Global and dynamic changes to fragmented populations of boreal ecosystems : BOREALP project

Greenhouse gases are responsible for higher surface temperatures, which will affect evapotranspiration patterns with an increase in drought and water stress conditions. These climate changes will influence plant physiology and phenology, in addition to the regime of natural disturbances such as insect epidemics and fires. If the composition of forest regions changes, two ecological trajectories are possible: in situ conservation, i.e. the replacement of dominant by sub-dominant species, or migration. Marginal populations, either located at the limit of their distribution areas or present in the form of fragmentary aggregates, are expected to play a predominant role in the ecological processes marking species' behaviour in response to climate changes. However, the quantification of the impact of the fragmentation process, as well as the capacity of species and populations to respond effectively to global warming, are still poorly documented.

• Improve our understanding of the relative influence of climate and disturbances on the dynamics of fragmented populations at their distribution limit;

• Characterize their ecological and genetic response to the climate changes under way.

• Characterize the impact of climate variations and disturbances on the contraction/expansion mechanisms of tree populations over different space and time scales.

• Evaluate the potential capacity of isolated populations to escape fire disturbances, to invade and colonize new sites brought to light by disturbances and to maintain themselves on a landscape scale in the form of metapopulations.

This project showed that marginal populations of several species, such as the sugar maple and the Eastern white cedar, display neither a reduction in the potential for growth and regeneration nor a decline in genetic diversity along the south-north latitudinal gradient. Relationships between climate and growth were essentially modulated by environmental variables, in particular the volume of precipitation, but also by variables related to soil and by the morphology of the trees. The responses of marginal populations to future warming will be highly dependent on the seasonality and volume of precipitation (Figure 1).

In addition, the growth synchronism between trees was influenced both by intra-population genetic diversity (Figure 2) and by the volume of precipitation. The reconstruction of the history of fires and the post-fire vegetation dynamics in mixed boreal forests highlighted the existence of two types of insular residual patches in mixed boreal forests during the Holocene: refuges and transient insular residual patches. Refuges are less susceptible to fire compared to transient insular residual patches, which escaped only the last fire, probably fortuitously. Refuges establish themselves in low, wet depressions that favour the accumulation of organic matter. A synthesis of the work resulting from charcoal analyses indicates two Holocene histories of fires (over the course of 7,000 years) that are significantly different when comparing the coniferous boreal forest (spruce–moss stands) and the mixed boreal forest (balsam fir–white birch stands). The establishment of generally wetter and colder conditions (Neoglacial) as of 3,000 BC resulted in a reduction in the length of the fire season, especially in its terminal (summer) part, which resulted in a decrease in the frequency of fires in the coniferous zone. In the mixed zone, the frequency of fires has remained unchanged for 7,000 years. Futher to this project, 15 articles have been published further explaining the different elements described herein.

Figure 1 : Low frequency variations in radial growth of Thuya occidentalis stands along the latitude gradient. The results demonstrate that despite recent warming, trees located in marginal areas show a decrease in growth, possibly due to drier conditions.

Figure 2 : Genetic structure of Thuya occidentalis populations as identified by STRUCTURE with K = 3. (Orange : MZ1, MZ2, MZ3, MZ4, et DZ6 ; jaune : MZ5, MZ6, MZ7, MZ8, MZ9, DZ1, DZ2, DZ3, DZ4, DZ5, CZ5, CZ6, CZ7, ET CZ8; bleu : CZ1, CZ2, CZ3, CZ4, ET DZ7).

• Fonds de développement académique du réseau (FODAR)

• Hydro-Québec

• Laboratoire international associé sur les forêts montagnardes et boréales (CNRS)

• Ministère de l'Économie, de l'Innovation et des Exportations (MEIE)

• Tembec

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