Abstract
Background: Black spruce (Picea mariana (Mill.) BSP)-forested peatlands are widespread ecosystems in boreal North America in which peat accumulation, known as the paludification process, has been shown to induce forest growth decline. However, the ecophysiological mechanisms that lead to growth reductions in black spruce remain unexplored. Trees growing in paludified forests have to deal with continuously evolving environmental conditions (e.g., water table rise, increasing peat thickness) that may require growth mechanism adjustments over time. In this study, we investigated tree ecophysiological mechanisms along a paludification gradient in a boreal forested peatland of eastern Canada by combining peat-based and tree-ring analyses. Carbon and oxygen stable isotopes in tree rings were used to document changes in carbon assimilation rates, stomatal conductance, and water use efficiency. In addition, paleohydrological analyses were performed to evaluate the dynamical ecophysiological adjustments of black spruce trees to site-specific water table variations.Results: Increasing peat accumulation considerably impacted forest growth, but no significant differences in tree water use efficiency (iWUE) were observed between the study sites. Tree-ring isotopic analysis indicates no iWUE decrease over the last 100 years, but rather an important increase at each site up to the 1980s, before iWUE stabilized. Surprisingly, inferred basal area increments did not reflect such trends. Our results suggest that the slower growth rates observed at the most paludified sites are attributable, at least partially, to both lower carbon assimilation rates and stomatal conductance. These findings show that iWUE variations do not necessarily reflect tree ecophysiological adjustments required by changes in growing conditions. Local water table variations induced no changes in ecophysiological mechanisms, but the synchronous shift in iWUE observed at all sites in the mid-1980s suggests a tree response to regional or global factors, such as increasing atmospheric CO2 concentration.Conclusions: Our study shows that paludification induces black spruce growth decline without, however, altering tree water use efficiency in boreal forested peatlands. This is the first attempt in exploring the complex interactions between stem growth, ecophysiological mechanisms, and environmental conditions in paludified sites. Additional research on carbon allocation strategies is of utmost importance to understand the carbon sink capacity of these widespread ecosystems and better predict their response to future climate change.
Paludification reduces black spruce growth rate but does not alter tree water use efficiency in Canadian boreal forested peatlands