|The Botanic Garden
of Smith College
An alarmingly high number of sugar maples (Acer saccharum) in the Smith College arboretum displayed decline symptoms during the summer of 2008. Symptoms included twig dieback, foliar chlorosis, premature leaf drop, and thinning canopies. While no one cause leads to decline, the combination of several long-term and short-term stresses can weaken a tree, making it more susceptible to plant health problems and eventual decline.1 Stressing factors may include nutrient deficiency, drought, soil compaction, even air pollution.1 Urban trees are often more prone to decline due to generally poor planting conditions. Sugar maples, in particular, have little tolerance for compaction, road salt, and excessive heat, all common urban stressors.2 Sugar maple decline remains largely unexplained, but past research suggests decline may be related to base cation deficiency (Ca, Mg, K), drought, deicing salt application, and winter thaws followed by freezing resulting in compromised root function.3
Eight mature sugar maples, four healthy and four declining, were examined in an attempt to pinpoint stressing factors leading to decline at Smith College. Sample trees were selected from one accession (3062PA), meaning all trees predate 1971 and all trees are straight species (Acer saccharum), no cultivars or varieties. Soil and foliage samples were collected for each individual tree and analyzed by the University of Massachusetts Soil and Plant Tissue Testing Lab. In addition to the standard soil test, a test for soluble salts was requested due to proximity to roadways and possible deicing salt contamination.
Soil test results revealed low soil pH for all trees, healthy and declining. Acer saccharum prefers soil pH from 6.0 to 7.5, but the Smith College samples ranged from 4.7 to 5.6. The group of declining trees showed a slightly low cation exchange capacity (CEC). CEC measures a soilís ability to hold nutrients and sufficient CEC ranges from 10 to 15 meq/100g. The average CEC was 10.025 for the declining group and 11.1 for the healthy group. While a CEC of 10.025 falls within the adequate range, it is on the lower end of the spectrum. Soil macronutrient (P, K, Ca, Mg) levels ranged from low to medium-high for all trees with no recognizable delineation between healthy and declining trees. Micronutrient levels were normal for all sites. Soluble salt measurements were negligible for all trees. Several sites, both healthy and declining, exhibited slightly elevated Lead (Pb) levels, though any connection to sugar maple decline is unclear.
Foliar analysis results revealed a low percentage of foliar Calcium (Ca) and Magnesium (Mg) among declining trees. Declining trees averaged 0.815 % Ca, a value well below the 1.01 to 2.42 % reference value range. Mg levels averaged 0.135 % in the same group, also below the 0.17 to 0.45 % reference value range. Macronutrient (N, P, K, Ca, Mg, S) percentages were lower over all for declining trees as compared to healthy samples. Foliar micronutrient levels fell within the reference value ranges for all trees with no clear delineation between sample groups. Both healthy and declining trees exhibited elevated foliar Aluminum (Al) levels with 61.25 ppm and 65.25 ppm respectively. Both values fell outside the reference value range of 35 to 56 ppm. Sodium (Na) levels were insignificant in all trees, suggesting deicing salt is not responsible for decline symptoms.
Soil test results suggest low soil pH and CEC may be associated with sugar maple decline, as soil acidity has a negative impact on nutrient availability, percent base saturation, and Al toxicity. However, because all sites showed low pH, long-term observation and soil analysis of a larger sampling group may yield more conclusive findings. The summer of 2008 was particularly wet and leaching may have altered mid-summer nutrient and soluble salt levels, so additional soil tests are recommended. Deeper soil samples of 18 to 24 inches should also be considered as the standard 6 to 8 inch sample depth may not accurately represent the sugar maple rooting zone. Based on initial soil analysis results, liming may be advantageous in raising pH levels. Foliar analysis results suggest a correlation between low foliar Ca and Mg and sugar maple decline. High foliar Al levels may also be a factor in decline, but the relationship is less clear as both sample groups exhibited the trend. Again, long-term sampling over a broader range of sites may yield more conclusive results.
Research supported by the Botanic Garden of Smith College Advisors: Michael Marcotrigiano, John Berryhill, Gabrielle Immerman
1 Harris, R.W., J.R. Clark and N.P. Matheny. 2004. Diagnosing plant problems. In: Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Upper Saddle River, New Jersey: Prentice Hall. p. 456-500.
2 Dirr, M.A. 1998. Manual of Woody Landscape Plants. 5th ed. Champaign, Illinois: Stipes Publishing. p. 53-57.
3 Horsley S.B., R.P. Long, S.W. Bailey, R.A. Hallett and P.M. Wargo. 2002. Health of eastern north american sugar maple forests and factors affecting decline. Northern Journal of Applied Forestry, 19(1): 34-44.
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