Postfire aspen presence, persistence and size in subalpine forests of Yellowstone National Park, USA. 1996 - 2014

Determining how ecological filters (e.g., climate, soils, biotic
interactions) influence where species succeed in heterogeneous
landscapes is challenging for long-lived species (e.g., trees),
because filters can vary over space and change slowly through time.
Stand-replacing wildfires create opportunities for establishment of
tree-species cohorts and can catalyze rapid shifts in where species
occur, facilitating unique opportunities for long-term study. We
quantified effects of multiple ecological filters on a colonizing
cohort of aspen (Populus tremuloides) that established from seed
throughout burned lodgepole pine (Pinus contorta var. latifolia)
forests after the 1988 fires in Yellowstone National Park (Wyoming,
USA) to ask: (1) How have aspen presence, density, and size varied
across the postfire landscape, and what filters explain these spatial
and temporal patterns? (2) How does aspen above-and belowground
biomass vary with postfire lodgepole-pine density? Aspen persisted to
postfire year 25 in 58% of the plots in which aspen were present in
postfire year 11 (n = 45), and mean stem density declined from 522 to
310 stems ha-1. Mean aspen height doubled (from 29 to 59 cm) over this
period. Ecological filters related to climate, competition, herbivory,
and soils all differentially affected aspen presence, persistence, and
size. Growing season temperature, inter-specific competition, and
herbivory also changed through time, altering their effects on the
colonizing cohort, and shifting where on the landscape aspen
persistence and growth were ultimately favored. Eleven years postfire,
aspen were favored at warmer, low elevations; ungulate browsing
strongly constrained aspen heights; and competition was unimportant.
By 25-years postfire, temperatures warmed nearly 1 C, and aspen were
more likely to persist at cooler, high elevations. Browsing pressure
declined, as ungulate populations decreased during this time, but
aspen height and basal diameters were constrained by dense, rapidly
growing postfire conifers. Landscape mosaics of ecological filters
shift over space and time and can facilitate or constrain the
persistence and growth of colonizing species. Long-term study of
post-disturbance colonizing cohorts uniquely reveal how species are
responding to real-time environmental change in heterogeneous
landscapes, which will help us better anticipate 21st century species
distributions and abundances.