Strong dispersal limitation in postfire regeneration of Baker cypress, a rare serotinous conifer.

Abstract

Dispersal capacity primarily determines the spatial establishment patterns that drive range expansions and contractions in tree species. Seedling establishment in Baker cypress (Hesperocyparis bakeri [(Jeps.) Bartel]) relies predominantly on fire events, due to its cone serotiny, shade intolerance, and small seeds that require the optimal conditions of fire-exposed, mineral soil seedbeds. We quantified the density and spatial distribution of post-disturbance seedlings following the 2014 Eiler Fire in northern California and compared the observed recruitment to predictions from a mechanistic seed dispersal model. Postfire recruitment was dense, averaging 11 seedlings/m2 , and occurred primarily in the first year after fire. We estimated the mean descent velocity of the wingless seeds as ~4 m/s, the highest value reported for any putatively wind-dispersed tree species. The rapid seed descent contributed to markedly spatially constrained recruitment. Most seedlings (~81%) established within 5 m of the parent tree, and 94% established within 10 m. The maximum observed dispersal distance was 48.5 m; dispersal distance scaled linearly with canopy height. Distributions of modeled seed dispersal distance and observed seedling establishment in Baker cypress did not differ, demonstrating that wind disperses seeds a short distance because of the lack of a wing, and secondary dispersal appeared to be minimal at this recently burned site. If seed dispersal is solely reliant on wind, migration in response to rapid climate change will be impeded and potentially present difficulties in sustaining populations of this and other obligate seeder species with equally constrained dispersal.