Changes in forest structure since 1860 in ponderosa pine dominated forests in the Colorado and Wyoming Front Range, USA

Management practices since the late 19th century, including fire exclusion and harvesting, have altered the structure of ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) dominated forests across the western United States. These structural changes have the potential to contribute to uncharacteristic wildfire behavior and effects. Locally-relevant information on historical forest structure can improve efforts to restore more fire adapted conditions. We used a dendrochronological approach to reconstruct pre-settlement era (ca. 1860) structure for 170, 0.5-ha plots in montane ponderosa pine-dominated forests of the Colorado and Wyoming Front Range. Historical reconstructions were quantitatively compared with current conditions to highlight key departures. In lower montane forests, historical basal area averaged 6.3 m2 ha−1, density averaged 97 trees ha−1, and quadratic mean diameter (QMD) averaged 26.5 cm, while current basal area averaged 17.6 m2 ha−1, density averaged 438 trees ha−1, and QMD averaged 24.3 cm. Similar trends were observed in upper montane forests, where historical basal area averaged 9.5 m2 ha−1, historical density averaged 163 trees ha−1, and historical QMD averaged 29.4 cm, while current basal area averaged 17.2 m2 ha−1, current density averaged 389 trees ha−1, and current QMD averaged 25.2 cm. Most differences between historical and current conditions were significant. Across the montane zone, ponderosa pine dominated historical (88% and 83% of basal area in the lower and upper montane, respectively) and current forests (80% and 74% of basal area, respectively), but pine dominance decreased primarily due to infilling of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Much of this establishment occurred around the period of settlement (1861-1920) and continued throughout the 20th century. Results from this study help inform ecological restoration efforts that seek to integrate elements of historical forest structure and aim to increase the resilience of Front Range ponderosa pine forests to future wildfires and a warmer climate.