Interactive effects of N deposition, O3 uptake, and drought on canopy health of mature Jeffrey pine

Background/Question/Methods

This research was conducted to test whether canopy health of Jeffrey pine would be improved by future, higher anthropogenic inputs of nitrogen (N).  A broad size and age cohort was studied in the southern Sierra Nevada in mesic or xeric microsites verified by topographic position, basal area increment, and level of drought stress. To simulate N deposition, half the trees in each microsite were amended with slow-release urea (50 kg N m^-2 s^-1) annually for 10 years. Ozone concentrations in this area averages ~68 ppb O₃ over the 6 month growing season and low N deposition rates (~6-7 kg ha^-1 yr^-1). Components of canopy health of each tree were assessed: level of foliar chlorotic mottle, needle and branch elongation and diameter growth, needle retention, and presence of foliar and bark insects.

Results/Conclusions

Statistical summaries of foliar and branch attributes associated with canopy health in each microsite and N level yielded few significant treatment effects, and a conclusion might have been drawn that N amendment had no effect on canopy health, or that insufficient time had passed to detect an effect. However, grouping trees into ‘unhealthy’, ‘average’, and ‘healthy’ based on canopy health attributes elucidated differing responses to both N levels and drought. There were shifts in the proportion of unhealthy, average, and healthy trees, as well as differences in specific canopy responses to N amendment, depending on microsite water availability. N amendment reduced the proportion of healthy trees, and increased the proportion of unhealthy trees in both microsites, but the increase in proportion of unhealthy trees was an order of magnitude higher in xeric microsites. Needle growth of unhealthy trees was improved in mesic microsites with N amendment, but retention was reduced. Needle growth of healthy trees in xeric microsites improved with N amendment, but the proportion of these trees decreased significantly. Because greater foliar biomass is associated with higher photosynthetic capacity, future anthropogenic increases in N deposition will likely improve C acquisition in unhealthy trees, as long as there is adequate moisture. Increased N deposition will likely decrease C acquisition in xeric microsites. With drought expected with climate change in the Sierra Nevada, we can expect a greater proportion of trees deleteriously affected by N deposition (with moderately high O₃ exposure), and reduced C acquisition in this extensive forest type.