This story highlights a current research project at the Hubbard Brook Experimental Forest. To read more about research projects at HBEF, visit Current Research page. Check back regularly to learn about new research projects.
Susceptibility of declining sugar maple to foliar damage by insects and diseases


  Contact Info:
  Dora Sugar Cornell University
Department of Natural Resources
Fernow Hall
Ithaca, New York 14853-3001
phone: (607) 255-2606
  Healthy sugar maple Declining sugar maple
  Figure 1. A healthy (left) and declining (right) sugar maple showing the symptom of crown dieback.

SUGAR MAPLE is in significant decline throughout the northeastern United States and southeastern Canada on soils that are easily leached of essential nutrients by acid precipitation (Fig. 1). Recent studies attribute symptoms of decline, including decreased growth, crown dieback, and eventual tree death, to the combination of nutrient imbalance and a secondary stressor like insect defoliation (Horsley et al. 2000) . Nutrient imbalance results when acid rain leaches cations from soils and foliage, causing increased soil acidity and a decrease in cationic nutrients like calcium. The lost cations may be replenished in areas underlain by cation-rich bedrock, such as limestone, otherwise, trees like sugar maple cannot get the nutrients they require for healthy growth.

  Damaged leaf Image analysis of damaged leaf  
  Figure 2. WINRhizo image analysis software was used to quantify the area of disease and insect damage on each leaf.  

The following hypotheses predict different effects of nutrient and decline status on tree susceptibility to damage by diseases and insects. 1) Nutrient imbalance can weaken trees, decreasing their ability to recover from natural events like insect defoliation or an ice storm, finally making them so weak that they cannot protect themselves against attacks by otherwise manageable diseases or insects (Manion 1991). 2) Healthier trees grow faster and may actually allocate more of their energy to growth than defense compared to slow-growing trees (Growth Rate Hypothesis). 3) Insects and diseases may prefer a diet of nutrient-rich trees. The first hypothesis would result in more disease and insect damage on nutrient imbalanced and declining trees, while the latter two predict more damage in non-declining, “healthy” stands. 4) Finally, the null hypothesis predicts no relationship between nutrient status or decline and damage by diseases and insects.

  Graphs of vigor, dieback and transparency  
  Figure 3. Frequency histograms (percentage of stems surveyed) of crown ratings (vigor, dieback, and transparency) for canopy sugar maple trees on Ca-treated W1 and reference W6, Hubbard Brook Experimental Forest, New Hampshire, USA. From Juice et al. 2006.

In the summer of 2005, I tested the relative susceptibility of nutrient imbalanced trees to disease and insect damage by comparing foliar damage between sugar maple growing on calcium-deficient soil and on adjacent calcium-amended soils within the Hubbard Brook Experimental Forest.

Acid rain has leached out significant quantities of calcium from the soil of the Hubbard Brook Experimental Forest, and sugar maple is declining, especially at high elevations. To better understand the affects of calcium deficiency on the ecosystem, scientists established several calcium fertilization and control plots in 1996 (NuPert) and in 1999, the entire Watershed 1 was fertilized with calcium-silica pellets.

We collected foliage from sugar maple seedlings, saplings, and canopies in the NuPert plots, along a transect within Watershed 1 and along a second transect adjacent to watershed 1 in an area not fertilized by calcium. We also surveyed saplings and canopy trees for severity of decline symptoms. Digital photos were taken of each collected leaf for subsequent image analysis and the presence of different damage morphotypes was recorded. Leaves were dried to prepare them for nutrient analysis.

  Graph of damaged tree vigor
  Figure 4. Trees equally damaged in 1998 are healthier on W1 than W6 in 2005. Individual trees were assigned to damage classes after the 1998 ice storm, and the same individuals were rated for vigor in 2005 (1- healthy to 5- dead). W1 was fertilized with calcium in 1999. W6 is the unfertilized reference watershed.  

At present, we are analyzing the foliage for 1) percent of leaf area damaged by insects or disease (Fig. 2), 2) nutrient concentrations (i.e., calcium), and 3) type of disease. Preliminary results from the decline severity survey show greater sugar maple decline in areas not fertilized with calcium compared to fertilized areas, especially among canopy trees (Fig. 3). This difference occurs even when comparing canopy trees equally damaged by the 1998 ice storm (Fig. 4). However, foliar disease and insect damage data do not show a clear pattern (Fig. 5). Nutrient imbalances and decline symptoms may be functionally independent of foliar susceptibility to disease and insect damage in sugar maple. In future work, I will use regression analysis to test for relationships between foliar nutrient status and disease and insect damage, and I will also compare the presence of different disease types (damage morphotypes) between fertilized and unfertilized areas, because different pathogen species may differ in their sensitivity to foliar nutrient concentrations and tree health.

  Graph of leaf disease area Graph of insect damage area  
Figure 5. Leaves (n = 5) from 60 saplings (dbh 5-15cm) from each treatment area were collected in June, July, and August of 2005 (W1 was fertilized, Control was not). Leaf images were analyzed using WINRhizo to calculate the percent of leaf area (+/- 1 SE) covered by disease and insect damage.  

Literature Cited
Horsley, S. B., R. P. Long, et al. (2000). "Factors associated with the decline disease of sugar maple on the Allegheny plateau." Canadian Journal of Forest Research 30(9): 1365-1378.

Manion, P.D. 1991. Tree disease concepts. Prentice Hall, Engelwood Cliffs, NJ (USA), 402 pp.

Date Prepared: November 2006