Data from: Reproduction as a bottleneck to treeline advance across the circumarctic forest tundra ecotone

The fundamental niche of many species is shifting with climate change,
especially in sub-arctic ecosystems with pronounced recent warming.
Ongoing warming in sub-arctic regions should lessen environmental
constraints on tree growth and reproduction, leading to increased success
of trees colonising tundra. Nevertheless, variable responses of treeline
ecotones have been documented in association with warming temperatures.
One explanation for time lags between increasingly favourable
environmental conditions and treeline ecotone movement is reproductive
limitations caused by low seed availability. Our objective was to assess
the reproductive constraints of the dominant tree species at the treeline
ecotone in the circumpolar north. We sampled reproductive structures of
trees (cones and catkins) and stand attributes across circumarctic
treeline ecotones. We used generalized linear mixed models to estimate the
sensitivity of seed production and the availability of viable seed to
regional climate, stand structure, and species-specific characteristics.
Both seed production and viability of available seed were strongly driven
by specific, sequential seasonal climatic conditions, but in different
ways. Seed production was greatest when growing seasons with more growing
degree days coincided with years with high precipitation. Two consecutive
years with more growing degree days and low precipitation resulted in low
seed production. Seasonal climate effects on the viability of available
seed depended on the physical characteristics of the reproductive
structures. Large-coned and -seeded species take more time to develop
mature embryos and were therefore more sensitive to increases in growing
degree days in the year of flowering and embryo development. Our findings
suggest that both moisture stress and abbreviated growing seasons can have
a notable negative influence on the production and viability of available
seed at treeline. Our synthesis revealed that constraints on predispersal
reproduction within the treeline ecotone might create a considerable time
lag for range expansion of tree populations into tundra ecosystems.