Longevity of needle fascicles of Pinus longaeva (Bristlecone pine) and other North American pines.

Variation in leaf longevity of gymnosperms has received surprisingly little attention despite its likely adaptive significance. Pinus longaeva, a pine of arid, subalpine environments in the western United States, has the record among conifers for needle longevity, with a maximum dwarf shoot (needle fascicle) retention time of up to about 45 years. Most low elevation pines have dwarf shoot retention times (DSRs) of two to four years. Literature data for the 37 species of Pinus native to the United States and Canada and field data for eight taxa (21 populations) of pines growing at various elevations in California each show a strong positive correlation between elevation and DSR, respectively, r=+0.65, df=35, p<0.001 and r=+0.82; df=19, p<0.001. We propose that extended needle fascicle longevity represents an adaptation to arid and especially high elevation environments. Field data from native stands and common gardens indicate that differences between taxa in DSR relate to both genetic and environmental factors. When grown at the same sites certain species (eg, P. longaeva, P. monophylla) had much longer DSRs than others, indicating a genetic basis for differences in needle fascicle longevity. For six of seven taxa that were each studied at more than one elevation there was a statistically significant increase in DSR in going from the lowest to the highest elevation site, indicating strong environmental control of needle fascicle longevity.The physiological control of dwarf shoot senescence and abscission is poorly understood. For P. longaeva dwarf shoots of a particular age class are not shed simultaneously; rather there is a more or less gradual attrition of dwarf shoots from the long shoot. Although different types of long shoots of pines are known to differ physiologically, for P. longaeva there was no consistent difference in DSR on various types of lateral long shoots (eg, vegetative, pollen cone-bearing, seed cone-bearing), nor was there a statistically significant difference in DSR on trunks versus on their lateral long shoots. In addition, for P. contorta ssp. bolanderi and P. muricata needle fascicle longevity was not affected by the degree of edaphically induced dwarfing (ie, stunting) of the trees.