Seed and seedling ecology of piñon and juniper species in the pygmy woodlands of western North America

Knowledge of the seed and seedling ecology of the piñon and juniper woodlands of western North America is essential for understanding both the northward migration and expansion of the woodlands during the Holocene (< 11,500 B.P.), and the accelerated expansion of the woodlands since settlement of the West by Anglo-Americans around 200 years ago. We follow the fates of seeds and seedlings of the different piñon and juniper species within the woodlands from seed development to seedling establishment, and discuss the implications of this information for the past and present expansion of the woodlands. While seed development requires about two and one-half years in pinons, it is species-dependent in junipers and can take one, two, or even three years. Substantial seed losses can occur during seed development due to developmental constraints, and before or after seed maturation as a result of insects, pathogens, or predatory animals. In piñon pines, the primary seed dispersers are scatterhoarding birds (corvids) and rodents that harvest seeds from the trees or after seed fall and cache them in the soil. In contrast, most junipers appear to be dispersed primarily by frugivorous birds and mammals that ingest the seeds and defecate them onto the soil surface. We have recently documented that scatter-hoarding rodents also disperse juniper seeds. Disperser effectiveness, or the contribution a disperser makes to the future reproduction of a plant population, may vary among species of piñons and especially junipers. Piñon seeds are short-lived and exhibit little dormancy, and they probably only germinate the spring following dispersal. Juniper seeds are long-lived and seed dispersal can occur over one or more years. Seed germination can be delayed for several years due to impermeable seed coats, embryo dormancy, or the presence of inhibitors. Seedling establishment of piñon pines is facilitated by nurse plants but, while junipers often establish beneath nurse plants, they are capable of establishing in open environments. In the southwestern United States, higher establishment of juniper occurs in open environments due to more favorable precipitation, and competition may be more important than facilitation in determining establishment.