Microclimate, water potential, transpiration, and bole dielectric constant of coniferous and deciduous tree species in the Continental Boreal Ecotone of Central Alaska

Tree canopy microclimate, xylem water flux and xylem dielectric constant have been monitored in situ since June 1993 in two adjacent natural forest stands in central Alaska. The deciduous stand represents a mature balsam poplar site on the Tanana River floodplain, while the coniferous stand consists of mature white spruce with some black spruce mixed in. During solstice in June and later in summer, diurnal changes of xylem water potential were measured to investigate the occurrence and magnitude of tree transpiration and dielectric constant changes in stems. A newly developed method for continuously monitoring bole dielectric constant was successfully applied to monitor changes in six trees during an entire growing season. At solstice, with continuous daylight of varying intensity, trees still showed a distinct diurnal pattern in both water potential and tree transpiration, indicating water stress and stomatal control in all trees observed. Dielectric constant in the xylem varied diurnally in black spruce and balsam poplar, while white spruce dielectric constant showed minimal response. Transpiration decreased with shortening daylight periods and lower vapor pressure deficits in the atmosphere. Dielectric constant gradually increased in all white spruce observed. Tree xylem flux stopped in all conifers at the end of October. Black spruce and balsam poplar had clear diurnal patterns of dielectric constant which correlated with changes in tree xylem flux. Tree xylem flux and dielectric constant in the black spruce was not correlated to diurnal environmental parameters and indicated strong depletion of water storage in its below canopy hydraulic system