Measuring the pulse of trees; using the vascular system to predict tree mortality in the 21st century

Abstract

Tree mortality during hot and dry conditions presents a stark reminder of the vulnerability of plant species to climatic extremes. The current global warming trend makes predicting the impacts of hot/dry events on species survival an urgent task; yet, the standard tools for this purpose lack a physiological basis. This review examines a diversity of recent evidence demonstrating how physiological attributes of plant vascular systems can explain not only why trees die during drought, but also their distributional limits according to rainfall. These important advances in the science of plant water transport physiology provide the basis for new hydraulic models that can provide credible predictions of not only how but when, where and which species will be impacted by changes in rainfall and temperature in the future. Applying a recently developed hydraulic model using realistic parameters, we show that even apparently safe mesic forest in central France is predicted to experience major forest mortality before the end of the century.

Keywords: Drought; extinction; hydraulic; tree mortality.

Figure 1

Different species have different vulnerability of xylem tissue to damage by cavitation. The ‘optical method’ (Brodribb et al., 2017) is a recently developed approach for quantifying the vulnerability of plant tissues to cavitation during water stress. The optical method is applied here in a Quercus robur plant to visualize cavitation in stem and leaf xylem tissues. (a) Custom-built miniaturized cameras are shown installed on a stem and a leaf of an oak plant to monitor xylem cavitation events during plant dehydration. (b) Colour map depicting the location of xylem cavitation events in leaf veins and the level of water potential that induces those events (colour scale) on a Q. robur leaf (scale bar, 1 mm). (c) Optical hydraulic vulnerability curve showing the cumulative embolisms visualized against the water deficit (water potential) in the plant. Plants typically die when > 50% of xylem tissue is damaged, so P₅₀ (blue dashed line) and P₈₈ (red dashed line) values (i.e. water potential at which 50% and 88% of xylem vessels are embolized, respectively) describe the range of water potentials (striped grey bar) where plant mortality is likely to occur.

Figure 2

Output from a hydraulic model used to estimate the temporal erosions of population and genetic diversity of Fagus trees under a future climatic scenario in central France. Depending on the amount of xylem damage expected to cause mortality two scenarios for tree decline are predicted. If 50% xylem failure is lethal (dashed line) then a collapse in adult tree number is predicted after 2020 due to xylem cavitation produced by increased transpiration and soil water deficit. A more conservative rate of tree decline is predicted if plants are able to survive up to 88% loss of xylem function (dotted line). The insert shows the distribution of tree genetic sensitivity to drought (xylem vulnerability: P₅₀) at four time points in the future, showing a shift to more resistant varieties. Under this RCP8.5 scenario (continuing increase in CO₂ emissions), a dramatic collapse of tree population is predicted by the middle of this century, with most vulnerable genotypes dying first.