Trends In GYE

The long-term GYE forest recovery trend also differs by ownership. Forests on national forest land generally grow back faster than forests in national parks and wilderness areas. During the two decades following the catastrophic fires in 1988, national forests consistently had higher values of forest recovery than national parks and wilderness areas. Despite the short-term similarity to the wilderness areas in the first few years after the 1997 fires, forest recovery in the GYE national forests rapidly outpaced the rest of the GYE forests.
The recovery of the forests following major harvesting years in the GYE NFs (1985-1990, Figure 6c) exhibits the following trends. (1) The percentage of forest that recovered from a harvest in the GYE by year 2011 generally depends on the year of the harvest, and the recovery trajectory can be grouped into two recovery periods, before the late 1980s (the earlier years) and the late 1980s. (2) For harvests in the earlier years (1985 to 1987), over 85% of the harvested area has returned to forest by 2011. Harvests that occurred in late 1980s (1988 to 1990) have lower percentages of forest recovery than the early years, and the recovery trajectories differ between the two time periods. (3) For the earlier
harvest period, two rapid recovery intervals occurred in the early 1990s and mid-2000s. In contrast, for the late 1980s harvesting period, the average recovery speed was slow for the first 15 years after the harvest, followed by rapid regrowth over the last decade.
Among different disturbance types, post-harvest forest recovery rates are consistently higher than post-fire forest recovery rates in the GYE.
The percentages of forest recovery reach 50-90% following the 1980s’ harvests, whereas the highest percent of forest recovery following the 1988 fires is less than 40% by the year of 2011. Even only considering the national forest land, post-harvest forest recovery rates (72% for harvests that occurred in 1988) are still much higher than the post-fire forest recovery rates (36% for fires that occurred in 1988) by year 2011. Further investigation into the potential causes of different forest recovery rates among different ownership and disturbance types is discussed in the following
sections.
An examination of the recovery percentages of major forest types in YNP following the 1988 fires (Figure 7b) shows that Whitebark Pine, Engelmann Spruce and Subalpine Fir at higher elevations have very low recovery percentages (less than 10%). Lodgepole Pine has the highest percent of forest recovery by 2011 (more than 30%), followed by Douglas-fir with more than 25% of the forest having recovered from the 1988 fires.
The Random Forest OOB error rates for all major species range from 4.5% to 18.2%, revealing a high prediction accuracy for modeling the impact of environmental variables on forest recovery conditions for the major forest types following the 1988 fires in YNP (Table 4). High-elevation Whitebark Pine and Spruce/Fir forests have the lowest OOB rates, i.e., 4.5% to 5.4%, whereas Lodgepole Pine and Douglas-fir have higher OOB rates, i.e., 13.4% to 18.2%.
Forest distributions in YNP are generally related to elevation and soil gradients, which also play important roles in predicting YNP forest recovery rates following the 1988 fires (Table 4). Using the stepwise ascending variable selection method, the most important variables have been selected to demonstrate the effects of environmental parameters on the binary forest regrowth conditions. The results suggest that topography (elevation and aspect) and post-disturbance climate play critical roles in the recovery of all four major forest species by 2011 in YNP.
We further examined the most important predictors for forest recovery between the 1988 fires and 2011 in YNP (Figures 8 and 9). Due to its spatial predominance in YNP (over 72% of YNP’s forested area), the Lodgepole Pine forest was selected for additional analysis. The five most important predictor variables were identified via the stepwise elimination approach: soil type, post-fire spring precipitation anomaly, slope, elevation, and northness. The impact of elevation on forest recovery in YNP (Figure 8a) is evident in the significantly lower recovery rates of Whitebark Pine, Engelmann Spruce and Subalpine Fir, which are located at higher elevations than Lodgepole Pine and Douglas-fir. The percentages of forest recovery for these higher-elevation forest types are approximately one-third those of the lower-elevation species. Soil also plays an important role in post-fire forest recovery, mainly because of the unique biophysical environment in YNP. The shallow and nutrient-poor Inceptisol and bedrock soils can only support Lodgepole Pine forests, a fire-dependent and fast-growing species, and these soils were associated with the highest percentages of forest recovery by 2011 (Figure 8b). The other forest types, especially Engelmann Spruce and Subalpine Fir, usually grow in relatively deep and nutrient-rich Mollisol soils.
After the most important variables were identified, partial dependence plots of the four continuous variables (Figure 9) were used to explore the relationships between these environmental conditions and the post-fire recovery of the Lodgepole Pine forest in YNP. The post-fire spring precipitation anomaly and northness factors generally had a positive impact on Lodgepole recovery after the 1988 fires, meaning sites with higher post-fire spring precipitation anomalies and sites that faced true north tended to recover faster. The number of years needed for Lodgepole Pine recovery exhibits more non-linear piecewise dependencies on slope and elevation. From 15 to 25 degrees, the number of years until recovery is strongly dependent on the slope and decreases with increasing slope. In the elevation range of 2200 to 2500 meters, the time required for recovery is strongly dependent on elevation and increases with increasing elevation.