Abiotic factors affecting sapflow rates and chlorophyll fluorescence in Juniperus virginiana, Lonicera maackii, and Pyrus calleryana on a roadcut in Kentucky

Background/Question/Methods

Roadcuts are challenging environments for woody plants, with thin, unfertile soils and frequent droughts. Three of the most successful species on roadcuts in the Ohio River Valley are redcedar (Juniperus virginiana), a small native evergreen tree, Amur honeysuckle (Lonicera maackii), an invasive deciduous shrub, and Callery pear (Pyrus calleryana), a small invasive deciduous tree. The purpose of this study was to determine the physiological strategies employed by these three species that may enable them to persist and coexist in roadcuts. Sapflow rates were determined in 2012-2013 at a roadcut in northern Kentucky when each species possessed foliage using Granier probes. The dark-adapted chlorophyll fluorescence F_v/F_m, a measure of stress, and light-adapted Φ_PSII, a measure of photosynthetic efficiency, were also measured every 1-2 weeks. Soil volumetric water content (VWC) and vapor pressure deficit (VPD) were measured hourly. Mean daily sapflow velocities were determined, as well as daily mean VPD and daily sum of hourly VPD during the daylight hours. The effects of VWC and VPD on mean daily sapflow velocity were modeled with time series analysis. Chlorophyll fluorescence measures were regressed against these measurements and mean daily temperature. 

Results/Conclusions

Redcedar and honeysuckle reached highest sapflow velocities in June; pear reached a maximum in July-August. Honeysuckle and pear had later peaks in September, and honeysuckle had another one in November; red cedar maintained sapflow while other species were leafless. Sapflow velocities in all species exhibited strong autocorrelation with the previous day, and they responded to previous-day VPD. Pear was the only species to not respond to VWC. Redcedar fluorescence increased with previous-day mean temperature but declined at higher temperatures. Honeysuckle fluorescence increased with temperature but was also affected by measurement time of day. Pear Φ_PSII responded similarly; F_v/F_m also declined with high previous-day VPD. Pear often had lower F_v/F_m (higher stress) but higher Φ_PSII (higher photosynthetic efficiencies). Redcedar generally had the lowest stress but the lowest photosynthetic efficiencies. The similar VPD responses suggest similar stomatal strategies among these three drought-tolerant species. Redcedar’s lower photosynthetic efficiencies are offset by its longest leaf-out period, with photosynthesis limited mainly by extreme temperatures. Pear has the highest photosynthetic efficiencies, is relatively sensitive to low temperatures, but is little affected by drought. Honeysuckle is less sensitive to low temperature and maintains photosynthetic efficiencies close to pear for a longer period.