4.1. Variables Significantly Linked to Competing Vegetation Cover
Our results contrast the nature of competing vegetation between two major ecosystem types differing in terms of climate and fertility. Ericaceous shrubs were more commonly associated with black spruce or jack pine ecosystems while the opposite was true for intolerant hardwoods and non-commercial hardwoods.
Ericaceous shrubs were more associated with colder ecological regions located to the north of the study area or sites located at higher altitudes. They were also associated with black spruce and jack pine stands that are typical in these conditions [43
]. In these stands, ericaceous shrubs are often present in the understory and can expand after harvesting [14
]. Cold climate slows down decomposition rates and favours the accumulation of organic matter [45
], a substrate more favourable to black spruce and ericaceous shrub growth. These species also often occur together on poorly drained soils [19
]. Poor drainage favours the accumulation of organic matter, hence reducing nutrient availability, a condition less favourable to hardwood development [46
]. The abundance of sphagnum mosses on poorly drained soils favours the creation of acidic, wet and cold soils that decrease the decomposition rate of the organic matter, nutrients availability, microbial activity and plant growth [47
]. At the other end of the drainage gradient, jack pine can grow on very xeric sandy sites that are also less favourable to hardwoods, but on which ericaceous shrubs like sheep laurel can also thrive [19
Both hardwood categories shared many influencing factors, which made it difficult to separate them clearly. They were more associated with well-drained sites located in warmer ecological regions located to the south of the study area. They were also less common in black spruce or jack pine ecosystems. Drainage and soil texture effects were closely linked, which explains that shade-intolerant hardwoods were more likely to be found on tills 50–100 cm thick and less on disintegration moraines and ice-contact deposits. Outwash generally feature low nutrient availability [49
], which can explain why this deposit had a negative effect on the probabilities to encounter hardwoods. Similarly, previous observations have shown that paper birch, trembling aspen, pin cherry, mountain maple and speckled alder are seldom found on fluvioglacial deposits such as ice-contact deposits and outwash [50
The link between the pre-harvest canopy abundance of intolerant hardwoods and their regeneration is not clear since some positive relationships between the abundance of paper birch in the canopy and regeneration of intolerant species were found in the RDA while negative relationships were observed in the linear mixed models. Even though paper birch can invade cutovers, it mostly regenerates through seeds that can disperse over relatively long distances [51
], making it less tightly linked to the harvest site. A positive relationship was found between the presence of aspen in the canopy and regeneration of intolerant hardwoods in the linear mixed models, but not in the RDA. Since trembling aspen mainly propagates via root suckering [52
], regeneration of this species is highly dependent on its presence within the canopy of the previous stand.
The abundance of non-commercial hardwoods in the regeneration layer was generally positively associated with pre-harvest hardwood stands. Although aspen and paper birch can have detrimental effects on conifer growth by intercepting light, their canopies can attenuate weather extremes and increase humidity for understory vegetation and regeneration [8
]. Their litter decomposes rapidly and increases nutrient availability [54
]. In addition, hardwoods let more light reach the understory than conifers [56
], which could help semi shade-tolerant species such as mountain maple to persist under their cover. They could then expand after canopy removal.
The difficulty of differentiating between conditions conducive to a high abundance of either intolerant or non-commercial hardwoods could also be explained by the fact that the non-commercial hardwoods group included species with very different behaviours. Mountain maple and pin cherry are generally present on relatively well-drained sites [50
]. Speckled alder, on the other hand, is commonly associated with poorly drained sites [50
]. Analyzing competing vegetation by functional groups might have affected our ability to predict the presence of hardwoods like speckled alder on imperfectly or poorly drained sites. In addition, both groups could respond to common factors.
The relationship with some variables may, however, not be direct. Hence, the effect of harvesting season may be linked to the fact that wet sites, favourable to ericaceous shrubs [57
] (e.g., bog Labrador tea and sheep laurel), are often selected for winter harvesting. It is also possible that the snow cover on the ground may help protect pre-established shade-tolerant regeneration during harvesting [58
], making it harder for shade-intolerant hardwoods to repopulate those sites. Full tree harvesting improved the odds to encounter commercial shade-intolerant hardwoods after harvesting and had the opposite effect on ericaceous shrubs. Dragging the trees disturbs the soil, which increases soil surface temperatures and creates mineral seedbeds [59
]. Higher soil temperatures stimulates trembling aspen suckering [52
] and the mixing of organic and mineral soil layers favour paper birch germination [60
]. We also observed that cut-to-length harvesting had a negative impact on the chances to encounter ericaceous shrubs after harvesting. This system has been reported to cause less damage to pre-established regeneration than full tree harvesting [59
], which could help regeneration to overcome ericaceous competition. However, a given harvesting approach is usually used over a large area, so the effects of harvesting system and harvesting season may well reflect the ecosystems in which they are applied. Winter harvesting, for instance, is often preferred for wet sites where trafficability is lower and road construction costs are higher.
The effect of original disturbance could also be indirect, reflecting the vulnerability of different stand types to specific disturbances. Non-commercial hardwoods were more likely to be present after the harvesting of stands that originated from partial windthrow or partial harvesting. Hardwoods are less vulnerable to windthrow in comparison with softwoods, so partial windthrows would be more frequent in mixedwood stands and total windthrow would be more common in coniferous stands [61
]. Partial cutting would also be more likely in complex stands, such as mixedwood stands. Non-commercial hardwoods were probably established before these disturbances and the small gaps created could have allowed semi shade-tolerant species like mountain maple to maintain themselves in the stand until the clearcut [62
] and then invade the cutovers [64
]. The negative effect of clearcutting and total windthrow on the probability of encountering commercial shade-intolerant hardwoods seems counter-intuitive at first, as the increase in light availability should have benefited these species. It is likely, however, that these disturbances were more common on cold or less fertile sites typical of black spruce or jack pine ecosystems, these being more favourable to ericaceous shrubs.
4.2. Temporal Impact of Competing Vegetation in Conditions of Marginal Regeneration
The different combinations of competing vegetation studied did not negatively affect the evolution of conifer stocking on sites with marginal regeneration, as this variable tended to improve in all combinations. In addition, significant increases of conifer free-to-grow stocking were observed on sites with either a low level of shade-intolerant hardwoods or a moderate level of ericaceous shrubs at the first inventory. In the first case, the higher availability of light provided better growth conditions for the regeneration. In the second case, this increase is likely linked to the characteristics of the sites supporting this specific combination. Most sites featuring low competition by ericaceous shrubs were imperfectly or poorly drained, while most sites featuring moderate competition by ericaceous shrubs had good or moderate drainage. Therefore, conifers happened to have better growing conditions on sites where moderate ericaceous shrubs competition occurred. Even though black spruce and balsam fir can grow on imperfectly drained sites, they have better growth rates on moderately drained sites [65
]. The general tendency for free-to-grow conifer stocking to improve, no matter the competition type or intensity, could be the result of the shade tolerance and nutrient requirement of the conifers. Almost every conifer encountered on the field were balsam fir or black spruce, and these species are respectively very shade-tolerant or with a broad spectrum [66
] and can tolerate relatively poor nutrient levels [67
]. They can grow under competing vegetation covers for many decades, until they finally overtop hardwoods or ericaceous shrubs. It must be remembered here that this analysis did not include sites that were scarified and planted between the first and the second sampling. However, these treatments would normally be applied in stands that do not reach 40% conifer stocking. Since free-to-grow stocking is almost always greater than total stocking, the impact should remain minor.
Our results for sites with marginal regeneration also show that the different combinations of competition levels and composition had no effect on medium-term regeneration vigour and basal area. At the second inventory, seedlings showed good vigour based on live crown ratio and apical index. Average live crown ratio was always greater than 66% and average apical index was always greater than 1, conditions that have been associated with vigorous seedlings [25
]. The shade tolerance of the regenerating species could again explain the good conditions of seedlings at the second inventory.