Increasing global temperatures, expected changes in the hydrological system, increasing probability of extreme events, and changing land use patterns will influence the management of forest systems [1
]. The genus Picea
is an important component of managed forest systems and is widely distributed throughout the northern hemisphere [5
]. Recent destructive insect outbreaks have occurred across Europe (spruce bark beetle (Ips typographus
) in Norway spruce (Picea abies
(L.) Karsten.) [6
] and western North America (spruce beetle (Dendroctonus rufipennis)
in Engelmann spruce (Picea engelmannii
Parry ex Engelm.)) [7
]. In western North America, the time for a spruce beetle to complete its life cycle is expected to decrease with warming temperatures, having potentially devastating effects on western spruce forests [10
The spruce beetle is a native insect endemic to spruce forests across North America [7
]. The spruce beetle can have a 1 to 3 year life cycle with adult spruce beetles boring into the main stem of the tree, feeding, and breeding in the live phloem; tree death occurs from girdling [11
]. Under endemic population levels, spruce beetles live and breed in recently windthrown trees. Destructive storms can greatly increase the amount of recently windthrown trees. Alexander [12
] recommended the removal of any large diameter Engelmann spruce trees that have fallen due to their increased susceptibility to a spruce beetle attack. Windthrow events in Europe also influence spruce bark beetle dynamics in Norway spruce forests [13
]. Effective removal of down trees can lessen future beetle impacts in many different spruce forest types [12
As population levels increase, spruce beetles will attack standing live trees, potentially transitioning to epidemic levels [7
]. Historical spruce beetle epidemics have been reported throughout the Central Rocky Mountains including the White River Mountains in Colorado [15
] and the Aquarius Plateau in Utah [16
]. Within the last 20 years, epidemic spruce beetle populations have been extremely destructive across the Central Rocky Mountains [17
] and outbreaks are expected to continue [18
]. Of the 3.8 million hectares in Colorado, Utah, and Wyoming in the spruce and fir forest types, FIA (Forest Inventory and Analysis) data suggest nearly half of the area has experienced some level of mortality due to insects [19
]. Recent spruce beetle epidemics have probably been driven by a combination of factors including susceptible stand structure and composition coupled with widespread drought [20
]. Spruce beetles favor large diameter (>25 cm diameter at breast height (dbh)) Engelmann spruce for both protection and nutrition [21
]. Recent dry conditions in high-density stands increase stresses on individual trees, decrease growth rates, and decrease defenses against the beetle, allowing spruce beetle populations to rapidly increase [8
]. Changing climatic factors, especially increased summer temperatures are associated with an increased shift from semivoltine to univoltine life-cycles [10
Management for the spruce beetle has focused on increasing resistance [24
]. Resistance (risk sensu
]), in this situation, is the decreased susceptibility to a spruce beetle outbreak through changes in structure and composition [2
]. DeRose and Long [28
] define this change in structure and composition at both the stand and the landscape level. A spruce beetle resistant stand has attributes, which decrease spruce beetle population growth and spruce beetle caused mortality within the stand. Landscape resistance is defined as the overall spatial heterogeneity and structure, which decreases the likelihood of a spruce beetle epidemic [28
]. When spruce beetles are at endemic levels, localized direct suppression techniques such as pheromone traps and trap trees can be used to reduce beetle populations [29
]. Schmidt and Frye [25
] developed a spruce beetle risk rating system that focuses on manipulating the stand structure and composition (i.e.
, reduction of overstory density and the amount of large diameter spruce) to reduce the risk of spruce beetle outbreaks and increase resistance [25
]. Manipulation of overstory structural characteristics has been shown to offer some short-term resistance to endemic spruce beetle populations [18
]. However, once spruce beetle populations transition from endemic to epidemic levels, all mature spruce-fir stands, even ones managed for density reduction are not resistant. For example, on the Markagunt Plateau in southern Utah, spruce beetle populations transitioned from endemic to epidemic levels, killing over 90% of the Engelmann spruce trees greater than 5 cm in dbh over an area of at least 250 km2
]. At the beginning of the epidemic, the spruce beetle attacked dense stands with large diameter Engelmann spruce [29
], as predicted by the Schmid and Frye [25
] risk rating system. Under this system, stands with low densities and small diameter spruce trees would be classified as potentially resistant; however, as the epidemic progressed the spruce beetle moved into these initially resistant stands [32
]. As a result of this extensive epidemic, forest composition has shifted towards subalpine fir (Abies lasiocarpa
(Hook.) Nutt.) and aspen (Populus tremuloides
Michx.) due to the limited number of mature live Engelmann spruce to serve as a seed source and the limited amount of spruce advance regeneration [18
]. Realistically, modification of stand composition and structure might only provide short-term resistance in the face of a spruce beetle epidemic [28
Managing for resistance is only a part of a comprehensive spruce beetle strategy, which should also include a provision for increasing resilience. Resilience at the stand level is associated with desired (or at least acceptable) structure and composition after a spruce beetle epidemic. At the landscape level, resilience reflects desired levels of heterogeneity in composition, structure, and age diversity [28
].We define resilience at the stand level as the adequate stocking of Engelmann spruce in the regeneration layer post spruce beetle epidemic. Successful Engelmann spruce regeneration can be a rare event with good cone crops occurring every two to five years [12
]. This sporadic cone production coupled with climatic variability creates long regeneration windows of 10 to 20 years [33
]. Since Engelmann spruce regeneration is not guaranteed, it is the most limiting factor in ensuring a resilient forest with a future spruce component. By ensuring a minimum amount of Engelmann spruce regeneration, if and when a spruce beetle epidemic occurs, Engelmann spruce would not be lost from the stand. This would reduce the likelihood of the forest type shift that was observed in southern Utah [18
]. Timely proactive forest management in spruce-fir stands can ensure a minimum level of stocking of Engelmann spruce, creating spruce beetle resilient stands.
In the Central Rocky Mountains, spruce beetle populations appear to be increasingly likely to transition from endemic to epidemic levels, and in some locations this has already occurred (e.g., southern Utah, [32
]). In northern Utah, an experimental silvicultural trial was established to compare three silvicultural treatments: single tree selection, group selection, and shelterwood with reserves. In this paper, we present results showing the short-term effect of these treatments on metrics of stand level resistance and resilience.
This study was initiated in 1998 to explore silvicultural treatments that could increase short-term resistance and long-term resilience to spruce beetle caused mortality at the stand level. Since the development of this study, there has been increased research, mostly retrospectively, on spruce beetle dynamics [18
]. However, our study is unique for two reasons: (1) it tests a pro-active management strategy and (2) utilizes explicitly defined metrics of resistance and resilience to spruce beetle outbreaks. Management focused on stand density reduction techniques intended to create “resistant” (sensu
]) stands is likely to be unsuccessful [26
On the TWDEF, there was little change between the pre-treatment risk rating and any of the post-treatment risk ratings. The prescriptions for the three treatments favored the retention of Engelmann spruce. Furthermore, across, all three treatments, the majority of Engelmann spruce basal area was in trees greater than 25 cm dbh which is characteristic of spruce-fir forests of the Central Rocky Mountains [35
]. Increasing resistance would require drastic changes in the structure and composition of these forests. Large diameter Engelmann spruce would need to be removed in order to decrease the QMD and proportion of live spruce [36
]. It is important to note, however, that during an epidemic, even in low risk stands, Engelmann spruce greater than 4 cm in dbh can be attacked by spruce beetles [18
]. Once at epidemic levels, it is no longer just the high-risk stands that are impacted but the entire landscape.
The shelterwood with reserves and group selection treatments implemented at the TWDEF resulted in substantial decreases in overstory basal area by 2008 from pre-harvest conditions. However, even with these significant decreases in basal area there were only modest changes in the risk rating. Between 2008 and 2013, there was a slight decrease in basal area due to mortality and this small decrease changed that specific metric from a two to a one. Within the spruce beetle risk rating system, small changes can influence the risk rating. While, the spruce beetle risk rating system can be sensitive to small changes, it does give managers a starting point when implementing forest management practices. Our study is one of the first to demonstrate that these three different silvicultural treatments resulted in relatively similar spruce beetle risk ratings. However, these results are in line with retrospective studies that found limited “resistance” to stands treated for density reduction [26
We propose that management of spruce-fir forests in the Central Rocky Mountains should focus on creating short-term resistance and long-term resilience. Short-term resistance is crucial to allow for the establishment of Engelmann spruce regeneration and is key to maintaining long-term resilience. Our study was conducted at the stand level, and we explicitly characterized a resilient stand as one with a minimum of 245 tph of Engelmann spruce regeneration. This metric was chosen because it will likely produce characteristic spruce-fir stand composition. As these trees mature and reach a QMD of 25 cm, the SDI of just Engelmann spruce will be 245 or approximately 16% of the maximum SDI. If we assume a stand basal area of 20 m2·ha−1 which is slightly lower than the stand basal area measurements for the group selection and shelterwood with reserves in 2013, Engelmann spruce basal area will be 12 m2·ha−1 or represent 60% of the total stand basal area. This stand composition would also produce a low (5) overall spruce beetle risk rating. The group selection had the second highest amount of Engelmann spruce regeneration with an average of 125 tph in 2013. Without any subsequent regeneration, Engelmann spruce would only compose about 30% of the basal area and 8% of the maximum SDI. As the study continues to be monitored in the future, this metric can be adjusted based on future recruitment and mortality of the regenerating Engelmann spruce. This is one of the first studies to put a lower limit on Engelmann spruce regeneration when timber management is not the primary goal.
The long regeneration windows of Engelmann spruce are a major barrier in building resilient spruce-fir forests in the Central Rocky Mountains [12
]. Resilience pre-harvest was very low due to the limited natural regeneration of Engelmann spruce. Natural regeneration of Engelmann spruce can be limited by irregular cone production, drought and extreme high and low temperatures, as well as, unfavorable microsite conditions [39
]. Planting of Engelmann spruce is the only way to ensure adequate stocking in the short-term. Because seeds in our study were collected from numerous overstory spruces at the TWDEF, these seedlings are presumed to be locally adapted and to represent a range of genetic variability. An additional benefit of supplemental planting of Engelmann spruce is that these small diameter trees (<4 cm dbh) are generally not attacked by spruce beetles, decreasing the likelihood of a potential vegetation shift to aspen and/or subalpine fir. The lack of resistance and the resulting vegetation shift to aspen and subalpine fir on the Markagunt Plateau, highlights how important resilience (adequate Engelmann spruce regeneration) is in maintaining the composition of spruce-fir forests in the Central Rocky Mountains. Resilience on the Markagunt Plateau will be low in the future due to the elimination of mature Engelmann spruce and limited spruce advanced regeneration [18
]. Proactive density reduction methods that increase short-term resistance coupled with supplemental spruce planting to increase long-term resilience can reduce the likelihood of a complete vegetation type shift after a spruce beetle epidemic.
Management for spruce beetle outbreaks currently and in the future at both the stand and landscape levels will need to be assessed in light of trade-offs between traditional management by group selection and silvicultural alternatives such as shelterwood with reserves (Table 5
). This study was conducted at the stand level. At the TWDEF, the shelterwood with reserves coupled with supplemental planting met many of the objectives. However, by thinning from below, the structure shifted from a wide diameter distribution, containing small to large diameter trees in various gaps and densities, to more uniformly spaced large diameter spruce trees. Although, these large diameter spruces are attractive to the spruce beetle [18
] they will produce large amounts of seeds and potentially supplement planted seedlings [28
Forest management activities in the Central Rocky Mountains can be delayed by appeals and ligation (5 years for our study). This potential delay must be taken into consideration when planning forest management activities. The shelterwood with reserves, once implemented could be used to treat the entire stand, potentially influencing landscape level resistance and resilience. By contrast, the small area treated at each entry is a limitation of the group selection. A larger group opening could be used but is not recommended due to limitation in natural regeneration and increased mortality due to extreme temperatures and sunscald [12
]. An additional issue with the group selection is time. Even with supplemental planting, the group selection treatment did not meet the minimum metric of resilience. In the absence of a spruce beetle epidemic, in future harvests, overstory density will be reduced and planting of Engelmann spruce will continue; entries every 20 years will create age class and structural diversity, characteristic of spruce-fir stands [12
]. Under this treatment, it will take another two cutting cycles to treat just half the stand. The cutting cycle could be reduced but due to the low productivity of the site would not be recommended because a 20-year cutting cycle is likely a minimum to ensure an economically viable harvest.
Trade-offs between the different treatments assessed at the stand and landscape level.
Trade-offs between the different treatments assessed at the stand and landscape level.
| ||Shelterwood with Reserves||Group Selection||Single Tree Selection|
|Stand Level|| || || |
|Reduced Basal Area||X||X|| |
|Retention of Groups & Gaps|| ||X||X|
|Diversity of Overstory Species||X||X|| |
|Minimum Levels of Spruce Regeneration||X|| || |
|Landscape Level|| || || |
|Ability to Treat Large Areas||X|| || |
The traditional structure of spruce-fir forests would be retained in the group selection treatment. However, composition may shift with a spruce beetle outbreak because resilience (i.e.
, adequate regeneration) would be limited in the short term. Given the increasing likelihood of stressed spruce trees due to increasing summer drought, the group selection method would not treat a large enough area of the stand quickly enough to provide adequate short-term resistance and long-term resilience at either the stand or landscape scale [20
An additional concern, in any treatment, but especially the shelterwood with reserves and the group selection is the potential for windthrow [66
]. Between 2008 and 2013, there were only minor differences in live basal area measurements and no discernible differences in incidence of windthrow between treatments (data not shown). While catastrophic windthrow did not happen in any of the silvicultural treatments on the TWDEF, any reduction in density has the potential for significant windthrow [12
]. Collection of pre-harvest data, including crown ratio, may aid in selecting and removing less vigorous trees which may be more vulnerable to windthrow.
Forest managers across the world are confronted with uncertainty about how changing climatic conditions and subsequent interactions with disturbances will influence forest composition and structure [67
]. Changing conditions in spruce-fir forests throughout the Rocky Mountains and the boreal forest are greatly influencing disturbance regimes [69
]. Managers will have to weigh trade-offs between traditional and novel management approaches [71
]. Long-term studies on experimental forests allow researchers and scientists to explore how different management approaches can influence both short and long-term forest dynamics.
Future climate change is expected to greatly influence spruce beetle dynamics across western North America and changing disturbance dynamics will greatly influence how spruce-fir forests are managed [13
]. Our study on the TWDEF is one of the first studies to test how different silvicultural treatments influence explicitly defined and quantified metrics of resistance and resilience to the spruce beetle. By using a long-term study design with permanent plots, both short (results presented here) and long-term forest dynamics can be explored. Additionally, when spruce beetle activity increases again on the TWDEF, our study will provide insight into potential differences in how spruce beetle populations build and spread in each of the different treatments. By using this long-term study design, these metrics of resistance and resilience can be tested and potentially adapted.
Managers will have to make difficult decisions as they plan for spruce beetle outbreaks. Traditional group selection harvests will maintain openings and groups, but potentially result in a loss of Engelmann spruce. Alternatively, the shelterwood with reserves will maintain a spruce component but with a novel structure. The shelterwood with reserves with supplemental planting was the only treatment to meet the resilience criteria on the TWDEF. If desired, increased structural variability could be built into this treatment by varying the type of reserve trees in the shelterwood (i.e., strip, uniform or clumped). To increase size diversity and decrease overall average diameter of Engelmann spruce, stands could be thinned from below to remove smaller diameter subalpine fir and thinned from above to remove some of the larger Engelmann spruce. However, as the planted Engelmann spruce mature, they will become susceptible to the spruce beetle with any of the treatments. The shelterwood with reserves and supplemental planting allows for the retention of Engelmann spruce in the future forest and time to plan future management activities which may include group selection. Our results suggest that in spruce-fir stands in northern Utah, shelterwood with reserves best meets the goals of short-term resistance and long-term resilience to the spruce beetle.