CLIMATE-GROWTH RESPONSES FROM PINUS PONDEROSA TREES USING MULTIPLE MEASURES OF ANNUAL RADIAL GROWTH

Abstract

When using old-growth trees from semiarid, open-canopy environments, basal area increment (BAI), an absolute measure of radial growth, is sometimes used instead of the more commonly used ‘conservative techniques’ (negative exponential or linear regression with a negative slope; NegX) because narrow rings have been shown to potentially bias results. In this study we explore the relationship between radial growth of ponderosa pine from four study sites in Montana and climate (temperature, precipitation, drought severity) using unstandardized raw ring width and BAI values, and standardized values generated via Friedman Super Smoother and NegX. All sites are minimally disturbed, and our selection criteria are limited to older (interior dates pre-A.D. 1850 at breast height) trees growing in open-canopy environments free of visible disturbance such as lightning strikes. We found the strongest relationships (r > 0.60) for radial growth with July and prior-year October Palmer Drought Severity Index values. Our results show that radial growth-climate responses generally fall within a narrow range regardless of the representation of annual growth (e.g. for July temperature r-values are largely –0.3 to – 0.4) and that site conditions determine which radial-growth values (i.e. unstandardized or standardized) optimize climate-growth responses.