Forest‐permafrost feedbacks and glacial refugia help explain the unequal distribution of larch across continents

Abstract

Aim The continental‐scale distribution of plant functional types, such as evergreen and summergreen needle‐leaf forest, is assumed to be determined by contemporary climate. However, the distribution of summergreen needle‐leaf forest of larch (Larix Mill.) differs markedly between the continents, despite relatively similar climatic conditions. The reasons for these differences are little understood. Our aim is to identify potential triggers and drivers of the current distribution patterns by comparing species' bioclimatic niches, glacial refugia and postglacial recolonization patterns. Location Northern hemisphere. Taxon Species of the genus Larix (Mill.). Methods We compare species distribution and dominance using species ranges and sites of dominance, as well as their occurrence on modelled permafrost extent, and active layer thickness (ALT). We compare the bioclimatic niches and calculate the niche overlap between species, using the same data in addition to modern climate data. We synthesize pollen, macrofossil and ancient DNA palaeo‐evidence of past Larix occurrences of the last 60,000 years and track differences in distribution patterns through time. Results Bioclimatic niches show large overlaps between Asian larch species and American Larix laricina. The distribution across various degrees of permafrost extent is distinctly different for Asian L. gmelinii and L. cajanderi compared to the other species, whereas the distribution on different depths of ALT is more similar among Asian and American species. Northern glacial refugia for Larix are only present in eastern Asia and Alaska. Main Conclusion The dominance of summergreen larches in Asia, where evergreen conifers dominate most of the rest of the boreal forests, is dependent on the interaction of several factors which allows Asian L. gmelinii and L. cajanderi to dominate where these factors coincide. These factors include the early postglacial spread out of northern glacial refugia in the absence of competitors as well as a positive feedback mechanism between frozen ground and forest.

Unraveling boreal forest composition and drivers across scales in eastern Siberia

“…Pearson and Dawson [85], discuss how a low realized versus potential ratio might indicate different phenomena such as biotic interactions, edaphic or other non-climatic environmental factors, and modeling misspecifications. Some Larix species have overlapping bioclimatic niches [96] which, in turn, affect the forest-type niche. For example, in the Khamra and Western Yakutia regions, Larix and Evergreen also occur in mixed classes (Mixed Summergreen-Evergreen; Mixed-Summergreen) and therefore Larix and Evergreen are present but, in the prediction, the realized versus potential niches are not aligned because we map them by their endmember classes of Evergreen or Larix only.…”

Variation and Driving Mechanisms of Bark Thickness in Larix gmelinii under Surface Fire Regimes

“…Nevertheless, as the plant matures and grows, the individuals become larger, leading to an increase in the proportion of outer bark thickness to total bark thickness. This suggests that thick bark is effective in protecting trees from surface fires [41], while the self-pruning capability of mature L. gmelinii trees, along with their towering height that occupies the canopy, minimizes the chance of fire penetration into the canopy. These fire-adaptive traits combine to make this tree highly resistant to surface fires.…”

The deglacial forest conundrum

“…For Siberia, misadaptation of the trees to the local glacial climate conditions in the refugia is discussed 52 , which probably has led to genetically exhausted and non-competitive populations and which would have necessitated a reinvasion of these taxa from southern populations 50 . This could explain the slow response in the reconstructions compared to the fast response in the model lacking a dispersal routine.…”