Population Condition of Populus x smithii, a Pleistocene Relict Aspen of the Niobrara River Valley, Nebraska, USA

At Smith Falls State Park in northern Nebraska, a relict population of Populus x smithii, a hybrid of quaking aspen (P. tremuloides) and bigtooth aspen (P. grandidentata), has been targeted for management due to its rarity and risk of extirpation. This relict occurs under favorable microclimatic conditions in springbranch canyons along the Niobrara National Scenic River, but factors such as climate change, lack of regeneration, and competition with invasive juniper (Juniperus virginiana) make its persistence tenuous. This study was designed to assess the overall health of the Smith Falls aspen population and determine whether individual stands (n = 10) differed in condition, whether these differences in condition were related to environmental or demographic variables, and whether the condition of the stands was worsening. We found evidence of a declining aspen population, with crown loss and limb dieback documented across all size classes and worsening over a period of two y. Trees from younger, smaller size classes displayed less damage than older, larger ones, but accrued more damage over the two-y interval. At the stand level, higher levels of damage occurred in stands with larger area, more individual boles, and larger, older trees. Our findings suggest that this unique relict population of aspen is showing significant signs of decline but has potential to survive if managed with ecological principles of succession in mind.