02532nas a2200193   4500008004100000245007500041210006900116300001500185490000700200520191400207100002002121700001402141700001602155700001702171700001702188700001502205700001902220856009902239       2008                        eng d00aScale-dependent responses of plant biodiversity to nitrogen enrichment0 aScaledependent responses of plant biodiversity to nitrogen enric  a2165 -21710 v893 aExperimental studies demonstrating that nitrogen (N) enrichment reduces plant diversity within individual plots have led to the conclusion that anthropogenic N enrichment is a threat to global biodiversity. These conclusions overlook the influence of spatial scale, however, as N enrichment may alter β diversity (i.e., how similar plots are in their species composition), which would likely alter the degree to which N-induced changes in diversity within localities translate to changes in diversity at larger scales that are relevant to policy and management. Currently, it is unclear how N enrichment affects biodiversity at scales larger than a small plot. We synthesized data from 18 N-enrichment experiments across North America to examine the effects of N enrichment on plant species diversity at three spatial scales: small (within plots), intermediate (among plots), and large (within and among plots). We found that N enrichment reduced plant diversity within plots by an average of 25% (ranging from a reduction of 61% to an increase of 5%) and frequently enhanced β diversity. The extent to which N enrichment altered β diversity, however, varied substantially among sites (from a 22% increase to an 18% reduction) and was contingent on site productivity. Specifically, N enrichment enhanced β diversity at low-productivity sites but reduced β diversity at high-productivity sites. N-induced changes in β diversity generally reduced the extent of species loss at larger scales to an average of 22% (ranging from a reduction of 54% to an increase of 18%). Our results demonstrate that N enrichment often reduces biodiversity at both local and regional scales, but that a focus on the effects of N enrichment on biodiversity at small spatial scales may often overestimate (and sometimes underestimate) declines in regional biodiversity by failing to recognize the effects of N on β diversity.1 aChalcraft, D.R.1 aCox, S.B.1 aClark, C.M.1 aCleland, E.E1 aSuding, K.N.1 aWeiher, E.1 aPennington, D.  uhttp://129.130.186.12/content/scale-dependent-responses-plant-biodiversity-nitrogen-enrichment