In the desert southwest, significant variations in moisture and temperature occur along steep altitudinal gradients. Ponderosa pine forests in the Santa Catalina Mountains, southeast Arizona, USA, consist of two partially sympatric species, the variable needle Pinus ponderosa var. brachyptera (3- and mixed-needle) that prefers cool and moist conditions, and the 5-needle P. arizonica that prefers hot and dry conditions. The objective of this research program is to determine how disturbance–that is variations in climate–contributes to shifts in population structure. Data was collected along two south-facing slopes (of similar elevation and aspect) for average needle number per fascicle, tree-ring widths, and gene frequencies, to examine limiting factors to growth, climatic shifts in growth, and population genetic structure. Our data show that co-occurring ponderosa pine needle-types (3-, mixed-, and 5-needle) have filled different ecological niches; their growth is limited by seasonal water availability, which also controls the length of the growing season. Three distinct genetic groups are present in the Santa Catalina Mountains. Genetic variability is reduced for the 5-needle type suggesting a possible bottleneck or founding event. Results from this study about moisture limitations will help land managers to determine range limits for seed planting zones.
Read
