Impacts of Changing Fire Regimes on Tree Regeneration and Diversity in California Forests

The historical patterns of fire type, size, severity, and seasonality that have long characterized western North American forests are rapidly changing. Factors that limited fire in the past have been altered by climate change and fire suppression across many forest types. In subalpine forests where high snowpack and cool summers generally resulted in infrequent fire, warmer, drier, and longer summers are giving rise to more burning. In mixed-conifer forests where frequent fire under indigenous burning practices reduced fuel loads and limited the severity of fire, a century of fire exclusion has resulted in denser forests and extremely high fuel accumulations. In this dissertation, I focus on the influence of changing fire regimes on understory biodiversity and tree regeneration in two distinct forest types. First, I consider the influence of fire in historically infrequent-fire subalpine forests, and second, the influence of fire suppression in historically frequent-fire mixed-conifer forests. In Chapter 1, I used an extensive subalpine forest plot network stratified across natural gradients of productivity and fire severity to investigate how the impacts of fire severity changed due to underlying forest productivity. Specifically, I sought to determine whether severe fire had a more positive effect on understory diversity in higher productivity forest where disturbance might result in greater release from competition for colonizer-type species. I found that plant species richness increased with increasing fire severity in subalpine forest and that indeed this relationship was stronger in higher productivity plots. Providing evidence for release from competition, the positive interaction between fire severity and productivity was driven by far-dispersing species, whose occurrence probability increased more than that of their near-dispersing counterparts in high productivity forest that burned at high severity.In Chapter 2, I used tree regeneration data from the same plot network to test theoretical predictions that disturbance would facilitate species range shifts into newly climatically available upslope habitat. While predicted habitat suitability increased for the two lower elevation tree species found at subalpine study sites (red fir [Abies magnifica], and Jeffrey pine [Pinus jeffreyi]), response to disturbance still depended on species traits. I found that disturbance reduced the probability of shade-tolerant red fir regeneration in subalpine forest, while it increased the probability of shade-intolerant Jeffrey pine. These results suggest that the impact of wildfire disturbance on species range shifts may depend on whether the postfire landscape aligns with a species’ regeneration niche.In Chapter 3, I studied the timing and drivers of understory change that has led to contemporary reductions in pine (Pinus spp.) in mixed-conifer forest overstories. I leveraged an historical tree regeneration experiment consisting of four large forestry plots that were mapped and logged in the early 1900’s and resampled in 2016. I found that the increased light availability from post-logging gaps was insufficient to promote post-logging pine regeneration and increase pine in the contemporary overstory. While the relative percentage of pine regeneration stayed low over time (9-6%), that of incense-cedar (Calocedrus decurrens) doubled (26-51%). Such changes in understory composition occurred despite lower relative basal area of incense-cedar in plot overstories at every time period. This persistent mismatch between overstory and understory indicates that the historical frequent-fire regime likely played an important role in advantaging pines at early demographic stages by differentially killing small cedars and firs. My results also serve to highlight that light alone is not sufficient to maintain co-dominance of pine in mixed-conifer forests in the west, and management efforts should strive to emulate multiple ecosystem conditions associated with the historical fire regime