Pollen contamination trends in a maturing Douglas-fir seed orchard

Pollen contamination was investigated in one block (block 4) of a 10-block Douglas-fir (Pseudotsuga menziesii (Mirb.) France) seed orchard complex in western Oregon. Blocks (25 clones each) represent different breeding zones; thus, contaminant pollen sources not only included the adjacent natural stand of Douglas-fir, but also other blocks within the orchard complex. Stored seed lots from five crop years (i.e., 1982, 1985, 1987, 1989, 1990) were sampled (200 seeds each) and assayed at 11 allozyme loci. One seed lot (1982) was found to have a high proportion of seed contaminants and was discarded, leaving five crop years for comparison (including 1980 from a previous study). Despite large increases (up to 6-fold) in pollen production from the first commercial crop in 1980, levels of pollen contamination (m) in block 4 remained high and did not differ significantly among years (mean (m) over cap = 0.489, range 0.421-0.606). On average, 79% of the contaminants since 1985 came from the natural stand; 21% from other orchard blocks. Without spatial isolation from nonorchard pollen sources or intensive pollen management to limit contamination (e.g., bloom delay and supplemental mass pollination), levels of pollen contamination can be quite high, even in mature Douglas-fir seed orchards.