Hydraulic adjustment of maple saplings to canopy gap formation |
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| Authors: | Hafiz Maherali Evan H DeLucia Timothy W Sipe |
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| Institution: | (1) Department of Plant Biology, University of Illinois at Urbana-Champaign, 265 Morrill Hall, 505 South Goodwin Avenue, Urbana, IL 61801-3707, USA Fax: 217-244-7246; e-mail: h-maher@uiuc.edu, US;(2) Department of Biology, Franklin and Marshall University, Lancaster, PA 17601, USA, MH |
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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.
Received: 18 February 1997 / Accepted: 23 June 1997 |
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| Keywords: | Hydraulic conductivity Biomass allocation Transpiration Acerrubrum Acerpensylvanicum |
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