Mechanisms defining ecological range limits in a natural plant hybrid zone

Hybrid zones can be used to study the mechanisms that isolate species.  We investigated the role of physiological trait differences in determining the range limits of parent species and hybrids.  Our study system is an extensive hybrid zone between Penstemon newberryi and P. davidsonii along an elevational gradient.  To determine which physiological traits may determine elevational range limits, we measured stem water potential and phenology of both species and naturally occurring hybrids, and gas exchange rates of seven different hybrid crosses and the two pure parent species.  Alpine P. davidsonii had less negative pre-dawn and mid-day water potential, lower photosynthetic rates, and took less time to produce mature fruits than its montane relative, P. newberryi.  Hybrids were intermediate for most characters, but gas exchange traits differed depending on the genotype of the hybrid.  We performed a large-scale reciprocal transplant experiment, planting both species and hybrids into three gardens at low, middle, and high altitudes. Mortality differed depending on the elevation of the garden and the plant type, and F₁ hybrids survived well in all gardens. Our work so far demonstrates that plant types in this hybrid zone have some morphological and physiological traits appropriate for the specific elevation where they grow.  It is this habitat specialization that seems to be responsible for maintaining genetically and morphologically distinct parent species at the elevational extremes of their ranges.