The response of black spruce to the climatic influence of Robert-Bourassa Reservoir in northern Quebec

Do large hydroelectric reservoirs in northern Quebec have an influence on r egional climate? In the absence of any climatic record covering the period prior to and after damming, the influence of a reservoir was studied by mea ns of tree rings. The objective of this research was to determine the react ion of black spruce to local climate change. The study was carried out on a n island in the second-largest reservoir of the La Grande complex in the Ja mes Bay area, the Robert-Bourassa Reservoir (2835 km(2)), which is also the oldest (dammed in 1978 and completely flooded in September 1979). Trees (n = 14) about 100 years old were selected by means of two perpendicular tran sects on the island. Cross sections were cut at l-m intervals at the base o f trees, and at 10-cm intervals in the top section of the stem which develo ped during the last 30 years. Densitometric analysis was performed on each section. Results were compared with two reference chronologies using the sa me tree-ring parameters. Reference stands were selected outside the area of climatic influence of the reservoir. The identified tree-ring indicators o f reservoir local climate were (i) absence of frost rings, despite their ab undance outside the reservoir's area of influence, due to a delayed growing season caused by local cool conditions; (ii) high frequency of light rings related to cooler summer conditions; (iii) large numbers of trees showing compression wood sequences indicating destabilization due to windier condit ions associated with the increased fetch; (iv) lower annual wood production ; (v) degradation of tree growth forms marked by massive foliage loss and b ranch mortality; and (vi) reduced wood density, despite increased proportio n of latewood. The number of trees that showed these six tree-ring indicato rs was a function of distance to water and stand density.