![]() ![]() In the field, shrub root collar sampling necessitates shrub excavation, a time-consuming and destructive manipulation that can interfere with the complex logistics of fieldwork in remote areas. From a dendrochronology standpoint, the root collar is thus the anatomical equivalent of the base of the primary axis of a tree as it integrates the growth of all stems. have a growth form characterized by multiple stems growing from an often-buried root collar, i.e., the interface between the stems and the roots ( Figure 1a). For example, seed-originating individuals of the subarctic shrub Betula glandulosa Michx. However, shrub growth and architecture present new challenges for dendrochronology. Therefore, dendrochronological studies on shrub species can provide precious insights on the drivers of climate and landscape change across the entire circumpolar region. North of the treeline, shrubs are the only woody plants and their growth rings are a valuable ecological archive with a yearly resolution. However, shrub encroachment in other ecosystems, such as arid and semi-arid grasslands, could lead to the decline of vegetation cover through its effects on the biophysical and biogeochemical processes affecting the regional climate. In such high-latitude ecosystems, shrub encroachment could alter the surface energy exchange, soil temperature, snow cover, and nutrient cycling, all of which could foster further shrub encroachment. The expansion of shrub species, either through the densification of existing stands or through the colonization of new habitats, contributes to this phenomenon. Although some studies have found browning trends in the boreal or lower Arctic regions due to changes in the natural disturbance regime, the improved climatic conditions during the growing season have resulted in widespread “greening” of the circumpolar region of the Northern Hemisphere. In recent decades, however, warming in Arctic and subarctic regions, which is nearly three times stronger than the global average, has substantially reduced the limiting effect of temperature on primary productivity. Cold temperatures reduce photosynthetic activity and slow down plant phenological processes such as bud burst and seed development. Harsh climatic conditions and short growing seasons limit plant productivity across the Arctic and subarctic. ![]() Although the use of BAI showed promising results for studying long-term climate signals in shrub growth chronologies, further studies focusing on different species and locations are needed before the use of BAI can become broadly used in shrub dendrochronology. The climatic drivers of radial growth were identical for stems and root collars when using BAI (July temperature and March precipitation), but were inconsistent when using detrended RW series (root collars: July temperature and March precipitation at all sites stems: April and June temperature, depending on the site). The climate sensitivity was, on average, 4.9 and 2.7 times higher with BAI than with detrended (mean-centered) RW chronologies for stems and root collars, respectively. Using stem and root collar samples collected from three sites in the forest–tundra ecotone of eastern Canada, we compared the climate–growth relationships of these two approaches for stems and root collars using mixed-effects models. We hypothesize that the conversion of the ring width (RW) to basal area increment (BAI) is better suited than traditional detrending methods to removing age/size-related trends without removing multi-decadal climate signals. In this paper, we propose that the lower climatic sensitivity of stems could be caused by the use of unsuitable detrending methods for chronologies spanning decades rather than centuries. Shrub stems may thus be considered as sub-optimal to study climate–growth relationships. Shrub architecture presents new challenges for dendrochronology, such as the seemingly lower and inconsistent climatic sensitivity of stems vs. Climate warming at high latitudes has contributed to the growing interest in shrub tree-ring analysis. ![]()
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