Stem xylem conductivity is key to plant water balance across Australian angiosperm species

Abstract

1. Plants must balance water expenditure from their crown with water supplied through root and stem tissues. Although many different combinations of hydraulic traits could accomplish water balance, we ask whether variation across species in stem hydraulic traits has been concentrated along few, or many, dimensions of trait variation. 2. We measured stem hydraulic traits for 120 woody dicot species across a range of different biomes in eastern Australia. Mean annual temperatures ranged from 10 to 27 °C and aridity (precipitation ⁄ potential evapotranspiration) from 0AE33 to 1AE02 across study sites. 3. Xylem-specific conductivity, species' height and ratio of leaf area to xylem area were positively correlated, manifesting as a single axis of trait variation, with other traits mostly orthogonal to this axis. Thus, as height and ratio of leaf area to xylem area increased across species and habitats (increasing resistance per leaf area), xylem-specific conductivity partially compensated for this resistance. Xylem-specific conductivity was well predicted by increasing height (r 2 = 0AE45) and ratio of leaf area to xylem area (r 2 = 0AE36). This three-trait axis was positively correlated with increasing precipitation (r 2 = 0AE28) and temperature (r 2 = 0AE15), but most of the explained variance lay within sites (39%) rather than across sites (10%). Thus, the spread of species' traits along this functional axis reflected structural and hydraulic differences among co-occurring species, at least as much as it reflected differences associated with contrasting climates. 4. High xylem-specific conductivity in stems was accomplished by high vessel diameter to number ratio (r 2 = 0AE32) and ⁄ or by high vessel lumen fraction (r 2 = 0AE13). Low midday water potential (higher xylem tension) was associated with low ratio of vessel diameter to number (r 2 = 0AE25), whereas low specific gravity (r 2 = 0AE18) and stiffness (r 2 = 0AE12) were associated with high vessel lumen fraction. 5. Light capture (i.e. increasing height and leafiness) may be facilitated by high xylem-specific conductivity, but marked increases in xylem-specific conductivity may also be associated with reduced hydraulic and mechanical safety. Although the trade-offs associated with increasing xylem-specific conductivity remain unclear, our data suggest that xylem-specific conductivity is important for maintaining water balance across a large range of species and biomes.

Coordination and trade‐offs between leaf and stem hydraulic traits and stomatal regulation along a spectrum of isohydry to anisohydry

“…In our study, higher stem capacitance was associated with higher fractions of vessels (Figure d). The high vessel fraction could contribute to high transport efficiency (Gleason et al, ). However, using a data set collected from chaparral shrubs growing at field sites in southern California, Pratt and Jacobsen () found no relationship between stem capacitance and stem‐specific conductivity.…”

Determinants of maximum tree height inEucalyptusspecies along a rainfall gradient in Victoria, Australia

“…Traditional views of hydraulic limitation include (1) declines in leaf area-specific hydraulic conductance with increasing tree height; (2) declines in w leaf with tree height due to decreased whole-plant hydraulic conductance and increased gravitational potential; and (3) resulting decreases in average stomatal conductance (Ryan and Yoder 1997, Koch et al 2004, Ryan et al 2006, Hinckley et al 2011, Mencuccini et al 2011. Factors that might contribute to a tighter relationship of maximum tree height to P/E p than expected based on hydraulic limitation (and thus D) alone might include (4) increased allocation to leaves vs. roots or stems at a given height with increasing P/E p , or conditions correlated with higher P/E p (e.g., greater soil silt or nitrate content); (5) higher photosynthetic rates per unit leaf mass at higher P/E p or under conditions correlated therewith, independent of the degree of stomatal limitation; (6) higher leaf area-specific conductance at higher P/E p in trees of a given height, reflecting differences in wood density and xylem diameter and length (Thomas 1996b, Thomas and Bazzaz 1999, Zach et al 2010, Fan et al 2012, Gleason et al 2012; (7) variation across species in the rate at which mesophyll photosynthetic capacity declines with decreasing w leaf (Givnish 1986, Tezara et al 2003, Lawlor and Tezara 2009); (8) variation across sites in which the rate at which evaporation from sunlit leaves increases with relative height at the top of the canopy; and (9) greater uncertainties in measuring average D than average height. The preceding factors are all plausible ways in which resource allocation or modified hydraulic limitation effects could constrain tree height along the Victoria transect, in response to the increase in P/E p toward the Yarra Ranges, the reduced heat load and cloudier conditions at higher altitudes and latitudes there, and the increasingly fine-grained, more P-and Nrich soils there (see Eq.…”

“…Moister, more fertile conditions should favor species with higher SLA, leaf N concentration per unit mass, stomatal conductance, D, leaf hydraulic conductance, and maximum photosynthetic rate per unit leaf mass, and lower percentage root allocation and wood density, with the latter leading to high values of stem hydraulic conductance. Indeed, in a recent analysis of gasexchange and hydraulic traits in 120 dicot species along an aridity gradient in eastern Australia, Gleason et al (2012) showed that plant height increased with xylemspecific hydraulic conductivity normalized by the ratio of total leaf area to total xylem area. The relationships observed accounted for 39% of height and conductivity differences within sites, but only 10% of the variance across sites.…”

“…Numerous studies have documented changes in tree anatomy or morphology that compensate, though only in part, for the expected declines in w leaf , K L , g, A mass , and exposure to runaway cavitation with tree height (Becker et al 2000, Mencuccini 2003, Zaehle 2005, Ryan et al 2006, Addington et al 2009, Ambrose et al 2009, Gleason et al 2012. Maintaining critical limitations constant with height, even if it were possible, would impose other trade-offs.…”

“…This pattern of whole-plant integration might suggest limited latitude for compensatory change in any single trait as height varies. Yet several studies have shown that maximum tree height among species locally increases with stem hydraulic conductivity per unit leaf area (Addington et al 2009, Zach et al 2010, Fan et al 2012, and therefore, often increases with xylem vessel diameter and the initial rate of height growth and decreases with wood density (King 1991, Thomas and Bazzaz 1999, Zach et al 2010, Fan et al 2012, Gleason et al 2012. Kempes et al (2011) present a flow-based model to account for variation in maximum tree height across the contiguous United States, supposedly also taking stem allometry into account.…”