Using long-term data to reveal the geographical variation in timing and quantity of pollen and seed production in silver and pubescent birch in Finland: Implications for gene flow, hybridization and responses to climate warming

Abstract

Silver (Betula pendula) and pubescent birch (B. pubescens) are the two main broad-leaved tree species in boreal forests and Subarctic areas, with great significance for both northern societies and ecosystems. Silver birch has more economical importance as it grows taller, but pubescent birch reaches much further North. The adaptability and genetic diversity of Subarctic birch populations are assumed to derive from inter- and intraspecific hybridization. Southern pollen clouds could in turn increase the adaptability of northern populations to warming climate. In the boreal forest zone of warmer climate, incompatibility reactions may prevent interspecific hybridization and much depends on the synchrony of flowering. Direct in situ observations are, however, mostly lacking and earlier results concerning the spatial and temporal match of flowering phenology between the species are contradictory. Conclusions based on pollen catches may also be biased as the pollen of silver and pubescent birch are notoriously difficult to sort out and the geographical origin of pollen is virtually impossible to determine. Here we employ direct flowering observations and reanalyze old pollen and seed production data, collected along a South-North gradient in Finland, to shed more light on these issues. Our results suggest that interspecific hybridization is an unlikely mechanism of adaptation in silver and pubescent birch as there is no significant overlap in flowering either near Subarctic or in more southern boreal areas (covering latitudes 60– 68° N). Long-distance southern gene flow also unlikely has importance in the adaptation of northern populations to a warming climate as heat sum requirements for flowering in northern and southern populations are equal and northern birches are therefore not receptive at the time of southern flowering. Long-term data of pollen and seed production in turn suggest that pubescent birch is more effective in seed production through the whole South-North gradient, but increasingly so towards the North. However, it appears that this difference is not due to silver birch flowering and regeneration being more sensitive to interannual variation as earlier suggested. Although there are more factors than reproduction alone that can affect species distributions, these two findings indicate that climate warming may not significantly alter the relative abundances of silver and pubescent birch in Subarctic Fennoscandia.