Abstract:
Effects of four inbreeding levels (F = 0, 0.125, 0.25, 0.5) and
two supplemental mass pollination (SMP) methods (agitation of
naturally shedding pollen with "air blaster" and pollen application
from large "pollen wand" atomizer) were evaluated on seed of trees
from eleven full-sibling Douglas-fir (Pseudotsuga menziesii [Mirb.]
Franco) families in a 16-year-old seed orchard. SMP responses
were tested at three crown levels on trees of three periods of
reproductive maturity. Cones covered during SMP treatments were
used as controls.
Inbreeding did not significantly affect flat and round seed
percentages or seed weights, but percent round, normal appearing
seeds that were empty (PE) had a positive, linear relation to F.
Filled seed yields after inbreeding did not differ among full-sib
families; but variation among trees within families, as well as
interactions between tree yields and inbreeding levels, were highly
significant. At F = 0.5, PE was greater for seed orchard trees (98.2)
than has been reported for trees in natural stands (90.9); juvenility
and off-site planting of orchard trees may contribute to this
difference.
Between F = 0 and 0.5, an increase in F of 0.1 was associated
with an increase of 12% PE. Self-pollination appears to have its
greatest impact on orchard seedset. Half- and full-sib matings also
give reduced seed yields (relative to outcrossing), but the relatively
high levels of inbred seed production should have their greatest
impacts upon the productivities of the resulting seedlings. This
contrast is discussed using inbreeding depression values from the literature.
In contrast with other reports, air blast SMP did not increase
seedset, but increased PE relative to the controls. Dense crowns of
seedling seed orchard trees appear to intercept more self pollen
than do the more open crowns of previously studied clonal orchards.
Wand application of pollen, which was used only in conjunction with
the air blast treatment, reduced PE to the level of the controls. The
effect of crown levels (upper, middle, lower), as well as an
interaction between crown levels and the phenology and SMP
treatment applied to specific trees, were highly significant; causes
of these results are speculated.
Practical implications are twofold. Because inbred seed
production is high at F = 0.25 and 0.125, seedling orchard designs
must minimize the possibility of within-family mating. Secondly,
crown form should be considered prior to use of some supplemental
mass polliantion treatments. In general, the dense crowns of young
seedling orchard trees do not appear suitable for air blast SMP.
