SCBG researchers reveal the mortality threshold for trees under drought on a global scale

Many areas on the earth are becoming drier under global warming. Meanwhile, in the past decades, drought-induced forest die-off has been recorded on every forested biome, exerting great effects on biodiversity and ecosystem functions. However, a general understanding of forest vulnerability to drought is lacking, because of our limited understanding of the variation in mortality threshold, i.e., the lethal water potential (Ψ_lethal), of trees.

In a recent article published in New Phytologist entitled "Wood density predicts mortality threshold for diverse trees", a research group led by Professor YE Qing from South China Botanical Garden of Chinese Academy of Sciences revealed the variation in mortality threshold of trees on a global scale, providing new insights into understanding and predicting tree mortality in a changing climate.

The researchers found the Ψ_lethal ranged widely from -1.5 to -14.7 MPa across 59 tree species from three biomes (Figure 1). On average, the Ψ_lethal was significantly lower for gymnosperms than angiosperms. For angiosperms, trees from temperate forests had a significantly less negative Ψ_lethal than those from tropical or subtropical forests.

Figure 1 The distribution of lethal water potential (Ψ_lethal) for trees under drought. One bar represents one species. The black dotted lines show the mean value for gymnosperm and angiosperm species, respectively.

They also found that tree Ψ_lethal was close to P50 (water potential at 50% loss of xylem conductivity) for gymnosperms, while always lower than P50 and closer to P88 (water potential at 88% loss of xylem conductivity) for angiosperms. Besides, they found highly significant correlations between Ψ_lethal and wood density in both angiosperms and gymnosperms (Figure 2), suggesting that wood density (which is very easy to measure) may be a robust proxy for the threshold. As the determination of mortality threshold is particularly time- and labor-consuming (it takes months to years), the results provide a new approach in the study of forest vulnerability to drought on a global scale.

Figure 2 Relationships between lethal water potential (Ψ_lethal) and (a) P50, (b) P88, and (c) wood density. The black dotted lines in (a) and (b) indicate the 1:1 lines.

"Water potential is a robust and direct indicator of the water status of trees. The time that takes a tree to reach the lethal water potential determines the tree survival during droughts, therefore models that aim to predict tree mortality under drought must include this threshold as a critical parameter," said Dr. LIANG Xingyun, the first author of this study.

For further reading please refer to: https://doi.org/10.1111/nph.17117.