COS 33-3
Do soil nutrients make maple sap sweeter?
Maple syrup is an important non-timber forest product in the northeastern United States and southeastern Canada. Sugar maples (Acer saccharum Marsh.) that are healthier and have higher growth rates often have higher sugar concentration in their sap. Soil N mineralization and base cations have been shown to be important to sugar maple health. However, the relationship between soil nutrients and sap sweetness is not well understood. The effect of soil and foliar nutrients on sap sweetness is economically important because it affects the amount of sap and the amount of fuel required to produce a gallon of syrup.
This study looked at what specific soil and foliar nutrients affect the sugar concentration of maple sap and whether addition of N, P, or Ca increases sap sweetness. We sampled 309 sugar maple trees for sap sweetness in five sites with N, P, and Ca additions in the White Mountains of New Hampshire using a digital refractometer. Gas exchange was measured on 84 trees using a LI-COR 6400. Soil nutrient characteristics were quantified through extractions and lab incubations of the top 10 cm of soil. Sap sweetness was compared to foliar and soil nutrients and photosynthetic potential through correlation analysis.
Results/Conclusions
Trees with higher sugar concentrations in their sap were growing in sites with higher soil N (p=0.007). Soil and foliar Ca were not correlated with sap sweetness (p>0.3), while foliar P had a negative correlation with sap sweetness (p=0.02). The foliar N:P ratio had a strong relationship with sap sweetness (p<0.001), stronger than the relationship with foliar N (p=0.12). Higher foliar N increased the potential photosynthetic rate in leaves (p=0.001), presumably allowing the trees to produce more sugars. Nitrogen addition plots increased sap sweetness 24% (p=0.09) By selecting sites with higher soil nitrogen or fertilizing with N, maple syrup producers may be able to collect maple sap with higher sugar concentration and reduce the energy needed to boil sap into syrup. Results of this study also help to understand carbohydrate production and transport in sugar maple.