Synergistic influences of climate and burning practices on tree regeneration in western Patagonia temperate rainforests

Background/Question/Methods

Temperate rainforest ecosystems in western Patagonia, dominated by fire-sensitive species have the potential to suffer long lasting changes in their resilience to recover from altered fire regimes mediated by interacting changes in climate and land use. However, little is known about the variability in the post-fire response of these species and links to climate and land use changes.  Stand structural and paleoecological data from temperate rainforests in ecotonal bogs in western Patagonia was analyzed to study the response of dominant fire-sensitive species to changes in fire regimes, and linked to recent changes in land tenure and burning practices (by Indigenous and Euro-Chileans), and to climate variability.  We focus on Pilgerodendron uviferum, a slow-growing and southernmost conifer which is the dominant of the rainforests and adjacent bogs, and is believed to be threatened because of apparently poor regeneration in some habitats. We used tree-age frequency distributions and tree-juveniles densities and plant cover to examine tree establishment patterns of all dominant tree species in 31 (0.04ha) plots. Tree-establishment patterns were compared to annually-resolved fire history reconstructed from local tree-ring fire scars, to decadal changes in human-impact on fire activity, and to decadal-scale changes in climate variability over the past 70 years.

Results/Conclusions

Over the last 70 years, fire-sensitive tree species successfully established following most of the reconstructed fire events in most study areas.  Stand structures and fire-scar frequencies suggest that these ecotonal temperate rainforests experienced a mixed-fire regime, as suggested by varying degrees of tree mortality associated with different fire events.  Fire frequency increased at all sites following permanent Euro-Chilean settlement.  Since the ca. 1970s however, most sites document a sudden and steady decline in tree of establishment of P. uviferum (the most fire-sensitive tree species) that cannot be explained by an obvious change in burning practices.   We argue that this change in tree establishment trend resulted from an increase in the frequency of ignition that coincided with warmer and drier climatic conditions, which in turn resulted in a few severe crown fire events that eliminated the seed bank. Our results place modern fire activity in these ecotonal temperate rainforests in the context of the projected warmer and drier climate conditions for the next century, and raise important questions about appropriate management responses due to the poor resilience of these ecosystems in response to continued high levels of fire activity.