Forest Restoration in a Surface Fire-Dependent Ecosystem: An Example from a Mixed Conifer Forest, Southwestern Colorado, USA

Fire suppression in the warm and dry mixed conifer forests of southwestern Colorado has created changes that have disrupted ecosystem feedback interactions between vegetation composition and the region’s natural fire regime. This fire suppression has made the ecosystem more susceptible to high intensity fires that were previously absent in this forest type. Thus Korb et al. (2012) established four replicated treatments, each about 16-ha, of 1) thin/burn, 2) burn alone and 3) control to quantify the effects of restoration treatments on forest structure. Thinning trees meant removing them usually by chopping them down  while control meant no treatment. They sampled the pre-treatment plots in 2003 and post-treatment plots in 2009. There were no meaningful changes between pre and post-treatment in the control and burn alone treatments for canopy cover, basal area, tree density, and tree regeneration. There were significant changes in the thin/burn treatments with basal area declining, tree density declining, tree canopy cover decreasing, white fir regeneration decreasing and aspen tree regeneration increasing. Analysis of tree basal area by species in the thin/burn treatments in 2009 revealed a strong shift away from 2003 pre-treatment data towards the reconstructed historical (1870) forest structure, while burn alone treatments were distinct from controls after treatment in 2009. Thin/burn treatments moved warm/dry mixed conifer forests in southwestern Colorado toward historical reference conditions by altering forest composition and structure. Forest restoration will make forests more resilient to stand-replacing fires and subsequent transitions to novel ecosystems under a warmer drier climate. –Loren Stutts
Korb, J.E., Fulé, P.Z., and Stoddard M.T. 2012. Forest restoration in a surface fire-dependent ecosystem: An example from a mixed conifer forest, southwestern Colorado, USA. Forest Ecology and Management 269, 10-18, doi:10.1016/j.foreco.2012.01.002.

            Korb et al. claim there is an abundance of evidence that suggests 20thcentury fire suppression in ponderosa pine and low elevation mixed conifer forests in the southwestern U.S. has created changes in forest structure, composition, and ecological processes. Temperature and moisture are the crucial drivers that influence fire regimes and species composition for the mixed conifer forests in the San Juan Mountains of Southwest Colorado. Ponderosa pine and Douglas fir dominate the warm/dry mixed conifer forest while cool/moist mixed conifer is dominated by white fir, Douglas-fir, aspen, and blue spruce. More than a century of fire suppression in warm/dry mixed conifer forests has shifted species composition toward more shade tolerant species such as Douglas-fir and white fir, increased surface and aerial fuels as well as increased tree density.
            Under a warmer and drier climate, Korb et al. assert that the use of site-specific reference conditions is a scientifically sound target for forest-stand conditions in fire-dependent forest ecosystems because they increase tolerance to uncharacteristic fire behavior. Thus they created a controlled experiment in the warm/dry mixed conifer forest of the San Juan Mountains of Colorado to evaluate forest change and to test restoration alternatives. They established four replicated blocks of three treatments, each about 16 ha: 1) thin/burn, 2) burn alone, and 3) control. Burn alone treatment was included to determine if restoration goals could be attained without tree thinning. All treatments were tested against site-specific reconstructed reference conditions.
            They attempted to quantify post-treatment differences in forest composition and structure among treatments and compare post-treatment stands with site specific reference conditions and quantify changes in untreated controls over a six year period (2003-2009) to evaluate the stability of warm-dry mixed conifer stands.
            The study area is located in the San Juan Mountains in southwest Colorado within the San Juan National Forest. Forest vegetation includes ponderosa pine, white-fir, Douglas fir, and aspen. Fire suppression has been the management policy in the region since the early twentieth century. The thinning prescription retained all living trees established in 1870 or earlier as identified by canopy architecture, size, and bark color. The trees designated for thinning were mostly white fir and a bit of Douglas fir. Logs and limbs were lopped and scattered while old growth trees were not raked to remove fuels around tree boles.
            Korb et al. produced 20 permanent study plots on a 60-m grid per unit to characterize forest structure and vegetation. They collected pre-treatment data in the summer of 2003 and post treatment data in the summer of 2009. Species, crown base height, condition, diameter at breast height, total height, field classification of pre-settlement or post-settlement origin were recorded for each tree encountered in the plot. Tree regeneration, or trees below breast height, was measured on a nested circular plot; species, height class, and condition were recorded for each seedling or sprout. Tree canopy cover was recorded using a vertical projection densitometer every 3 meters along a permanently marked 50 m line transect upslope through the plot center. They measured dead woody biomass and forest floor on a permanently marked planar transect in a random direction from each plot center.
            For analysis the team compared tree density, canopy cover, mortality, basal area, and regeneration density among treatments with a Kruskal-Wallis test. They conducted post-hoc tests with pair wise Kruskal-Wallis two-sample tests following a statistically significant result for a total variable. They used Wilcoxon signed-ranks tests to quantify changes over time between 2003 and 2009 data to include the repeated measurements on the permanent plots. The team used nonmetric multidimensional scaling to examine changes in basal area of all tree species over time and among treatments. They compared stress value of the final solution to 50 random solutions using a Monte Carlo test. They examined differences between reconstructed 1870, 2003 pre-treatment, and 2009 post-treatment forest structure using multivariate analysis of variance to quantify differences in basal area and tree ha across time and among treatments. They used indicator-species analysis to identify species that were consistent indicators to the analysis dates of 2003 or 2009. They compared the maximum indicator value and random trials for occurrence of a given species to produce an approximate P-value.
            After running their experiment, Korb et al. found that in terms of forest structure, there were no differences in total tree density or basal area among treatment units prior to restoration in 2003. Following treatments, (2009) total density and basal area were lower in thin/burn units than the control and burn alone units. Thin/burn units had the only significant change in tree density and basal area with significant declines in both. After treatment diameter distributions in the controls were relatively unchanged. In terms of canopy cover, there were no differences among treatment units in tree canopy cover prior to restoration treatments in 2003. Tree canopy post treatment had the highest average in controls. In the thin/burn treatment, tree canopy cover decreased between 2003 and 2009. There were no differences in tree mortality for young trees post-treatment. The highest variability in tree mortality occurred in the thin/burn treatments. Total tree regeneration was no different before or after treatments. There were also no differences in forest floor depth prior to restoration treatments.
            Compared to reconstructed 1870 forest structure, there was a difference in tree basal area and tree density between reconstructed 1870, 2003 pre-treatment and 2009 post-treatment data. Tree basal area by species in 2009 in the thin/burn treatments showed a strong shift away from 2003 pre-treatment data toward the reconstructed 1870 forest structure. White-fir, aspen, and Douglas fir served as indicator species and dominated more highly in 2003 than in 2009.
            This experiment was the first to apply dendrochronologically reconstructed data on historical reference conditions to the design and testing of replicated ecological restoration treatments in a mixed conifer forest. The combination of thinning and burning in the study moved warm/dry mixed conifer forests in southwestern Colorado close to the historical reference condition, while burn alone treatments moved forests in the same direction towards the historical range. Fire had a thinning effect since younger (smaller) trees were most likely to be burned and die. Forest composition also shifted in the thin/burn treatments. For example ponderosa pine represented about 63% of the basal area in the historical reference condition, compared to 73% of the basal area in thin/burn treatments and only 44% in burn alone treatments. Old-growth trees represent a genetic and structural legacy that has largely vanished. Differential mortality amongst the treatments reveal that over time the reintroduction of the repeated surface fire regime will continue to shift composition away from less fire resistant species. Aspen regeneration in thin/burn treatments increased about fivefold over pre-treatment level while aspen regeneration in burn alone treatments did not double.
            The findings of this study support restoration studies in mixed conifer at other biogeographic locations where limited treatments did not restore stand composition and structure within historical reference conditions over the short-term. Creating forest conditions that increase forest heterogeneity at the landscape scale to replicate historical reference conditions will make forests more tolerant to altered fire regimes under a warmer drier climate. Since it has been revealed that widespread increases in mortality of old trees across the western U.S. was linked to regional warming, it is critical that restoration ecologists incorporate natural mortality into restoration treatment design because of the ensuing implication that background tree mortality has on forest stand structure and thus restoration goals. Fires are becoming easier to ignite and spread, the fire season is longer and extreme fire behavior is more common with warmer temperatures, longer growing seasons, and drier soils. Thus it is crucial to enact management actions to mitigate trajectories in species composition and ecological processes by restoring the self-regulating attributes of fire dependent forests.

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