Fine-root growth and chemical composition in declining Central Ontario sugar maple stands

Abstract

This study examines the condition of fine-root systems of healthy and declining sugar maples (Acersaccharum Marsh.) at two sites in Central Ontario, one moderately declining and the other severely declining. Roots are frequently sensitive indicators of soil nutrition, thus ingrowth cores (soil transplants) were used to assess the effect of bulk surface (F-layered Ah horizon) soils collected from beneath healthy or declining trees on fine-root chemistry and growth. Soil at each site was collected from healthy and declining trees and reciprocally transplanted (buried) in mesh bags, and roots were allowed to grow into the soil cores for 8 weeks. In addition to transplants of untreated soils, additional transplants of "healthy" and "decline" soils treated by either (i) steam sterilization to remove pathogens or (ii) fertilization with superphosphate were made. Root dry weight of healthy and declining trees was reduced 25–70% in untreated decline soils compared with healthy soils (p ≤ 0.007). This was consistent with lower Ca, Mg, Mn, and Fe in the soil solution of declining trees at the severely declining site and a lower Mn concentration and Ca/Al and Mg/Al ratios in the soil solution at the moderately declining site. Compared with roots of healthy trees, roots of declining trees had significantly lower Ca concentrations for both stands, and in addition, significantly lower concentrations of P, Mg, S, K, Mn, Al, Fe, and Zn in the more severely declining stand. The experimental soil transplants established that lower nutrient concentrations in soils from declining trees are reflected in the chemistry of roots growing in them. On the whole, the fertilizer treatment significantly increased root growth and the steam sterilization treatment had no effect on root growth. While this study shows that reduced nutrient availability in the rooting substrate is an important factor in decline, it is not known how recently this nutrient depletion has occurred, nor what is the cause. Although other mechanisms may be involved, reduced nutrient availability in the rooting substrate is consistent with the speculation that deposition of acidic salt solutions of sulphate and nitrate to these forest soils has caused accelerated soil base cation leaching losses that are reflected in nutritional deficiencies and growth decline.