Phytophage effects on primary production, nutrient turnover, and litter decomposition of young Douglas-fir in western Oregon

We tested the effect of defoliating and sap-sucking phytophages on young Douglas-fir at the H.J. Andrews Experimental Forest in western Oregon. Experimental trees were subjected to manipulated abundances of a sap-sucking insect at 0-1 insect g^-1 foliage or a defoliating insect at 0 -0.06 g^-1 foliage. Tree mass, throughfall, litterfall, litter decomposition, and N, K and Ca turnover were measured for each tree over a 3 year period. Herbivore abundance had no effect on calculated tree growth or nutrient content. These data suggest compensatory growth and replacement of lost nutrients. Herbivory also did not significantly affect decomposition rate for exogenous Douglas-fir needle litter. Throughfall volume, N, K and Ca content, and litterfall mass were positively related (P < 0.05) to defoliator abundance during the early growing season (April-June). At the highest defoliator abundance (causing about 20% foliage removal), turnover of N, K and Ca amounted to 15-25% of the total inputs to litter during this period. Throughfall Ca was significantly related defoliator abundance for the entire growing season (April-September). Sap-sucker feeding significantly influenced K turnover during the growing season. The results of this study support results from an eastern deciduous forest. Our study relates nutrient turnover rates to herbivore abundances, a prerequisite for modeling phytophage effects on nutrient flows.