Stéphane Boudreau

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra

Shrub expansion at the forest-tundra ecotone: spatial heterogeneity linked to local topography

Recent densification of shrub cover is now documented in many Arctic regions. However, most studies focus on global scale responses, yielding very little information on the local patterns. This research aims to quantify shrub cover increase at northern treeline (Qu´ ebec, Canada) in two important types of environment, sandy terraces and hilltops (which cover about 70% of the landscape), and to identify the species involved. The comparison of a mosaic of two aerial photographs from 1957 (137 km 2 ) and one satellite image taken in 2008 (151 km 2 ) revealed that both hilltops and terraces recorded an increase in shrub cover. However, the increase was significantly greater on terraces than on hilltops (21.6% versus 11.6%). According to ground truthing, the shrub cover densification is associated mainly with an increase of Betula glandulosa Michx. The numerous seedlings observed during the ground truthing suggest that shrub densification should continue in the future.

Recent expansion of erect shrubs in the Low Arctic: evidence from Eastern Nunavik

In order to characterize shrub response near the treeline in Eastern Nunavik (Qu´ ebec), a region under extensive warming since the 1990s, we compared two series (1964 and 2003) of vertical aerial photos from the vicinity of Kangiqsualujjuaq. Our study revealed a widespread increase in erect woody vegetation cover. During the 40 years spanning the two photo series, erect shrub and tree cover increased markedly on more than half of the land surface available for new colonization or infilling. Within the 7.2 km 2 analysed, areas with dense shrub and tree cover (>90%) increased from 34% to 44% whereas areas with low cover (<10%) shrank from 45% to 29%. This increase in cover of trees and shrubs occurred throughout the landscape regardless of altitude, slope angle and exposure, although to varying extents. The main shrub species involved in this increase was Betula glandulosa Michx. (dwarf birch), which was present in 98% and dominant in 85% of the 345 plots. In addition, numerous seedlings and saplings of Larix laricina (Du Roi) K Koch (eastern larch) were found above the treeline (25% of the plots), suggesting that the altitudinal treeline might shift upslope in the near future. Sites that remained devoid of erect woody vegetation in 2003 were either characterized by the absence of a suitable seedbed or by harsh local microclimatic conditions (wind exposure or excessive drainage). Our results indicate dramatic increases in shrub and tree cover at a Low Arctic site in Eastern Nunavik, contributing to a growing number of observations of woody vegetation change from various areas around the North.

Caribou-induced changes in species dominance of lichen woodlands: an analysis ofplant remains

Plant communities in northern Quebec-Labrador, Canada have been severely grazed and trampled since the early 1980s by the increasingly large George River caribou herd (GRCH). To evaluate changes in species dominance associated with caribou disturbance, we compared past and present ground vegetation from 14 lichen woodlands. Plant remains from superficial organic horizons indicate that ground vegetation was largely dominated by lichens (especially Cladina) before the onset of caribou disturbance. In enlargments of aerial photos taken before 1975 (i.e., prior to maximum size of the GRCH), all sites were free of caribou trails and were dominated by a continuous lichen (Cladina) carpet. Principal components analysis showed that partial or complete destruction of the Cladina-dominated lichen carpet was the most striking change in ground vegetation. Severe trampling degraded superficial organic horizons, subsequently exposing mineral soil in heavily used sites. With reduced caribou activity in the 1990s, exposed ground was colonized by crustose lichens and Cladonia. Sites that faced severe grazing but light trampling were recolonized mainly by small podetia of Cladina stellaris sprouting from the lichen litter. However, patterns of post-caribou disturbance lichen succession differed from those of post-fire succession, because species from different successional stages are present at the same time in a stand and also because caribou can modify the successional trajectory at any time.

Recent Decline of the George River Caribou Herd as Revealed by Tree-Ring Analysis

Abstract Dendroecological analysis is often used in animal ecology to infer population fluctuations. In this study, we used scars produced by caribou hooves on superficial roots of conifers to evaluate the recent activity of the George River caribou herd (GRCH). In 1999 and 2000, we sampled a minimum of 300 trampling scars at each of 31 lichen woodland sites distributed over the summer habitat of the GRCH. Among the 31 selected sites, 18 had been previously sampled in 1992–1993 and showed a good agreement in trends inferred in caribou activity with the 1999–2000 data set at the same sites. We evaluated the recent activity pattern of the GRCH using mean values of pooled scar-age data from the 31 sites. We inferred two major trends from the tree-ring data: that the GRCH experienced an important increase from 1975 to the late 1980s, and that this growth was followed by a major decline that began in the early 1990s. Radio-collar data from 1991 to 1998 also support the decline as no major change was found in the geographical distribution of the GRCH during this period.

Long-term interactions between migratory caribou, wildfires and Nunavik hunters inferred from tree rings.

Abstract Barren-ground caribou (Rangifer tarandus) herds in North America may reach considerable size and undertake large-scale seasonal migrations from the Arctic tundra to the boreal forest. To test the caribou decline hypothesis associated with native harvesting and fire, we have documented the long-term trends of caribou activity based on a novel approach which uses tree-ring dated trampling scars produced by caribou hooves in the extensive trails distributed over the summer and winter ranges of the Rivièreaux-Feuilles herd (RAF herd, east of Hudson Bay in northern Quebec). The age structure data of trampling scars from lichen woodlands distributed over the entire RAF range confirmed the overall trends of caribou activity from the late 1700s to present time. Over the last 200 years, the RAF herd has undergone two highs in the late 1700s and 1900s separated by a moderate activity pattern in the late 1800s. Native harvesting was possibly involved in the early 1900s decline, although at a moderate level. The reduced magnitude of caribou activity during this period has not modified the natural cycle of highs and lows, which suggests that other demographic factors were controlling the changing caribou abundance. Our data also show that only exceptionally large fires may have a minor, short-lived impact on caribou migrations but not on caribou numbers.

The importance of ecological constraints on the control of multi-species treeline dynamics in eastern Nunavik, Québec

UNLABELLED PREMISE OF THE STUDY Treelines are temperature-sensitive ecotones that should be able to expand in response to global warming; however, they are also controlled by ecological constraints. These constraints can create bottlenecks for tree regeneration, hindering treeline advances. Near Kangiqsualujjuaq (Nunavik, subarctic Québec), previous studies suggested successful recruitment of Larix laricina above the altitudinal treeline, while Picea mariana establishment remains scarce. We studied regeneration of both species to identify factors responsible for such contrasting responses. • METHODS We measured seeds and wings to evaluate species dispersal potential. We compared seed viability and tolerance to shrub leachates with germination trials. To evaluate seedbed preferences, we compared seedling occurrence on the different seedbeds with seedbed relative abundance in the field. • KEY RESULTS Seed germination was similar between L. laricina and P. mariana, whereas dispersal potential was higher for the latter. Germination of P. mariana seeds was more strongly inhibited by shrub leachates than were L. laricina seeds. In the field, we found only a few Picea seedlings, but numerous seedlings of Larix had established disproportionally on several seedbeds. While Betula glandulosa, mosses, and Vaccinium uliginosim impeded Larix establishment, numerous seedlings were found on lichens, mineral soil, and liverworts. The low occurrence of suitable seedbeds for Picea, mainly mineral soil, could explain the seedling scarcity of this species. • CONCLUSIONS This study highlighted that allelopathy and unsuitable seedbeds could contribute to regeneration failure of P. mariana in eastern Nunavik and emphasizes the need to consider ecological preferences of species before predicting treeline expansion under a warmer climate.

Population dynamics of Empetrum hermaphroditum (Ericaceae) on a subarctic sand dune: Evidence of rapid colonization through efficient sexual reproduction

The importance of sexual reproduction for clonal plant species has long been underestimated, perhaps as a consequence of the difficulty in identifying individuals, preventing the study of their population dynamics. Such is the case for Empetrum hermaphroditum, an ericaceous species, which dominates the ground vegetation of subarctic ecosystems. Despite abundant seed production, seedlings are rarely observed. Therefore, prevalent seedling recruitment on a subarctic dune system provided an opportunity to study the population dynamics and spatial pattern of the colonization phase of this species. We established a 6-ha grid on the dune systems that extended from the shoreline to the fixed dunes and mapped and measured all E. hermaphroditum individuals in the grid. Moreover, we sampled 112 individuals just outside the grid to identify any allometric relationship between the size and age of the individuals, which allowed us to reconstruct population expansion. The overall size structure suggests that the population is still expanding. In the last 50 yr, E. hermaphroditum advanced more than 200 m in the dune system. Expansion started in the 1960s simultaneously at different distances from the shoreline. Colonization did not proceed gradually from the fixed dune toward the shoreline but instead individuals established earlier in the troughs between the dunes, with an increasingly clumped spatial pattern as the population filled in with time.

Phenological patterns of Quercus ilex, Phillyrea latifolia, and Arbutus unedo growing under a field experimental drought

Abstract: Cladina stellaris is a dominant lichen species of old-growth lichen-spruce woodlands in subarctic Québec, where the continuous lichen carpet persists for several decades in the absence of disturbance. The lichen carpet is, however, vulnerable to caribou (Rangifer tarandus) disturbance under dry site conditions. Through grazing and trampling, large herds of migratory caribou reduce C. stellaris abundance and modify ground vegetation composition. Here, we have evaluated the age and growth performance of C. stellaris in relation to the recent activity of the George River Caribou Herd (GRCH). We measured the age, length, and growth rate of C. stellaris podetia in and near caribou trails in 15 lichen-spruce woodlands across the summer habitat. The age of lichens, used to evaluate the initial period of lichen recruitment following the decline of the GRCH, indicated that lichen recovery began in the early 1990s in heavily disturbed sites, whereas comparisons between the age structure of border and trail lichens also suggested that the southern part of the calving ground was still heavily used in the early 1990s. Length and growth rate of C. stellaris thallus were closely associated with severity of caribou disturbance, as inferred from the residual ground cover of terrestrial lichens. Growth performance of C. stellaris during the initial stage of lichen recovery varied according to the spatial use of the summer habitat by caribou, with better performance at the edge of the summer and winter habitats and reduced performance in the extensively used calving ground. The analysis of growth performance of C. stellaris provides new insights on lichen-spruce stand recovery associated with caribou disturbance.

Different parts, different stories: climate sensitivity of growth is stronger in root collars versus stems in tundra shrubs

Abstract Shrub densification has been widely reported across the circumpolar arctic and subarctic biomes in recent years. Long‐term analyses based on dendrochronological techniques applied to shrubs have linked this phenomenon to climate change. However, the multi‐stemmed structure of shrubs makes them difficult to sample and therefore leads to non‐uniform sampling protocols among shrub ecologists, who will favor either root collars or stems to conduct dendrochronological analyses. Through a comparative study of the use of root collars and stems of Betula glandulosa, a common North American shrub species, we evaluated the relative sensitivity of each plant part to climate variables and assessed whether this sensitivity is consistent across three different types of environments in northwestern Québec, Canada (terrace, hilltop and snowbed). We found that root collars had greater sensitivity to climate than stems and that these differences were maintained across the three types of environments. Growth at the root collar was best explained by spring precipitation and summer temperature, whereas stem growth showed weak and inconsistent responses to climate variables. Moreover, sensitivity to climate was not consistent among plant parts, as individuals having climate‐sensitive root collars did not tend to have climate‐sensitive stems. These differences in sensitivity of shrub parts to climate highlight the complexity of resource allocation in multi‐stemmed plants. Whereas stem initiation and growth are driven by microenvironmental variables such as light availability and competition, root collars integrate the growth of all plant parts instead, rendering them less affected by mechanisms such as competition and more responsive to signals of global change. Although further investigations are required to determine the degree to which these findings are generalizable across the tundra biome, our results indicate that consistency and caution in the choice of plant parts are a key consideration for the success of future dendroclimatological studies on shrubs. &NA; Growth at the root collar is much more sensitive to climate than growth of the stem for a widely distributed shrub species, Betula glandulosa. These differences were maintained across common tundra ecosystems in Northern Canada and have important implications for sampling strategies when trying to quantify tundra change at a global scale. Figure. No caption available.