Graduation date: 2006
Growing societal demand for forest products is pressuring managers to
increase productivity from a finite land area, and it is expected that increased
supply will come mostly from expansion of intensively managed stands. The
USDA Forest Service and numerous collaborators created the Long-Term
Soil Productivity (LTSP) network of research sites across North America to
investigate the implications of intensive management. The purpose of the
LTSP research program is to examine effects of management disturbances
on soil productivity, evaluate standards for soil quality monitoring, understand
fundamental relationships between soil, forest management practices, and
long-term productivity, and to examine ways to mitigate adverse disturbance
effects. Research in this thesis was conducted at the Matlock, WA and
Molalla, OR LTSP affiliate sites, which were specifically designed to examine
effects of contemporary management practices on growth and productivity of
Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco) as well as the driving soil
factors associated with productivity. Three levels of logging-debris
manipulations (bole-only, whole-tree, bole-only with debris piling) and two
herbicide treatments (initial and sustained control) were replicated using a
randomized complete block design at each study site. A stratified sampling
scheme was used to characterize needle-litter decomposition (using a
litterbag study) and associated nutrient dynamics as well as net nitrogen (N)
mineralization (using in situ sequential coring) in the mineral soil. Rates of
needle-litter decomposition were highest in conditions characterized by thick debris coverage. Needles acted as an N sink, immobilizing 14 to 40 kg N ha-1
y-1 initially. Needles were a source of potassium (K) and calcium (Ca), and a
minimal source of phosphorus (P) and magnesium. Soil net N mineralization
results were highly variable, likely reflecting extensive soil variability, although
some patterns were observed. Nitrate-N was the dominant inorganic N form
in soils, and accumulations between 25-45 mg N kg-1 soil yr-1 were found at
sites. A faster Douglas-fir needle-litter decomposition rate was observed in
bole-only logging-debris treatments at Matlock (lower productivity site), which
also retained higher N, P, K, and Ca percentages than other treatment types.
No treatment differences in needle-litter decomposition and nutrient-release
dynamics were observed at Molalla (higher productivity site), possibly
resulting from greater resource availability. At both sites, a combination of
soil net N mineralization in the range of 25-75 mg N kg-1 soil yr-1 and high
rates of initial N immobilization in decomposing needle materials suggests
that N is being retained in response to the three logging-debris and two
vegetation control treatments. Early dynamics of these two soil processes
suggest that soil N pools are initially conserved.