How do climate and topography influence the greening of the forest‐tundra ecotone in northern Québec? A dendrochronological analysis ofBetula glandulosa

Abstract

Summary1. NDVI analysis and repeated aerial photographs have revealed significant shrub expansion in many subarctic regions. While the recent increase in temperature is usually considered to be the main driver of this phenomenon at regional scales, very little is known about the local heterogeneity of shrub responses across the landscape. 2. In this study, we aim to identify the climatic factors controlling the growth of the largely distributed shrub species Betula glandulosa in three types of environments (terrace, hilltop and snowbed). We also aim to evaluate the relationship between B. glandulosa growth and the NDVI data for the Boniface River region, in north-western Qu ebec, where the study took place. 3. In the field, we harvested 180 B. glandulosa individuals (20 per site, 3 sites per type of environment). We constructed specific growth-ring width chronologies and mean axial growth rate chronologies for each site and used them for dendroclimatic analysis (response functions). We also used linear regressions to evaluate the relative influence of dwarf birch growth on the NDVI trend. 4. We found a sharp increase in B. glandulosa radial growth in the 1990s followed by a sharp decreasing trend from 2002 on terraces and hilltops, while growth remained high in snowbeds. Betula glandulosa growth was positively correlated with summer temperatures on terraces and hilltops, whereas winter precipitation promoted growth on snowbeds. The NDVI trend was largely correlated to B. glandulosa growth on terraces and hilltops for the period between 1986 and 2002 (71-80% explained variance). 5. Synthesis. Our results suggest that topography plays a major role in B. glandulosa growth and therefore in shrub community dynamics. Because terraces and hilltops represent 70% of the land surface, the sharp B. glandulosa growth increase at these sites promoted an important overall expansion of the shrub community in the region. However, the decline in B. glandulosa growth observed after 2002 suggests that the expansion could be slowed down in the near future, therefore limiting shrub growth contribution to the regional NDVI signal.

Arctic shrub growth trajectories differ across soil moisture levels

“…However, a relative scarcity of landscape‐scale investigation has made it difficult to connect plot‐level experimentation to regional greening trends (Hobbie & Kling, ). Landscape factors like topography, disturbance, and biotic interactions may modulate rates of shrub expansion (Ackerman & Breen, ; Christie et al., ; Olofsson et al., ; Ropars, Levesque, & Boudreau, ; Schuur, Crummer, Vogel, & Mack, ). A poor understanding of the heterogeneity of shrub expansion across arctic landscapes inhibits accurate incorporation shrub feedbacks into mechanistic climate models such as dynamic global vegetation models (e.g., Sitch et al., ).…”

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

“…Shrubs either have expanded in patch size (Sturm et al, 2001a;Tape et al, 2006;Ropars & Boudreau, 2012;Tremblay et al, 2012), recruited in areas where they were less abundant in the past (Sturm et al, 2001a;Frost et al, 2013;Frost & Epstein, 2014;B€ untgen et al, 2015) or have increased in height (Myers-Smith et al, 2011b;Paradis et al, 2016). Though this increase in tundra shrubs has been attributed to recent warming in tundra ecosystems (Elmendorf et al, 2012;Ropars et al, 2015), the climate sensitivity of shrub growth has been demonstrated to be variable across the tundra biome (Myers-Smith et al, 2015a).…”

“…Dendrochronological approaches have successfully been applied to shrub species to analyze temporal and spatial processes in fast-changing tundra ecosystems (e.g., B€ ar et al, 2008;Forbes et al, 2010;Blok et al, 2011;Myers-Smith et al, 2015a;Ropars et al, 2015). Like trees, shrubs form annual growth rings that can be related to climate variables (e.g., Myers-Smith et al, 2015a) or disturbances in ecosystem processes such as landslides (e.g., Gers et al, 2001) and permafrost degradation (e.g., Gaertner-Roer et al, 2013).…”

Simulated caribou browsing limits the effect of nutrient addition on the growth of Betula glandulosa, an expanding shrub species in Eastern Canada

“…Warmer summers and longer growing seasons enhance shrub growth directly by increasing their photosynthetic rate (Chapin, ; Hallinger, Manthey, & Wilmking, ; Post et al., ). This is supported by dendrochronological studies on tundra shrubs that have shown a positive relationship between radial growth and summer temperature (Boudreau & Villeneuve‐Simard, ; Forbes, Fauria, & Zetterberg, ; Ropars, Lévesque, & Boudreau, ). Warmer temperatures also increase shrub growth indirectly by warming the soil, which results in increased microbial soil decomposition and greater nutrient availability for shrub growth (Loffler, Cypionka, & Loffler, ; Rustad et al., ).…”

“…The absence of a significant effect of our warming treatment on B. glandulosa radial growth should not be used to discard the importance of warmer temperatures for shrub growth. In fact, summer air temperatures and growing season degree‐days have been shown to control shrub growth both in temperature‐enhancement experimental studies (Arft et al., ; Wahren et al., ; Walker et al., ) and in dendrochronological studies (Myers‐Smith et al., ; Ropars et al., ). Our results rather suggest that either our OTCs did not increase air temperature sufficiently to influence radial growth or that they resulted in unwanted negative effects (extreme temperatures or lower air humidity).…”