Cs. Cronan et Df. Grigal, USE OF CALCIUM ALUMINUM RATIOS AS INDICATORS OF STRESS IN FOREST ECOSYSTEMS, Journal of environmental quality, 24(2), 1995, pp. 209-226
The calcium/aluminum (Ca/Al) molar ratio of the soil solution provides
a valuable measurement endpoint or ecological indicator for identific
ation of approximate thresholds beyond which the risk of forest damage
from Al stress and nutrient imbalances increases. The Ca/Al ratio can
also be used as an indicator to assess forest ecosystem changes over
time in response to acidic deposition, forest harvesting, or other pro
cesses contributing to acid soil infertility. Based on a critical revi
ew of literature on Al stress, we estimate that there is a 50:50 risk
of adverse impacts on tree growth or nutrition when the soil solution
Ca/Al ratio is as low as 1.0, a 75% risk when the soil solution ratio
is as low as 0.5, and nearly a 100% risk when the soil solution Ca/Al
molar ratio is as low as 0.2. The Ca/Al ratio of the soil solution can
be corroborated with other complementary indices. Our analysis found
that threshold conditions for potential forest impacts from Al stress
are indicated by four successive measurement endpoints: (i) soil base
saturation less than 15% of effective CEC; (ii) soil solution Ca/Al mo
lar ratio less than or equal to 1.0 (for 50% risk); (iii) fine root ti
ssue Ca/Al molar ratio less than or equal to 0.2 (for 50% risk); and (
iv) a foliar tissue Ca/Al molar ratio less than or equal to 12.5 (for
50% risk). With appropriate precautions and caveats, these sequential
indices based on the Ca/Al ratio provide a means of distinguishing sit
e conditions where Al stress is likely to affect tree growth adversely
.