A short-interval reburn catalyzes departures from historical structure and composition in a mesic mixed-conifer forest

Abstract

Recent increases in fire frequency and severity across the western US are triggering abrupt changes in ecosystem structure and composition, especially in lower montane forests, but consequences of fire-regime change for mesic, mixed-conifer forests remain uncertain. Glacier National Park (Montana, USA) is characterized by a complex mosaic of species typical of Rocky Mountain, Pacific Northwest and boreal floras, creating opportunities for species composition to shift as species respond individualistically to environmental change. We quantified prefire and postfire stem density and composition in a recent anomalously short-interval fire (15 yrs) and historically typical long-interval fire (88–282 + yrs) to answer two questions: (1) How do the structure and composition of tree regeneration differ after long- and short-interval fires that burned in close proximity? (2) What drivers explain differences in postfire regeneration densities among species? We additionally compared understory cover in each fire to anticipate consequences of contemporary fire for the whole plant community. Mean postfire tree seedling density was 45 times higher after the long- than short-interval fire, and many stands in the short-interval fire lacked any tree regeneration. Postfire tree seedling densities were above prefire stem densities in the long-interval fire but well below in the short-interval fire. After both fires, fire-avoiding conifers (e.g., Picea engelmannii, Abies lasiocarpa) were generally replaced by fire-tolerating or fire-embracing species (e.g., Larix occidentalis, Pinus contorta var. latifolia). Some stands remained dominated by fire-sensitive conifers after long-interval fire but never after short-interval fire. Seedlings of Thuja plicata and Tsuga heterophylla were present after the long-interval fire but absent before and after the short-interval fire, despite historically occupying both burned areas. For all species, seedling density increased with indicators of proximate seed supply. Seedling density of dispersal-dependent species declined with distance to live trees, whereas P. contorta responded to serotiny. Seedlings of drought-intolerant species were more abundant on northerly aspects. Total understory cover was 50% lower after short-interval than long-interval fire, but shrub cover was twice as high. Our study suggests that persistent seed sources will allow L. occidentalis to expand following sequential high-severity fires, while dispersal-limited, drought-intolerant conifers may decline due to loss of seed sources and elevated postfire aridity. Increased fire activity will likely produce sparser forests that are dominated by fire-tolerating and fire-embracing conifers at the expense of fire-avoiding and fire-refuge species. Complete regeneration failure may be delayed by high fire-trait syndrome diversity, but the structure and composition of extant forests will shift.