Unexpected warming-induced growth decline in Thuja occidentalis at its northern limits in North America
Résumé
Aim - Towards the cold margins of the Northern Hemisphere boreal zone, continuing warming should theoretically provide a longer vegetative season, favouring growth and a northward shift in tree species distribution. The north- ern distribution of Thuja occidentalis L. (eastern white cedar) is marked by the presence of isolated marginal populations distant from the continuous distribu- tion. If those populations proved to be well adapted to their future local cli- matic conditions, their expansion could accelerate cedar poleward migration. We tested the hypotheses that (1) there will be a growth increase in cedar northern marginal populations as a result of global warming, and (2) the edaphic conditions and regional precipitation regimes will modulate their response to warming.
Location - Canadian boreal forest, western Québec (47–50° N, 74–80° W).
Methods - We investigated radial growth using tree-ring measurements from dominant and co-dominant eastern white cedar trees (n = 723) distributed along a latitudinal gradient from the species’ northern margin to the centre of its natural range. First, low-frequency growth variations were analysed on whole chronologies (ad 1720–2010). Second, inter-annual growth variations were tested against ad 1953–2010 monthly temperature and precipitation time series with a bootstrapped correlation function. Finally, the impact of environ- mental variables on the growth–climate relationships was assessed.
Results - Unexpectedly, a growth decline was observed starting in 1980 in mar- ginal sites. Dendroclimatic analyses revealed that radial growth was not only limited by short growing seasons but also by summer droughts in the marginal zone. This response was exacerbated in sites that received less summer precipi- tation. Counterintuitively, autumn and spring precipitation negatively impacted on growth, especially in wet soil stands.
Main conclusions - Northern marginal populations of cedar may have already reached their optimum temperature threshold for radial growth. Our results suggest that they will probably be facing increasing hydric stress selection pres- sure under the assumptions of climate change. Their responses to future warm- ing will be highly dependent on the seasonality and magnitude of variation in precipitation regimes.