|
The interaction of light with moisture and nutrients may play a major role in the interior dry-belt Douglas-fir forest (IDF). Variability of moisture across the study site affects regeneration success and overall tree growth, and interacts to
some degree with the physical features of microsite and substrate. Spatial variation in moisture stress could be caused by uneven snow accumulation and melt, grass evapotranspiration as related to leaf area and energy availability, soil depth and
water storage variability, water use by large trees and patterns of shade and light.
Net carbon accumulation is a function of absorbed light energy and is
constrained by such factors as total leaf area, nutrient content, temperature,
internal leaf CO2 concentration and water. Both water and CO2 are exchanged to
the air via leaf stomata, thus a close relationship exists between the amount of
water used and the amount of CO2 assimilated (i.e., growth). For dry interior
sites, we need to know how the available moisture on the site is being used by
the trees and by other vegetation. Understanding the relationship between water
availability and the stand leaf area could also help determine the maximum
current annual increment.
Several research
projects that examine the role of water in this ecosystem have been completed at
the Pothole Creek Study Area:
1) Sap flow
sensors were used to measure water use by Douglas-fir of various sizes and under
differing levels of competition. These sensors are drilled into the tree stems
and monitor changes in sap flow in response to moisture availability. A close
relationship was found between stem basal area increment and early summer
individual tree water use (Simpson 2000).
2) The effects
of competition from grass and large trees on saplings was studied by isolating
planted seedlings with trenches to remove large tree root competition, and
scalping areas around planted seedlings to remove grass competition. However, as
with other studies at the site that made use of planted seedlings, browsing by
ungulates and grouse spoiled the results by destroying too many subject trees.
Ironically, the browsers seemed to prefer the seedlings in the scalped plots,
perhaps because grass removal made the seedlings easier to find.
3) The spatial
pattern of soil moisture availability was examined by installing 16 moisture
blocks in a line at three metre intervals. The line ran through two clumps of
young trees and a grassy area in order to compare the moisture availability
under these two cover types. The soil moisture readings indicated some
association with the position of overstory trees, but this relationship was
uncertain due to high variability among spatial locations and among repeated
measurements at the same location. A finer grid of moisture readings over a
larger area might be able to overcome these measurement errors and repeatability
problems in future studies. As it was, we were unable to demonstrate definitive
spatial patterns of moisture availability (Simpson 2004).
References
Simpson,
D.A. 2000. Water use of interior Douglas-fir. Can. J. For. Res. 30:534-547.
Simpson,
D. A. 2004. Soil moisture content: spatial patterns. Pothole Creek Study Area
file report, 9 pp.
|
