The objectives of this study were to assess the range of genotypic variation in the vulnerability of the shoot and root xylem of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings to water-stress-induced cavitation, and to assess the trade-off between vulnerability to cavitation and conductivity per unit of stem cross-sectional area (k(s)), both within a species and within an individual tree. Douglas-fir occupies a broad range of environments and exhibits considerable genetic variation for growth, morphology, and drought hardiness. We chose two populations from each of two varieties (the coastal var. menziesii and the interior var. glauca) to represent environmental extremes of the species. Vulnerability curves were constructed for shoots and roots by plotting the percentage loss in conductivity versus water potential. Vulnerability in shoot and root xylem varied genetically with source climate. Stem xylem differed in vulnerability to cavitation between populations; the most mesic population, coastal wet (CW), was the most susceptible of the four populations. In the roots, the most vulnerable population was again CW; the interior wet (IW) population was moderately susceptible compared with the two dry populations, coastal dry (CD) and interior dry (ID). Root xylem was more susceptible to cavitation than stem xylem and had significantly greater k(s). The trade-off between vulnerability to cavitation and k(s), however, was not evident across populations. The most vulnerable population (CW) had a shoot k(s) of 0.534 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1), compared with 0.734 +/- 0.067 &mgr;mol m(-2) s(-1) MPa(-1) for the less vulnerable CD stems. In the roots, IW was more vulnerable than ID, but had the same k(s).

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Diameter prediction models based on the Weibull distribution function and stand-table projection models based on changes in relative diameter were developed for 2- to 10-year-old Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) plantations. Both modeling approaches incorporated the effects of site preparation, animal protection, and competing vegetation. The diameter distribution approach is appropriate when information on initial diameters is not available. The stand-table projection approach may be applied when tree diameters in a plantation are measured two or more growing seasons after planting. At young ages, the stand-table approach provided more accurate representation of observed diameter distributions than the diameter distribution approach. At age 10 the two methods provided comparable diameter distributions. The equations derived for predicting survival, height growth of dominant trees, height–diameter relationships, and the development of woody vegetation over time will facilitate the study and comparison of stand structure and dynamics after various site-preparation and animal-protection treatments.

A parameter recovery procedure for the Weibull distribution function, based on diameter percentiles, was modified to incorporate the effects of interfering vegetation in young Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco var. menziesii) plantations. The applicability of the system was tested by using data from sites in the Coast Ranges of Oregon and Washington and in the Siskiyou Mountains of southwestern Oregon. Four percentiles (0, 25th, 50th, 95th) of the cumulative probability distribution were predicted as functions of quadratic mean diameter and age. In the Siskiyou study, cover and total vegetation control affected quadratic mean diameter and all four percentiles; intensity of the vegetation treatments affected the 0 and 25th percentiles, and the interaction between intensity and timing of treatment affected quadratic mean diameter. In the Coast Ranges study, only quadratic mean diameter was affected by cover of woody vegetation, while quadratic mean diameter and the 25th percentile were significantly affected by total vegetation control. The predicted distributions showed decreasing variance with increasing cover, particularly in the Siskiyou Mountains. In the Coast Ranges study, the coefficient of variation increased with increasing cover, indicating that the variance of stem diameters was affected by average size. On xeric sites in the Siskiyou Mountains, high diameter variability in plots with total vegetation control suggests that interspecific competition may inhibit the expression of microsite variation.