Northern forest tree populations are physiologically maladapted to drought

Abstract

Northern forests at the leading edge of their distributions may not show increased primary productivity under climate warming, being limited by climatic extremes such as drought. Looking beyond tree growth to underlying physiological mechanisms is fundamental for accurate predictions of forest responses to climate warming and drought stress. Within a 32-year genetic field trial, we analyze relative contributions of xylem plasticity and inferred stomatal response to drought tolerance in regional populations of a widespread conifer. Genetic adaptation leads to varying responses under drought. Trailing-edge tree populations produce fewer tracheids with thicker cell walls, characteristic of drought-tolerance. Stomatal response explains the moderate drought tolerance of tree populations in central areas of the species range. Growth loss of the northern population is linked to low stomatal responsiveness combined with the production of tracheids with thinner cell walls. Forests of the western boreal may therefore lack physiological adaptations necessary to tolerate drier conditions.

Acute Drought Is an Important Driver of Bark Beetle Infestation in Austrian Norway Spruce Stands

“…This is alarming in view of the projected increased exposure of temperate climate zones to drought events and associated sensitivity of conifer forests to biotic disturbance (Lindner et al, 2010). Despite high vulnerability to water deficits in terms of cavitation risks, northern populations of Pinus spp., and P. abies seem to lack adequate strategies of xylem and stomatal control in order to protect against hydraulic constraints (Isaac-Renton et al, 2018;Peters et al, 2018). For the first time directly relating tree transpiration deficits to attack by I. typographus, Matthews et al (2018) found clear hints for compromised tree defense.…”

Range margin populations show high climate adaptation lags in European trees

“…reproduction, survival) remains unexplored at this large geographical scale in trees (but see Gárate‐Escamilla, Hampe, Vizcaíno‐Palomar, Robson, & Benito Garzón, ). Moreover, the introduction of pre‐adapted alleles to warmer climates in cold margin populations may mitigate their physiological maladaptation to drought (Isaac‐Renton et al, ) but at the same time increase the risk of frost damage (Montwé et al, ). The same is true for warm margin populations where an increase in adaptation lag for the future is only translated into a decrease in tree height, and cannot be extrapolated to other fitness‐related traits in trees.…”

Widespread drought‐induced tree mortality at dry range edges indicates that climate stress exceeds species' compensating mechanisms

“…This mortality may be due to drier populations crossing beyond the species' “fundamental niche” and existing outside where the species can physiologically survive, either temporarily (representing die‐off with recovery) or permanently (representing local extirpation/geographic range contraction). In other species, drought impacts have been more severe at “drought naïve” populations in the range core or wetter populations that do not often experience drought (Isaac‐Renton et al, ; Lloret & Kitzberger, ; Zuleta, Duque, Cardenas, Muller‐Landau, & Davies, ), hinting that these non‐drought adapted or acclimated populations may be more vulnerable to extreme water deficits due to some combination of physiological vulnerability and “structural overshoots” at the ecosystem level (Jump et al, ). In these scenarios, compensating mechanisms such as within‐species drought response trait variation, for instance more embolism resistant xylem in drier populations (Anderegg, ; López et al, ), and/or lower ecosystem‐level tree density or leaf area, may have allowed drier populations to suffer less mortality than wetter populations during the same drought.…”

“…This mortality may be due to drier populations crossing beyond the species' "fundamental niche" and existing outside where the species can physiologically survive, either temporarily (representing die-off with recovery) or permanently (representing local extirpation/geographic range contraction). In other species, drought impacts have been more severe at "drought naïve" populations in the range core or wetter populations that do not often experience drought (Isaac-Renton et al, 2018;Lloret & Kitzberger, 2018;Zuleta, Duque, Cardenas, Muller-Landau, & Davies, 2017), hinting that these non-drought adapted or acclimated populations may be more vulnerable to extreme water deficits due to some combination of physiological vulnerability and "structural overshoots" at the ecosystem level .…”