Cone opening phenology, seed dispersal, and seed predation in southwestern white pine (Pinus strobiformis) in southern Colorado

Abstract

Southwestern white pine (Pinus strobiformis), a little-studied pine of the southwestern United States and northern Mexico, is closely related to and often confused with limber pine (Pinus flexilis). Like limber pine, southwestern white pine has large, wingless seeds, which are considered an adaptation for avian seed dispersal. We studied southwestern white pine in the San Juan Mountains of southern Colorado, an area of sympatry with Clark’s nutcracker (Nucifraga columbiana). First, we present morphological evidence confirming that the pines in our study area are southwestern white pine. Second, we present evidence that nutcrackers are important seed dispersers for the pine in this region and show that the cone opening phenology of southwestern white pine is asynchronous both among and within trees, a pattern adapted to avian seed dispersal. Red squirrels (Tamiasciurus hudsonicus) proved to be the major predator of southwestern white pine seeds, removing 93% of the cones in two of our study stands. Southwestern white pine cones show a combination of traits that facilitate avian seed dispersal and discourage squirrel predation. However, anti-squirrel defences fall short of those in limber pine; this may be due, in part, to gene flow from more southern populations without squirrels.