Figure 1. Sugar Maple – American Beech stand at the Hubbard Brook Experimental Forest.	Figure 2. We had five paired plots (sugar maple and American beech) within Watershed 1 and west of Watershed 6 during May and July 2006.

Contact Info:
Pamela Templer
Department of Biology
Boston University
5 Cummington Street
Boston, MA 02215
phone: (617) 353-6978
email: ptempler@bu.edu

Linda Pardo
US Forest Service
705 Spear St
S. Burlington, VT 05403
phone: (802) 951-6771 x1330
email: lpardo@fs.fed.us

Date Prepared: November 2006

Human activity has led to significantly elevated amounts of nitrogen (N) being deposited onto temperate forests of the eastern U.S. (Galloway et al. 1995). Acid rain, in the form of N and sulfur deposition, has led to significant base cation depletion in forests throughout the northeastern U.S. (Driscoll et al. 2001). Widespread, but patchy, sugar maple mortality has been associated with deficiencies in base cation nutrition such as calcium (Mader and Thompson 1969), which predisposes trees to secondary stresses such as winter freezing and insect defoliation (Horsley et al. 2000).

Figure 3. Fine roots of each tree species from the forest floor were identified and removed from the soil, while remaining intact and connected to the tree.

Previous research shows that under current levels of N deposition, the forest floor of sugar maple stands retains significantly less N compared to stands of other dominant tree species (Templer et al. 2005). One possible consequence of calcium depletion could be reduced N uptake by sugar maple and other dominant tree species, a development that could lead to reduced vigor of trees, a reduction in the size of the plant sink for incoming N, further losses of N from forests, and greater losses of calcium.

We began our study at the Hubbard Brook Experimental Forest in summer 2006 to examine the relationship between soil cation depletion and plant uptake of nutrients. Using the N depletion technique (BassiriRad et al. 1999), we measured N uptake by intact roots of sugar maple and American beech trees in the calcium fertilized watershed, Watershed 1, as well as reference plots outside of this watershed. In the coming years we plan to expand our study to measure in situ plant uptake of nutrients (e.g., N, cations) across plots treated with varying amounts of calcium added to them and across a gradient of healthy to declining trees. Results from this study will contribute to our understanding of the mechanistic links between soil cation depletion and plant nutrient uptake in temperate forest ecosystems. If N that is not taken up by sugar maple is not taken up by individuals of other tree species, forested stands throughout the eastern U.S. could retain even less N over time as the trees become more stressed due to soil base cation depletion. Understanding the combined effects of increased N inputs and changes in plant species composition on forest N cycling is critical to understanding forest plant-soil interactions and forest nutrient budgets.

Figure 4. Using the nutrient depletion method (BassiriRad et al. 1999), intact fine roots were incubated in solution containing inorganic nitrogen for 90 minutes. Nitrogen uptake was determined by the change in inorganic N within the solution over the incubation period.

References

BassiriRad H, SA Prior, RJ Norby and HH Rogers. 1999. A field method of determining NH4+ and NO3- uptake kinetics in intact roots: Effects of CO2 enrichment on trees and crop species. Plant and Soil 217: 195-204.

Driscoll CT, G Lawrence, A Bulger, T Butler, C Cronan, C Eagar, K Lambert, G Likens, J Stoddard and K Weathers. 2001. Acidic deposition in the northeastern US: Sources, inputs, ecosystem effects, and management strategies. BioScience 51:180-198.

Galloway JN, WH Schlesinger, H Levy II, A Michaels and JL Schnoor. 1995. Nitrogen fixation: atmospheric enhancement-environmental response. Global Biogeochemical Cycles 9:235-252.

Horsley SB, RP Long, SW Bailey, RA Hallett and TJ Hall. 2000. Factors associated with the decline disease of sugar maple on the Allegheny plateau. Canadian Journal of Forest Research 30:1365-1378.

Mader DL and BW Thompson. 1969. Foliar and soil nutrients in relation to sugar maple decline. Soil Science Society of America Proceedings 33:794-800.

Templer PH , G Lovett, K Weathers, S Findlay and T Dawson. 2005. Influence of tree species on forest nitrogen retention in the Catskill Mountains, New York, USA. Ecosystems 8:1-16.