Canadian Forest Service Publications
Changes in growth of pristine boreal North American forests from 1950 to 2005 driven by landscape demographics and species traits. 2012. Girardin, M.P.; Guo, X.J.; Bernier, P.Y.; Raulier, F.; Gauthier, S. Biogeosciences 9:2523-2536.
Available from: Laurentian Forestry Centre
Catalog ID: 33966
Available from the Journal's Web site. †
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In spite of the many factors that are occurring and known for positively affecting the growth of forests, some boreal forests across North America have recently felt the adverse impacts of environmental changes. Knowledge of causes for productivity declines in North American boreal forests remains limited, and this is owed to the large spatial and temporal scales involved, and the many plant processes affected. Here, the response of pristine eastern boreal North American (PEBNA) forests to ongoing climatic changes is examined using in situ data, community ecology statistics, and species-specific model simulations of carbon exchanges forced by contemporary climatic data. To examine trends in forest growth, we used a recently acquired collection of tree-ring width data from 252 sample plots distributed in PEBNA forests dominated by black spruce (Picea mariana [Mill.] B.S.P.) and jack pine (Pinus banksiana Lamb.). Results of linear trend analysis on the tree growth data highlight a dominating forest growth decline in overmature forests (age > 120 years) from 1950 to 2005. In contrast, improving growth conditions are seen in jack pine and mature (70–120 years) black spruce stands. Multivariate analysis of climate and growth relationships suggests that responses of PEBNA forests to climate are dependent on demographic and species traits via their mediation of temperature and water stress constraints. In support of this hypothesis, the simulation experiment suggests that in old-growth black spruce stands the benefit to growth brought on by a longer growing season may have been low in comparison with the increasing moisture stress and respiration losses caused by warmer summer temperatures. Predicted increases in wildfire frequency in PEBNA forests will likely enhance the positive response of landscape-level forest growth to climate change by shifting the forest distribution to younger age classes while also enhancing the jack pine component.
Plain Language Summary
Global warming extends the tree growing season. However, in the old-growth boreal forests of North America, this warmer climate has negative effects. This conclusion was reached following an analysis of tree growth in the boreal forests of northeastern North America between 1950 and 2005. The trends observed were consistent with the modeling results: the age and size of a tree affect its ability to sequester atmospheric CO2 by decreasing its efficiency in using available water and carbon. As it ages, a forest loses its capacity to sequester carbon; this was found to be particularly true of old-growth black spruce forests. As trees age, water in the soil which is necessary for growth also becomes a limiting factor.