Assessing inter- and intraspecific variability of xylem vulnerability to embolism in oaks

Abstract

The genus comprises important species in forestry not only for their productive value but also for their ability to withstand drought. Hence an evaluation of inter- and intraspecific variation in drought tolerance is important for selecting the best adapted species and provenances for future afforestation. The presence of long vessels makes it difficult to assess xylem vulnerability to embolism in oak. Thanks to the development of an in situ flow centrifuge equipped with a large rotor, we quantified (i) the between species variability of embolism resistance in four native and two exotic species of oaks in Europe and (ii) the within species variability in. Embolism resistance varied significantly among species, with the pressure inducing 50% loss of hydraulic conductivity ( ) ranging between - 7.0 and -4.2 MPa. Species native to the Mediterranean region were more resistant than pan-European species. In contrast, intraspecific variability in embolism resistance in was low within provenances and null among provenances. A positive correlation between and vessel diameter among the six oak species indicates that the more embolism resistant species had narrower xylem vessels. However, this tradeoff between hydraulic efficiency and safety was not observed between provenances.

Hydraulic acclimation in a Mediterranean oak subjected to permanent throughfall exclusion results in increased stem hydraulic capacitance

“…However, and contrary to our expectations, differences were observed during phase I of the desorption curve, when trees were still well‐hydrated, instead of during phase II, when reliance on stored water might play a more important role in maintaining xylem hydraulic functionality. These results suggest that capacitive water release to limit Ψ reductions represents a secondary strategy to face drought in Q. ilex , as this drought‐tolerant species is highly resistant to drought‐driven embolism, with P 50 values of −7.13 MPa (Lobo et al, ). Contrarily to drought‐avoidant species, the significance of C S in drought‐tolerant species such as Q. ilex might be more important to maximizing carbon gain during well‐hydrated conditions than to dampening Ψ reductions under drought stress.…”

“…Therefore, capillary water might be physiologically irrelevant during phase I of the desorption curve (Ψ PD between −0.45 and −1.10 MPa) and C S_I could be primarily attributed to elastic living cells. The subsequent reduction in C S during phase II might result from the limited contribution of largely depleted elastic pools together with a small capacitive water release from vessel embolism given the remarkable embolism resistance of Q. ilex , with P 12 values down to −4.93 MPa (Lobo et al, ). Remarkably, the breakpoint of the desorption curve closely matched the threshold for stem growth interruption imposed by water deficit at the study site (Ψ PD = −1.1 MPa; Lempereur et al, ) and was not modified by TE.…”

Large hydraulic safety margins protect Neotropical canopy rainforest tree species against hydraulic failure during drought

“…Vulnerability to embolism was measured following the flow-centrifugation technique ) in a large Cavitron equipped with a 1-m diameter custom-built rotor (Cavi1000; DGMeca, Gradignan, France). This large rotor was designed to process species with long vessels (Lobo et al 2018). This limits the occurrence of 'open-vessel' artifacts if the vessel length is much shorter than the diameter of the rotor .…”

No local adaptation in leaf or stem xylem vulnerability to embolism, but consistent vulnerability segmentation in a North American oak

“…In addition, interannual variation in rainfall within sites occasionally produces severe drought events known to cause adult mortality (Brown et al, 2018), suggesting that selection pressure for drought tolerance traits is likely to be strong. Although uniform selection pressure could be acting in geographically disparate populations (possibly because populations are adapting to local extremes rather than the average; Gutschick & BassiriRad, 2003), we sampled across sites covering a substantial range in aridity, equal to the range over which other species display local adaptation (Wortemann et al, 2011;Lamy et al, 2014;Lobo et al, 2018) and over which we see variation in vulnerability between congeneric oak species (Skelton et al, 2018, see also Larter et al, 2017. Thus, we suggest that low local adaptation in Q. douglasii leaf and stem xylem vulnerability to embolism is caused either by high gene flow between populations or canalization of vulnerability to embolism, like species such as F. sylvatica and P. pinaster (Lamy et al, 2011;Wortemann et al, 2011).…”