Fire-Climate-Vegetation Interactions in Subalpine Forests of the Selway-Bitterroot Wilderness Area, Idaho and Montana, USA

The long term patterns of fire-climate interactions and forest recovery processes in subalpine forests are poorly understood. This study used a suite of dendrochronological techniques to identify tree growth-climate relationships, assess the interactions of fire with interannual climate variability, and reconstruct summer temperature in subalpine forests of the Selway-Bitterroot Wilderness Area on the border of Idaho and Montana, USA. Comparison of ring-width chronologies from whitebark pine ( Pinus albicaulis Engelm.) and subalpine larch ( Larix lyallii Parl.) with modern climate data indicated that summer temperatures were most limiting to growth in these conifers. Warm summers were generally conducive to radial growth. However, the temporal stability of the climate-tree growth relationship weakens from the early to later periods of the record. Alterations to growing season length, possibly modified by snow pack, may be related to the reduction in climate-growth relationships. A 748-year reconstruction of average summer temperature was developed that explains [approximate]36% of the variance of the instrumental record. Positive values of the coefficient of efficiency and reduction of error verification statistics indicated that the reconstruction was of good quality. Warm and cool periods in the reconstruction include a warm decade around the 1650s and prolonged cooling around 1700. Peaks in variance in reconstructed average summer temperature occurred at 87, 15, and 2 years. More than 2000 fire scar and age structure samples were used to evaluate fire-climate relationships. Comparison of widespread fire events to climate variables indicated dry conditions both during the fire year and one year before a fire. Multiple spatial patterns of drought and El Niño were related to widespread fire occurrence. Forest recovery following fires generally proceeds from lodgepole pine ( Pinus contorta var. latifolia Dougl.) toward spruce-fir forests ( Picea engelmannii Parry- Abies lasiocarpa (Hook) Nutt.). Two successional pathways occur, one beginning with an initial lodgepole pine stage, the other a spruce-fir stage. Initial composition was related to the presence of overstory lodgepole pine at the time of fire occurrence as well as the intervals between successive fires. Collectively, these results suggest a strong multi-year drought linkage between climate and fire, and dependence on fire intervals for structuring forest communities.