Abstract
Tree species responses to climate change will be greatly influenced by their evolutionary potential and their phenotypic plasticity. Investigating tree-rings responses to climate and population genetics at the regional scale is therefore crucial in assessing the tree behaviour to climate change. This study combined in situ dendroclimatology and population genetics over a latitudinal gradient and compared the variations between the two at the intra- and inter-population levels. This approach was applied on the northern marginal populations of Thuja occidentalis (eastern white-cedar) in the Canadian boreal forest. We aimed first to assess the radial growth variability (response functional trait) within populations across the gradient and to compare it with the genetic diversity (microsatellites). Second, we investigated the variability in the growth response to climate at the regional scale through the radial growth-climate relationships, and tested its correlation with environmental variables and population genetic structure. Model selection based on the Akaike Information Criteria revealed that the growth synchronicity between pairs of trees of a population covariates with both the genetic diversity of this population and the amount of precipitation (inverse correlation), although these variables only explained a small fraction of the observed variance. At the regional scale, variance partitioning and partial redundancy analysis indicate that the growth response to climate was greatly modulated by stand environmental variables, suggesting predominant plastic variations in growth-response to climate. Combining in situ dendroclimatology and population genetics is a promising way to investigate species’ response capacity to climate change in natural stands. We stress the need to control for local climate and site conditions effects on dendroclimatic response to climate to avoid misleading conclusions regarding the associations with genetic variables.
Highlights
Climate change will alter ecological gradients, notably via poleward shifts of isotherms and locally-dependent changes in precipitation patterns (IPCC, 2013)
A latitudinal transect was established from 47.3 to 50.0◦N and divided into three zones based on cedar abundance: The continuous zone (CZ), where cedars are common, the discontinuous zone (DZ), which marks the northern edge of the continuous distribution where it becomes less common in the forest matrix and, the marginal zone (MZ) where only a few isolated stands were found (Figure 1A)
Over the 1953–2010 period, climate was colder in the MZ, with a mean (±sd) annual temperature (MAT) of −0.03 ± 0.37◦C, than in the DZ and CZ, which had a MAT of 1.40 ± 0.25◦C and 2.40 ± 0.33◦C, respectively (Table 1)
Summary
Climate change will alter ecological gradients, notably via poleward shifts of isotherms and locally-dependent changes in precipitation patterns (IPCC, 2013). While transplant experiments in common gardens are the most direct way of controlling for environmental differences among populations, these experiments are not available for most non-commercial species In such cases, investigations of tree responses to climate in natural stands are required. Numerous dendroclimatic studies report that growth-climate relationships are modulated by environmental variables such as regional precipitation or temperature regimes, soil variables and forest stands characteristics (e.g., Babst et al, 2013; Gewehr et al, 2014; Housset et al, 2015; Brienen et al, 2016) The effect of those ecological gradients could be confounded with a genetic effect on the growth responses to climate and need to be controlled. We test for the correlation between population genetic structure and tree-rings responses to climate, while controlling for ecological gradients through an approach combining model selection and variance partitioning. The objective is to test for a correlation between neutral population genetic structure and tree-rings, while accounting for environmental variables, at both the intrapopulation and the inter-population levels
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