REGULATION OF PONDEROSA PINE FOLIAR PHYSIOLOGY AND INSECT RESISTANCE MECHANISMS BY BASAL AREA TREATMENTS

We compared foliar physiology and several measures of tree resistance to insect attack among ponderosa pine (Pinus ponderosa var. scopulorum Engelm.) trees growing in thinned stands. Measurements were made in a second-growth ponderosa pine forest in northern Arizona where the bas al area treatments (6.9, 18.4, 27.6, 78.2 m(2) ha(-1)) have been exper imentally maintained by frequent thinnings for 32 years before our mea surements began in 1994. Most of the physiological characteristics mea sured were affected by the basal area treatments. As stand basal area increased from 6.9 to 78.2 m(2) ha(-1), predawn water potential, midda y water potential, net photosynthetic rate, resin production, phloem t hickness, and foliar toughness decreased. Foliar nitrogen concentratio n was greatest in trees in the intermediate basal area treatments. Our results show that the physiological condition of second-growth ponder osa pine can be manipulated by silvicultural control of stand basal ar ea, and support the hypothesis that high stand basal area increases tr ee stress and decreases tree resistance to insect attack.