Sugar maple health is significantly affected by defoliation stress, particularly when nutritional thresholds of foliar Ca (>5500 mg kg^-1), Mg (>700 mg kg^-1) and Mn (<1900 mg kg^-1) are not met.   On unglaciated sites in northern Pennsylvania, sugar maples meeting these thresholds could withstand defoliation and drought stress without showing symptoms of decline disease.  Here, we examine sugar maple basal area growth in relation to foliar nutrient thresholds over short (1987 to 1996) and long (1937 to 1996) periods in two regions in the northeastern U.S. with contrasting defoliation histories.  A total of 35 foliage and soil nutrient, topographic, climatic, stand structure and health variables from each of 76 stands was used for this assessment.  PA and NY stands (PANY; n=46) some of which had drought and defoliation stress from 1987-96 were compared with NH and VT stands (NHVT; n=30) which had minimal stress from drought and defoliation.  Foliage samples from 3-6 trees/stand were obtained from healthy dominant and codominant sugar maples in mid-August from 1995 to 1997, and were used to evaluate nutrient content (Ca, Mg, P, K, Mn, Al, and N).  Increment cores were collected from each tree and individual ring widths were measured and crossdated in order to calculate stand mean basal area increment (MBAI).  

Results/Conclusions

Stepwise linear regression yielded an equation using foliar Ca or Mg and an expression of stand health described by crown condition ratings (SMVIG) that accounted for 46% of the variation in MBAI from 1987-96.  Defoliation history was not a significant factor in the model so all stands within the region were used in the analysis despite different defoliation histories between PANY stands and NHVT stands.  To examine long-term growth trends, 5 year MBAIs were used with repeated measures analysis of variance to assess the impact of foliar nutrient thresholds over the 60 year period from 1937-96.  Significant (P ≤ 0.05) nutrient threshold by time interactions for Ca (P=0.010), Mg (P=0.003), and Mn (P=0.043) indicate changing nutrient conditions in this time period that affected MBAI across the entire region.  Stands with below threshold amounts of Ca and Mg and above threshold Mn, alone or in combination, exhibited decreased MBAI starting in the early 1970s.  This decrease in MBAI for below threshold stands is likely related to stand topographic position and soil parent material, but its timing also suggests altered soil chemistry due to long-term acidic deposition inputs.