Use of Calcium/Aluminum Ratios as Indicators of Stress in Forest Ecosystems

Abstract

The calcium/aluminum (Ca/M) molar ratio of the soil solution provides a valuable measurement endpoint or ecological indicator for identification of approximate thresholds beyond which the risk of forest damage from M stress and nutrient imbalances increases. The Calm ratio can also be used as an indicator to assess forest ecosystem changes over time in response to acidic deposition, forest harvesting, or other processes contributing to acid soil infertility. Based on a critical review of literature on A! stress, we estimate that there is a 50:50 risk of adverse impacts on tree growth or nutrition when the soil solution Calm 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 Calm molar ratio is as low as 0.2. The Calm ratio of the soil solution can be corroborated with other complementary indices. Our analysis found that threshold conditions for potential forest impacts from Ai stress are indicated by four successive measurement endpoints: (i) soil base saturation less than 15% of effective CEC; (ii) soil solution Calm molar ratio <-1.0 (for 50% risk); (iii) fine tissue Ca/M molar ratio ~0.2 (for 50% risk); and (iv) a foliar tissue Calm molar ratio <_ 12.5 (for 50% risk). With appropriate precautions and caveats, these sequential indices based on the Ca/M ratio provide a means of distinguishing site conditions where M stress is likely to affect tree growth adversely.

Status of soil acidification in North America

“…Red spruce forests in the highest elevations of the southern Appalachians have shown an increase in mortality in recent decades and a significant decline in crown condition (PEART et al 1992). Foliar Ca levels, and soil and root Ca/Al ratios are considered low to deficient over most of the southern spruce-fir region and portions of the northern region (JOSLIN et al 1992;CRONAN, GRIGAL 1995). Reduced ratios of photosynthesis to dark respiration were found in red spruce in the southern Appalachians at high elevation sites where Ca/Al ratios in needles were low (MCLAUGHLIN, KOHUT 1992).…”

Effects of lime-induced differences in site fertility on fine roots of oak

“…Root health appeared to be improved by liming, as shown by the increased Ca/Al ratio [6], a higher live/dead ratio of fine roots in the initial years after liming and indications of a higher root life-span [1]. The Ca/Al molar ratio in the fine roots expresses the inability or ability to take up nutrients owing to presence or absence of Al stress.…”

“…A way of evaluating the root function, that is root health and thus uptake potential, is the use of Ca/Al ratios in the tissue of fine roots [1,6]; Ca/Al ratios below the threshold levels indicate potential toxicity and physiological disturbance. As allocation of nutrients, after uptake, is often preferential to the canopy relative to the roots, fine roots can be good additional indicators of change in nutritional conditions in forest stands [12].…”

“…In a previous paper [1] CaCO 3 or CaO. In addition to the lime amendment some complementary compounds were used: N (sites [4][5][6], N, P and K (site 8) and Mg (3.5 % in the lime product on sites 9 and 10). The levels of these additions were low, especially for the N which was only applied in the oldest sites (26-27 years ago), and it was assumed that it would not have an influence on the present tree response [3].…”

“…The fine roots (< 2 mm in diameter) were separated from the soil by wet sieving above a 4-and 2-mm sieve followed by short flotation to rinse the roots and root length was estimated by the line intersection method [16]. The [6], and that liming increased this Ca/Al ratio in many soil layers.…”