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What Limits Productivity in Northern Hardwoods?  Surprising Findings from the First Long-Term N x P Addition Experiment in a Temperate Forest

The productivity of temperate forests on glaciated soils has been assumed to be nitrogen limited, although theory suggests that ecosystem productivity should be co-limited by multiple nutrients, including phosphorus. Experimental tests of N vs. P limitation in temperate forests are needed to better understand the capacity of ecosystems to balance the acquisition of limiting resources. In 2011, in the White Mountains of New Hampshire, USA, we began a full-factorial N x P fertilization experiment in 13 stands of young, mid-aged, and mature northern hardwoods in three sites that span a range of native soil fertility. Contrary to past assumptions, but credible after decades of anthropogenic N deposition, after 4 years we found a greater response of aboveground growth to P addition than to N addition. Consistent with aboveground P limitation, retranslocation of foliar P was proportionately higher than retranslocation of N, and P addition decreased foliar N, suggesting an alleviation of growth limitation by P. Surprisingly, this aboveground response was not associated with reduced allocation to root production under P addition; instead root growth increased in response to N addition, at the plot scale. Soil respiration, similarly, which should reflect effort allocated belowground for nutrient acquisition, was higher pretreatment where N availability was low and was also most reduced by N addition where N availability was low. There was no effect detected of P addition on soil respiration. In summary, both diameter growth and foliar responses suggested P limitation, while both root growth and soil respiration suggested N limitation. The combination of N limitation belowground and P limitation aboveground may contribute to the maintenance of co-limitation of forest production in these northern hardwood ecosystems.