Graduate Thesis Or Dissertation

Management effects on nitrogen nutrition and long-term productivity of western hemlock stands : an exercise in simulation with FORCYTE

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  • Regression equations applicable to biomass components of standsof western hemlock [Tsuga heterophylla (Raf.) Sarg.} were developedby destructive sampling of a thinned and an unthinned stand ofwestern hemlock near Seaside, Oregon. Equations predicted more live-branchbiomass and less dead-branch biomass per tree for the thinnedstand, but equations for biomass of foliage, twigs, stern wood andstem bark did not differ significantly between stands. Concentrationsof N, F, K, S, Ca, Mg, and Mn in tree components were determined and aboveground tree biomass and nutrient content were estimatedfor both stands.The biomass data was used in conjunction with published data tomodify an existing computer model in order to simulate growth andnutrient cycling of western hemlock stands. The FORCYTE computermodel, originally developed for Douglas-fir forests in BritishColumbia by J. P. Kimmins and K. Scoullar, was used. Calibration runs indicated that yield in FORCYTE was extremely sensitive to theparameter defining the rate of mineralization of soil organic matter,a process which supplied the majority of N available for tree uptake.The mineralization rate was set initially so that yield remainedconstant in three succesive 90 year rotations, which resulted in a4.1% loss of soil organic matter over the 270 year period. Simulationsof 6 different 270 year management scenarios of varying intensityindicated that more intensive management (e.g., whole-tree harvestingand commercial thinning) caused faster depletion of soilorganic matter and site N capital, resulting in an eventual declinein site productivity in later rotations. Simulations suggested thathemlock forests may not begin to accumulate soil organic matter untilthey approach old-growth status. Predicted declines in soil organicmatter caused by intensive management were compared to documentedlosses of organic matter in agricultural systems. FORCYTE predictedsoil organic matter would eventually equilibrate in about 1500 yearswhen inputs to the soil organic matter pool balanced decomposition,resulting in an equilibrium yield level well below that of the firstrotation, but sustainable in perpetuity. The strengths and weaknessesof the FORCYTE model are discussed. The predicted trends arebased on currently available information, but it must be realizedthat as more information becomes available, predictions will inevitablychange.
  • Keywords: FORCYTE (Computer program), Western hemlock
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