Mature interior Douglas-fir (Pseudotsuga menziesii var. glauca) leave trees show inconsistent survival when retained after harvest in the Sub-Boreal Spruce biogeoclimatic zone of BC. Government policy is to maintain appropriate levels of leave-trees on cut blocks to meet biodiversity objectives. Douglas-fir is utilized because populations are naturally fragmented, at the northern edge of their natural distribution, and potentially sensitive to adverse management practices. Dominant and veteran trees that survive fire persist while new cohorts of spruce, lodgepole pine, and sub-alpine fir come and go beneath them. Such stands have unique vertical structure and provide habitat for numerous wildlife species: ungulate winter range for mule deer. Studies in 2002 provided knowledge on the geographical range where leave tree mortality is occurring and revealed potential factors related directly to mortality. In 2003, we focused on the measurement of possible changes in pre and post harvest water relations around large and/or old Douglas-fir leave-trees, and the potential stresses such changes place on them. Douglas-fir trees may be functioning at the maximum limit of their hydraulic conductance potential in this latitude. If this is true, sudden changes in moisture availability may exert lethal stresses. Plant moisture stress (twig water potential), soil water content, soil water potential, bole temperature, carbon isotope discrimination and micro climate data were collected in 2003 for typical Douglas-fir leave-trees in harvested (?cc?) and unharvested (?un?) treatment units on two sites: harvested winter 2003 and harvested winter 1998. Preliminary analysis indicates differences in water relations between ?cc? and ?un? treatments. Sample trees in harvested treatments, in some cases, reach water potentials that may potentially be harmful. Some of the responses are due to the genetic variability among trees. Although not as frequently as in the ?cc?, some ?un? trees reached lower water potentials than expected. This suggests their post harvest survival rate may be marginal. These trees would be identified by a Douglas-fir Leave Tree Risk Index.
Christopher David Hawkins.
Forest Investment Account (FIA). 2004. An evaluation of Douglas-fir leave-tree retention practices in central British Columbia. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report. FIA2004MR254