Pinus monophylla establishment in an expanding Pinus-Juniperus woodland: Environmental conditions, facilitation and interacting factors

The tree species comprising Pinus-Juniperus woodlands are rapidly expanding into shrub-grasslands throughout their range. Observational studies indica te that establishment is facilitated by nurse plants, but little informatio n exists on the mechanisms involved. I examined both abiotic and biotic fac tors influencing Pinus monophylla establishment in Artemisia tridentata ste ppe with expanding populations of P. monophylla and Juniperus osteosperma. I also examined the effects of seed burial and predation on seedling establ ishment. Microhabitats under trees and shrubs had higher extractable P and K, higher organic matter, total nitrogen and cation exchange capacity than interspace microhabitats. Soil water contents (0-15 cm) were lower in inter spaces than under shrubs or trees due to dry surface (0-5 cm) soils. Soil t emperatures (at 1 and 15 cm) were lowest under trees, intermediate under sh rubs, and highest in interspaces. Timing and rate of seedling emergence wer e temperature dependent with the order of emergence paralleling mean growin g season temperatures: tree interspace = shrub interspace > under shrub > u nder Juniperus greater than or equal to under Pinus. Seed burial was requir ed for rooting and the highest emergence occurred from depths of 1 and 3 cm indicating that caching by birds and rodents is essential and that animals bury seeds at adequate if not optimal depths for emergence. Seedlings requ ired microenvironmental modification for survival; all seedlings, including those that emerged from seeds and transplants, died within the first year in interspace microhabitats. Survival in under-tree or under-shrub microhab itats depended on soil water availability and corresponded closely to soil water contents over the 3-yr study. Under-shrub microhabitats had more favo urable soil and micro-environmental characteristics than under-tree microha bitats and had the highest seedling life spans for the first-year seedling cohort. predation of Pinus seedlings by rodents was a significant cause of mortality with caged transplants exhibiting life spans that were 74 % longe r overall than uncaged transplants. Emergence and survival of P. monophylla within the expanding woodland were dependent upon a complex set of interac ting factors including growing season conditions, microhabitat characterist ics. and animal species.