Seed Dispersal and Seedling Establishment of Pinon and Juniper Species within the Pinon-Juniper Woodland Jeanne C. Chambers Eugene W. Schupp Stephen B. Vander Wall Abstract-Understanding the prehistoric and historic dynamics of pinon-juniper woodland requires knowledge of the seed dispersal mechanisms and seedling establishment requirements of the tree species. Here, the types and effectiven-ess of the different seed dispersers and the environmental requirements for seedling estab- lishment are compared and contrasted for the various pinon and juniper species within the woodlands. The importance of long- distance vs. short-distance dispersal and the roles of ecotones and disturbance in woodland dynamics are discussed. Recommenda- tions for future research are given. The distribution of pinon and juniper species within the woodlands has undergone dramatic changes in both prehis- toric and historic times. Since the end of the Wisconsin Ice Age 12,000 years ago, some species have moved upward in elevation as much as 1,000 to 1,500 m and northwards as much as 6° latitude. For example, singleleafpinon (P. mono- phylla) has migrated from the warm deserts of southern Arizona and New Mexico and northern Mexico northwards through the Great Basin as far as southern Idaho. Colorado pinon (P. edulis) has migrated over most of the Colorado Plateau and southern Rockies (Betancourt 1987). Recent expansions of the woodlands appear to be influenced by human activities. Prior to settlement ofthe West in the early 1800's, fires burned through much ofthe woodlands as often as every 50 to 100 years, resulting in a mosaic of early seral grasses, mid-seral shrublands, and late seral woodlands (West and Van Pelt 1987). However, overgrazing by live- stock and a severe reduction in fire frequency has resulted in an increase in relatively unpalatable and fire-intolerant shrub and tree species throughout the woodland (West and Van Pelt 1987; Miller and others 1994). Trees are expanding into adjacent grasslands and shrub lands throughout their range (Johnsen 1962; Tausch and others 1981; Miller and Rose 1995), and tree density is increasing within existing stands (Tausch and others 1981). Information on seed and seedling ecology of the tree species is essential for understanding both the long- and In: Monsen, Stephen B.; Stevens, Richard, comps. 1999. Proceedings: ecology and management of pinyon-juniper communities within the Interior West; 1997 September 15-18; Provo, UT. Proc. RMRS-P-9. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station. Jeanne C. Chambers is Research Ecologist, USDA Forest Service, Rocky Mountain Research Station, 920 Valley Road, Reno, NV 89512. Eugene W. Schupp is an Assistant Professor of PI ant Ecology, Department of Rangeland Resources, Utah State University, Logan, UT 84322. Stephen B. Vander Wall is an Assistant Professor of Biology, Department of Biology, University of Nevada, Reno, NV 89512. USDA Forest Service Proceedings RMRS-P-9. 1999 short-term dynamics of pinon-juniper woodlands. Despite the fact that seed dispersal and seedling establishment processes are critical determinants of both the prehistoric migration and historic expansion of the woodlands, rela- tively little research has focused on this area. Here, we provide an overview of the state of our knowledge of seed dispersal and seedling establishment of pinon and juniper species within the woodlands. We then discuss the impor- tance of long-distance versus short-distance dispersal and the roles of ecotones and disturbance in woodland expan- sion. Finally, we suggest areas for future research. Seed Dispersal Processes ___ _ To understand plant dispersal processes, information on both the types and behaviors of the seed dispersers and the effectiveness of dispersal for plant establishment is neces- sary. Disperser effectiveness has been defined as "the contri- bution a disperser makes to the future reproduction of a plant" population (Schupp 1993). Effectiveness has a quan- titative component (the number of seeds dispersed) and a qualitative component (the likelihood that a dispersed seed will survive to produce a new plant in the population). Here, we examine the information on both the types and behaviors of the animals that disperse pinon and juniper and the effectiveness of those dispersers. Seed dispersal of both pinon and juniper is probably much more complex than the literature to date indicates. We have the best information on seed dispersal of pinon pines by birds. Pinon pines are dispersed by several species of corvids (jays and nutcrackers) that store seeds in shallow caches in the soil (Vander Wall and Balda 1981; Vander Wall 1990). Pinon pine cones and seeds are well adapted for dispersal by birds. The seeds are large and nutritious (Botkin and Shires 1948), and the cones are weakly constructed so that the seeds can be easily extracted by corvids with long pointed beaks, such as Clark's nutcracker (Nucifraga columbiana) and pinon jays (Gymnorhinus cyanocephalus). Seeds are enclosed in deep pockets and held by thin flanges so that they do not fall readily from the cones. Unlike many conifers, cones are primarily pointed to the side and upward, which not only retards seed loss, but increases the visibility and availability of the seeds to avian dispersers. Birds typically disperse seeds from several meters to 5 km (Vander Wall and Balda 1981). Differences in dispersal distances exist among different bird species with the more solitary scrub jays (Aphelocoma coerulescens) seldom dispersing seeds more than 1 km, the gregarious pinonjays 29 This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain.