Spatiotemporal fire dynamics in mixed-conifer and aspen forests in the San Juan Mountains of southwestern Colorado, USA
Ecological Society of America
Mixed-severity ﬁre regimes may be the most extensive yet poorly understoodﬁre regimes of western North America. Understanding their long-term spatiotemporal dynamics is central to debates regarding altered ﬁre regimes and the need for restoration in the context of changing climate and nearly a century of active ﬁre suppression. However, the complexity of ﬁre patterns and forest stand and landscape structures characteristic of mixed-severity regimes poses a substantial challenge to understanding their long-term dynamics. In this study, we develop analysis methods aimed at understanding the ﬁre-driven forest dynamics of mixed-severity systems and apply them in the San Juan Mountains of southwestern Colorado. We sampled ﬁre scars, stand structure, and >4300 tree ages across two 1340-ha landscapes (Williams Creek and Squaretop Mountain) that span the environmental gradient of montane mixed-conifer and aspen forests. New approaches were applied to identify pulses of tree recruitment, evaluate climate and ﬁre as potential drivers of synchronous recruitment pulses, and combine ﬁre scar and recruitment data to reconstructﬁres. The reconstructions provided detailed ﬁre history for each stand, which in turn was used to develop a ﬁre-severity metric, compare ﬁre frequency and severity by forest type, and develop a simulation procedure to evaluate the degree to which tree regeneration depended onﬁre by species within each forest type.
Twenty ﬁres were reconstructed since 1685 at Williams Creek and 13 ﬁres since 1748 at Squaretop Mountain. Patterns of ﬁre severity varied within each ﬁre and over successive events, including high-severity patches of hundreds of hectares in both study areas. Dry mixed-conifer forests experienced relatively short ﬁre intervals (mean 21 years) and low ﬁre severity, and regeneration of the main conifer species was largely dependent on open conditions sustained over successive ﬁres. Moist mixed-conifer forests experienced longer ﬁre intervals (mean 32 years) and a broader range of severities, and ﬁre-caused canopy openings were important for initiating pulses of tree recruitment. Most (83%) aspen stands included two or more post-ﬁre cohorts. The methods presented here can be adapted to other mixed-severity systems to better understand their long-term spatial and temporal dynamics and develop restoration priorities.
Tepley, A. J., and T. T. Veblen. 2015. Spatiotemporal fire dynamics in mixed‐conifer and aspen forests in the San Juan Mountains of southwestern Colorado, USA. Ecological Monographs 85:583-603.