Study Area

The study was carried out in a mixed conifer forest within the NWG of LVNP, in a 540-acre area near Manzanita Lake in the northwest corner of the park (fig. 1). The project area surrounds high value National Park Service infrastructure. Historically, the project area is expected to have had a mean fire return interval of approximately 32 years (Taylor, 2000; Skinner, 2009). No major wildfires have burned in the project area since 1918 (Skinner, 2009). In the absence of frequent fires, surface fuel loads and tree densities have increased, favoring small, shade tolerant white fir (Abies concolor [Gordon & Glend.] Lindl. ex Hildebr.). Insect and disease mortality also has become increasingly common. Aspen (Populus tremuloides Michx.) was historically abundant in the project area, though quite rare today. The goals of the Northwest Gateway project included (1) reducing surface and canopy fuels, (2) restoring the aspen population, and (3) maintaining desired fire regimes. Common species in the project area include white fir, ponderosa pine, Jeffery pine, with incense cedar (Calocedrus decurrens [Torr.] Florin), and sugar pine (Pinus lambertiana Douglas) intermixed.

Treatments

Mechanical thinning treatments were carried out between January and April of 2014 (fig. 2). A combination of thinning prescriptions was applied throughout the project area. Whole tree yarding was used for tree removal in all prescriptions. Although the overall treated area is known, the boundaries of the specific treatment types within each unit were not recorded. Mechanical treatments were usually followed by hand piling of fuels. The hand piled fuels were burned during the winter of 2014–16 as conditions permitted (fig. 3). The mechanical thinning prescriptions are described in table 1.

The thinning prescriptions were applied across the study area using a designation by description approach (that is, conditional based on field reconnaissance; fig. 1). A subset of the thinned area was subsequently treated with prescribed broadcast burning, separate from the pile burns (Unit 4 in 2018 and parts of Unit 5 in 2019; see fig. 1). Different parts of the project area were burned in 2018 and 2019. Note that the thinning treatments in Unit 5 were only partially completed because of time constraints.

Field Data Collection

Data Processing

Analysis

We performed statistical analysis on changes in surface fuels and growth of cored trees. We used a Bayesian approach for all analyses. We fit all models using the brms (Bürkner, 2018) package in the R statistical programming environment (version 4.0.4, R Core Team, 2022). We fit all models with four chains that ran for a minimum of 2,000 iterations each. We assessed model convergence with trace plots of the Markov chain Monte Carlo (MCMC) chains, and examined convergence with rank plots of MCMC chains for all parameters with the R-hat convergence diagnostic ($\widehat{R}$) above 1.01. Model fit was assessed using posterior predictive checks (Gabry and others, 2019).

Stand Structure

Average basal area and stem density following treatments were lower than pre-treatment baselines for pole and overstory trees (figs. 4, 5). The notable exception was in overstory Populus, which had no change before and after thinning.

Absolute and proportional change in live basal area after thinning was highest for pole-size Abies, with more than 80-percent basal area and stem density removed on average (figs. 4, 5). Overstory Abies basal area also saw substantial reductions averaging more than 60 percent (fig. 4). Pole-size Populus basal area was also reduced more than 60 percent on average. However, Populus trees were not targeted for removal, regardless of size, suggesting that these trees died via other mechanisms. This is consistent with observations of aspen mortality in the study area between 2006 and 2014. Compared to Abies, Pinus structural attributes changed much less on average after treatment. Pinus trees only were removed in areas of “radial release” for old growth legacy trees and in heavy infestations of mistletoe. Basal area was reduced 50 percent on average for pole-sized trees and 25 percent in overstory trees. Unsurprisingly, proportional basal area change was negligible for both size classes of all genera in the untreated plots, with average changes of less than 5 percent (fig. 6).

Average SDI and stem density was notably lower following thinning treatments in all treatment units (fig. 7). We did not observe notable changes in either average SDI or stem density in the untreated plots.

Average live crown base height was lowest in the untreated plots (fig. 8). As expected, live crown base height increased following thinning and burning treatments, with the highest average values recorded following the burn treatments. Interestingly, live crown base height seemed to continue to increase over time following thinning treatments, even when no additional treatments were applied (data not shown).

Surface Fuels

Surface fuel loads were relatively unchanged between samples in untreated plots for most size classes, suggesting that fuels either remained stable or increased over time in the absence of treatment (fig. 9). There was little support for a consistent effect of thin-only treatments on fuel loads relative to the untreated reference condition (fig. 10). Model predictions indicated little observed change between treatment types, with nearly equal support for negative and positive values for all fuel size classes except for the one hour and litter/duff size classes. There was some support for decreased fuel loads following treatment relative to the untreated reference condition in the smaller size classes, though there was more uncertainty in the litter and duff class.

In contrast, mechanical thinning followed by prescribed fire had a more consistent effect on subsequent fuel loading. Predicted post-treatment medians were below zero for all size classes except 1,000-hour and total woody fuels (fig. 10), with almost all support for a negative effect in the smaller size classes (litter/duff, 1-, and 10-hour fuels). Pairwise posterior contrasts (fig. 11) also indicated that there was fewer surface fuels following thin and burn treatments than thin-only treatments for the three smallest size classes. However, the models had much less support for a consistent effect in the other size classes, with posterior medians closer to zero and pairwise posterior contrasts suggesting considerable uncertainty in the direction of these effects. All models indicated that there is strong evidence that higher pre-treatment loads could result in large differences between pre- and post-treatment fuel mass, suggesting that areas with more initial fuels experienced a greater reduction in fuel load following treatment.

Growth

Plots of average BAI documented considerable growth differences in the post-treatment years for trees with and without thinning treatments (fig. 12). The model echoed these results, indicating good support for a positive effect of treatment on BAI (fig. 13). The model also had strong support for a positive effect of precipitation on growth, though it was weaker than the treatment effect.

Competition also seemed to affect growth. There was good support for a negative relationship between competitor basal area and average growth (fig. 14). Additionally, including competitor basal area in the model had a substantial moderating effect on the importance of the treatment variable, suggesting that the removal of competitors was largely responsible for the treatment effect.

Stand Structure

As expected, most of the changes in stand structure occurred in pole-size Abies, with more than 80 percent of the basal area removed for trees ≤15-cm DBH. This stands in stark contrast to the untreated plots, which indicated little change in basal area or stem density. The removal of small trees can reduce the available ladder fuels and raise the height to live crown, decreasing the probability of crown fire and subsequent mortality in larger trees (Stephens and Moghaddas, 2005). We observed increases in live crown base height following treatment where data was available.

The loss of small aspen following treatment was surprising, given that the preservation of aspen was a primary management goal of NWG. This is likely explained by two factors: (1) the degraded condition of remnant aspen within the high-density conifer stands that have overtopped them (losses have occurred continuously since monitoring began in 1998), and (2) the relatively old age of the last remaining aspen (which likely originated following a 1918 fire). Furthermore, an ice storm in 2006 weakened many trees by splitting and breaking trunks. However, the number of aspen in our sample was small to begin with (21 trees across all plots in the pre-treatment read), so the loss of only a few trees will create large proportional changes. Regardless, the small population size suggests that any loss may threaten the continued viability of the aspen population in the project area.

Surface Fuels

Though we expected some variability in the effect of treatment on surface fuels, we were surprised that mechanical thinning treatments seemed to produce limited changes in most surface fuel size classes in the absence of prescribed fire. Previous work has documented that mechanical treatments may increase surface fuel loads in the short term unless they are followed by prescribed fire (Raymond and Peterson, 2005). However, our results indicated little support for increasing fuels following mechanical thinning. The lack of a thinning effect on surface fuels may be a result of two independent mechanisms. First, trees were removed from the project area via whole tree yarding, which can be effective at limiting the amount of accumulated slash within the project units (Stephens and others, 2009). Second, there were subsequent mitigation measures applied after thinning, most notably the hand piling and burning of heavier pockets of residual fuels (fig. 3). These treatments are mentioned in the prescription, and post-treatment photographs suggested that they were widely applied across the project area. However, there are no systematic records available concerning the extent of these secondary treatments, so their overall effect is unquantifiable.

The combined thinning and burning treatment resulted in larger reductions in surface fuel loading than the thin-only treatment. These effects were best supported for the 1-hour, 10-hour, and litter and duff size classes. Reduced fuels in these size classes following burn treatments is unsurprising, because fuels in the smaller size classes are more easily consumed than their larger counterparts in prescribed fire (Knapp and others, 2005) and wildfire (Cansler and others, 2019). The two prescribed burns in this study were completed during cool conditions in October and were low intensity and patchy but were still locally effective at reducing surface fuel loading (figs. 2, 3).

The change in duff and litter depth after the combined thin and burn treatments may not be attributable to fuel consumption alone. Similar to our findings, Chiono and others (2012) documented that litter and duff were reduced following treatment, even in the absence of prescribed fire. Chiono and others (2012) postulated that litter and duff reductions following thin-only treatments may have been the result of fuel bed compaction or displacement by machinery. Fuel bed compaction was observed within the Northwest Gateway project, lending support to this phenomenon. There is uncertainty in our analysis because of the potential for misclassification and imprecision in the burn treatment specification. Plots were categorized as simply burned or not, with no additional information on whether the whole plot was burned, burn treatment effectiveness, or other attributes. It is possible, and perhaps likely, that some plots and their associated fuels transects were either unburned or burned at low enough intensity that fuels were largely unaffected. Assessing burn severity at such fine scales is beyond the scope of this work but represents an avenue for future research to understand mechanisms driving plot-level treatment effects.

There does not seem to be substantial differences in the effect of treatment on surface fuels in NWG and other projects that used whole tree harvesting methods in younger (non-old growth) stands (Stephens and others, 2009). These results suggest that similar treatments are likely to obtain similar results in mature versus old growth stand conditions.

Growth

As expected, we observed a strong growth release in Pinus following thinning treatments. This has important implications for the future persistence old growth legacy trees in this landscape because increased growth rates are correlated with vigor and lower rates of mortality (Dobbertin, 2005; Cailleret and others, 2019). Support for the post-treatment growth model including competitor basal area suggests that this release is likely because of reduced competition in treated area. These effects are consistent with prior research. Vernon and others (2018) reported that lower competition after thinning resulted in increased growth of Douglas-fir and ponderosa pine near Whiskeytown National Recreation Area, California. Fajardo and others (2007) reported that ponderosa pine growth increased following thin and combined thin and burn treatments, though the growth release was smaller in burned areas. We cored trees before the application of the burning treatment, so we were unable to characterize any effect of prescribed fire on growth at NWG. The relative effects of the two prescribed burns on growth was likely minimal given the cool prescription and patchy burns. There were few observations of extensive bole or crown scorch following the burns.

The effects of weather on annual growth are well established. Therefore, our finding that increased precipitation results in greater annual growth was unsurprising. Indeed, thinning treatments have been reported to lead to increased drought resistance (Vernon and others, 2018; Low and others, 2021), defined as the reduction or reversal of negative growth trends during periods of drought (Lloret and others, 2011). Additionally, annual precipitation was reported to positively affect growth for ponderosa pine and white fir in a separate study of prescribed fire effects also in LVNP (Wenderott and others, 2022). However, it is of some interest that the relative effect of precipitation on growth was weaker than the treatment effect. This phenomenon suggests that competition for other resources such as light and access to groundwater may have a greater effect on annual growth than interannual variability in local climate.

Our results demonstrate that thinning treatments were effective at reducing stand density and encouraging radial growth at the Northwest Gateway project area at Lassen Volcanic National Park. Our results also indicated that thinning combined with subsequent prescribed burning was the most effective at reducing surface fuel loads. Our observations seemed to indicate that harvest techniques such as whole tree yarding used at Lassen Volcanic National Park reduced the initial addition of post-treatment residual fuels, especially when combined with hand piling and other targeted treatments. This result was encouraging given the concern that thinning operations might increase surface fuel loading throughout the project area. Our results indicate that continued cycles of prescribed burning would maintain the current low surface fuel loadings and prevent the establishment of another dense cohort of fir in the understory.

Thinning treatments resulted in a growth release of large residual Pinus, indicating that even these well-established trees were affected by competition under pre-treatment conditions. The enhanced growth of the residual Pinus also indicates that these trees are more vigorous following treatments and may be more likely to survive future disturbances such as drought or bark beetle outbreaks, though some pine species may be more susceptable to beetles during sever outbreaks, especially following prescribed fire (Stephenson and others, 2019; Steel and others, 2021). The duration of this growth release will be dependent on environmental and biological conditions, including how stands are managed in the future.

Additionally, the post-treatment loss of small aspen underscored the importance of limiting mortality in small populations. Aspen are rare throughout the project area. Results indicate that continued monitoring may be necessary to determine if the treatments have been effective at preserving this population.