Acclimation of Betula alleghaniensis Britton to moderate soil water deficit: small morphological changes make for...

In the context of the predicted increasing frequency of summer droughts in the northeastern deciduous forest of North America due to climate change, we investigated the acclimation capacity of yellow birch, an economically important native tree species, to soil water deficit. We carried out an integrated examination of allocation of biomass, leaf physiology, branching pattern and in situ 3D crown display. Potted seedlings were subjected to moderate soil water deficit for four consecutive months during their second growing season. Individuals under soil water deficit showed a 40% decrease in biomass accumulation but no change in the relative allocation of biomass to the different plant compartments. Net CO₂ assimilation rates at leaf level decreased under water deficit (~15%) but could not alone explain the total reduction in growth, excluding the carbon starvation hypothesis. The observed reduction in net CO₂ assimilation rates was related to a decrease in stomatal conductance and chlorophyll content. STAR_zen (in situ silhouette to total leaf area ratio; a proxy for light interception efficiency) decreased under soil water deficit due to shifts in biomass allocation within the branch compartment from long upper axes to short bottom axes. Despite the fact that the understanding of the processes involved in growth reduction and branching pattern alteration will need more attention in future research, we conclude that under water deficit yellow birch at young stages will: (i) experience a substantial loss of growth and biomass; and (ii) acclimate through architectural plasticity rather than through changes in the relative allocation of root biomass to enhance its water management.