Hydraulic adjustment of maple saplings to canopy gap formation

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Abstract
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 Acerrubrum 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 KL 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 (r 2=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 (r 2=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.