Aim

Non?climatic constraints on species northern range boundaries are often overlooked in attempts to predict climate?induced range shifts. Here, we examined the effects of habitat availability and fire disturbance on the distribution of a species that transitions from being common to being found only in marginal populations at the northern boundary of its range.

Location

North?western Quebec, Canada (46–51° N and 74–79° W).

Taxon : Eastern white cedar (Thuja occidentalis L.)

Methods

We used forest inventory data (n = 4,987) to characterize white?cedar habitat based on edaphic and topographic conditions at sampled sites along a 600?km latitudinal gradient. Non?metric multidimensional scaling was used to assess habitat similarity of sites in the south, where white?cedar stands are abundant, and sites in the north, where white?cedar stands are rare. We constructed ensemble white cedar distribution models based on habitat variables in the south and compared ensemble forecast projections of white cedar in the north with observed occurrences to determine if habitat availability was limiting. We independently estimated the age of white?cedar stands and adjacent stands without white cedar along the gradient. ANOVA was performed to test the age difference between white?cedar and adjacent stands to determine whether the location of white?cedar stands was influenced by disturbance, primarily stand?replacing fire.

Results

Habitat availability was not limiting the distribution of eastern white cedar at its northern range boundary. White cedar did not occupy most sites with suitable habitat in the north, suggesting that other factors prevent white cedar from establishing more stands northward. White?cedar stands were older than adjacent stands without white cedar all along the gradient, but the difference was more pronounced in the north. This suggests that white?cedar stands in the north are restricted to undisturbed areas.

Main conclusions

Fire disturbance, more than habitat availability, limits the distribution of white cedar at its northern range boundary. Projections of white cedar distribution under climate change that ignore fire could overestimate the ability of warming temperatures to extend its northern range limit.

Landscape-level forest management has long been hypothesized to affect forest insect outbreak dynamics, but empirical evidence remains elusive. We hypothesized that the combination of increased hardwood relative to host tree species, prevalence of younger forests, and fragmentation of those forests due to forest harvesting legacies would reduce outbreak intensity, increase outbreak frequency, and decrease spatial synchrony in spruce budworm Choristoneura fumiferana outbreaks. We investigated these hypotheses using tree ring samples collected across 51 sites pooled into 16 subareas distributed across a large ecoregion spanning the international border between Ontario (Canada), and Minnesota (USA). This ecoregion contains contrasting land management zones with clear differences in forest landscape structure (i.e. forest composition and spatial configuration) while minimizing the confounding influence of climate. Cluster analyses of the 76-yr time-series generally grouped by subareas found within the same land management zone. Spatial nonparametric covariance analysis indicated that the highest and lowest degree of spatial synchrony of spruce budworm outbreaks were found within unmanaged wilderness and lands managed at fine spatial scales in Minnesota, respectively. Using multivariate analysis, we also found that forest composition, configuration, and climate together accounted for a total of 40% of the variance in outbreak chronologies, with a high level of shared variance between composition and configuration (13%) and between composition and climate (9%). At the scale of our study, climate on its own did not explain any of the spatial variation in outbreaks. Outbreaks were of higher frequency, lower intensity, and less spatially synchronized in more fragmented, younger forests with a lower proportion of host species, with opposing outbreak characteristics observed in regions characterised by older forests with more concentrated host species. Our study is the first quantitative evaluation of the long-standing ‘silvicultural hypothesis’ of spruce budworm management specifically conducted at a spatio-temporal scale for which it was intended.

Wildfire simulations were carried out using the Prescribed Fire Analysis System (PFAS) to study the effect of landscape composition on fire sizes in eastern Canadian boreal forests. We used the Lake Duparquet forest as reference, plus 13 forest mosaic scenarios whose compositions reflected lengths of fire cycle. Three fire weather risks based on duff moisture were used. We performed 100 simulations per risk and mosaic, with topography and hydrology set constant for the reference. Results showed that both weather and landscape composition significantly influenced fire sizes. Weather related to fire propagation explained almost 79% of the variance, while landscape composition and weather conditions for ignition explained ~14 and 2% respectively. In terms of landscape, burned area increased with increasing presence of shade-tolerant species, which are related to long fire cycles. Comparisons among the distributions of cumulated area burned from scenarios plus those from the Société de Protection des Forêts contre le Feu database archives showed that PFAS simulated realistic fire sizes using the 80–100% class of probable fire extent. Future analyses would best be performed on a larger region as the limited size of the study area could not capture fires larger than 11 000 ha, which represent 3% of fires but 65% of the total area burned at the provincial scale.

Forest management has been criticised in the last 20 years for its negative impact on the native species, structures and functions of the forest. Of many possible alternatives proposed to minimize these effects, the functional zoning (or TRIAD) approach is gaining popularity in North America. The goal of this approach is to minimize the negative environmental impacts of forestry while maintain timber supply by dividing the forest into three broad land-use zones: (1) conservation, (2) ecosystem management, and (3) wood production. In this study, we used a spatially explicit landscape model to simulate the effects of fire and six different forest management scenarios on a boreal mixedwood forest management unit in central Quebec. The management scenarios examined included the current practices scenario, a scenario proposed by the provincial government, and four TRIAD scenarios varying in the amount of forest allocated to each of the three zones. For each scenario, we examined the harvest volume, percentage old-growth forest or old forest managed to favour old-growth attributes, and effective mesh size of forest patches by 20-year age classes. With more area set aside for conservation and high-retention partial cut harvesting techniques designed to maintain the attributes of old-growth stands, all TRIAD scenarios resulted in higher percentages of stands with old-growth attributes than the current practices scenario and the government proposed scenario, and two of the four TRIAD scenarios also resulted in higher harvest volume over the long term. All forest management scenarios resulted in significantly lower effective mesh size than the fire-only scenario, but this difference was not as pronounced for the four TRIAD scenarios as for the current practice and government proposed scenarios. We conclude that the TRIAD approach has the potential to minimize some of the negative impacts of forestry on the landscape, while maintaining timber supply over the long term.

Aim Present northern distribution limit of jack pine (Pinus banksiana Lamb.) follows the northern limit of continuous open boreal forest in western Canada, but not in eastern Canada where it is located further south. We tested the hypothesis that fire plays a more important role than climate in explaining the present position of the northern distribution limit of jack pine.

Location An experimental jack pine plantation was set up in 1992, c. 300?km north of the present distribution limit of the species, in the Boniface river area of northern Québec (57°43??N, 76°05??W).

Methods Climate and fire data were used to compare sites at and north of the present distribution limit of jack pine. In 2001, surviving individuals from the plantation were measured (total height, annual shoot elongation, basal diameter, and presence/absence of cones).

Results Climate data from the ten weather stations used in this study did not show major differences. The northern limit of jack pine distribution is closely associated with the occurrence of fires larger than 200?ha. Survival of the planted jack pines was 31%. About 25% of the surviving pines qualified as normal, single?stem individuals; the others were slightly uprooted and/or showed marks of erosion or foraging. Cones were produced, although no viable seeds were found.

Main conclusions The low number of degree?days above 5?°C at the plantation site could explain why the seeds were not viable. However, such climate conditions are not sufficient to prevent growth, as was shown by annual shoot elongation measurements. Most of the surviving jack pines from the Boniface river plantation are relatively healthy and follow a normal developmental programme. Low fire frequency and small fire size are amongst the main factors that prevented P. banksiana from migrating further north or east following deglaciation in northern Québec and Labrador.

Résumé :

La production annuelle moyenne de graines est proportionnelle à la surface terrière pour les arbres composant la canopée. Cependant, il reste à démontrer que des individus croissant sous couvert produisent moins de graines que prévu (en fonction de leur taille) en raison de la faible disponibilité de lumière. La production de cônes a été examinée pour le sapin baumier (Abies balsamea (L.) Mill.) et l'épinette blanche (Picea glauca (Moench) Voss) en 1998 et pour le sapin baumier en 2000 dans l'ouest du Québec. Les arbres étudiés poussaient sous couvert, près ou loin de la bordure forestière, ou (pour un site) faisaient partie d'une plantation. Un modèle très simple de transmission de la lumière à travers la canopée de peuplier faux-tremble (Populus tremuloides Michx.) et à travers les troncs en bordure de la forêt a été développé afin de quantifier l'effet de la disponibilité de lumière sur la production de cônes. L'augmentation de disponibilité de lumière près d'une bordure (par exemple une coupe forestière récente) entraîne un doublement de la production de cônes chez les arbres croissant sous couvert. La hauteur minimale pour la production de cônes chez des arbres croissant sous couvert et loin de la bordure forestière est d'environ 10 m pour le sapin baumier et 14 m pour l'épinette blanche, ces valeurs diminuant près de la bordure. En comparaison, la hauteur minimale pour la production de cônes pour les épinettes blanches d'une plantation (lumière = 100%) est d'environ 3 m. Une augmentation de la variance expliquée est obtenue en tenant compte de la disponibilité de lumière.

Abstract

Mean annual seed production is assumed to be proportional to basal area for canopy trees, but it is not known if subcanopy trees produce fewer seeds than expected (given their size) because of low light availability. Ovulate cone production was examined for balsam fir (Abies balsamea (L.) Mill.) and white spruce (Picea glauca (Moench) Voss) in 1998 and for balsam fir in 2000 in western Quebec using subcanopy stems, near or far from forest edges, or (at one site) planted white spruce trees in fully open conditions. A very simple light model for transmission through mature trembling aspen (Populus tremuloides Michx.) crowns and through boles near forest edges was developed to account for the effect of light receipt on cone production. The enhanced light near forest edges (e.g., recent clearcuts) leads to about a doubling of cone production for subcanopy stems. The minimum subcanopy height for cone production far from an edge is about 10 m for balsam fir and 14 m for white spruce, with these minima decreasing near edges. By contrast, the minimum height for white spruce in a plantation (full light) is about 3 m. Accounting for light receipt leads to an increase in the explained variance © 2008 NRC Canada.

Remnant tree stands left intact following a disturbance constitute the sole seed banks available for regeneration of coniferous species that neither bear serotinous cones nor reproduce vegetatively. The success of regeneration of tree species on these disturbed sites is, therefore, dependent on the distance from potential seed sources. The regeneration of balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss) and white cedar (Thuja occidentalis L.) was studied at two sites in Quebec's southwestern boreal forest in order to quantify the influence of remnant stands on spatial distribution of regeneration. The first site is located in an area that burned in 1944 while the second site is located in an area that was clear-cut in the mid-1980s. Canonical correspondence analyses were used to determine the respective contributions of environmental data and spatial variables to the pattern of spatial distribution of regeneration. The results reveal that distance from a remnant stand is the most important variable in explaining spatial distribution of regeneration when compared to environmental variables such as soil type, drainage, slope and altitude. The plots of regeneration density against distance from a remnant stand for both the burned site and logged site show that regeneration density decreases abruptly with distance from a remnant stand. Furthermore, spatial autocorrelation analyses (Moran's I) indicate that even small remnant zones can significantly influence the pattern of spatial distribution of regeneration for the three species studied. The results presented here suggest that where preestablished regeneration is not abundant enough, alternative silvicultural systems such as strip clear-cutting or seed-tree systems could be used instead of cuts with protection of regeneration and soils (CPRS). (C) 2001 Elsevier Science B.V. All rights reserved. © 2002 Elsevier B.V. All rights reserved.

In this paper, we investigate spatial relationships between vegetation boundaries and environmental boundaries from a second-growth forest in southwestern Quebec, Canada. Four statistics that quantify the amount of direct spatial overlap and the mean minimum distance between boundaries are introduced and used to compute the degree of spatial co-occurrences between boundaries. The significance of these statistics is determined using randomized and restricted permutation tests. Boundaries based on tree species density are found to significantly overlap the locations of boundaries delineated by the environmental data at the study site. Significant overlap is also found using boundaries defined by tree presence-absence data and environmental variables. Vegetation boundaries based on tree species density and on tree presence-absence data are not, however, at the same locations. This suggests that for the study site the two types of vegetation boundaries (tree density and presence-absence) reflect different responses to underlying environmental processes. Vegetation boundaries determined using species diversity and species richness, although spatially related to the presence-absence boundaries, did not overlap the environmental boundaries. Results of the two permutation tests (randomized and restricted) agree only when the spatial relationship between the two boundary types is strong. Overall, randomization is found to be a more conservative test for detecting boundary spatial relationships, rejecting the null hypothesis of no spatial relationship fewer times than the restricted permutation test.