Cloud pattern and water relations in Picea rubens and Abies fraseri, southern Appalachian Mountains, USA

The spruce-fir [Picea rubens Sarg.-Abies fraseri (Pursh) Poir.] forests of the southern Appalachian Mountains are considered refugial, endangered communities that exist on only seven mountaintop areas in Virginia and North Carolina, USA. These relict forests continue to be threatened by stress factors such as logging, acid rain deposition, attacks from invasive insects, and climate change. It has been suggested that these communities have persisted because of frequent cloudiness and periods of cloud immersion (fog), although few studies have examined corresponding effects on microclimate and tree ecophysiology. Incident sunlight (PPFD), air temperature, vapor pressure deficit (VPD), and xylem water potentials were measured throughout the summer growing season in Mount Mitchell State Park, NC (35 degrees 45'53 '' N, 82 degrees 15'54 '' W), along with continuous camera recordings of the forest canopy and accompanying cloud conditions. Approximately 60% of all summer days had at least 2 h of cloud immersion with the large majority (80.3%) of immersion events occurring during morning hours. Cloud-immersed days had the greatest reduction in cumulative daily PPFD compared to clear days (11.09 mol m(-2) day(-1) vs. 38.03 mol m(-2) day(-1) respectively), as well as substantially reduced mean, VPD (0.98 kPa vs. 1.81 kPa) but only slightly lower mean air temperatures (14.5 degrees C vs. 14.9 degrees C, respectively). Moreover, xylem water potential (Psi) increased significantly (similar to 0.2 MPa; values) from morning to afternoon on cloud-immersed days. In contrast, clear days showed no afternoon recovery in Psi, but a continued decrease during the afternoon. juvenile Psi was more responsive to daily cloud regime compared to adult Psi and had a strong negative correlation with vapor pressure deficit. When all measurement days were sorted by the cloud pattern of the previous day, there was a strong response in juveniles trees, i.e. Psi increased following previous afternoon cloud-immersion. juveniles of both species also had greater seasonal decreases in Psi than adults (P. rubens, adult: 0.05 MPa, juvenile: 0.13 MPa; A. fraseri, adult: 0.06 MPa, juvenile: 0.20 Mpa). For climate change models that predict a higher cloud base (resulting in less immersion) and dryer conditions, the water relations of Abies fraseri and Picea rubens could be substantially and negatively influenced. (C) 2012 Elsevier B.V. All rights reserved.