Recruitment of ponderosa pine seedlings in the Cascade Range

Abstract

Abiotic stresses on seedling regeneration in xeric ecosystems are great, hence recruitment processes can be facilitated by stand factors that ameliorate the germinant-scale microenvironment. An experiment was conducted on the eastern slope of the Cascade Range to test the effects of shrub cover, simulated seed caching, and substrate on the recruitment of Pacific ponderosa pine ( Pinus ponderosa var. ponderosa Dougl. ex Laws.) seedlings. Failure rates of seeds sowed in exclosures were large, with less than 30% emerging as germinants in the spring following fall sowing. Simulated seed caching improved emergence rates by more than sevenfold and was responsible for 88% of all spring germinants. Emergence rates were lowest from uncached seeds on litter. Just 16% of the crop survived the summer and fall to the month of November, or less than 5 months after emergence. Shrub cover did not affect emergence rates, but establishment rates were higher: seedlings beneath shrubs succumbed to desiccation at a slower rate than unshaded seedlings. By August there were 2.3 times more survivors at shrub-shaded sites than unshaded sites, and by the end of fall, when seedlings were considered established, more than 78% existed beneath shrubs. This study provides evidence that the natural recruitment of ponderosa pine seedlings is facilitated by the occurrence of the species’ common shrub associates.