Hybrid Larch of the Carlton Ridge Research Natural Area in Western Montana

Alpine larch (Larix lyallii Parl.) and western larch (L. occidentalis Nutt.) are deciduous conifers of the northern Rocky Mountains. The species usually are separated from each other by elevation, but they converge on a rocky outcrop in the Carlton Ridge Research Natural Area in western Montana. We compared six features of trees sampled in homogeneous populations of each species and in the population in the overlap area. Summer and fall foliage color, pubescence of current-year twigs, needle luster, roughness of three-year-old bark, and tree form separated the species. Fifteen of twenty trees sampled in the overlap area appeared to be hybrids, possessing features of both western and alpine larch or intermediate between them. The diversity in characteristics of the apparent hybrids suggests extensive introgression. At least one of the hybrids was exceptionally tall and large in diameter, suggesting the possibility of hybrid vigor. The Carlton Ridge Research Natural Area serves as a unique outdoor laboratory for study of natural hybridization between these two larch species and illustrates the value of the Research Natural Area System for conducting research in forest ecosystems.