Moisture, nutrient, and insect constraints on plantation growth: The Garden of Eden study

Abstract

Factorial combinations of insect, nutrient, and weed control treatments were applied repetitively to eight Pinus ponderosaDougl. ex Laws. var. ponderosa(ponderosa pine) plantations across a broad span of site qualities in California. Findings from the first 6 years showed that plantation growth potential was several times greater than previously realised. On the average, tree volumes from combined treatments were 4 times greater than for untreated controls. Sites of poor quality were limited primarily by soil drought, and fertiliser offered no further advantage over weed control alone. However, droughty sites often are infertile. Weeding not only improved moisture availability, but improved nutrient availability as well, and the joint effects were not easily separated. Controlling vegetation repeatedly with herbicides tripled volume growth and produced significant increases in foliar nutrient concentrations. Sites of medium quality also were limited by soil drought as well as by soil infertility. However, fertiliser application without vegetation control boosted weed growth which in turn blocked trees from a fertiliser response. Plantations on the best sites responded positively to both herbicides and fertilisers and effects were additive when treatments were combined. Despite an extreme range of site and plant stress conditions, insects had little influence on plantation development. Enhanced soil fertility and elevated foliar nutrient concentrations did not render trees more susceptible to insects. Thus, insecticides had no effect on tree growth. Findings suggest that weed control is essential for plantation development on poor sites under the Mediterranean climatic conditions, but not on better sites. Fertiliser responses are expected to continue for several years.

Citation

Powers, Robert F.; Ferrell, George T. 1996. Moisture, nutrient, and insect constraints on plantation growth: The Garden of Eden study. New Zealand Journal of Forestry Science. 26(1/2): 126-144.