The leaf-specific hydraulic conductivity (K(L)) of plant stems can control leaf water supply. This property is influenced by variation in leaf/sapwood area ratio (A(L)/A(S)) and the specific hydraulic conductivity of xylem tissue (K(S)). In environments with high atmospheric vapor pressure deficit (VPD), K(L) may increase to support higher transpiration rates. We predicted that saplings of Acer rubrum and A. pensylvanicum grown in forest canopy gaps, under high light and VPD, would have higher K(L), and lower A(L)/A(S) than similar sized saplings in the understory. Leaf-specific hydraulic conductivity and K(S) increased with sapling size for both species. In A. rubrum, K(S) did not differ between the two environments but lower A(L)/A(S) (P = 0.05, ANCOVA) led to higher K(L) for gap-grown saplings (P < 0.05, ANCOVA). In A. pensylvanicum, neither K(S), A(L)/A(S), nor K(L) differed between environments. In a second experiment, we examined the impact of sapling size on the water relations and carbon assimilation of A. pensylvanicum. Maximum stomatal conductance for A. pensylvanicum increased with K(L) (r2 = 0.75, P < 0.05). A hypothetical large A. pensylvanicum sapling (2 m tall) had 2.4 times higher K(L) and 22 times greater daily carbon assimilation than a small (1 m tall) sapling. Size-related hydraulic limitations in A. pensylvanicum caused a 68% reduction in daily carbon assimilation in small saplings. Mid-day water potential increased with A. pensylvanicum sapling size (r2 = 0.69, P < 0.05). Calculations indicated that small A. pensylvanicum saplings (low K(L)) could not transpire at the rate of large saplings (high K(L)) without reaching theoretical thresholds for xylem embolism induction. The coordination between K(L) and stomatal conductance in saplings may prevent xylem water potential from reaching levels that cause embolism but also limits transpiration. The K(S) of the xylem did not vary across environments, suggesting that altering biomass allocation is the primary mechanism of increasing K(L). However, the ability to alter aboveground biomass allocation in response to canopy gaps is species-specific. As a result of the increase in K(L) and K(S) with sapling size for both species, hydraulic limitation of water flux may impose a greater restriction on daily carbon assimilation for small saplings in the gap environment.
- Acer pensylvanicum
- Acer rubrum
- Biomass allocation Transpiration
- Hydraulic conductivity
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics