In the Klamath region of Oregon and California stabilizing feedbacks between a mixed-severity fire regime and successional dynamics maintain two distinct ecological communities: conifer forests and shrub-chaparral-hardwood matrix. Decades of fire suppression, conifer planting, and cooler and wetter climatic conditions have favored the conifer state, resulting in a conifer dominated landscape. There is great concern that climate change will alter the fire regime and the post disturbance recovery dynamics in ways that will favor the shrub-chaparral-hardwood state. Such a shift in the Klamath region could impair the unparalleled botanical diversity of the region and release massive amounts of greenhouse gases as some of the most carbon-dense forests in North America transition to low-biomass shrub-chaparral-hardwood. Our collaborative multi-institutional study combines empirical field studies of post-fire recovery dynamics across aridity and time-since-fire gradients with mechanistic landscape simulations to better understand if anticipated changes in climate may alter the disturbance-recovery dynamics and force a regional-scale critical transition from mature conifer forest to shrub-chaparral-hardwood.
Our field data show that post-fire conifer recruitment has been limited to a narrow window after fire, with 89% of recruitment in the first four years. Early recruitment confers a competitive advantage whereby height growth decreases as the lag between the fire year and the recruitment year increases. This advantage of early recruitment is more pronounced at drier sites, suggesting that a future warmer and drier climate could inhibit conifers. Correspondingly, our landscape simulations show a reduction in conifer establishment associated with all four of the climate scenarios we examined. Simulated climate change reduced the mean fire rotation age by as much as twenty percent. Somewhat surprisingly, however, longer growing seasons and greater winter and spring precipitation that are associated with some climate change scenarios increased the conifer growth rate. This had the effect of offsetting some of the negative effects of increased fire frequency and decreased conifer establishment on simulated regional-scale conifer cover by the end of the century (i.e. 2100). Also surprisingly, the landscape simulations suggest that current widespread regional conifer cover is higher than would be maintained without management intervention (i.e. fire suppression and planting), and that, without management, shrub-chaparral-hardwood would expand under the contemporary fire regime and successional trajectories, even without climate change. In addition, simulations showed that shrub-chaparral-hardwood would expand under the contemporary fire regime and successional trajectories, even without climate change, suggesting the current expansive conifer cover is influenced by past management.