Does sexual reproduction limit treeline ecotone expansion in the circumpolar north?

Background/Question/Methods

The fundamental climatic niche of many species is shifting with global change, pronouncedly so in sub-arctic ecosystems where recent warming has been more dramatic than at lower latitudes. The sub-arctic is largely defined by transitions from southerly to northerly ecosystems, predominantly by the shift from forest cover to tundra. Summer air temperature warming in sub-arctic regions should lessen environmental constraints on the northern distribution of boreal forest species. Thus, we would expect treeline regions to be undergoing structural and positional changes, colonizing tundra ecosystems. However, range expansion, stand infilling, no response, or even the retraction of the treeline ecotone associated with warming temperatures has been documented across the circumboreal. A leading explanation for a time lag between increasingly favourable environmental conditions and treeline expansion is reproductive limitations caused by low productivity or poor dispersal of seeds. Our objective was to assess the reproductive potential of the dominant tree species at the treeline ecotone in the circumpolar north. We sampled reproductive structures (cones and catkins) and stand attributes in 13 regions around the circumpolar north (Canada, Norway, Sweden, and Russia). We used mixed-modelling techniques to relate i) seed production and ii) seed viability to regional climate, stand structure, and species-specific characteristics. 

Results/Conclusions

The availability of viable seed, not seed production, constrains treeline expansion in the circumpolar north. Two general patterns were detected: 1) seed production per cone decreased from forest to tundra; and 2) high seed production in a region did not necessarily result in a large number of viable seed available for recruitment. Our broad geographic approach revealed both biotic and abiotic factors driving these patterns. Reproductive potential of circumpolar tree populations was closely tied to favourable climatic conditions; however, biotic constraints in the form of pollen limitation and pre-dispersal seed predation outweighed the favourable conditions created warmer air temperatures in some regions, thereby limiting the amount of viable seed available for expansion. Predictions of treeline ecotone expansion with climate change will improve with further quantification of the species-specific relationships between climate and embryo maturation, a better understanding of how seed predators and herbivores limit reproduction at treeline, and further investigation into the next steps to colonization: seed germination and survival in tundra habitats.