Different growth patterns lead to mortality and survival in Pinus edulis

Background/Question/Methods

Pinus edulis (piñon pine) has suffered very high mortality in the recent drought in the Southwest; in some areas, up to 90% of trees have died. Previous studies have found that climate sensitivity was related to mortality under drought conditions. We used dendrochronological and stem growth analysis to determine whether patterns of past growth and climatic sensitivity were correlated with the probability of mortality. We examined radial growth rings for 78 trees and stem growth for 96 trees of similar age (c. 65yrs) in two piñon populations growing on a young and and old soil differing in physical and chemical properties.

Results/Conclusions

Based on stem growth rates over the past 20 years and tree-ring records over the most recent 50 years, we determined that trees that died at the low stress site had been growing more slowly. Trees that died at the high stress site had been growing faster or equivalently to those that survived. Tree ring growth (and stem growth) at the low stress site was 0.8mm (40mm) for dead trees and 1.0mm (49mm) for live trees. Tree ring growth (and stem growth) at the high stress site was 1.1mm (38mm) for dead trees and 0.8mm (37mm) for live trees. The probability of death was not related to past climatic sensitivity, calculated using a ratio of ring growth in wet and dry years (W:D). At both sites, W:D was 1.5 for dead trees and 1.4 for live trees. The soil at the younger site is drier, warmer, and contains less nitrogen than the soil at the older site. Moreover, piñon pine have been present at the younger site for far less time than at the older site. We are examining a number of possible explanations for these patterns including tradeoffs of growth rate and drought resistance due to xylem hydraulic properties.