Increased water use by sugar maple trees in the presence of a dense beech understory layer

The formation of a recalcitrant understory vegetation layer is a phenomenon observed in various forests around the world, which can limit tree regeneration and in the long term, modify the composition, succession process and water balance of forests. In temperate forests of southern Quebec (Canada), the proliferation of American beech (Fagus grandifolia) in stands dominated by sugar maple (Acer saccharum) can be related to the recalcitrant vegetation phenomenon. With a projected increase in the severity and duration of droughts, a better understanding of its effect on water fluxes is crucial to understand the trajectory of impacted forests. The objective of this study was to understand how recalcitrant-type vegetation, in this case beech proliferation in sugar maple stands, influence tree water use. We compared transpiration i) by overstory trees (i.e. sugar maples) and ii) by understory saplings (i.e. American beech) in sites with and without beech understory dominance. At each of the six sites, we measured sap flux density (Fd) of two sugar maple trees (diameter at breast height > 9 cm) and one beech sapling (1 cm < diameter at breat height ≤ 9 cm) with thermal dissipation sensors during the growing season. At tree level, Fd of sugar maple trees was significantly larger in beech-dominated sites compared to control ones, indicating greater water consumption by sugar maple when understory is dominated by beech. At stand scale, total transpiration varied between 140 and 296 mm for the study period, with no significant difference between beech-dominated and control sites. We provide two hypotheses to explain our results at tree scale: i) reduced cover by forest floor vegetation would limit transpiration through this layer, thus allowing increased availability of water resources to supply tree transpiration or ii) increased tree transpiration rate would be a mechanism to satisfy nutrient requirements in beech-dominated stands often associated with lower soil fertility. Further research is needed to better understand the mechanisms explaining tree water use given beech proliferation.