Growth and genotype × environment interactions in Betula pendula: can tree genetic variation be maintained by small-scale forest ground heterogeneity?

Abstract

Forest ground heterogeneity can affect interactions among tree species and control the assembly of local forest communities. Less is known of the effects of spatial heterogeneity on the maintenance of tree genetic variation through small-scale geno- type 9 environment (G 9 E) interactions. We measured growth variation within and among 17 Betula pendula genotypes, planted in a clear-cut forest site through the summers 2009-2011. We assessed the spatial heterogeneity at two scales: among forest stands having history of the same or different tree species combinations (treated as replicate blocks), and along a gradient of within-block forest density, revealed by the stump density. To add a temporal perspective, we distinguished between old (cut 50 years earlier) and new stumps (cut one year earlier). The broad-sense heritabilities for growth were 0.093-0.055 and the coefficients of genotypic variation 0.37-0.21 in 2009-2011. The growth difference among the genotypes was 3.5-5.5 fold, significant in all years, and the rank of genotype means correlated positively between the years. The most favourable block had 106 % higher growth than the least favourable block and the amount of total variation explained by block increased from 0.4 % in 2009 to 6.9 % in 2011. Geno- type 9 block interaction was marginally significant in 2009, but not later. Similarly, the response of growth to old stump density in sapling vicinity varied among the genotypes in 2009, but not later. In 2010 and 2011, the mean growth increased by 50-91 % along the old stump density gradient. Our results suggest that despite creating significant variation in sapling growth the small-scale forest ground heterogeneity, which reflects the recent forest history, may not significantly contribute to the maintenance of genetic variation in B. pendula populations.