Plant recruitment in a northern Arizona ponderosa pine forest: Testing seed- and leaf litter- limitation hypotheses

Seed availability and leaf litter limit plant establishment in some ecosystems. To evaluate the hypothesis that these factors limit understory plant recruitment in Pinus ponderosa forests, I conducted a seeding and litter removal experiment at six thinned sites in the Fort Valley Experimental Forest, northern Arizona. Experimental seeding of four native species (Penstemon virgatus, Erigeron formosissimus, Elymus elymoides, and Festuca arizonica) and raking of litter occurred in 2005. Seeding resulted in a substantial recruitment of 14 to 103 seedlings/m2 (1 to 10/ft2) one month after seeding for two species (P. virgatus and E. elymoides), but these densities subsequently declined by 13 and 27 months after treatment to near control densities. No P. virgatus adults established, and seeding also did not significantly increase densities of E. elymoides adults. Litter removal and seeding did not interact, as seedling density on raked + seeded plots did not differ from density on seed-only plots. Consistent with a previous experiment in these forests, litter removal also had no effect on plant richness or cover. Results suggest that (i) factors other than seed availability limited recruitment of adult plants of the four seeded species, and (ii) leaf litter did not limit plant recruitment.