Among-provenance diversity and phenotypic plasticity of water-use efficiency in sessile oak populations growing in a mesic common garden.

Abstract:

As a widespread species, sessile oak (Quercus petraea) populations occupy a wide range of ecological conditions with different local selection pressures, especially different drought exposure, which would have favoured different locally adapted populations. Water-use efficiency (WUE), which is defined at the tree level as the ratio between the biomass produced and the quantity of water transpired during the same period of time, is an interesting candidate trait for adaptation to drought. Six hundred trees from sixteen different provenances planted in 1993 in a common garden in the North-Eastern of France were harvested during the 2014-2015 winter. Intrinsic WUE (WUE_i), estimated from carbon isotope composition (δ¹³C) measurements of tree-rings, was compared among and within provenances for three contrasted years: (i) 2000, a wet year; (ii) 2003, a severely dry year; (iii) and 2005, a moderately dry year. The main purpose was to assess the drought-adaptive character of WUE_i for sessile oak trees. For this, (i) the adaptive character of WUE_i was evaluated by relating population mean WUE_i to the mean pedoclimatic conditions of their provenance sites. (ii) The phenotypic plasticity of WUE_i to drought was evaluated by comparing  the values observed in 2003 and 2005 to those of  2000 ; this plasticity was also related to the mean pedoclimatic conditions of their provenance sites. (iii) The contribution of WUE_i to tree and population fitness was assessed from the relationship between WUE_i and tree growth. Significant differences in δ¹³C (thus WUE_i) were found among populations. However, no linear relationship was established between mean population δ¹³C and the mean pedoclimatic conditions of the provenance sites. Based on these results observed on juvenile sessile oak trees in the relatively wet conditions of the common garden, no local adaptation in terms of WUEi was detected. An increase in drought intensity resulted in an increase in population WUE_i and all provenances displayed a similar plasticity of WUE_i to drought, suggesting no among population diversity for drought responses. A significant correlation between WUE_i and tree growth was detected only during the wet year, when populations with a higher WUE_i also had a higher growth index. Moreover, a much larger variability in WUE_i was demonstrated within populations (2–4‰) than among-population (0.6‰).

Key words : Climate change, assisted migration, local adaptation, water-use efficiency, fitness, diversity