Forest Nutrient Cycling in a North Idaho Conifer Stand
Garrison-Johnston, M.T.. 2003. Ph.D. Dissertation. Dept. Forest Resources, University of Idaho, Moscow, ID
Abstract
Overstory, understory, forest floor and soil elemental contents and litterfall flows of all
macronutrients and most micronutrients and aluminum were measured during one growing
season in a north Idaho conifer stand. These data were analyzed for differences which might
explain past variation in fertilization response. The first phase of the study involved the
examination of soil nutrient availability as measured by ion exchange resins. Goals were to
detect possible causes of variation in past fertilization response, to develop an improved
understanding of seasonal nutrient dynamics, and to examine changes in soil nutrient
availability throughout the soil profile. Using ion-exchange resins, differences in elemental
availability were detected by experimental block, as well as with soil depth and throughout
the growing season. Results were discussed in light of findings by other researchers and used
to help explain possible reasons for past variation in forest fertilization response. Soil
nutrient availability was also compared to tree nutrient uptake throughout the growing season
using graphical interpretation, and correlation analysis was performed between Douglas-fir
foliar nutrient concentrations and ion-exchange nutrient availability. Several interesting
findings regarding interpretation of ion-exchange data in light of tree nutrient uptake were
discussed. During the second phase of this study, differences in macronutrient, micronutrient
and aluminum contents of various forest ecosystem components were analyzed and evaluated
during one growing season. Block differences in overstory and litterfall contents that may
help explain past fertilization response at this site are discussed. Needle attributes of
overstory trees showed significant differences by species, crown class, crown position, foliar
age class and sampling date. Foliar weight varied with the degree of foliage exposure to
sunlight, and elemental foliar contents generally followed the samepattern. Understory vegetation showed significant nutrient content differences between growth forms and during
the growing season for several elements. Litterfall dry weight and elemental contents
showed seasonal variation, and forest floor content of most elements was within expected
ranges. The final phase of this study entailed the compilation of overstory, understory, forest
floor and soil elemental contents and litterfall flows into a systems analysis model, which
was then projected over a three-year period. Model simulations were performed for three
species composition scenarios, including pure grand fir, pure Douglas-fir and mixed conifer.
For all species simulations, model components and flows were examined, and both seasonal
and annual behavior were evaluated in biological and mechanistic terms. Significant
differences in overstory elemental content and related flows were revealed through
simulation of various species compositions. Overall, systems analysis was a useful tool for
evaluating forest elemental cycling, and provided a better understanding of seasonal and
short-term nutrient dynamics and component interactions of our experimental stand.
Suggestions for future research are discussed in light of these findings.
