Human activities, via atmospheric acidic deposition, have increased the availability of nitrogen (N) and potentially decreased the availability of phosphorus (P) in Appalachian forests. Because plants acquire nutrients based on stoichiometric proportions, an increase in N would also increase the demand of P. However, the available of P declines with acidity. Therefore, it is possible that soil acidification due to N deposition can be promoting P limitation, especially on the weathered, base-poor unglaciated soils. Based on the nutrient economy hypothesis, we might expect different responses from arbuscular mycorrhizal (AM)-associated trees compared to ectomycorrhizal (ECM)-associated tree species. The purpose for this experiment was to determine if P, or acidity, is mediating the growth of Appalachian mixed-mesophytic forests and to determine if this is widespread or is dependent on mycorrhizal associations. We hypothesized that these forests will show modest P limitation, but P availability should have disproportional positive influence on AM-associated tree growth in comparison to ECM-associated tree growth. This hypothesis was tested by measuring tree growth for 7 years on plots where lime was added to elevate pH from 4.7 to 6.8 to indirectly increase P and/or phosphate fertilizer was added to directly quadruple readily available P.
Results/Conclusions
The forest community as a whole was insensitive (P = 0.41) to the treatments, but there was a significant (P < 0.01) interaction with mycorrhizal association. Growth rates for tree species associated with ECM were suppressed by 26% under Elevated pH and Elevated pH+P, but showed modest (-9%) change with Elevated P when compared to the control. On the contrary, AM-tree growth increased (15% to 9%) under those same treatments, compared to the control. When the trees were further grouped into three size classes, we observed that only the small and medium size trees were responding to the treatments and large canopy trees showed no significant (P = 0.09) response. The medium size trees showed the greatest difference between AM and ECM tree species. For example, elevated pH+P significantly (P < 0.001) increased AM tree growth by 29%, but suppressed ECM tree growth by 38%, when compared to the control. Results support the hypothesis that AM associated trees are better suited under an inorganic nutrient economy caused by the treatments, while elevating pH and P can actually be detrimental to the ECM associated tree growth.