Influence of tree size on shoot structure and physiology of Pinus contorta and Pinus aristata

This study examined the morphological and physiological characteristics of shoots of large and small mature trees of two pine species with different growth patterns and tree longevities: Rocky Mountain bristlecone pine (Pinus aristata Bailey) and lodgepole pine (Pinus contorta Dougl. ssp. latifolia Bailey). Large trees of P. aristata had 21% less annual shoot growth but retained more total foliar biomass per shoot than small trees because of a 95% longer leaf life span. Large trees of P. contorta had 69% less annual shoot growth and less foliage production per shoot, and even with a 23% longer leaf life span, they retained less total foliar biomass per shoot than small trees. Needles of both species and sizes did not begin to abscise until they reached approximately 40-45% of their expected maximum life span. Foliar nitrogen and phosphorus concentrations, Nmass, and Pmass, of 1-year-old P. aristata leaves were significantly lower and higher, respectively, in the large than in the small trees. In P. contorta, Nmass and Pmass of 1-year-old leaves were similar in large and small trees. Pinus aristata also had a more consistent Nmass with leaf age than P. contorta, and the pattern differed for small and large trees, Photosynthetic capacity and stomatal conductance of 1-year-old leaves of P. aristata were less in large trees than in small trees. The increased plasticity in leaf life span with tree size in P. aristata compared with P. contorta enabled an increase in foliar biomass per shoot in P. aristata that may offset the decrease in photosynthetic performance and contribute to the smaller reduction in shoot growth with tree size in P. aristata compared with P. contorta. Similarities in needle biomass distribution between species and tree sizes suggest that relative needle age may be of greater importance to the retention of the leaves than their chronological age. The magnitude of the changes in shoot characteristics with increasing tree size differed between the species and may reflect alternative strategies of resource allocation and use.