Relationships of tree growth to nitrogen and water availability in a sheep-tree-pasture system in Douglas County, Oregon : field case study and shadehouse simulation

Abstract

This thesis consists of three parts: 1) a field case study involving tree growth, moisture stress, and foliar nitrogen response to sheep-grazed pasture treatments; 2) a shadehouse (potted-plant) study of simulated grazing effects on tree growth and moisture use; and 3) a summary, synthesizing results of the field and shadehouse studies and relating both to previous research. Part I. Field Case Study In a two-year-old agroforestry planting near Roseburg, Oregon, tree growth in grazed forb-dominated, grazed grass-dominated pasture, and bareground treatments was compared. Grazing by sheep was intensive. Trees were the KMX pine hybrid (Pinus attenuata X P. radiata) and Douglas-fir (Pseudotsuga menziesii). Significantly greater height and diameter growth of trees was found on the bareground treatment. KMX pine absolute growth was always superior to that of Douglas-fir. On a relative basis, however, both species were growing at about the same rate. Predawn tree xylem potential did not differ significantly among pasture treatments, but KMX pine values were significantly greater (less stress) than those of Douglas-fir during summer drought. Tree foliar nitrogen concentrations of both species were consistently high; species and treatment differences were generally insignificant. Soil total nitrogen likewise did not differ between treatments. Superior growth of KMX pine, compared with that of Douglas-fir, appeared related to lower summer xylem moisture stress. KMX pine produced superior growth, compared with Douglas-fir, due to lower xylem moisture stress during summer months. For the site and conditions investigated, moisture rather than nitrogen appear to be limiting growth. On sites similar to the one investigated, it was concluded nitrogen recycled in animal waste is unlikely to induce a tree foliar N response in the establishment phase (0-3 years) of tree plantations. Part II. Shadehouse Grazing Simulation Effects of simulated grazing of interplanted forage plants on growth and water use of three tree species were evaluated in a semicontrolled environment (open shadehouses). Varied proportions of perennial ryegrass (Lolium perenne) and subterranean clover (Trifolium subterraneum) were planted in pots with individual KMX pine, Douglas-fir, and Eucalyptus glaucescens. A tree-only treatment was also included. Forage in pots was clipped monthly for one growing season (May until October 1986). To simulate animal waste nitrogen return, 80 percent of nitrogen removed was returned as urea after each clipping. A second set of forage treatments was clipped but received no urea. KMX pine showed significantly (p=O.O5) greater diameter growth and total biomass than eucalyptus or Douglas-fir. Eucalyptus had the greatest height growth of the three species. Generally, trees with clover only or with no competing vegetation showed greater (p=O.1O) growth than trees with grass or mixed clover-grass competition. High grass competition had a depressing effect on tree growth. Eucalyptus appeared most affected by forage treatments, followed by Douglas-fir. KMX pine was least affected. Fertilization had no effect on tree growth, although it significantly (p=O.O5) increased eucalyptus shoot/root ratio. Moisture stress experiments indicated trees with no competing vegetation lost the most water over time. Because of a watering regime predisposing trees to stress, soil moisture content could not be correlated with tree predawn xylem potentials. A comparison of tree foliar nitrogen (N) in October 1985 (forage establishment) and October 1986 (harvest) showed no significant difference between forage/fertilization treatments at either time. Total soil nitrogen likewise did not change during the study period. Ryegrass biomass production consistently exceeded that of subterranean clover in grass-clover mixtures. Ryegrass dominated clover when ryegrass proportion was 20 percent or greater (unfertilized) and 10 percent or greater (fertilized). Fertilization approximately doubled ryegrass biomass yield but had no effect on clover yield. Forage growth in association with KMX pine markedly decreased. Douglas-fir had no effect on forage growth. Eucalyptus was intermediate. I conclude that tree growth in the simulation was limited by moisture. Added urea nitrogen benefited ryegrass growth. Trees with the least amount of vegetative biomass competition produced the greatest growth. Clover was neutral in effect on tree growth. Results suggest young tree plantations in grazed western Oregon pastures are unlikely to benefit from animal waste nitrogen return. On dry sites, summer moisture stress will limit tree growth and inhibit uptake of animal waste nutrient return.