Abstract
Forest seed dispersal is altered after fire. Using seed traps, we studied impacts of fire severity on timing of seed dispersal, total seed rain, and seed rain richness in patches of high and low severity fire and unburned Douglas-fir (Pseudotsuga menziesii) forests in the Fischer and Tyee fire complexes in the eastern Washington Cascades. Unburned plots had the lowest average seed production. The high severity fire patches in the Fischer Fire Complex had a higher total seed production than low severity fire patches of the same complex. At the Tyee Fire Complex, the total seed production for each of the two fire severities was similar, but the period of maximum seed dispersal was later for high severity than low severity fire. Seed rain at the Fischer Fire patches (sampled one year after the fire) was predominantly composed of annual species, while that of the Tyee Fire patches (sampled nine years after fire) was predominantly perennial species. Seed rain richness was greater in Tyee high severity patches than paired low severity fire patches. In these paired Tyee patches the average number of new seed species (species not found in the extant plot vegetation) was greater for high severity than low severity fire. Our results suggest that high severity fire plots are more porous to seed rain than low severity plots. Intact forest canopies may filter seed rain and reduce seed influx, while high severity fires are more open to invasion by seed dispersal.
Highlights
Forest fires and other ecological processes produce complex patterns of vegetation on the landscape
Hartnett and Richardson (1989) found evidence that seed rain from Florida lady’s nightcap (Bonamia grandiflora) was 10 to 30 times greater after a fire, an increase they ascribed to increased stem densities from both asexual growth and new genet recruitment from seed in the year after fire
We hypothesized that seed dispersal into burned forests would be affected by fire severity, and that more dispersal would occur in high severity than low severity fire patches owing to filtering effects of intact canopies
Summary
We studied impacts of fire severity on timing of seed dispersal, total seed rain, and seed rain richness in patches of high and low severity fire and unburned Douglas-fir (Pseudotsuga menziesii) forests in the Fischer and Tyee fire complexes in the eastern Washington Cascades. Solbreck and Andersson (1987) trapped fireweed (Epilobium angustifolium) seeds using a vacuum pump 90 m above the ground in Sweden They showed that 20 % to 50 % of dispersing fireweed seeds may reach 100 m or more above the ground, suggesting that newly burned sites some distance away may be readily colonized by this species. We quantified seed production and seed species richness at two fires, in patches of different burn severity and in unburned forest. We hypothesized that seed dispersal into burned forests would be affected by fire severity, and that more dispersal would occur in high severity than low severity fire patches owing to filtering effects of intact canopies.
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