title: Vegetation-zonation patterns across a temperate mountain cloud forest ecotone are not explained by variation in hydraulic functioning or water relations.
abstract: Many studies have demonstrated linkages between the occurrence of fog and ecophysiological functioning in cloud forests, but few have investigated hydraulic functioning as a determining factor that explains sharp changes in vegetation. The objective of this study was to compare the plant water status during cloud-immersed and non-immersed conditions and hydraulic vulnerability in branches and roots of species across a temperate, mountain fog ecotone. Because cloud forests are often dark, cool and very moist, we expected cloud forest species to have less drought-tolerant characteristics (i.e., lower Pe and P50-the pressures required to induce a 12 and 50% loss in hydraulic conductivity, respectively) relative to non-cloud forest species in adjacent (lower elevation) forests. Additionally, due to the ability of cloud forest species to absorb cloud-fog water, we predicted greater improvements in hydraulic functioning during fog in cloud forest species relative to non-cloud forest species. Across the cloud forest ecotone, most species measured were very resistant to losses in conductivity with branch P50 values from -4.5 to -6.0 MPa, hydraulic safety margins (Ψmin - P50) >1.5 MPa and low calculated hydraulic conductivity losses. Roots had greater vulnerabilities, with P50 values ranging from -1.4 to -2.5 MPa, leading to greater predicted losses in conductivity (∼20%). Calculated values suggested strong losses of midday leaf hydraulic conductance in three of the four species, supporting the hydraulic segmentation hypothesis. In both cloud forest and hardwood species, Ψs were greater on foggy days than sunny days, demonstrating the importance of fog periods to plant water balance across fog regimes. Thus, frequent fog did not result in systemic changes in hydraulic functioning or vulnerability to embolism across our temperate cloud forest ecotone. Finally, roots functioned with lower hydraulic conductivity than branches, suggesting that they may serve as more sensitive indicators of hydraulic functioning in these mesic, foggy ecosystems.
authors.affiliation: Department of Biology, Wake Forest University, Winston-Salem, NC 27109, USA Present address: Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH 03824, USA zcberry@gmail.com.
authors.forename: Z Carter
authors.initials: ZC
authors.lastname: Berry
authors.affiliation: Department of Forest, Rangeland and Fire Sciences, University of Idaho, Moscow, ID 83844, USA.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Johnson
authors.affiliation: Department of Biological Sciences, Idaho State University, Pocatello, ID 83209, USA.
authors.forename: Keith
authors.initials: K
authors.lastname: Reinhardt
mesh_terms: D028202:Abies / Q000502:physiology*; D002940:Circadian Rhythm / Q000502:physiology; D065928:Forests; D028222:Picea / Q000502:physiology*; D018515:Plant Leaves / Q000502:physiology; D013045:Species Specificity; D014867:Water
chemical_list: D014867:Water
keywords: embolism; fog; hydraulic conductivity; southern Appalachians; vulnerability