In North American temperate and boreal forests, granivorous rodents (Order Rodentia, Families Sciuridae, Heteromyidae, Cricetidae) and jays and nutcrackers (Order Passeriformes, Family Corvidae) are important seed dispersers for forest trees, shrubs, and understory plants. These animals exhibit “directed seed dispersal,” which entails burial of seeds for retrieval during times of food scarcity in microsites that facilitate plant recruitment. In general, rodents are not major dispersers of forest tree seeds to post-fire landscapes. Jays and nutcrackers often disperse seeds farther than 1 km, connecting fragmented landscapes and fostering recovery from disturbance. In contrast, most winged conifer seeds are dispersed by wind within 100 m of the parent tree. Here, we review the roles of jays and nutcrackers in post-fire forest community development. We focus on post-fire forest regeneration effected by Clark’s nutcracker (Nucifraga columbiana), a keystone seed disperser and mobile link among populations of white pines (Pinus, subgenus Strobus) with large, wingless seeds but also among conifers with winged seeds (Pinus, Pseudotsuga). In particular, whitebark pine (Pinus albicaulis) obligately depends on nutcrackers for seed dispersal, whereas other nutcracker-dispersed pines exhibit mixed seed dispersal strategies.
Results/Conclusions
Jays cache tree nuts and pine seeds in burned chaparral, pine, and oak communities, initiating regeneration. Clark’s nutcrackers are highly effective seed dispersers in post-fire landscapes, carrying 100 seeds at a time up to 30 km, caching seeds from tundra to lower treeline elevations, and facilitating early colonization and gene flow. Whitebark pine seeds remain viable for years within a soil seed bank, including burned seedbeds, but typically require two or more years to germinate. Whitebark pine regeneration densities in ~25-year-old post-fire landscapes declined with distance from the seed source following a negative exponential curve, with regeneration extending >8 km into the burn. The oldest whitebark pine in these burns and after the 1988 Yellowstone fires established 3 to 6 years after fire. In the Northern Rocky Mountains, where whitebark pine decline from mountain pine beetle (Dendroctonus ponderosae) and white pine blister rust (Cronartium ribicola) has reduced cone production and nutcracker visitation, we see lower whitebark pine regeneration rates across burned landscapes. Given predictions of decreasing fire return intervals and larger, more severe wildfire with climate change, we modeled the probability of future whitebark pine extirpation. We found that long distance seed dispersal by nutcrackers provides a “rescue effect.”