Genotypic variation and phenotypic plasticity in the drought response of fine roots of European beech

How temperate trees respond to drier summers, as predicted by climate change models for parts of Europe and eastern North America, will depend on the drought susceptibility of the root systems. We investigated the importance of the genetic constitution for the belowground drought response of European beech (Fagus sylvatica L.), in four populations from regions differing in precipitation (520-970 mm year(-1)). Saplings were grown at ample (10 vol.%; well-watered) or reduced (5 vol.%; drought treatment) soil water content in the Göttingen Rhizolab Facility for two consecutive summers, and the responses of fine root biomass, root morphology, root depth distribution, and fine root production and turnover were investigated by a combined mini-rhizotron and harvest technique approach. In the drought treatment, total root mass per plant was reduced by 30-40% as a result of: (1) a reduction in median fine root lifespan by roughly 50% and hence an increase in fine root turnover; and (2) a 10-fold reduction in relative fine root growth rate (productivity per standing root biomass). The root:shoot ratio did not increase with drought. Although beech plants originating from drier climates tended to reduce their root biomass in response to drought less than those from wetter climates, analyses of variance revealed no significant influence of genotype on root mass, morphology, growth rate or turnover. However, most fine root traits showed marked differences between the well-watered and drought treatments. We conclude that beech saplings respond to summer drought primarily by shortening root lifespan, whereas root system structure and root:shoot carbon partitioning pattern are unaltered. Beech fine root growth and turnover exhibited high phenotypic plasticity, but genotypic variation was of minor importance. In contrast, genotype had a strong influence on leaf and shoot morphogenesis and growth.