Theme 1 | Forest Resilience and Vulnerability

Forests are increasingly exposed to numerous disturbances (drought, wildfires, pests and pathogens attacks, windthrow etc). Each tree species has developed strategies in response to these stressors. However, if disturbances are too frequent and/or intense, trees might not have to ability to cope with them, therefore making forest ecosystems more vulnerable.

Forest vulnerability can be assessed via three distinct components: exposure, sensitivity, and adaptive capacity. Exposure is defined as the degree of environmental change a species will experience, in terms of character, magnitude, and rate. Sensitivity refers to the degree to which a species might be affected by these changes. Adaptive capacity is the ability of a species to tolerate or cope with environmental changes.

The aim of theme 1 is to assess Canadian forests vulnerability to the main global change stressors. This will be done by building species-specific sensitivity indices based on functional traits; these indices are a cornerstone to the assessment of Canadian forests’ vulnerability.

Forests are increasingly exposed to numerous disturbances (drought, wildfires, pests and pathogens outbreaks, windthrow etc). Each tree species has developed strategies in response to these stressors. However, if disturbances are too frequent and/or intense, trees might not have to ability to cope with them, therefore making forest ecosystems more vulnerable.

Forest vulnerability can be assessed via three distinct components: exposure, sensitivity, and adaptive capacity. Exposure is defined as the degree of environmental change a species will experience, in terms of character, magnitude, and rate. Sensitivity refers to the degree to which a species might be affected by these changes. Adaptive capacity is the ability of a species to tolerate or cope with climate change impacts.

Think of the human body and sunburn. Skin can be sensitive, due to intrinsic features such as skin carnation: a pale skin will be more sensitive than a darker skin. If you are sensitive but don’t go in the sun or don’t get out too long (if you don’t expose yourself), you won’t get burn, so you will not be vulnerable. If you go outside in the sun, your body may have the capacity to adapt by producing melanin, which could in turn protect you from the sun ray and reduce your vulnerability to future sunburns. 

When faced with a stressor (drought, fire or changes in growing conditions), species can either tolerate its effects, avoid stressor-induced damage, or re-establish after impacts. The different strategies of trees to cope with disturbances depends on some intrinsic features called functional traits. Functional traits are morphological, physiological, and phenological attributes that determine an organism’s response to a stressor. For instance, some species have a deep taproot which allow them to access soil water at greater depths. They therefore avoid being stressed by a lack of water.

To sum up, functional traits will influence whether species’ exhibit tolerance or sensitivity when faced with a stressor. The problem is that, for most of the species, the values of functional traits are still poorly known. Determining which functional traits are the main drivers of species sensitivity to different disturbances and measuring them, will improve our understanding of species vulnerability.

DIVERSE seeks to improve our knowledge about the different components of forests vulnerability to global change. This will help us to inform about forest resilience, a property of forest ecosystem to keep its main functions (such as regulating the water cycle, host biodiversity, store carbon) during or after disturbances.

Informing species sensitivity to global change stressors requires lots of data about functional traits. Yet, there is a lack of trait data for North American tree species: some traits are only available for a few commercial species and/or measurements have only been taken in some well-studied areas.

First, we need to collect functional traits data on tree species, in a way that include the variability of traits values. Indeed, a given species does not present a unique functional trait value, but rather a range, which depends on its genetics and on its growing conditions. Many Canadian tree species have a wide subcontinental distribution spanning large environmental gradients that lead to large intra-trait variation. This variation is important to take into account when assessing tree sensitivity and species adaptive capacity to environmental variations; two components of vulnerability. For instance: leaf area and leaf thickness, two traits that influence species response to drought, will change according to mean temperatures. This intra-specific variation is rarely included in models which reduce our capacity to accurately assess forest vulnerability to global change.  

Measuring tree species functional traits across their distribution represents a huge work, beyond the capacity of a single lab. This is why we are organizing a collaborative field campaign during summer 2024, across Canada and some regions of the United States of America. The FunTree campaign seeks to coordinate the efforts of research teams to investigate the intraspecific variability of key traits related to tree vulnerability to global change. The campaign will focus on filling knowledge gaps on tree species sensitivity to drought, by considering two components of drought response: drought induced mortality of mature trees and regeneration failure.

FunTree relies on the expertise of several researchers for methodological design and for data analysis; and on the collaboration of fifty teams across Canada and North Esat US for field sampling. The data collected on the field will be first used into fundamental ecophysiology projects which aim at better understand the link between key hydraulic and anatomical wood traits. It will then be used to fill knowledge gap on trees sensitivity. A ready-to-use dataset will be developed for integration in models, subsequently improving models’ predictions. Ultimately, the collected data will be integrated into international traits databases to foster and support future larger scale projects.

Species-specific indices of sensitivity to the main global change stressors will be developed based on the functional traits data. These indices will be built around the main mechanisms by which trees cope with each stressor. For instance, drought sensitivity encompasses the extent to which species can cope with drought via avoidance, resistance, and recovery. Trees may avoid drought by staying hydrated, either by maintaining access to water (e.g., through deep roots), or by limiting water loss (e.g., through rapid control of stomata). Other tree species exhibit drought resistance, typically via xylem resistance to embolism. Finally, trees can recover from drought at the individual level, through xylem recovery, or at the population level, through strong vegetative propagation.

These indices will be integrated into a regional assessment of vulnerability to climate change, which is necessary for the development of suitable adaptation strategies for sustainable forest management. Indeed, trends in tree response observed at the global scale (from boreal forests to tropical forests and dry savannas) may not remain when working at a smaller scale (boreal and temperate forests only). It is therefore important to study this at a regional scale, the scale at which forest management is applied.

For these tasks, theme 1 will collaborate with Audrey Maheu, Morgane Urli, Julie Messier, Olivier Villemaire-Côté, Anne Ola and Alison Munson.