Silvicultural practices such as thinning and fertilization influence the availability of carbon and nutrients for whole-tree physiological processes. Changes in resource availability influence partitioning of resources for tree growth and defense. Understanding the responses to such environmental changes can aid in devising silviculture practices that enhance tree growth and resistance against pests. Phenotypic response models such as growth differentiation balance hypothesis (GDBH) (Herms and Mattson, 1992), carbon-nutrient balance hypothesis (Bryant et al., 1983) and optimal allocation theory (Bloom et al., 1985) are leading alternative models for understanding phenotypic changes in plant growth and defense. In 1993, we performed a factorial manipulation of the availability of carbon and nutrients to loblolly pine (Pinus taeda) in the Kisatchie National Forest of Louisiana. We investigated short-term (1-3 years) and long-term (13 years) post-treatment effects of thinning and fertilization on loblolly pine growth, constitutive resin defenses and inducible resin defenses.

Results/Conclusions

Both thinning and fertilization increased tree growth in height and diameter -- especially in the short-term, but patterns persisted in the long-term. In the short-term, increased growth from fertilization was associated with reduced constitutive resin defenses (as predicted by GDBH). However, in the long-term, effects of fertilization on resin flow disappeared (as predicted by optimal allocation theory). Previous thinning, 13 years ago, enhanced the capacity of trees for rapidly inducible synthesis of resin following depletion of constitutive resin. Apparently, long-term increases in ability to acquire carbon, as occurs in thinned stands, can enhance the efficacy of inducible resin defenses. The high susceptibility of overstocked pine stands to bark beetles could be partly a result of low capacity for inducible resin defenses during the early stages of beetle attacks. Thus, silvicultural thinning may reduce long-term risks from the southern pine beetle by enhancing tree defenses.