by André Boily | 8 October 2020
Published in Tree Physiology, tpaa096. https://doi.org/10.1093/treephys/tpaa096
Late frost can cause damage to trees, especially to the developing bud of broadleaf species in spring. Through long-term adaptation, plants adjust leaf phenology to achieve an optimal trade-off between growing season length and frost avoidance. In this study, we aim to assess ecotypic differentiation in leaf development of sugar maple populations planted in a common garden. A total of 272 sugar maple seedlings from 29 Canadian provenances were planted at the northern boundary of the natural range and the phenological phases of bud and leaf development were monitored during spring 2019. The wide geographical area under evaluation showed a complex seasonal pattern of temperature, with spring warming occurring later in the north and close to the sea. Overall, leaf development lasted between 20 and 36 days, from the end of May to end of June. We observed different timings and rates of leaf development among provenances, demonstrating the occurrence of ecotypes in this species. Minimum April temperatures of the original sites were able to explain such differences, while maximum April temperatures were not significant. Seedlings from sites with colder minimum April temperatures completed leaf development earlier and faster. On average, leaf development diverged by up to 6 days among provenances with minimum April temperatures ranging from −3 to 3°C. Our results demonstrated that the avoidance of late spring frost is a driving force of leaf development in sugar maple populations. In the colder sites, the growing season is a limiting factor for tree growth. Thus, when thermal conditions become favorable in spring, an earlier growth reactivation and high metabolic activity ensure a fast leaf emission, which maximizes the period available for photosynthesis and growth. These patterns demonstrate the long-term phenological adaptation of sugar maple populations to local climatic conditions and suggest the importance of frost events for leaf development.
by Marie-Claude Boileau | 8 October 2020
Published in Forest Ecology and Management 466: 118115. https://doi.org/10.1016/j.foreco.2020.118115
The success of partial cut applications to northern hardwood stands is based upon the contribution of harvested and residual trees to overall stand growth, which can be captured by a stand-level criterion, i.e, the growth dominance coefficient (GDC). The main objective of this study was to test the relevance of GDC in evaluating the success of partial cuts in even-aged and uneven-aged stands that were composed of sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britt.). We first determined whether GDC that was calculated with two indicators of light capture, stem mass and tree leaf area, produced similar results after partial cutting in both stand structures. Second, using linear mixed models, we determined the effect of selection cutting on GDC in uneven-aged stands over the short- and medium-term, and evaluated effects of tree vigour and size before cutting on diameter growth response of trees after cutting. GDC based upon stem mass was similar to GDC based upon tree leaf area in even-aged stands (p = 0.7200), but was lower in uneven-aged stands (p < 0.001). Selection cutting in uneven-aged stands did not change GDC in the short- (p = 0.7721) and medium- (p = 0.8363) term. Yet, several trees that grew rapidly before cutting responded negatively to partial cutting, while many trees that grew slowly before cutting responded positively. Overall, assessing the success of partial cuts appears to be difficult using GDC, a stand-level criterion. Such assessments, as well as a better understanding of the effects of partial cuts, could be obtained through more detailed growth analyses that are performed at the tree level.
by Claire Morin | 8 October 2020
Published in Forest Ecology and Management 466: 118137. https://doi.org/10.1016/j.foreco.2020.118137
In northeastern North America, mixed hardwood-conifer forests have commonly been harvested with selective practices such as diameter-limit cutting. By removing trees of highest commercial value, these cuts often left stands with highly variable density and reduced volume, less species diversity, and lower wood quality. The lack of care to the regenerating layer also resulted in regeneration deficiencies, especially on productive sites supporting yellow birch (Betula alleghaniensis Britton)-conifer stands. We assessed the 15-yr effects of four rehabilitation scenarios on stand growth, quality, vigor and regeneration in an experiment established in Quebec, Canada. The trial compared untreated experimental units (e.g. control), uniform shelterwood method (50 overstory trees/ha), strip clearcutting (20-m strips), and seed-tree method (10 overstory trees/ha), all combined with 3 site preparation treatments (no scarification, mechanical raking, and spot scarification). After 15 years, the ongoing recovery of growth, vigor and regeneration in the control and untreated part of the strip clearcut plot reflected the resilient nature of these mixedwood stands, with conifer (Abies balsamea [L.] Mill., Picea rubens Sarg., Picea glauca [Moench] Voss) saplings and poles contributing to the recovery of total BA and that of vigorous trees (i.e. acceptable growing stock [AGS]). The control and strip clearcut treatments, however, had high BA of unacceptable growing stock. Shelterwood plots recovered more slowly in BA and AGS, but had greater BA of high-quality trees than the control. Seed-tree method was less optimal because of its slow recovery and high understory competition. The lack of synchronization with a good seed year limited the efficiency of scarification, and increased the abundance of non-commercial species. An active rehabilitation strategy combining shelterwood system and ground disturbance (e.g. raking) during a good seed year should help to improve stand quality and regeneration. Yet, our results indicate that retaining conifers in the understory and overstory would ensure a seed source and accelerate recovery. Where seed sources and conifer advance regeneration are lacking, enrichment planting may be necessary to maintain a mixedwood composition over time.
by Claire Morin | 8 October 2020
Published in Global Change Biology 26(4): 2072-2080. https://doi.org/10.1111/gcb.14991
Abstract Climate change is altering phenology; however, the magnitude of this change varies among taxa. Compared with phenological mismatch between plants and herbivores, synchronization due to climate has been less explored, despite its potential implications for trophic interactions. The earlier budburst induced by defoliation is a phenological strategy for plants against herbivores. Here, we tested whether warming can counteract defoliation-induced mismatch by increasing herbivore-plant phenological synchrony. We compared the larval phenology of spruce budworm and budburst in balsam fir, black spruce, and white spruce saplings subjected to defoliation in a controlled environment at temperatures of 12, 17, and 22°C. Budburst in defoliated saplings occurred 6–24 days earlier than in the controls, thus mismatching needle development from larval feeding. This mismatch decreased to only 3–7 days, however, when temperatures warmed by 5 and 10°C, leading to a resynchronization of the host with spruce budworm larvae. The increasing synchrony under warming counteracts the defoliation-induced mismatch, disrupting trophic interactions and energy flow between forest ecosystem and insect populations. Our results suggest that the predicted warming may improve food quality and provide better growth conditions for larval development, thus promoting longer or more intense insect outbreaks in the future.
by Marie-Claude Boileau | 8 October 2020
Published in Annals of Forest Science 77(2): 38. https://doi.org/10.1007/s13595-020-0929-5
A model describing species composition, density and diameter distribution of saplings was developed from operational inventory data. It could be used as an input into growth models calibrated exclusively with merchantable trees to correct some recruitment bias. Important differences in distributions were found between plantations and naturally regenerated stands. Longer-term monitoring would be required to observe the effects of thinning treatments on saplings.
