Numerous long-term, free-air plant growth facilities currently explore
vegetation responses to the ongoing climate change in northern
latitudes. Open top chamber (OTC) experiments as well as the
experimental set-ups with active warming focus on many facets of plant
growth and performance, but information on morphological alterations of
plant cells is still scarce. Here we compare the effects of in-situ warming on leaf epidermal cell expansion in dwarf birch, Betula nana in Finland, Greenland, and Poland. The localities of the three in-situ warming experiments represent contrasting regions of B. nana
distribution, with the sites in Finland and Greenland representing the
current main distribution in low and high Arctic, respectively, and the
continental site in Poland as a B. nana relict Holocene microrefugium. We quantified the epidermal cell lateral expansion by microscopic analysis of B. nana
leaf cuticles. The leaves were produced in paired experimental
treatment plots with either artificial warming or ambient temperature.
At all localities, the leaves were collected in two years at the end of
the growing season to facilitate between-site and within-site
comparison. The measured parameters included the epidermal cell area and
circumference, and using these, the degree of cell wall undulation was
calculated as an Undulation Index (UI). We found enhanced leaf epidermal
cell expansion under experimental warming, except for the extremely low
temperature Greenland site where no significant difference occurred
between the treatments. These results demonstrate a strong response of
leaf growth at individual cell level to growing season temperature, but
also suggest that in harsh conditions other environmental factors may
limit this response. Our results provide evidence of the relevance of
climate warming for plant leaf maturation and underpin the importance of
studies covering large geographical scales.