The likelihood that subalpine forests will reburn before reaching reproductive maturity is growing as climate warms. For forests that historically burned at high severity every 100-300 years, regeneration after short-interval fires (< 30 years) may decline if in situ seed supply is low and surrounding forests are young. Seed dispersal from live trees into reburned forest supports ongoing tree seedling establishment, but how dispersal distances vary with the age/structure of nearby forests is poorly understood. We studied short-interval fires in subalpine forests to ask: (1) How does seed rain within live forest vary with age? (2) How does dispersal into burned areas vary with surrounding forest structure? (3) What variables best explain patterns of lodgepole pine dispersal? (4) How would spatial patterns of seed delivery into burned forest vary with age of surrounding forest? We expected that seed delivery would increase with tree height, cone abundance, and parallel winds. During summer 2018, we established 100-m transects (n = 18) extending from unburned forest of varying age (18, 30 and > 100 yrs) into burned areas in Grand Teton National Park. Seed traps were deployed for 3 months. Forest structure and cone abundance were measured at the origin of each transect.
Results/Conclusions
Seed rain was lower in 18-yr-old Pinus contorta var. latifolia forests (8 seeds m-2) compared to 30- and >100-yr-old forests (35 and 29 seeds m-2, respectively), which did not differ. Total seed delivery in burned patches was greater from >100-yr-old vs. 18-yr-old edges. More seeds traveled farther from >100-yr-old edges than from 30-yr-old and 18-yr-old edges (11.3 seeds m-2 at 30-m distance vs. 4.6 and 1.3 seeds m-2, respectively). Postfire seed delivery was primarily a function of distance from live forest edge and tree height. Empirical dispersal kernels estimate that seed delivery from ≥30-yr-old forest edges exceeds 0.06 seeds m-2 for greater distances (≥ 500 m) than seeds from 18-yr-old edges (< 265 m), which represents a biologically significant threshold of seeds necessary to maintain forest cover. Many locations with low seed dispersal occurred in areas of short-interval fire (< 30 yrs), where prefire forest lacked an in situ canopy seedbank. As increased fire frequency creates larger proportions of young postfire forests, immaturity risk will arise not only because in situ seed sources are low, but also because dispersal from young forests of short stature will be limited. Thus, tree regeneration after young forests reburn may face double jeopardy.