Are maples limited by phosphorus in acidic unglaciated forests?

Background/Question/Methods

Human activity has increased the atmospheric input of nitrogen (N) and acidic compounds to eastern deciduous forests. As N accumulates, other nutrients like phosphorus (P) may have a greater influence on limiting ecosystem processes. Available soil P also might become scarcer due to soil acidification promoting the formation recalcitrant P compounds. We hypothesize that chronic acidic deposition is promoting P limitation in acidic hardwood forests, but it will be more severe on older unglaciated soils in comparison to younger glaciated soils. We tested this hypothesis by manipulating the pH and P availability in oak-maple-beech dominated forests from two Ohio ecoregions, Erie Drift Plain and Western Allegheny Plateau. These regions have similar soil pH and texture, but differ in soil properties due to age and climate. The experiment was established in 2009 where lime was added to increase soil pH from 4.2 to 6.5 (elevated pH); P was added to triple readily available phosphate pools (elevated P); and a cross-treatment of lime and P (elevated pH+P). In this study, our efforts were focused on Acer spp. Tree productivity was determined via annual DBH measurements, and canopy foliar N/P ratio was used as a proxy of P limitation. 

Results/Conclusions

We observed no significant difference in Acer spp. growth, however unglaciated annual Acer spp. basal area did increase by 28% (P = 0.14) in the elevated pH+P treatment when compared to the controls. In the glaciated site, the elevated pH+P treatment increased foliar P by 18% when compared to the controls.  Foliar P in the unglaciated site increased by 56% and 72% for the elevated P and elevated pH+P, respectively, when compared to the controls. Treatments had no effect on foliar N, but glaciated leaves had 18% more N than unglaciated leaves. Control foliar N/P ratio was 15.6 for glaciated site, but 18.0 for the unglaciated site. Results support our hypothesis, and indicated P limitation in the unglaciated forests, and mild P limitation in the glaciated forests. In the glaciated forests, the elevated pH+P treatment decreased foliar N/P by 18% and in the unglaciated forests, the elevated P and pH+P treatments decreased foliar N/P by 35% and 43%, respectively. These results suggest adding P shifted Acer spp. from being P-limited to N limited (i.e. N/P < 12). It remains to be seen whether these results will apply to other tree species or result in greater aboveground growth and/or shifts in NPP allocation.