Abstract
Conventional methods for estimating the current annual increment of stand volume are based on the uncertain assumption that height increment decreases with tree age. Conversely, size, rather than age, should be accounted for the observed senescence-related declines in relative growth rate and, consequently, implemented in silvicultural manuals. Results stem from a study on Abies alba Mill. at its southern limit of distribution. Many factors limit height increment when age and size increase in large-statured tree species. Height–diameter allometric relationships are commonly used measures of tree growth. In this study, we tested if tree age was the main factor affecting the reduction in height increment of silver fir trees (Abies alba Mill.), verifying also whether tree size had a significant role in ecophysiological-biomechanical limitations to tree growth. The study was carried out in a silver fir forest located in Southern Italy, at the southernmost distribution limit for this species. Through a stratified random sampling, 100 trees were selected. All the selected trees were then felled and the total tree height, height increments (internode distances), diameter at breast height, and diameter increments (ring widths) were measured. The analyses of allometric models and scaling coefficients showed that the correlation between tree age and height increment was not always significant. We may conclude that tree age did not statistically explain the decrease in height increment in older trees. Instead, the increase in tree size and related physiological processes (expressed as product between diameter at breast height and tree height) explained the reduction in height increment in older trees and was the main factor limiting height growth trends in marginal population of silver fir.
Highlights
The observation that for most tree species mass growth rate of individual trees increases continuously with tree size has triggered new interest in the ecology of old and tall organismsHandling Editor: John M
The main objectives of this study were (i) to test if tree age is the main factor affecting the reduction in height increment and (ii) to verify if tree size has a significant role in declining height increment as tree matures
Limited to silver fir growing at its southernmost distribution limit, we showed that an appropriate choice of allometric models and scaling coefficients allows the resolution of uncertainties found in silvicultural manuals about the variation in the relationship between tree height and stem diameter as tree age and size increase
Summary
The observation that for most tree species mass growth rate of individual trees increases continuously with tree size has triggered new interest in the ecology of old and tall organismsHandling Editor: John M. The observation that for most tree species mass growth rate of individual trees increases continuously with tree size has triggered new interest in the ecology of old and tall organisms. This outcome implies that accumulation of carbon would continue as trees increase in size and age rather than decrease as trees mature. Evidence exists that size accounts for the observed age-related declines in relative height growth and net assimilation rates (Mencuccini et al 2005). The growth of trees continues for years by continuously increasing both diameter and height, though it is unclear whether mature trees have static linear diameter–height trajectories or curved trajectories in relation to neighbor effects (Henry and Aarssen 1999). The allometric relationship between stem diameter at breast height and tree height does not always represent stem-mass growth patterns (Sillett et al 2010)
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