Leaf longevity of western red cedar (Thuja plicata) increases with depth in the canopy

Leaf longevity determines the annual cost of replacing foliage biomass and influences water and nutrient budgets. Longevity is readily estimated in most evergreen species by annual bud scars along the shoot. However, some species with indeterminate growth do not show these annual markers, making estimation of longevity difficult. One of these species is the widespread and economically valuable western red cedar (Thuja plicata J. Donn ex D. Don), for which no dependable estimates of leaf longevity exist. In this study, we estimated leaf longevity for western red cedar by counting growth rings in shoots at the point of leaf abscission. Estimates were obtained on 26 dominant or codominant trees growing in natural stands in a montane forest in northern Idaho, USA. Leaf longevity averaged 8.9 (SE = 0.2) years, but it strongly increased with depth in the canopy (0.3 year m(-1); mean crown depth was 15 m), increasing from a mean of 6.8 years in the upper third of the canopy to 10.6 years in the lower third. The increase in longevity with depth in the crown is consistent with many reports showing that longevity increases in resource-limiting environments. Longevity did not vary significantly with altitude or solar insolation in these montane forests. Among stand-level variables, longevity was correlated only with leaf area index: it increased slightly in stands with high leaf area indices. This approach to longevity estimation may be useful for any species that produces annual rings but no obvious bud scars, including many Cupressaceae species.