Evaluating topographic variation as a guide to Cassia crossbill refugia

Abstract

The Cassia crossbill (Loxia sinesciuris) feeds exclusively on seeds held in serotinous cones of lodgepole pine (Pinus contorta latifolia). Its population, confined to 67 km2 of pine in southern Idaho, declined by ~ 80% between 2003 and 2011 apparently because an increase in the occurrence of hot summer days (≥32 °C) caused the serotinous cones to open and shed their seeds. We hypothesized that the propensity for serotinous cones to open varies across the landscape depending on the maximum temperatures cones experienced. We tested a suite of abiotic variables for predicting the occurrence of open serotinous cones among 1547 trees at 170 sites. The best-supported model indicated that trees with open cones were less frequent on northeastern aspects and as slope increased. Fewer open cones were expected on northeast aspects because they receive less direct solar radiation. However, the decrease in trees with open cones as slope increased on southwest aspects was unexpected. Increased convection on slopes could reduce maximum cone temperatures and account for the decrease of open cones on steeper slopes, regardless of aspects. Although cones are less likely to open on steeper slopes, slopes with southerly aspects are likely the least favorable for pine recruitment and survival. The most favorable sites for both lodgepole pine and the persistence of closed cones are slopes on northeast aspects. Such locations are often dominated by subalpine fir (Abies bifolia). Reforestation of fir-dominated areas with lodgepole pine, therefore, provides a means to expand microclimatic refugia for Cassia crossbills.