Influence of landscape on the apportionment of Ca nutrition in a Boreal Shield forest of Saskatchewan (Canada) using 87Sr/86Sr as a tracer

Abstract

A 87Sr/86Sr tracer technique was used to apportion the supply of calcium (Ca) between atmospheric deposition and soil mineral weathering in a pristine Boreal Shield forest of northern Saskatchewan. To assess the impact of landscape variability on soil mineral weathering 87Sr/86Sr ratios, the watershed was divided into six study plots at low, middle, and high elevations along two toposequences - 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 (31-98%), 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 (84-98%) than trees in middle (31-86%) and low (37-82%) 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. Key words: Boreal forest, calcium cycling, atmospheric deposition, soil mineral weathering, strontium isotopes, 87Sr/86Sr, calcium to strontium ratios, Sr/Ca; forest floor, tree calcium pools