David Pothier

Impact of dominant tree dynamics on site index curves

Site index curves were modeled for two species of different shade tolerance, black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.), from an extended network of permanent sample plots (PSP) that covers periods of time varying from 10 to 30 years, in the province of Quebec. A data set reserved for validation allowed us to compare the site index curves derived from PSPs with published site index curves fitted to temporary sample plots (TSP) and stem analyses (SA). For both species, the site index curves calibrated from PSPs and TSPs behave similarly as they have comparable average bias and accuracy. The major difference is seen with the SA curves that strongly overpredict the dominant height growth of the PSPs. The similar pattern of change of site index curves calibrated from TSP and PSP data reinforces their validity as both types of curves were calibrated with independent data sets and methodologies. The differences observed between SA and PSP curves were likely produced by the dynamics of dominant height related to tree mortality and change in social status. For both species, approximately one tree out of five (22% for black spruce and 16% for jack pine) was replaced every 10 years in the tree group that was used to estimate dominant height. Consequently, the trajectory of dominant height through time for a particular plot is saw-toothed, the size of the “teeth” being, among other things, a function of stand regularity, as measured by an evenness index. Due to this tree replacement dynamic, stand dominant height curves are also more rapidly asymptotic than those of individual trees.

Using the shelterwood method to mitigate water table rise after forest harvesting

The groundwater level of a conifer stand established on a lowland in eastern Canada was periodically measured using water wells installed in an experimental design composed of four completely randomized blocks and five levels of cutting (0, 40, 50, 60, and 100% of basal area (BA) removed). The three partial cutting treatments were applied following the principles of low thinning, but with the seed cutting objectives of the shelterwood method. Before cutting, highly similar values for groundwater level were recorded for plots targeted to receive the planned treatments. During the first growing season after cutting, the water table rise was linearly related to the percentage of cutting, and this effect was more apparent at the lower levels recorded for the control water table. This finding is partly explained by the leaf biomass of residual trees that intercepted an increasing proportion of rainfall with decreasing cutting intensity. Five years after cutting, although the water table of clearcut experimental units (EU) was still higher than that of the controls, it was no longer related to cutting intensity. During the 5 years following cutting, the slopes of the relationship between the water table depth of the control plots and those of any treatment gradually approached the value calculated before cutting. This water table recovery was related to the increasing leaf biomass of the regeneration stratum over time, rather than to the crown expansion of residual trees. The shelterwood method should be considered for forest management of wetlands, since it mitigates water table rise after the first cut and promotes a vigorous regeneration stratum which should also mitigate water table rise following the final cut.

Deer browsing and soil disturbance induce cascading effects on plant communities: a multilevel path analysis

Understanding how large herbivores shape plant diversity patterns is an important challenge in community ecology, especially because many ungulate populations in the northern hemisphere have recently expanded. Because species within plant communities can exhibit strong interactions (e.g., competition, facilitation), selective foraging by large herbivores is likely not only to affect the abundance of palatable species, but also to induce cascading effects across entire plant communities. To investigate these possibilities, we first tested the effects of deer browsing and soil disturbance on herbaceous plant diversity patterns in boreal forest, using standard analyses of variance. Second, we evaluated direct and indirect effects of deer browsing and soil disturbance on the small-scale richness of herbaceous taxa using a multilevel path analysis approach. The first set of analyses showed that deer browsing and soil disturbance influenced herb richness. Path analyses revealed that deer browsing and soil disturbance influenced richness via complex chains of interactions, involving dominant (i.e., the most abundant) browsing-tolerant (DBT) taxa and white birch (Betula papyrifera), a species highly preferred by white-tailed deer (Odocoileus virginianus). We found no evidence that an increase of white birch in fenced quadrats was the direct cause of a decrease in herb richness. However, we found strong evidence that a higher abundance of DBT taxa (i.e., graminoids and Circium arvense), both in fenced and unfenced quadrats, increased herb layer richness. We propose an empirical model in which competitive interactions between white birch and DBT taxa regulate the strength of facilitative relationships between the abundance of DBT taxa and herb richness. In this model, deer browsing and the intensity of soil disturbance initiate a complex chain of cascading effects in boreal plant communities by controlling the abundance of white birch.

Photosynthetic Light Response and Growth Analysis of Competitive Regeneration After Partial Cutting in a Boreal Mixed Stand

Abstract. The ecophysiological development of two competitive species naturally regenerated after partial cutting was studied in a boreal mixed stand of eastern Canada. Photosynthetic light response curve and growth analyses were performed on balsam fir [Abies balsamea (L.) Mill.] and trembling aspen (Populus tremuloides Michx.) natural regeneration growing at different light intensities. Five levels of light intensity (14, 21, 26, 30 and 76% of full light) were obtained by applying the following treatments: three partial cutting intensities (removal of 35, 50 and 65% of the initial basal area), a clearcut (removal of 100% of the initial basal area) and an uncut control. Leaves of aspen suckers showed higher values of photosynthetic capacity, apparent quantum yield, light compensation point and dark respiration in comparison to fir seedlings at all light levels. In response to light availability, photosynthetic plasticity of aspen was greater than that of fir. Aspen was less able to maintain a positive net photosynthesis at low light than fir, and this could reduce its survivorship, particularly at 14% and 21% of full light. Leaf area ratio of balsam fir was higher than for aspen at all light levels, but both species showed a general increase in leaf area ratio as percent of full light decreased. Above-ground relative growth rate of aspen was higher than for fir at all light levels. At 14% and 21% of full light, aspen suckers had higher height-to-stemwood weight ratios than at higher light intensities. This higher allocation to height growth formed etiolated seedlings that can hinder the long-term survivorship of aspen in low light.

Impact of deer browsing on plant communities in cutover sites on Anticosti Island

Abstract Browsing by Anticosti Islands large white-tailed deer (Odocoileus virginianus) population reduces the abundance of the deers preferred species and increases the abundance of less preferred species. This alteration of plant communities can lead to degradation of deer habitat. However, the centres of large clear-cuts may be spared from deer browsing given that this animals instincts regarding predation risk impel it to remain near escape cover. This project assessed the impact of high deer populations on plant communities in large clear-cuts harvested by cutting with protection of regeneration and soils (CPRS). To this end, vegetation growing within fenced-off areas was compared with that growing in adjacent unfenced plots following application of large CPRS in 1995 and 1996. The results show that deer browsing does not have an impact on the number of plant species but does dramatically reduce total species cover. Furthermore, although the number of species was the same in the fenced and unfenced plots, some species were only found in one or the other type of plot. Rubus idaeus, Betula papyrifera, Epilobium angustifolium, Abies balsamea, Maianthemum canadense, Cornus Canadensis, and Prunus pensylvanica were all associated with fenced plots, while the cover of true grasses and Cirsium arvense increased in the unfenced plots. We conclude that large CPRS-harvested clear-cuts cannot serve to preserve the initial plant communities on Anticosti Island.

A comparison of mortality rates between top height trees and average site trees

Abstract• Although comparisons between methods of selecting trees for site index estimates are well documented in the literature, little is known on mortality rates of different canopy tree cohorts used for that purpose.• This study was initiated to test the hypothesis that the mortality rates of top height trees are lower than those of codominants only or a combination of codominant and dominant trees. To test this hypothesis, we used records from a network of permanent sample plots in Québec and studied the fate of different cohorts of site trees for five different species.• Our results did not show clear evidence of lower mortality rates for top height trees. Instead we found that depending on the species, top height trees have lower (Populus tremuloides, Pinus banksiana), higher (Picea mariana, Abies balsamea) or equal mortality rates (Betula papyrifera) than codominant trees or codominant and dominant trees combined.• These results suggest a tendency for shade intolerant species to maintain lower top height tree mortality rates over time when compared to shade tolerant species. In the latter case, it is also shown that spruce budworm epidemics (Choristoneura fumiferana) did not change the pattern of mortality rates of site trees of A. balsamea.Résumé• Bien que des études comparatives sur les méthodes servant à déterminer l’indice de qualité de station sont disponibles dans la littérature, on connaît peu de choses sur les taux de mortalité de différentes cohortes d’arbres utilisées à cette fin.• Cette étude a été initiée dans le but de tester l’hypothèse selon laquelle les taux de mortalité de arbres dominants sont plus faibles que ceux des arbres codominants ou de ceux d’une combinaiso d’arbres codominants et dominants. Afin de tester cette hypothèse, nous avons utilisé la base de don nées des placettes-échantillons permanentes du Québec et nous avons suivi la destinée de différente cohortes d’arbres indicateurs de la station pour cinq espèces différentes.• Nos résultats n’ont pas montré de façon tranchée que les arbres dominants ont un taux de mortalit inférieur à celui des autres cohortes à l’étude. Nos résultats démontrent plutôt que selon les espèces, les arbres dominants ont un taux de mortalité inférieur (Populus tremuloides, Pinus banksiana), suérieur (Picea mariana, Abies balsamea) ou égal (Betula papyrifera) au taux de mortalité des arbre codominants ou à une combinaison d’arbres codominants ou dominants.• Ces résultats suggèrent que les taux de mortalité des espèces classées intolérantes à l’ombre ont ten dance à demeurer plus faibles dans le temps lorsqu’on les compare aux taux de mortalité des espèce tolérantes à l’ombre. Pour ces dernières, les résultats démontrent de plus que l’effet des épidémie de la tordeuse des bourgeons de l’épinette (Choristoneura fumiferana) n’ont eu aucun effet sur le patrons de mortalité des différentes cohortes d’arbres indicateurs de la station en ce qui concern A. balsamea.

Predicting decay and round-wood end use volume in trembling aspen (Populus tremuloides Michx.)

Abstract• In Quebec (Canada), predicting net merchantable volume of standing trees is essential to adjust stumpage fees. Furthermore, round-wood end use is important in the provincial forest management context because it is used to split the allowable annual cut among the different mill types.• A method relying on linear, binomial and cumulative logit regressions is proposed to predict both decay volume and round-wood end use volume. Tree age, height and quality, as well as ecological region, stand origin and presence of Phellinus tremulae (Bond.) Bond. & Boriss. and Ceratocystis fimbriata (Ellis & Halst.) fungi are the main factors that contribute to the presence and the proportion of decayed merchantable volume. Once the net merchantable volume is estimated, its division into round-wood end use is estimated through a series of steps involving the presence of Phellinus tremulae, saw log height, stem quality and size as explanatory variables. The first step is a multinomial regression which predicts the number of end uses (pulp wood, low-grade saw logs, saw logs, low-grade veneer, and veneer) that are present in the stem. A series of logistic regressions then determines the presence of each end use, with linear regressions predicting the round-wood volume of each end use.Résumé• Au Québec (Canada), la prévision du volume marchand net des arbres sur pied est indispensable pour déterminer les droits de coupe. En outre, la répartition par produits du volume marchand net est essentielle dans le contexte de la gestion forestière régionale car c’est cette étape qui détermine la portion de la possibilité forestière qui est attribuée aux différentes usines de transformation.• Une méthode utilisant des régressions linéaires, binomiales et logistiques cumulatives est proposée pour prédire à la fois le volume de carie et sa répartition par produits. L’âge de l’arbre, la hauteur et la qualité, comme la région écologique, l’origine du peuplement et la présence de champignons Phellinus tremulae (Bond.) Bond. & Boriss. et Ceratocystis fimbriata (Ellis & Halst.) sont les principaux facteurs qui contribuent à la présence et à la proportion du volume de bois carié. Une fois le volume marchand net estimé, sa répartition par produits est estimée à travers une série d’étapes impliquant la présence de Phellinus tremulae, la hauteur de bois d’oeuvre, la qualité et la taille des tiges comme variables explicatives. La première étape est une régression logistique cumulative qui prédit le nombre d’utilisations finales (bois de pâte, sciage de basse qualité, bille de sciage, placage de basse qualité et placage) qui sont présentes dans le tronc. Une série de régressions logistiques détermine alors la présence de chacune de ces utilisations. Enfin, des régressions linéaires prédisent le volume marchand net de chaque utilisation.

Forest Structure and Understory Plant Communities Inside and Outside Tree Retention Groups in Boreal Forests

Abstract: Alternatives are being sought to the widespread use of clear-cut logging in boreal forests. Group retention harvesting is a silvicultural treatment in which well-distributed but relatively small residual forest patches (ca 10 m wide) are left inside cutover sites. The objective of this study was to compare vascular plant communities, tree species regeneration, and dead wood retention in tree retention groups and adjacent clear-cuts with soil protection. Our results indicate that plant diversity is relatively similar inside tree retention groups and the adjacent clear-cut area. This result may be explained by the important spatial variability observed among the stands, which were located in different geographical locations, the fact that soils were little disturbed during harvesting in clear-cuts and few opportunities were present for the establishment of pioneer species, and the relatively short time span since harvesting. Using a functional trait approach, we found that shade tolerance still plays a significant but relatively minor role in explaining species abundance between the 2 environments. Tree retention groups also retain a greater quantity and greater variability of dead wood materials compared with clear-cuts, and a higher abundance of regenerating trees, which is likely explained in part by the direct damage caused by the harvesting operations outside tree retention groups. Overall, retention groups do not appear to confer superior protection for late-successional plants compared with traditional clear-cut logging with soil protection. Their ecological value mostly consists in ensuring a minimal input of dead woody materials for saproxylic species.

Effects of heartwood formation on sugar maple (Acer saccharum Marshall) discoloured wood proportion

Key messageDiscoloured heartwood proportion (DHP) decreases with an increasing rate of heartwood formation for vigorous sugar maple trees, suggesting that age-related increase in DHP is due to increasing likelihood of injuries with tree age.AbstractSugar maple heartwood is more susceptible to decay and discolouration than the sapwood. To understand heartwood formation, foliage, sapwood, heartwood, and discoloured wood areas as well as other biometric variables were measured on 79 trees sampled in two sites in south-eastern Quebec, Canada. Tree growth was related to heartwood formation and discolouration with a modelling approach. Heartwood formation increased with tree height, age, and crown size, but decreased with increasing leaf area to stem basal area ratio. In general, the proportion of discoloured heartwood increased with an increasing rate of heartwood formation. However, for trees visually classified as vigorous, the proportion of discoloured heartwood tended to decline with an increasing rate of heartwood formation. This indicates that the size/age-related increase in discoloured wood proportion in sugar maple is possibly due to older trees being more likely to have injuries and inoculations by fungi. Thus, residual stands composed of high vigour trees can likely maintain higher growth while minimizing discoloured wood proportion.

Large-Scale Variations in Lumber Value Recovery of Yellow Birch and Sugar Maple in Quebec, Canada

Silvicultural restoration measures have been implemented in the northern hardwoods forests of southern Quebec, Canada, but their financial applicability is often hampered by the depleted state of the resource. To help identify sites most suited for the production of high quality timber, where the potential return on silvicultural investments should be the highest, this study assessed the impact of stand and site characteristics on timber quality in sugar maple (Acer saccharum Marsh.) and yellow birch (Betula alleghaniensis Britt.). For this purpose, lumber value recovery (LVR), an estimate of the summed value of boards contained in a unit volume of round wood, was used as an indicator of timber quality. Predictions of LVR were made for yellow birch and sugar maple trees contained in a network of more than 22000 temporary sample plots across the Province. Next, stand-level variables were selected and models to predict LVR were built using the boosted regression trees method. Finally, the occurrence of spatial clusters was verified by a hotspot analysis. Results showed that in both species LVR was positively correlated with the stand age and structural diversity index, and negatively correlated with the number of merchantable stems. Yellow birch had higher LVR in areas with shallower soils, whereas sugar maple had higher LVR in regions with deeper soils. The hotspot analysis indicated that clusters of high and low LVR exist across the province for both species. Although it remains uncertain to what extent the variability of LVR may result from variations in past management practices or in inherent site quality, we argue that efforts to produce high quality timber should be prioritized in sites where LVR is predicted to be the highest.