A negative relationship between growth rate and lifespan is frequently observed across taxa, yet despite its importance to life history theory and demographic processes, the mechanisms that link slow growth to longevity are still unresolved for most organisms. In trees, the vascular system is a prime source of critical functional trade-offs that affect both growth and survival, especially under drought. Conifers may be particularly advantageous for detecting these trade-offs, because of their ancestral secondary xylem design, in which specialized structures that connect xylem cells (pit membranes) have a strong but conflicting effect on two critical hydraulic functions: cavitation resistance and hydraulic conductivity. To determine if functional trade-offs due to vascular anatomy explain patterns of growth and longevity in conifers, we compared growth rates and xylem anatomical traits directly related to hydraulic function (i.e. tracheid diameter, pit frequency, and pit structure) in old (>350 years at sampling) and young (<190 years) ponderosa pine trees from two mixed-age stands in Northern Idaho. Previous work has shown that old trees in this study had the slowest lifetime growth, even when growth rates at the same age (60-140 years old) in both old and young trees were compared.
Results/Conclusions
Among young trees, we also identified relatively fast-growing (mean lifetime BAI > 30 cm2/year) and slow-growing (< 25 cm2/year) individuals. Pit structure had the strongest relationship to growth rates of all traits examined. Specifically, torus overlap (a measure of the degree of overlap between the inner valve-like structure and the opening within a pit membrane) was higher in the slow-growing trees (both old and young) relative to fast-growing young trees (p=0.06; 0.005 respectively). Moreover, torus overlap was negatively correlated with early-life growth rates when both old and young trees were pooled (R2= 0.501). However, torus overlap also had an overall significant negative relationship with tree size (R2=0.39), raising potential confounding effects of tree size on the relationship between pit structure and growth rates. We are now disentangling the scaling relationship between pit structure and tree size in order to determine if functional trade-offs due to vascular structure underlie the relationship between growth rates and survival in conifers. Understanding the mechanisms behind the relationship between growth and longevity will address a crucial gap in life history theory and enhance our predictions of forest demographic processes under climate change.