Contrasting carbon allocation strategies related to wood porosity converge toward similar growth responses to drought

In mixed forests, diffuse-porous and ring-porous species represent two distinct functional groups undergoing similar environmental variations, but allegedly displaying different growth responses due to their anatomical features. We hypothesized that in sympatric species, functional groups-specific carbon allocation strategies result in different relationships between wood traits and canopy architecture, mirroring contrasting sensitivity to drought.
We selected 2 diffuse-porous species (sugar maple and yellow birch) and 2 ring-porous species (red oak and American ash) growing in a mixed forest in Quebec, Canada. In 8 individuals per species, we measured traits linked to wood anatomy and canopy architecture, covering the period 2008–2017. We finally assessed tree-growth sensitivity to water balance by relating all growth variables with the standardized precipitation evapotranspiration index (SPEI).
Stem elongation in diffuse-porous species depended upon the number of ramifications and the xylem vessels’ hydraulic diameter, while in ring-porous species stem elongation depended upon the productivity of the current year. In April, cold and wet soil conditions could explain the negative relationship between SPEI and tree growth. In late spring, greater water availability affected carbon partitioning, by promoting the formation of larger vessels in all species.
Drought responses in both functional groups were determined by different growth-related factors, but ultimately resulted in a similar overall loss of biomass. Growth responses to drought showed intra-annual patterns specific to the functional groups, suggesting that in the future their vulnerability will depend on the timing, duration, severity and frequency of droughts