Regeneration patterns reveal contraction of ponderosa forests and little upward migration of pinyon-juniper woodlands

A recent severe drought caused widespread mortality of ponderosa pine (Pinus ponderosa) and pinyon pine (Pinus edulis) in forests and woodlands in the southwestern United States. The sustainability of these tree species in the region depends on adequate regeneration and perhaps movement to more climatologically favorable locations. We investigated tree regeneration and species migration along elevation gradients in three community types (pinyon-juniper woodlands, woodland-forest ecotones, and ponderosa pine forests) and three soil parent materials (sedimentary, flow basalt, and volcanic cinder) at 27 sites in northern Arizona. We measured stand characteristics, historic cone production, and density of tree regeneration and small trees. All species produced cones at sites with cinder soil parent material but had little regeneration. Woodlands and woodland-forest ecotones had regeneration of pinyon pine and juniper despite earlier mortality of some mature pinyon trees. Regeneration of ponderosa pine was nearly absent in woodland-forest ecotones, suggesting eventual loss of ponderosa pine and transition to pinyon-juniper woodland. Forests had little regeneration and few small trees of any tree species, indicating little upward migration of pinyon pine and juniper. Occurrence of pinyon pine and juniper regeneration was associated with a suite of biotic and abiotic factors, including a negative relationship with temperature and a positive relationship with precipitation. Regeneration failure of ponderosa pine at woodland-forest ecotone and low-elevation forest sites coupled with limited upward migration of pinyon pine and juniper portends future losses of tree cover as climate continues to warm.