Establishment of sugar maples (Acer saccharum Marsh.) clonally propagated by rooted cuttings at Heiberg Forest, New York, in 1983 presented a unique opportunity to study the genetic control of sap sweetness and mercury (Hg) concentration. Maple sugar is the most important non-timber forest product in the northeastern United States and southeastern Canada; the production of maple sugar products depends on the sweetness and volume of sap collected. Little is known about mercury cycling in trees, in spite of the growing interest in Hg as an environmental pollutant and a threat to human health. There have been few studies that report the concentrations of Hg in plant tissues, and only in pine and spruce have concentrations in xylem sap been reported. Sap was collected during the spring sap flow by inserting 16 gauge syringe needles 1 cm into the side of a tree in April 2014, and sugar concentrations were measured with a refractometer in the field. The sap was freeze-dried and analyzed for Hg using thermal decomposition, catalytic conversion, amalgamation, and atomic absorption spectrophotometry. Tree stems were mapped and a competition index from the basal area of surrounding trees calculated for each sampled tree.
Results/Conclusions
Sap sweetness differed among clones (p<0.001) when analyzed with a randomized complete block design ANOVA. Mercury concentrations in sugar maples differed across ten different genetic clones (p = 0.03) with an average of 0.0050 ± 0.0003 ng mL. The effect of genetic control was greater than the effect of site difference on maple sap sweetness (p=0.7) and Hg concentration (p = 0.12). Competition from surrounding trees did contribute to some variability in sap sweetness (p=0.01) but the effect was not as great as the genetic control. If the Hg concentrations we observed are characteristic of xylem sap, then the amount of Hg moving within the tree with the transpiration stream would amount to 2.4 μg m-2 yr-1. This is the first study to report Hg in hardwood tree sap and it is also the first study to report sap sweetness in sugar maples propagated by rooted cuttings. Further research may reveal the mechanisms by which tree genotypes differ in the uptake, transport, or exclusion of Hg. Clonal propagation of trees genetically programmed to produce sweeter sap could be important to the maple sugar industry.