Natural and anthropic disturbances determine the dynamics, structure, and composition of forests and control the functioning of forested ecosystems [1
]. In forests with long fire cycles, disturbances such as insect outbreaks and windthrow play major roles in forest landscapes [3
]. Insect outbreaks must be considered in forestry planning due to the important economic and ecological implications of these disturbances [6
]. Insect outbreaks affect timber supplies and have a marked impact on overall forest productivity. For this reason, many studies have evaluated the vulnerability of mature trees in boreal forests to this type of disturbance (e.g., References [8
]). However, there is still a lack of information regarding the impact of insect outbreaks on seedling regeneration. In the last decades, the greater intensity of harvest practices has increased pressure on boreal forests to respond to the high demand for wood in the international market [11
]. Between 1990 and 2016, the harvested area in Canada reached 24 million ha, most of this area (86%) being harvested using clearcutting methods [13
]. Consequently, a large surface of the North American boreal forest exists at an early development stage; the post-disturbance regeneration phase deserves more attention as it may provide early warning of ecosystem processes degrading [14
]. Therefore, assessing the vulnerability of seedlings to insect outbreaks is critical for evaluating the persistence, productivity, and resilience of forest ecosystems, especially in the context of climate change with the expected increase in the frequency and severity of natural disturbances in the boreal biome [1
Spruce budworm (Choristoneura fumiferana
(Clem)) (SBW) is the main defoliator of North American boreal forests [15
]. Between 1990 and 2016 in the Canadian boreal forest alone, 93 million ha were affected by SBW activity [13
]; this extent is equivalent to an area that is more than 5× that of the state of Florida (USA). Damage to the forest occurs during the larval budworm stage when this insect consumes annual foliage, thereby reducing the foliar area of conifers that is available for photosynthesis [16
]. This lepidopteran has a 10-year cycle and an outbreak frequency of 30–40 years [17
]. In the last decades, the severity and frequency of SBW outbreaks have increased and spatial patterns have changed [18
]. Outbreaks now reach latitudes to the North of previously-observed infestations, possibly because of modified stand structure, species composition, and host species distribution, as well as climate warming [20
SBW outbreaks reflect a complex phenomenon influenced by multiple factors. Tree vulnerability depends on species, stage development, height, spatial location, and regional climatic conditions [22
]. The most vulnerable species are balsam fir (Abies balsamea
(L.) Mill.), followed by white spruce (Picea glauca
(Moench) Voss.) and black spruce (Picea mariana
(Mill.) BSP) [25
]. Balsam fir budburst occurs 14 days earlier than that of black spruce and is synchronized with SBW emergence; this synchrony explains the greater vulnerability of balsam fir to defoliation [26
]. Phenological asynchrony between black spruce and the insect improves the resistance of this host to defoliation and provides some protection from severe SBW defoliation [27
]. SBW not only has an impact on mature trees but also affects the regeneration phase, i.e., seedlings [22
]. The nutritional proprieties of foliage vary between species, but they are influenced mainly by the phase of stage development, directly affecting vulnerability to defoliation due to the differing chemical composition of leaves [29
]. The foliage of seedlings has higher concentrations of nitrogen, sugar, and secondary compounds, e.g., tannins, than mature trees. Lower concentrations decrease the nutritional quality of foliage and reduce the suitability for the development of SBW larvae [30
]. Similar to mature trees, taller seedlings generally have a larger crown that can intercept larvae; thus height could influence larval density on seedlings [31
]. The proximity of seedlings to residual patches and competition related to stand density could also be crucial in the seedling vulnerability to SBW activity, especially within clearcut stands where the sheltering effect provided by mature trees is almost nonexistent [32
]. Thus, a better understanding of the vulnerability of conifer regeneration to SBW outbreaks requires that these factors be examined as they may strongly influence seedling defoliation [22
To mitigate the projected future impacts of insect disturbance, several studies have examined the effect of SBW outbreak under different silvicultural treatments [35
]. These studies focused on mature stands; few studies, however, have investigated seedling vulnerability. Although the role of forest overstory composition on seedling defoliation [22
] and post-outbreak seedling response [39
] have been evaluated, SBW effects on boreal forest seedlings remain understudied. Thus, as much of the Eastern Canadian forest area previously harvested by clearcutting is affected by SBW activity, a better understanding of the impacts of SBW outbreaks on seedlings in these early-stage stands should be a major priority in forest management strategies.
Here we investigate the vulnerability of conifer regeneration to SBW outbreak within clearcut areas of the Eastern Canadian boreal forest. We aimed to quantify cumulative and annual defoliation levels on seedlings based on (i) conifer species, (ii) seedling height classes, and (iii) distance to the residual forest. We hypothesized that:
Balsam fir will be more affected by SBW than black spruce due to the phenological synchrony of balsam fir with SBW.
Taller seedlings will have a higher level of defoliation due to the sheltering effect provided to smaller seedlings.
Defoliation will be more intense as the distance between seedling and residual forest increases because the refugee effect of the mature stand is increasingly limited with distance.
In recent years, the Eastern Canadian boreal forest has experienced a major SBW outbreak phase, resulting in severe damage to a vast expanse of very productive forest areas. This SBW outbreak has important implications at the ecological (forest dynamics) and economic levels (economic losses). Natural regeneration is a key component of forest management in the boreal biome, given its major role in ensuring the persistence and resilience of forest ecosystems [55
]. Currently, balsam fir and black spruce stands in the regeneration stage cover a large surface area in Canada due to the strong harvest ratio over the last 20 years. Thus, knowing the impacts of SBW activity on regeneration come to the fore. In this study, we evaluated the cumulative and annual defoliation of conifer seedlings and the factors influencing the vulnerability of conifer regeneration to insect outbreaks, e.g., species, seedling height, and distance from residual forests. We quantified SBW-related defoliation within plots that had undergone clearcutting forestry management. This study was an initial step in improving a methodology for upcoming research regarding the impacts of SBW defoliation on conifer regeneration after silvicultural treatments. Our research, therefore, represents a major contribution in providing a first diagnosis of the vulnerability of conifer seedlings during periods of insect outbreak in clearcut boreal forests.
First, seedling vulnerability differed between balsam fir and black spruce seedlings in terms of cumulative defoliation; balsam fir seedlings were more affected by SBW than black spruce seedlings. These observations confirm our first hypothesis and agree with previous studies (e.g., References [57
]) that had compared levels of defoliation between species and demonstrated balsam fir’s greater vulnerability to SBW activity [25
]. The phenological synchrony between balsam fir budburst and the emergence of SBW larvae could explain this heightened vulnerability [59
]. Black spruce budburst occurs 10–14 days after balsam fir, and therefore the SBW larvae must feed on older black spruce foliage that is less suitable for larval development [27
]. On the other hand, a later black spruce budburst would provide an excellent food supply for later-emerging larvae [26
]. Fuentealba et al. [29
] highlighted that black spruce foliage had a lower nutritional quality than that of balsam fir. However, this situation could change in the future as climate exerts a strong influence on black spruce phenology [28
]. As such, the vulnerability of black spruce to SBW could increase as higher springtime temperatures could decrease the gap between black spruce budburst and the emergence of SBW larvae [61
]. Consequently, we recommend that regeneration vulnerability to SBW outbreak be accounted for in forest management strategies, e.g., sites and species selection, when adapting to projected climate change scenarios. Therefore, our study was located in black spruce stands, where balsam fir was the secondary conifer (almost absent in some plots). Even if our results between both species were significantly different, we recommend the development of future research with more replications to better understand the differences between species vulnerability.
Second, the level of defoliation differed depending on conifer seedling height. Thus, the composition of the forest overstory and seedling height influence understory regeneration vulnerability to SBW activity, as observed by Cotton-Gagnon (2018). In agreement with our second hypothesis, defoliation level was positively correlated with seedling height. Similar results were obtained by Nie and MacLean [22
] who observed greater defoliation in balsam fir seedlings having a height >30 cm than the smaller seedlings [22
]. The relationship between seedling height and defoliation could be explained by the wider crowns that increase larvae density on taller seedlings [62
]. SBW larvae fall from the upper to lower branches of mature trees before reaching the understory; thus, overstory vegetation and taller seedlings provide a protector effect for smaller seedlings [38
Stand composition, stand density, and seedling location can also influence defoliation and mortality of regeneration [22
]. Nie et al. [22
] observed that softwood or mixedwood stands favored higher defoliation of balsam fir seedlings. Swaine [63
] demonstrated that seedlings protected by a canopy are less susceptible to defoliation than seedlings situated within an open area. Our observations of black spruce seedlings were similar, as seedlings within clearcut areas were twice as defoliated as seedlings within the residual stands. Thus, the open canopy conditions created by clearcutting affected the distribution of species, the type of regeneration [64
], and regeneration mortality caused by defoliation. We could not confirm the influence of distance from the residual forest on seedling defoliation. We used a medium-long distance transect (40 m); this distance may be insufficient to identify distance effects on defoliation within open areas. Further research is required to better understand the effect of distance on seedling defoliation using both a longer distance, more plot replications and a greater number of transects within each plot.
Clearcutting is the most widely used harvesting method in Canada [13
]. Clearcutting leads to highly fragmented landscapes, declines in habitat diversity, and losses of productivity [65
]. For this reason, ecosystem-based management proposes partial cuttings as a means of timber harvesting that attempt to (i) preserve the long-term structure and ecological processes responsible for maintaining forest productivity and (ii) ensure ecosystem integrity, biodiversity, and sustainability [55
]. Recently, these silvicultural treatments have been adopted within the boreal forest [69
]. Based on our findings, we consider evaluating the vulnerability of seedlings to SBW outbreaks within partial-cutting sites to be an essential future study requirement. We would hypothesize that, relative to clearcutting, partial cutting would result in lower defoliation of seedlings as the residual stand could protect seedlings from SBW activity.
Much effort is being placed on the study of insect outbreaks in the boreal forest to better understand the spatial patterns, future scenarios, insect-climate interactions, and past dynamics of insect outbreaks [71
]. Under climate change scenarios, disturbance regimes in boreal forests are expected to be highly affected; for example, scenarios forecast an increase in the frequency and severity of fire, insect outbreaks, and windthrows [1
]. Improving our understanding of the variability of natural disturbance cycles at multiple scales will be a major, yet important, challenge in mitigating the effects of climate change on boreal forests and adapting forest management in consequence. Insects outbreaks are a major disturbance agent in forest ecosystems. Tree defoliation affects productivity through reduced growth [72
], increases tree mortality [73
], decreases ecosystem resilience [74
], modifies forest structure and dynamics [75
], and heightens the vulnerability of the forest to other disturbances, e.g., windthrow [76
]. Most studies focus on mature trees; nonetheless, the vulnerability of seedling regeneration to insect outbreak and the selection of silvicultural practices that minimize the effects of insect outbreak on stand regeneration remain understudied aspects of forest ecology. Our research demonstrated how species and seedling height were the main factors that explained seedling defoliation levels. SBW affected balsam fir more than black spruce seedlings, and defoliation was greatest for taller seedlings. Although black spruce seedlings within residual stands experienced less defoliation than seedlings in open clearcutting areas, distance from the residual stand did not influence the level of seedling defoliation. These results improve our understanding of the effects of insect outbreaks on conifer regeneration. We suggest that studying the effect of SBW on seedlings should be a priority for the management strategies in Eastern Canadian boreal forests, particularly as boreal forests are expected to undergo marked change due to future warming.