1. Introduction
Wildfires in the western United States have become more severe, larger, longer lasting, and more destructive [
1,
2] in overstocked forests due to heavy fuel loads. Planted forests may be more susceptible to higher severity fire compared to surrounding natural stands [
3,
4,
5]. This increased risk can be attributed to their single species and dense, homogenous structure, which differs greatly from fire-resilient, pre-fire suppression conditions found in areas that historically had frequent fires, like the mixed conifer forests in the Sierra Nevada mountains [
6,
7]. Younger plantations are especially at risk; their increased density leads to a high accumulation of surface and canopy fuel, their lower canopy base height increases the likelihood of crown fires, and their thinner, less fire-resistant bark results in higher post-fire mortality [
8].
Science-based, active management of plantations has been employed to reduce their risk of high-severity fire. First, planting density has been significantly reduced to break a high continuity of surface and canopy fuels. Second, a larger growing space with fewer trees and additional silvicultural treatments such as control of competing vegetation and fertilization have proved to effectively enhance rapid tree growth and stand development [
9]; this in turn indirectly increases stand resistance to disturbances by quickly increasing canopy height and bark thickness and controlling shrubs between trees to break continuity of ground fuels. Third, foresters can apply thinnings early to manipulate stand structure to promote stand resilience. Fourth, planting trees in clustered groups has been proposed—though yet to be tested—by resembling century-old forest stand tree patterns evolved from natural regeneration patterns to reduce fuel connectivity and slow fire spread [
10,
11]. Finally, fuel reduction treatments, such as overstory thinning, mastication, and prescribed fire, are often necessary for effective mitigation efforts [
12]. Overstory thinning reduces crown density, thus slowing the spread of fire through a canopy, but it does little to affect how fire spreads along the surface fuels [
12]. Mastication of small trees and shrubs can reduce connectivity from the surface to the crown via ladder fuels [
13,
14,
15,
16,
17]. However, the addition of the small, chipped fuels to the surface fuel bed can increase flame lengths and spread rate [
15]. Therefore, the effectiveness of using only mastication is contested, and likely, dependent on pre-treatment and residual stocking levels. Studies found it can both reduce [
15] and increase risk of crown fire [
14]. Other studies found mastication helped moderate some fire behavior metrics while exacerbating others [
13,
17]. Therefore, mastication is often combined with prescribed fire [
15,
18,
19]. Prescribed fire simultaneously reduces surface fuels (via consumption) and crown fuels (via consumptions and post-fire mortality), while promoting understory diversity and releasing nutrients back into the soil [
19,
20]. While some damage from prescribed fire is inevitable in young plantations due to their low canopy base height and thin bark, the effect of fire on fuel loading, and thus, future fire behavior, often outweighs most of the damage it causes [
21,
22].
Simulation modeling allows managers and researchers to test the efficacy of different management techniques on planted forest yield and reduction in wildfire risk over long periods without going through the time, costs, and logistics of implementing them in the field. The Forest Vegetation Simulator (FVS) and its Fire and Fuels Extension (FFE) is one effective tool to evaluate growth and fire behavior. FVS is a free, keyword-based, spatially independent tree-based model, developed by the U.S. Forest Service which models stand level growth and mortality over time using tree and plot level variables collected in the field [
23]. There are 20 different variants of FVS, each calibrated to specific regions in the United States. FVS allows the user to perform different management actions in the stand, including many types of thinning and fuels treatments. Additionally, the model is customizable, allowing the user to calibrate growth and mortality relationships. FVS–FFE is used to model fires in the stands, calculate simulated and potential fire behavior, and calculate fuels in the stand [
24].
Creating and calibrating a model of stand development under different fuel treatment scenarios using FVS–FFE can provide a more comprehensive understanding of how stand growth influences fire behavior over time and help identify how different treatments will influence stand development and fire risk. Specifically, the objectives of this work are to (1) determine what combination of thinning intensity and fuel treatments best reduces crown fire danger and maximizes growth in mixed conifer forests, and (2) determine the persistence of early management in the younger planted stands. From an ecological standpoint, this study makes an important contribution to post-fire restoration of conifer-dominated forest systems. From a management standpoint, this study provides important fire management insight for addressing fuel treatment concerns. Overall, improvements in understanding how to effectively manage fuel conditions will ultimately lead to reduced fire risk, and thereby, improve safety of firefighting professionals.
4. Discussion
Thinning treatments impacted stocking levels predictably, as overstory thinning decreased basal area but increased QMD (
Table 6). All the thinning and fuel simulations were from below, making it unlikely for any regeneration to make it to the overstory, resulting in a lower overstory basal area as thinning intensity increased. While the overall stocking decreased with thinning intensity, residual mean tree size after thinning increased as overstory thinning intensity increased. Ponderosa pine and mixed conifer stands show a positive growth response to thinning because thinning reduces competition for water, light, and nutrients and allows for more growing space [
37,
49,
50,
51].
Mortality from the mastication with burning treatment provided growth benefits to the stands (
Table 7). Smaller trees have a lower chance of surviving fires than larger trees, so prescribed fires can shift diameter distributions upward [
52]. Mortality from prescribed burning is unavoidable; in fact, one of its benefits is that it reduces stem density providing a competition release for the remaining trees [
52,
53,
54]. The effect of mastication with burning was only seen in the 618 SDI and no overstory thin simulations for basal area and only the no overstory thinning simulations for final QMD (
Figure 5 and
Figure 8A). This could be due to the mortality response of different thinning intensities to prescribed burning. The prescribed fires killed more trees in the 618 SDI and the no thin simulations compared to the 370 SDI simulations (
Table 8). The percent mortality from prescribed burning is in line with other studies. Percent mortality from the simulations of prescribed fire was the highest in the earliest fire (2029), ranging from 33 to 64%, decreased to 19–44% for the next prescribed fire in 2039, and stayed below 22% for all subsequent prescribed fires (
Table 8). Reiner et al. [
15] performed a mastication and burning study in a 25-year-old plantation, and found mortality from prescribed burning between 27 and 49%, which overlaps with the 33–64% mortality observed in this study.
One unexpected result was the effect of original management on final QMD. When comparing the stands in 2017, the PCT plantations had larger diameter than the non-PCT plantations and natural regeneration stands. However, at the end of the simulation, the natural regenerating stands’ QMD were about 8 cm larger than the PCT and non-PCT plantations (
Table 7). In general, the thinned stands had steeper slopes than the other two original management groupings. High slopes often have a negative effect on tree growth due to decreasing soil depth [
55]; this relationship is expressed in the diameter growth equations of FVS—that is, it predicts more diameter growth on gentler slopes [
23,
30]. The small differences in slope could manifest themselves over time. Furthermore, projections of stand characteristics using forest simulation models can start to unravel with long-term projection time periods [
56].
While the fuels and overstory thinning treatments had a large effect on fire behavior, all stands, regardless of silvicultural prescription, experienced a similar pattern in fire behavior over 100 years. Fire severity and intensity reached a maximum usually within the first 10 to 50 years of the simulation, but eventually decreased so that surface fires were more common, flame lengths were below canopy-base height, and mortality was below 25% (
Figure 5,
Figure 6 and
Figure 7). This pattern is a consequence of the stand structure and development in even aged planted forests. When the stands are young, the trees have low canopy base heights, leaving them susceptible to crown scorch, even with low flame length. The horizontally homogenous nature of a plantation allows for the fire to spread throughout the stand, resulting in high mortality [
10]. This pattern has been seen in young ponderosa pine and mixed conifer plantations (under 50 years), both modeled and observed [
5,
8,
57]. However, as the stand grows, those canopy fuels move away from the ground, increasing canopy base height (
Figure 10). The uniform distribution of growth in planted forests usually results in one size/age class, so there will not be several layers of vertical stratum lowering the position of canopy base height. Regeneration can also affect canopy base height. If regeneration is dense enough it can lower canopy base height and help carry a fire from the surface to the canopy, torching and killing mature trees. The simulated overstory thinning and fuels treatments removed regeneration, as they all focused on small trees, preventing them from becoming a ladder fuel. Regeneration in the simulations without any management did not keep canopy base height low enough to maintain severe fire behavior though the full simulation, though it did delay the onset of canopy base height overtaking flame length (
Figure 7).
High canopy base heights have been noted in an even-aged mixed conifer forest before. Stephens and Moghaddas [
57] studied mature (80–100 years old), even aged stands that naturally regenerated after railroad logging and did not experience any silvicultural treatment effects on height to crown base. They found high canopy base heights in these stands, and therefore, low potential for crown fire [
58]. While not a plantation, the stand structure is like what one would find in plantations; in addition, this study did include some even aged stands that naturally regenerated after a fire. However, there are other factors besides canopy base height that control whether a fire will travel into a crown. Downed logs and snags can also be ladder fuels, and extreme winds can also carry a surface fire to the crown [
5]. Creating a fire resilient forest stand cannot simply rely on the fact that canopy base heights will eventually increase over time in a plantation.
The main variables that influence fire behavior in FVS–FFE such as surface fuel loading, canopy base height, and canopy bulk density can be modified by various silvicultural treatments [
12]. A consistent interaction between thinning intensity and fuel treatment was observed for most of the fire behavior variables (
Table 5,
Figure 8). There were more significant effects of fuel treatment in no overstory thinning simulations. This could be due to the nature of the stand structure and how the thinning was performed. All thinning treatments were from below, so they removed the smallest trees first. Smaller trees can act as ladder fuels which can carry fire up into tree canopies [
12]. Simulations without overstory thinning needed something else to reduce ladder fuels, which mastication and prescribe burning can do [
17].
Mastication with prescribed burning was the most effective fuel treatment because of how it altered surface fuels and flame lengths (
Figure 5,
Figure 6,
Figure 7 and
Figure 8). After a prescribed burn, most of the surface and ladder fuels have been consumed [
12,
19,
59]. This decreases flame length and reduces the risk of crown fires as fires on the surface cannot travel up the canopy [
19]. This reduction crowning drastically reduces fire caused mortality (
Figure 6). The effectiveness of prescribe burning can be seen in the interaction among thinning and fuel treatments in the transition to surface fires (
Figure 8C). Using prescribed fire with mastication caused the transition to surface fires to happen so quickly and consistently, that tree density did not matter in the range observed. Prescribed burning is often found to be the most effective treatment for reducing surface fuel loading and thus reduce fire risk in Sierra Nevada plantations [
14,
15,
17,
44].
The differences between the mastication only and the no fuel treatment simulations were minimal (
Figure 2,
Figure 4 and
Figure 6). One of main benefits of mastication is how it removes ladder fuels [
13,
17]. However, in an even aged plantation, where most trees are about the same size, there are not many ladder fuels, diminishing the benefits of mastication [
14]. Additionally, mastication does not remove the fuels from the stand, it just moves them to the surface and decreases their size. Both Kobizar et al. [
14] and Reiner et al. [
15] found that masticated fuel beds produced longer flame lengths than stands without fuels treatments when modeling fire behavior in young Sierra Nevada pine plantations.
Despite high total flame lengths, masticated fuels reached basal area mortality under 25% earlier than the no fuel treatment simulations (
Figure 6). Both the average surface flame length and average surface spread rate across all years in masticated fuel beds were smaller than the no fuel treatments, while there were no differences among total flame length and spread rate (
Table 9). These differences in surface fire behavior could result in less scorch damage in the scenarios with masticated fuel beds, and thus, less mortality. Both laboratory and field studies suggest that masticating fuels results in denser fuel beds than can dampen surface flame lengths and spread rates [
13,
60]. Masticated fuel beds can be quite difficult to model [
13,
43]. The fuel particles often have an irregular shape which can have complicated interactions with fuel moisture and decay [
43,
61]. A development of a fuel model specifically designed for masticated fuel beds would improve subsequent studies modeling fire behavior under different fuel treatments.
The largest impact of overstory thinning intensity on fire behavior was seen on crowning index. As thinning intensity increased, crowning index increased (
Figure 9). As the stand density decreases from thinning, the density of fuels in the canopy will also decrease simply due to less trees being present (
Figure 11). A decrease in canopy bulk density results in less canopy fuel continuity, which ultimately, decreases the occurrence and severity of crown fires [
12]. While thinning target influenced other fire behavior variables due to removing ladder fuels, the only differences found between the targets were usually among the most intense thinning target, 370 SDI, and no thinning. Additionally, the difference between them was usually only a 5–10 year improvement when fire behavior reached low risk levels, while mastication with burning often provided a 15–20 year improvement. The more the stands were thinned, the larger the trees became (
Table 7). Larger trees will have thicker bark which is more resistant to fire [
62,
63]. Furthermore, repeated thinning in ponderosa pine plantation alone to 320 SDI enhances DBH to about 70 cm in a Sierra Nevada site at age 60 [
51], which was not affected by a fire [
64]. Thinning the overstory to below full site capacity was required to produce the most effective changes in mortality from fire, suggesting it is not a viable option to reduce crown fire risk. Several other studies have found similar results of overstory thinning having minimal reductions in fire behavior alone [
12,
17,
65]. This is likely due to the fact that overstory thinning does not decrease surface fuels; in fact, it can increase surface fuels when logging slash is left on the ground.
In addition to the silvicultural impacts on fire behavior, several differences among the original management scenarios were found. These differences can be attributed to original stand structure and how they grew over time. The stands with PCT reached low mortality and achieved a canopy base height above flame length sooner than the other original management scenarios (
Figure 3 and
Figure 4). While at the end of the simulation the natural regenerating stands had larger trees, the stand with PCT started out with larger trees. This switch from the PCT trees to the natural regenerating trees as the largest likely happened after fire behavior decreased. Like with thinning intensity, this response could also be linked to bark thickness. Larger trees have thicker bark, which protects the vascular cambium from heat and scorch damage from fire and is a common adaptation in trees in fire dependent ecosystems [
63].
There are some modeling limitations with FVS–FFE that should be taken into consideration. As mentioned earlier, the lack of a full establishment model for all variants and a fuel model for masticated fuel beds create complications for accurately modeling fire behavior [
23,
65]. Another limitation of FVS is its spatial independence. The spatial arrangement of trees can greatly affect growth and fire behavior [
6,
7,
66,
67]. In addition to this, post-fire planted forests have included experimentation with planting trees in a clustered arrangement to mimic this pattern [
10]. Although the plantations used in this study were a mix of clustered and evenly spaced trees, they were not analyzed along these lines since it is difficult to incorporate spatial dependency in FVS. Another spatial variable which is not included in FVS is landscape fire behavior dynamics. The spatial arrangement of stands and silvicultural treatments across a landscape can affect how a fire spreads [
68]. The landscape aspect of fire behavior was outside of the scope of this project but is an important factor consider when interpreting results. Stands that are predicted to have conditional crown fires are more likely to have crown fire spread if an adjacent stand has an active crown fire [
26]. Lastly, shrubs have been proven to be a factor that can impact plantation growth in the Sierra Nevada; often controlling for shrubs can be one of the most important factors in plantation survival and growth [
36,
69]. FVS has a submodel for shrubs and understory cover, but it is not currently developed for the western Sierra variant or linked with the FFE extension, and therefore not used in this project. An expansion of this submodel would greatly help in modeling plantation and post-fire growth in the Sierra Nevada.
5. Conclusions
One of the objectives of the study was to determine what combination of thinning intensity and fuel treatments best reduces crown fire danger and maximizes growth. The results indicated that stand development and the various silvicultural treatments all interacted to create a variety of simulations outcomes. While the overall pattern of increasing canopy base heights over time eventually lead to a decrease in fire behavior metrics regardless of treatment, the amount of time required to reach these decreased crown fire risks changed with treatment. Using prescribed burns reduced flame lengths so drastically, that canopy base height quickly exceeded flame length. Additionally, performing intensive thinning reduced risk of active crown fires spreading through the stand. These results suggest that treating stands early is important for reducing fire risk, as that is when the risk is the highest. Further research into specific timing of treatments will help answer this. Prioritizing prescribed burning, when possible, and thinning from below, are the most effective ways to quickly improve fire resistance in mixed conifer plantations. However, the most effective treatments are not without disadvantages. The prescribed burns killed many trees, reducing overall stock. The most intensive thinning treatment provided the best reduction of fire behavior but was also below full stocking level.
The second objective of the study was to determine the persistence of early management in the younger planted stands, some of which received early pre-commercial thinning. Identifying the most effective early stand management techniques to create fire resilient stands has become increasingly important in the past few decades. More than half of the Forest Service’s plantations in the Sierra Nevada mountains established from 1998 to 2016 have not received PCT, and 38% of them have not experienced any competition release [
10]. These young, dense stands pose a large crown fire risk, and if they are left unmanaged, this risk will continue for several decades. Our study’s findings indicated that the pre-commercial thinning helped reduce crown fire risk and reduce the extent of basal area mortality.
While the study was conducted in the Sierra Nevada region of northern California in the Pacific southwest region of the United States, the findings of this study have implications for other forest systems in other countries sharing similarities in physiography and vegetation. In particular, the findings of our study have the potential to be extrapolated to other areas which share a montane forest environment, a climate that is Mediterranean in nature, and a forest composition that is dominated by conifer species. For example, there are many similarities between the Sierra Nevada mixed-conifer forest system and the Mediterranean pine forests of Spain dominated primarily by
Pinus pinaster Ait. (e.g., [
70]).
The methodology used in our study has both advantages and disadvantages. In terms of advantages, we did a comprehensive consideration of three treatment factors. Namely, we considered three different categories of original management (i.e., natural stand, planted with not pre-commercial thinning, and planted with no pre-commercial thinning), crossed with different three categories of fuel treatments (i.e., no treatment, mastication only, and mastication with prescribed burning), which were further crossed with four thinning targets. This provides insight into how to manage the post-fire regeneration of mixed conifer forests to ensure resilience against crown fire risk.
In terms of disadvantages in our methodology, our study only simulated expected fire behavior based on different treatment categories, but did not spatially model fire spread across the landscape. Thus, future modeling efforts could examine the spatial propagation of fire which could include incorporation of landowner objectives and degree of investment in fuel treatment activities between neighboring parcels of land [
71,
72]. FVS is not without drawbacks, either. It can be sensitive to certain inputs, like regeneration and fuel models, and cannot incorporate all the complexity of fire, like spatial arrangement of trees. However, it provides a good tool for evaluating overall trends of stand development and how to alter them to reduce fire risk. Furthermore, future re-inventorying of stand and fuel conditions will allow more rigorous validation of our simulations and help identity appropriate management intervals and approaches. Moving forward, continued tracking of forest growth dynamics across forest and habitat types recovering from the Power Fire can help in future validation of new extensions of forest growth and fuel management models and software.