Decreased needle longevity of fertilized Douglas-fir and grand fir in the northern Rockies

Changes in nutrient availability significantly affect canopy dynamics in conifers. To elucidate these effects, we experimentally fertilized mixed conifer stands at several sites across the northern Rocky Mountains. We measured needle longevity, total branch length and foliated length along the main branch axis, and determined mean retained cohort length on mid-canopy branches of shade-intolerant Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) and shade-tolerant grand fir (Abies grandis Lindl.). Needle longevity ranged from 6 to 8 years in Douglas-fir and from 7 to 8 years in grand fir on unfertilized plots. Fertilization significantly decreased needle longevity by 26 and 27% in Douglas-fir and grand fir, respectively. However, the foliated branch length remained unchanged following fertilization and was similar for both species, indicating a 33% increase in mean branch length per needle cohort in Douglas-fir and a 27% increase in grand fir. These data are consistent with the theory that foliated branch length and needle longevity are a result of the ecological light compensation point (ELCP), which links the inherent physiology of the leaf with the availability of resources in the leaf environment. Mid-canopy ELCP was approximately 74 and 71 cm from the branch terminus in Douglas-fir and grand fir, respectively, regardless of fertilization. We hypothesize that fertilization-enhanced needle production and annual shoot growth resulted in a higher rate of shading of older needles. The shaded needles were unable to maintain a positive carbon balance and abscised. The results demonstrate that foliated branch length of Douglas-fir and grand fir in the northern Rocky Mountains can be treated as a homeostatic response to fertilization, whereas foliar turnover is plastic.