Dispersal and fire limit Arctic shrub expansion

Abstract

Arctic shrub expansion alters carbon budgets, albedo, and warming rates in high latitudes but remains challenging to predict due to unclear underlying controls. Observational studies and models typically use relationships between observed shrub presence and current environmental suitability (bioclimate and topography) to predict shrub expansion, while omitting shrub demographic processes and non-stationary response to changing climate. Here, we use high-resolution satellite imagery across Alaska and western Canada to show that observed shrub expansion has not been controlled by environmental suitability during 1984–2014, but can only be explained by considering seed dispersal and fire. These findings provide the impetus for better observations of recruitment and for incorporating currently underrepresented processes of seed dispersal and fire in land models to project shrub expansion and climate feedbacks. Integrating these dynamic processes with projected fire extent and climate, we estimate shrubs will expand into 25% of the non-shrub tundra by 2100, in contrast to 39% predicted based on increasing environmental suitability alone. Thus, using environmental suitability alone likely overestimates and misrepresents shrub expansion pattern and its associated carbon sink.

Nitrogen fixing shrubs advance the pace of tall-shrub expansion in low-Arctic tundra

“…Spatial limits on shrub expansion. In contrast to coarse-scale dispersal models 33 unable to resolve fine-scale shrub-patch dynamics, our models find dispersal limitation to only influence local patch infilling, whereas environmental conditions were strong predictors of both locally and regionally heterogeneous patterns of shrub habitat and expansion. Therefore, we attribute the difference in modeled shrub habitat and the present remotely sensed shrub cover (Supp.…”

“…To determine the influence of dispersal limitation on our shrub extent model output (only used edaphic predictors), we tested whether shrubs failed to expand across our study domain due to limits on propagation rather than environmental factors. We added an omnidirectional minimum distance from existing shrub canopy metric by using the Google Earth Engine distance function on the 1950 classification, which served as a proxy for dispersal limitation, as seed propagation decreases with distance 33 . Although this simplified approach was necessary to contextualize the relative importance of our results, understandably, it does not account for the complexities of shrubspecific sexual or asexual reproduction that can modify the spatial patterns of dispersal limitation that varies with environmental conditions 59 .…”

Wildfire‐induced increases in photosynthesis in boreal forest ecosystems of North America

“…The first is the long‐term increase in annual burned area (Figure S6) that is pushing more of the landscape into early and mid‐successional states where shrubs and deciduous broadleaf trees are more abundant. A second mechanism is connected to a more permanent replacement of evergreen conifers by deciduous broadleaf forests (Figures 4 and 11) that may be a consequence of poor post‐fire seedling establishment from warmer (and drier) summers (Baltzer et al., 2021; Frelich et al., 2021; Reich et al., 2022), the compound effect of multiple disturbance agents (Anoszko et al., 2022; Liu, Riley, et al., 2022), or interactions between warming and nutrient availability that favors the growth strategy of deciduous trees (Foster, Shuman, et al., 2022; Mekonnen et al., 2019; Walker et al., 2023). Together, the rapid decline of evergreen conifers in fire‐affected areas and the slower recovery (if any) of evergreen conifers in undisturbed areas (Figure 2; Figure S3) provide evidence that the boreal landscape is far from a steady state, likely as a consequence of increasing fire and warming.…”

Kilometre-scale simulations over Fennoscandia reveal a large loss of tundra due to climate warming

“…as predicted for the Abisko area, is, therefore, probably overestimated, and instead parts of this area would become some type of tundra associated with shallow soils. Dispersal capacity and fire disturbance are also factors that may restrict vegetation expansion, as integration of these processes into an extrapolation of current trends in Alaska and western Canada reduced the predicted shrub expansion on non-shrub tundra from 39 % to 25 % by 2100 (Liu et al, 2022). In LPJ-GUESS, new PFTs can establish when climatic conditions are met, as we assume seeds are always present.…”