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

Knowledge of the seed and seedling ecology of the pinon and juniper woodlan ds of western North America is essential for understanding both the northwa rd 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 pinon and juniper species within the woodla nds from seed development to seedling establishment, and discuss the implic ations of this information for the past and present expansion of the woodla nds. 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 lasses can occur during seed development due to dev elopmental constraints, and before or after seed maturation as a result of insects, pathogens, or predatory animals. In pinon pines, the primary seed dispersers are scatter-hoarding birds (corvids) and rodents that harvest se eds from the trees or after seed fall and cache them in the soil. In contra st, 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 junipe r seeds. Disperser effectiveness, or the contribution a disperser makes to the future reproduction of a plant population, may vary among species of pi nons and especially junipers. Pinon seeds are short-lived and exhibit littl e dormancy, and they probably only germinate the spring following dispersal . Juniper seeds are long-lived and seed dispersal can occur over one or mor e years. Seed germination can be delayed for several years due to impermeab le seed coats, embryo dormancy, or the presence of inhibitors. Seedling est ablishment of pinon pines is facilitated by nurse plants but, while juniper s 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 es tablishment. When considering the mechanisms involved in the past and present expansion of the woodlands, short-distance dispersal, local population growth, and lo ng-distance dispersal are all important. Different classes of dispersers, s ome of which appear to have coevolved with the tree species, appear to be r esponsible for local (short-distance) vs. long-distance dispersal in pinons and junipers. Because ecotones form the interface between the woodlands an d adjacent communities, they can provide valuable information on both the s eed dispersal and seedling establishment processes responsible for tree exp ansion. Disturbance regimes and, recently, the effects of humans on those regimes h ave major effects on the expansion and contraction of the woodlands. Before Anglo-American settlement, fires occurred as frequently as every 50-100 ye ars throughout much of the woodlands. During this century, fire frequencies have been reduced due to the indirect effects of livestock grazing and the direct effects of removing Native Americans from the ecosystem and impleme nting active fire-prevention programs. The result has been an increase in t ree-dominated successional stages at the expense of grass-dominated stages. Various management techniques, including controlled burning and chaining, have been implemented to reduce tree dominance, but their effects depend la rgely on the life histories of the tree species and the disturbance charact eristics. Several areas relating to the seed and seedling ecology of the pi non and juniper require additional research if we are to truly understand t he dynamics of the woodlands.