Biogeography and Diversity of Pines in the Madrean Archipelago

Pines are important dominants in pine-oak, pine and mixed-conifer forests across the Colorado Plateau, southern Rocky Mountains, Sierra Madre Occidental, and in the intervening Sky Islands of the United States-Mexico borderlands. All 17 native species of pines in the Sky Islands region or their adjacent mountain mainlands reach the northern or southern margins of their geographic ranges and most (12) have Madrean vs. Petran affinities. We compiled data on the occurrence and diversity of pines across 31 Sky Islands based on extensive fieldwork that complemented data from museum collections. Lower and smaller mountains supported zero to four species of pines whereas higher ones with greater area and thus larger population sizes supported five to seven species. Diversity of pines increased by 0.34% with each 1% increase in area (km2) of forest in a mountain range with similar effects for maximum elevation (P < 0.001); diversity decreased by 0.70% with each 1% increase in distance (km) to the nearest mainland (P < 0.015). Forest area explained >2 times more variation in pine diversity (R2 = 0.42) than distance from the nearest mainland (R2 = 0.19), suggesting that larger population sizes of pines on bigger mountains reduce rates of stochastic extinctions and that this process is more important than dispersal limitation in driving pine diversity. Although pine species have no legal conservation status in the Sky Islands, pine forests, especially those on small isolated mountaintops, are among the most threatened plant communities in this region due to increased re disturbance, insect outbreaks, and climate change.

(Conference Proceedings: Merging science and management in a rapidly hanging world: biodiversity and management of the Madrean Archipelago III, RMRS-P-67.)