Abstract
The recent massive dieback of forest trees due to drought stress makes assessment of the variability of physiological traits that might be critical for predicting forest response and adaptation to climate change even more urgent. We investigated xylem vulnerability to cavitation and xylem specific hydraulic conductivity in seven species of three principal conifer genera (Juniperus monticola, Juniperus deppeana, Juniperus flaccida, Pinus pseudostrobus, Pinus leiophylla, Pinus devoniana, and the endangered Picea chihuahuana) of the Mexican mountains in order to identify the species most vulnerable to future warmer and drier climates. Hydraulic traits were examined using the in situ flow centrifuge technique (Cavitron) on branches collected from adult trees of natural populations and seedlings growing in a common garden. We found evidence of significant differences in xylem safety between genera (P50: pressure inducing 50% loss of hydraulic conductance): the three juniper species exhibited low P50 values (ranging from -9.9 to -10.4 MPa), relative to the much more vulnerable pine and spruce species (P50 ranging between - 2.9 to - 3.3 MPa). Our findings also revealed no variation in P50 between adult trees assessed in the field and seedlings growing in a common garden. We therefore propose that if, as projected, climate change makes their natural habitats much warmer and drier, populations of Mexican pines and the studied spruce will be likely to decline severely as a result of drought-stress induced cavitation, while the juniper species will survive.
Highlights
Concerns are increasing because of the mounting evidence of forest decline related to drought stress, apparently linked to ongoing climatic change (Breshears et al, 2005; Peñuelas et al, 2007; Mátyás, 2010; Allen et al, 2010)
Previous studies have reported the high variability of pressure inducing 50% loss of conductance (P50) among conifer species, ranging from -3 to -19 MPa (Delzon et al, 2010; Pittermann et al, 2010; Larter et al, 2015)
Study area and study species We focused on seven conifer species, six of which occur across a wide altitudinal gradient along the central Mexican mountains (Trans-Mexican Volcanic Belt, TMVB) and one in the Sierra Madre Occidental (Figure 1)
Summary
Concerns are increasing because of the mounting evidence of forest decline related to drought stress, apparently linked to ongoing climatic change (Breshears et al, 2005; Peñuelas et al, 2007; Mátyás, 2010; Allen et al, 2010). Hotter drought periods are inducing massive tree mortality (Allen et al, 2015) and, by year 2050, there is likely to be a substantial reorganization of vegetation (McDowell and Allen, 2015), with a plant community composition unfamiliar to modern civilization (Williams et al, 2013) In such a context, the study of variation among and within forest species in terms of cavitation resistance is very important in order to predict the potential of adaptation to climatic change (Choat et al, 2012). We aim to assess differences in hydraulic safety (drought-induced cavitation resistance) and conductivity (water-transport efficiency) among seven Mexican conifer species belonging to three genera: Juniperus, Pinus and Picea Some of these frequently co-occur in the highly biodiverse Mexican mountains (Quiñones-Perez et al, 2014; Figure 1) and share similar climatic habitat conditions. We hypothesize highly species-specific responses, with a high risk of mortality for species of greater vulnerability to cavitation as a response to drought events
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