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 assessed 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, carbon isotope discrimination and micro climate data were collected in 2004 for typical Douglas-fir leave-trees in harvested (cc) and unharvested (un) treatment units on five sites. In the SBSmk1 and wk1 subzones cutblocks harvested the previous winter and 5-6 years previously were sampled while in the SBSvk1 subzone, a cutblock harvested 1 year previously was sampled. 2003 and 2004 data indicate differences in water relations between cc and un treatments. Twig water potential and stable carbon isotope ratio values indicating possible water stress were seen most often in trees in the cc treatments with the most extreme values observed in the wettest subzone. Vapour pressure deficit around leave-trees in harvested cutblocks is substantially greater than in the unharvested stand. Sample trees in harvested treatments, in some cases, reach water potentials that may potentially be lethal. Some of the responses are due to genetic variability among trees. Although not as frequently as in the cc, some un trees reached lower water potentials than expected. This suggested their post harvest survival rate may be marginal. These trees could be identified by a Douglas-fir Leave Tree Risk Index.
Bruce Rogers, Nicole Balliet and Chris Hawkins.
Rogers, Bruce, Hawkins, Chris D.B.; Balliet, Nicole. 2005. Evaluation of Douglas-fir leave-tree retention practices in Central British Columbia. Forest Investment Account (FIA) - Forest Science Program. Forest Investment Account Report
Topic: FLNRORD Research Program
Keywords: Forest, Investment, Account, (FIA), Pseudotsuga, Menziesii, British, Columbia
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