Seasonal dynamics of tree growth, physiology, and resin defenses in a northern Arizona ponderosa pine forest

During periods of resource stress, such as drought, allocating limited photosynthate between growth and defense is a crucial component of tree survival. Our objectives were to describe the seasonal dynamics of physiology, growth, and resin defense of southwestern ponderosa pine (Pinus ponderosa Dougl. ex P. & C. Laws.) which undergoes regular seasonal drought. We measured leaf-level net photosynthetic rate, leaf water potential, resin flow after phloern wounding, and growth in 24 consecutive months in 2002 and 2003. Precipitation was below average in both years, and 2002 was an extreme drought. In both years, the highest resin flow occurred when tree water stress was highest and photosynthesis was low. Belowground growth was highest in August in both years. Aboveground growth occurred primarily between May and August and was greater in 2003 than in 2002. Temporal variation in resin flow was positively related to temporal variation in needle and radial growth but was not related to temporal variation in root nor shoot growth. Thus, trade-offs in carbon allocation between resin and growth were weak. We discuss these results in the context of water stress in trees, current hypotheses of carbon allocation in plants, and the historical lack of pine bark beetle epidemics in northern Arizona.