Tree Species Vulnerability

Overcoming Drought Stress

Drought is expected to become more frequent and intense, possibly affecting plant growth and survival over the coming century (Allen et al. 2010; Clark et al. 2016). A complex set of mechanisms interact within a tree to mitigate water stress during drought in order to survive.

Four broad strategies include:

1) Drought avoidance by maintaining water access: As water levels drop within the soil profile during drought, the ability of roots to access deeper soil horizons is crucial to avoiding drought (Breda et al. 2006). Shallow rooted species will therefore be the most sensitive.

2) Resistance to drought-induced damage by maintaining circulation when internal water levels are low: When soil water access decreases but demand remains constant, mechanisms such as xylem resistance to cavitation and stomatal sensitivity become important to avoid mortality.

3) Tree recovery after drought: Several mechanisms influence a tree’s ability to recover after drought. These include the capacity to resume hydraulic conductivity after xylem embolization and/or the ability to produce new conductive tissues. While recovery ability is important to characterize tree response to drought, further research is needed to develop an index comparable across species.

4) Population recovery after mortality: The ability of a species to rapidly recolonize an area after drought-induced mortality influences its persistence on the landscape. Population recovery is influenced by several mechanisms such as prolific stem resprouting and proximity of nearby seed sources as well as the ability of regenerating stems to start producing seed rapidly.

How to Interpret the Index

Index scores

Indices were developed for three strategies for which information was available: Drought avoidance, Resistance to drought-induced damage, Population recovery after mortality. An index value was attributed to each species based on its relative ability to use a given strategy when faced with drought. Five classes of sensitivity-tolerance were defined: highly sensitive, medium sensitive, intermediate, medium tolerant, highly tolerant.

A data confidence value is also attributed to each sensitivity index value. Data confidence is based on the number of independent sources, whether the data were quantitative or qualitative, whether there is known variation in values and the source of the data. This information was ranked into five confidence classes: low, medium-low, intermediate, medium-high, high.

Explore the Drought Indices

For direct access to data visualization tool on Tableau Public, please visit Tree Sensitivity to Climate Change - Drought Index Tool

For more information on index development and how sensitivity classes were defined, see the supplementary material from the publication “Using a Trait-Based Approach to Compare Tree Species Sensitivity to Climate Change Stressors in Eastern Canada and Inform Adaptation Practices” (PDF download available here: https://www.mdpi.com/1999-4907/11/9/989/s1).

Glossary - Glossary of terms used in vulnerability assessments

Another set of strategies that characterise species’ ability to adapt in the face of climate change are presented on this page.

Literature cited

Allen, C.D.; Macalady, A.K.; Chenchouni, H.; Bachelet, D.; McDowell, N.; Vennetier, M.; Kitzberger, T.; Rigling, A.; Breshears, D.D.; Hogg, E.H.; et al. A global overview of drought and heat-induced tree mortality reveals emerging climate change risks for forests. Forest Ecology and Management 2010, 259, 660–684. Available online: https://cfs.nrcan.gc.ca/publications?id=30987

Clark, J.S.; Iverson, L.; Woodall, C.W.; Allen, C.D.; Bell, D.M.; Bragg, D.C.; D'Amato, A.W.; Davis, F.W.; Hersh, M.H.; Ibanez, I.; et al. The impacts of increasing drought on forest dynamics, structure, and biodiversity in the United States. Global Change Biology 2016, 22, 2329–2352.

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