Disjunct and decoupled? The persistence of a fire-sensitive conifer soecies in a historically frequent-fire landscape

Local and regional species extirpations may become more common as changing climate and disturbance regimes
accelerate species’ in situ range contractions. Identifying locations that function as both climate and disturbance
refugia is critical for biodiversity conservation. Here, we investigate the persistence of a disjunct, fire-sensitive
conifer population, yellow-cedar (Callitropsis nootkatensis), in the historically frequent-fire landscape of the Blue
Mountains in eastern Oregon, USA. We used tree rings to reconstruct multi-century fire histories, which were
then used to compare historical mean fire return intervals (MFRIs) inside of the cedar grove to the surrounding
dry forest matrix, as well as to examine relationships between historical fire occurrence and reconstructed climate.
We also examined trends in post-fire yellow-cedar mortality and regeneration between 2006, when the
grove burned in a wildfire, through 2017. Results indicated that fire was less frequent in the cedar grove
(MFRI=35.8 years) than in the surrounding dry mixed-conifer forest (MFRI=14.4 years). Historical fire occurrence
was associated with hotter and drier conditions. Following the 2006 fire, cedar mortality was high
(>90 %), but by 2017 post-fire regeneration was abundant (median=8125 seedlings ha−1). The eastern
Oregon cedar grove appears to occupy a fire refugium historically decoupled from the frequent-fire regime of the
broader landscape. The topographic position of the grove suggests it is also a climate refugium. Managing for
climate and disturbance refugia has been identified as an important option for conservation in the context of
rapid global change, and this study provides evidence that locations where climate and disturbance refugia
overlap may be disproportionately important for the maintenance of regional biodiversity. However, active
management may be required for populations to persist in some of these locations as anthropogenic change is
capable of overwhelming the underlying buffering capacity of refugia.