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The degree to which seed dispersal or microsite requirements are limiting regeneration on this site,
the variation in seed production over time, and the relationship between
Douglas-fir regeneration and proximity to adults were investigated with
one study design. Seed traps and germination plots were paired and set out at random points throughout the stand, allowing for the correlation of seed fall and germination the following year. Also, microsite information was recorded for all the
germination plots to analyze the requirements for germination and establishment. By following seed counts in the traps for a number of years, we
hope to characterize variation in long-term seed production. Finally, by mapping adults within a 25-m radius
of the plots we can relate the production of seeds and/or seedlings to the position of adults with maximum likelihood analysis.
Thirty random coordinates were generated to locate the regeneration plots within the Permanent Sample Plot. The seed traps are 0.54 m2 wooden frames, with window screening on the bottom to allow drainage, and hardware cloth covering
the top to exclude predators but allow seeds to fall through. They were placed as flush with the ground as possible. The germination plots
were 1-m2 quadrats, half covered by a 0.5-m2 predator exclusion cage and the other
half exposed, to characterize predation in natural regeneration.
 The
traps and germination plots were established by August 1998 in
order to be in place before the beginning of seed fall. All first
year germinants in the germination plots were mapped and marked
with bright orange metal washers so that they would not be confused
with new germinants the following spring.
The
germination plots were first examined in May 1999, at which time an unforseen
confounding factor was identified. Cattle graze on the site every October, and
the exposed part of the germination frame was found to be free of long grass but
the covered part was not. This created very different germination conditions
that became increasingly disparate over time. Since we did not have the resources
to artificially clip the covered section of each germination plot every year, we
had to abandon this part of the study. The seed traps are emptied every spring. The samples are taken to
the Research Branch Lab, where the seeds are sorted from the litter
and counted. Then the
seeds are scanned by X ray to determine which have viable embryos. The data from
seed years 1998 to 2006 is summarized in Extension Note 85: "The
Pothole Creek Study Area: Dry uneven-aged Douglas-fir stand development".
Data collection from the seed traps will continue for at least fifteen years,
to allow us to estimate the variation of seed production from year to year and
potentially estimate a cycle of crop production. This cycle can then be compared
with data from the nearby
climate station
to study how seed production responds to weather patterns. The location of adults in relation to the seed traps should allow us to determine the average reproductive output and mean dispersal distance of
seeds from average adults in the stand by maximum likelihood analysis. Research contact: Catherine Bealle Statland.
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