Nitrogen cycling and spatial heterogeneity following fire and restoration treatments in the Ponderosa pine/Douglas-fir ecosystem.

摘要

Lower elevation ponderosa pine ecosystems of the Rocky Mountain West (U.S.) historically experienced a frequent, low-intensity fire regime that promoted dominance of large diameter ponderosa pine (Pinus ponderosa ). An abrupt change in this historical disturbance regime occurred upon Euro-American settlement of the West in the late 1800s and early 1900s. A century of fire exclusion likely allowed less fire-tolerant species to become more dominant and C rich organic matter to accumulate. Some investigators hypothesize that these changes in forest structure and composition have resulted in reduced nutrient turnover relative to historical conditions. Land managers throughout the West are introducing surrogates of natural disturbance into the ponderosa pine community in an effort to reduce the risk of stand replacing wildfire and to restore historical stand structure and function. Within this dissertation I present an introduction and four manuscripts of original research that focus on how fire and restoration treatments influence various aspects of ecosystem function, with emphasis on the internal N cycle. In the first manuscript, I report numerous soil physical, chemical and biological parameters measured following four restoration treatments, and find that N, more than any other soil parameter, was influenced by restoration treatments. In the second manuscript I determine that spatial heterogeneity of available N following restoration treatments has a positive relationship with understory diversity, which was driven by divergence in species composition on high and low N patches. In the third manuscript, I investigate the potential role of charcoal on soil solution chemistry and growth of a native species that thrives following wildfire, Koeleria macrantha. Data in this manuscript suggests that charcoal can have a large effect on soil solution chemistry, including increased N cycling, and altered growth of K. macrantha. In the last manuscript I present data that leads to a better understanding of how several charcoal properties vary as a function of temperature and substrate.