Sr ratios, the watershed was divided into six study plots at low, middle, and high elevations along two toposequences Á one consisting of mixed woods and the other consisting of black spruce. Apportionment analysis shows that none of the trees in the study plots depend entirely on soil mineral weathering as a source of Ca. Calcium pools in trees are shifted towards the atmospheric end-member (31Á98%), probably because of the low soil mineral weathering fluxes combined with relatively high atmospheric deposition fluxes. These results need to be considered as the consequence of buildup and recycling of atmospherically derived Ca in the soil-vegetation system rather than the direct and large use of current atmospheric Ca inputs by the trees on an annualized basis. We also find that the trees in the high-elevation plots contain more Ca from atmospheric sources (84Á98%) than trees in middle (31Á86%) and low (37Á82%) elevation plots. Two mechanisms are proposed to explain these results. First, if each plot is assumed to receive a constant supply of atmospherically derived Ca, then hillslope changes in soil mineral weathering rates are required to explain the changing proportions of atmospherically derived Ca cycling internally within each plot. Second, trees on hilltops may capture more dry aerosols than trees that are more shielded from atmospheric circulation in the valleys. Our work supports previous Ca cycling models in other forested ecosystems suggesting that much of the Ca requirement in trees is met via the flux from the atmosphere, but adds knowledge with regards to the role of topographical effects and forest canopy height as local factors that can influence the balance of Ca captured from atmospheric sources and released from soil mineral weathering sources. Sr a e´te´utilise´e pour de´terminer les contributions des de´poˆts atmosphe´riques et de l'alte´ration des mine´raux dans les sols quant aux re´serves de calcium (Ca) dans une foreˆt vierge du bouclier bore´al du nord de la Saskatchewan. Pour e´valuer l'impact de la topographie sur les ratios 87 Sr/ 86 Sr de l'alte´ration des sols, le bassin versant a e´ted ivise´en six parcelles, a`partir de l'interfluve jusqu'au milieu et au bas de pente, le long de deux topose´quences, l'une domine´e par l'e´pinette noire et l'autre repre´sentant une foreˆt me´lange´e. L'analyse de re´partition re´ve`le qu'aucun des arbres e´tudie´s de´pendent entie`rement sur l'alte´ration des mine´raux dans les sols comme source de Ca. Les re´serves de Ca dans les arbres tendent vers les de´poˆts atmosphe´riques (31 a`98%), probablement a`cause des faibles taux d'alte´ration des mine´raux dans les sols en concomitance avec des flux atmosphe´riques relativement e´leve´s. Les re´sultats doivent eˆtre interpre´te´s comme la conse´quence d'une accumulation et d'un recyclage du Ca atmosphe´rique dans le syste`me plante-sol plutoˆt qu'une importante utilisation directe des apports atmosphe´riques contemporains de Ca par les arbres sur une base annuelle. Les re´sultats re´ve`lent aussi q...