Background/Question/Methods Recent publication of a large, long term study of tree demography in Sierran mixed confer forests attributed mortality to increased temperature (van Mantgem & Stephenson 2007). Results from a 10 year manipulative experiment support elevated mortality rates reported, but clarify the role of elevated nitrogen (N) deposition and Jeffrey pine beetle in Jeffrey pine mortality. To simulate elevated N deposition, N was amended at the rate of 50 kg/ha yr above background (6 – 9 kg/ha yr) around individual Jeffrey pine trees for 10 years. There were 3 control and 2 N amended sites for trees in mesic microsites, and 1 control and N amended site each for trees in xeric microsites. Replicate control sites were 8 and 40 km apart. The study was initiated with 224 mature Jeffrey pine in 1998, and survival, canopy growth attributes, and presence of needle scale and Jeffrey pine beetle were assessed biennally. Mesic microsites were defined based on topographic position and tree growth response to increased water availability. Basal area growth of Jeffrey pine in xeric microsites responded little to years of increased water availability.

Results/Conclusions

In years of average precipitation, elongation growth (needles, branchlets) showed little response to N amendment. In below-average precipitation years, N amendment significantly depressed elongation growth only in xeric microsites. The frequency of needle scale was 3% in mesic microsites trees exposed only to background N deposition, but increased to 14% with amended N. Decadal mortality rates due to Jeffrey pine beetle was 0% in control trees, and 8% in N-amended trees in mesic microsites. In xeric microsites, the frequency of trees with needle scale was twice as great as that in mesic microsites (28%), but N amendment decreased the frequency to 17%. Decadal mortality in xeric microsites was 22% with background N deposition and 9% with N amendment. In mesic microsites, simulated N deposition improved canopy growth attributes, but increased needle scale and success of Jeffrey pine beetle. In xeric microsites, simulated N deposition decreased the frequency of needle scale and success of Jeffrey pine beetle.