Biophysical gradients and performance of whitebark pine plantings in the Greater Yellowstone Ecosystem

Abstract

The efficacy of planting efforts for species of conservation interest is important for ecosystem managers. Planting efforts represent an opportunity to conserve and manage species during a population crisis. Although federal agencies have been planting whitebark pine (WBP) in the Greater Yellowstone Ecosystem (GYE) for three decades, these efforts have been met with varying success. In this study, we use a combination of field sampling and remote sensing approaches in order to investigate local biophysical gradients as explanatory variables for WBP performance in GYE planting units. Present-day field sampling affords an opportunity to evaluate WBP performance relative to earlier planting and monitoring records. We used remotely-sensed temperature and precipitation alongside field measurements of elevation, aspect, slope, shading, and soils to utilize an adapted Thornthwaite-type water balance model to explain individual growth rates and site density change ratios (essentially survival and natural recruitment). We found that planting sites varied greatly in their biophysical characteristics and WBP performance. Five of twenty-nine sites had higher present-day density than at date of planting, therefore indicating some amount of natural regeneration occurring within those sites since time of planting. These sites were often higher in elevation, not south or southwest facing, and had soils that could hold moisture later in the season and for longer periods following precipitation events. Sites that experienced reductions in the density of WBP were often lower in elevation, with poorer soils, and facing south or southwest. They therefore experience greater potential evapotranspiration, and also greater water deficit when water demands are not being met. Notably, our two response variables, individual growth rate and site density change ratio represent short and long-term performance variables respectively. Although our results suggest that individual growth rates are likely more often limited by energy than water demands, the site density change ratio associated with this late to mature, long-lived species is likely a better benchmark for success. If they make it to maturity, trees planted this season will not begin to produce cones until the end of this century or the beginning of the next. Therefore, they must overcome forecasted periods of greater water stress in the coming decades and centuries. We strongly recommend planting efforts that seek to reduce the effects of increased drought stress by planting at sites with soils of greater water holding capacities (non-rhyolitic), planting on northerly and easterly aspects, and utilizing microsites particularly when planting at sites with potentially higher water stress. We also detected a negative relationship between the density of local competitors and WBP performance, but only at higher densities. Ecosystem managers will continue to plant WBP in the GYE for years to come, and this research helps to inform and identify high quality habitat during a period of changing climate and high GYE WBP mortality rates.