title: Changes to the N cycle following bark beetle outbreaks in two contrasting conifer forest types.
abstract: Outbreaks of Dendroctonus beetles are causing extensive mortality in conifer forests throughout North America. However, nitrogen (N) cycling impacts among forest types are not well known. We quantified beetle-induced changes in forest structure, soil temperature, and N cycling in Douglas-fir (Pseudotsuga menziesii) forests of Greater Yellowstone (WY, USA), and compared them to published lodgepole pine (Pinus contorta var. latifolia) data. Five undisturbed stands were compared to five beetle-killed stands (4-5 years post-outbreak). We hypothesized greater N cycling responses in Douglas-fir due to higher overall N stocks. Undisturbed Douglas-fir stands had greater litter N pools, soil N, and net N mineralization than lodgepole pine. Several responses to disturbance were similar between forest types, including a pulse of N-enriched litter, doubling of soil N availability, 30-50 % increase in understory cover, and 20 % increase in foliar N concentration of unattacked trees. However, the response of some ecosystem properties notably varied by host forest type. Soil temperature was unaffected in Douglas-fir, but lowered in lodgepole pine. Fresh foliar %N was uncorrelated with net N mineralization in Douglas-fir, but positively correlated in lodgepole pine. Though soil ammonium and nitrate, net N mineralization, and net nitrification all doubled, they remained low in both forest types (<6 μg N g soil(-1) NH(4) (+)or NO(3) (-); <25 μg N g soil(-1) year(-1) net N mineralization; <8 μg N g soil(-1) year(-1) net nitrification). Results suggest that beetle disturbance affected litter and soil N cycling similarly in each forest type, despite substantial differences in pre-disturbance biogeochemistry. In contrast, soil temperature and soil N-foliar N linkages differed between host forest types. This result suggests that disturbance type may be a better predictor of litter and soil N responses than forest type due to similar disturbance mechanisms and disturbance legacies across both host-beetle systems.
authors.affiliation: Department of Zoology, University of Wisconsin, Madison, WI 53706, USA. jgriffin2@wisc.edu
authors.forename: Jacob M
authors.initials: JM
authors.lastname: Griffin
authors.forename: Monica G
authors.initials: MG
authors.lastname: Turner
mesh_terms: D000818:Animals; D001517:Coleoptera; D058458:Nitrogen Cycle; D028223:Pinus / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism; D011157:Population Dynamics; D028224:Pseudotsuga / Q000378:metabolism*; D012987:Soil / Q000737:chemistry; D013696:Temperature; D014197:Trees / Q000254:growth & development*; D014959:Wyoming
chemical_list: D012987:Soil