf emergence. Lowest E_L during the year occurred during July and August. Soil moisture, which also declines with canopy leaf emergence due to increased canopy E_L, may be the primary limiting factor to R. maximum water use during the growing season. Leaf temperature limited R. maximum gas exchange during early spring when light levels were highest as leaf level stomatal conductance approached zero when leaf temperatures were below 10° C. Stand leaf area index during the growing season was ~4.2 with ~25% of total leaf area from evergreens and >80% of that from R. maximum. Stand LAI during winter was ~1 with R. maximum accounting for the majority. The high relative contribution of Rhododendron to leaf area index for many Appalachian forests, combined with an extended growing season, means that Rhododendron may contribute substantially to total forest water use.