Climatic and human influences on fire regimes and forest dynamics in temperate rainforests in southern Chile

Fire regimes were investigated in the context of interannual and multidecadal changes in climate, long-term land-use changes, and post-fire tree establishment. The fire-scarring capacity of Pilgerodendron was explored and tested for reconstructing fire activity in temperate rainforests since the late 1500s A.D. The suitability of Pilgerodendron as a tree-ring fire recording species creates a significant opportunity to extend research on multi-century fire activity in relation to climate variability at mid- to high-latitudes (i.e. 43--55°S) in southwestern South America.The relative impacts of Native American and Euro-Chileans on fire activity were examined in six sample areas spanning latitudes 42 to 48°S in southern Chile. Pilgerodendron fire scars provide robust evidence that fire frequency increased after the arrival of Euro-Chilean settlers. Recent human-ignitions appear to be acting in concert with warming conditions to explain an increase in wildfire activity in southern Chile since the 1950--70s.The influences of climate on wildfires were investigated using Pilgerodendron tree-ring records of fire and variability of climate at interannual and multidecadal-scales. Relationships between fire and large-scale climate modes, including El Nino Southern Oscillation (ENSO), Pacific Decadal Oscillation (PDO), and the Southern Annular Mode (SAM) were established, in addition to the effects of phase interactions among these oscillations. Significant associations of wildfire activity at c. 48°S at interannual scales with positive SAM, and at multidecadal-scales with positive SAM and with negative PDO. When phase combinations of SAM, ENSO and PDO are considered, at both interannual and multidecadal time scales, the dominant role of positive SAM in creating climate conditions conducive for widespread fire becomes apparent. Compared to the pre-1950 period, the association of years of widespread fire with positive values of SAM strengthened after 1950. Climate models predict that the upward trend in SAM is likely to continue during the 21st century and consequently future climate conditions are likely to be conducive to increased wildfire activity in southern Chilean rainforests.Results from 31 sites sampled with 0.04 ha plots show strong pulses of Pilgerodendron uviferum mortality and establishment associated with fires in the early 1940s and 1950s that also coincide with abrupt changes in tree-ring widths. Spatially variable fuel loads and depth to water table in these habitats of Pilgerodendron bogs and adjacent tall forests, appear to account for highly variable fire severity and the patchy effects of fire-caused tree mortality and post-fire tree regeneration. This spatial heterogeneity of fire severity and fire effects indicates that the fire regime of Pilgerodendron vegetation should be classified as a variable severity (a.k.a. mixed severity) fire regime. Scarcity of post-fire Pilgerodendron regeneration after the 1970s is consistent with conservation concerns about the future extent of this species. Potential reasons for the relatively recent decline in the extent of post-fire regeneration of Pilgerodendron uviferum include a shift to less favorable climate conditions after the 1970s, unfavorable changes in water table levels induced by burning and loss of transpiring biomass, as well as possible synergisms between shortened fire intervals and cumulative effects of fire that have reduced seed sources. These are not mutually exclusive hypotheses, and each may have some applicability in different habitat types or in different parts of the range of Pilgerodendron. Although the mechanisms explaining local declines in post-fire regeneration of Pilgerodendron remain uncertain, observations and evidence collected over a large part of the range of this threatened tree species indicate that it is likely to continue to decline in abundance under present trends in climate and burning.