Influences on vegetation structure 55 years after clearcutting and broadcast burning, and wildfire in a western larch forest

Abstract Background Experimental forests present unrivaled opportunities to understand the factors influencing vegetation structure many decades after silvicultural treatments or natural disturbances. In this study, we use the western larch-dominated moist mixed-conifer forests at Miller Creek Demonstration Forest (Miller Creek) to better understand the relationship between current vegetation structure, 55 years after two forms of high-intensity disturbance (clearcutting plus broadcast burning, and a high-severity wildfire) and characteristics of the site, the disturbance, and pre-disturbance vegetation. We also utilize measurements of fire intensity to test the hypothesis that moderate intensity fires promote greater fine-scale vegetation spatial heterogeneity, many decades disturbance, than low or high fire intensities. Results Fifty-five years after high-intensity disturbance, forest cover has returned to all sample sites at Miller Creek. Further, these sites support the full suite of tree species common to this forest type; although western larch is generally dominant in the overstory, its more shade-tolerant associates are abundant in the understory. Our results suggest that fire intensity and heat load index, used as summaries of disturbance and site characteristics, respectively, are associated with current overstory vegetation. By contrast, our data implies that understory vegetation at Miller Creek is more strongly influenced by a variety of site factors. Our results provide no evidence of a relationship between fire intensity and spatial heterogeneity. Conclusions Our findings suggest that characteristics of both the initiating disturbance and the site influence vegetation structure at Miller Creek. Moreover, five-plus decades post-disturbance, the understory plant community is more closely aligned with habitat type than is the overstory. However, while western larch is likely to maintain overstory dominance at Miller Creek for many decades, our findings suggest that perpetuating this species alongside a diverse understory may call for future moderate- to high-intensity harvests with overstory retention and broadcast burning.