- Temperate coniferous forests, such as those that cover vast areas of the western US and Canada, have evolved to depend on cycles of disturbance for succession of species and overall ecosystem maintenance. Many of these forest systems are managed, often for timber production, where disturbances are of anthropogenic origin and implemented to create space for regeneration of a single species. Forest soils control many ecosystem functions which enable and support primary forest productivity. Therefore, maintaining the health of the soil is critical to sustainable forest management. The concept of forest soil health relies on the interactions between chemical, physical, and biological soil properties to retain an environment conducive to the function and maintenance of an ecosystem. Many soil health attributes are dynamic and largely controlled by the soil microbial community. The motivation of this thesis was to investigate how soil health and the microbial community are impacted by forest disturbances of all types and specifically, how these properties respond to harvest management practices.
The first objective of this research was to determine the current state of knowledge about how soils change following an array of forest disturbances, including both natural and anthropogenic. This was achieved through a thorough review of the published literature and synthesizing findings related to changes in microbial community structure, activity, and related soil health properties in response to disturbance. Overall, it was determined that disturbances are largely detrimental to the soil biological community, although the extent of this was determined by the severity, continuity, and duration of the disturbance event. Some disturbances, such as wildfire and harvesting, directly impacted the microbial community; others, such as insect infestations, were more likely to indirectly impact soil microbes by disrupting flows of carbon and nutrients involved in microbial processes. This synthesis also illuminated some of the gaps in knowledge which exist in regard to the response of the soil microbial community to disturbance, including a lack of longitudinal studies and studies that combine phylogenetic data with activity and soil health property data, for a comprehensive understanding of how the entire soil system responds to the perturbation.
The second objective of this research was to measure soil properties related to both soil nutrient pools (total C and N, active C) and microbial-activity related processes (mineralization, enzymatic activity) in order to investigate whether the manipulation of organic matter and compaction would have apparent effects despite 15-25 years having passed since the initial disturbance. For this experiment, we took advantage of Long-term Site Productivity installations already in place across the Pacific Northwest and Northern California. We sampled locations with a range of unique site attributes, including soil texture, dominant vegetation, and climate. This allowed us to compare the response of soil properties to treatments in a variety of ecosystems and gain insight into whether treatment effects were unique to a site, or universal. Overall, we found little effect of management treatments on either nutrient pool status or microbial activity indicators. One exception being treatment plots where the forest floor was removed, in which a decline was observed across nearly all soil measurements. In many cases, sites responded differently to management and therefore few common trends in soil response were observed. Depending on overall site characteristics and severity of the disturbance event, the microbial community appears resilient, and recovery may be possible within a few decades subsequent to disturbance.