Abstract
The rarity of symbiotic nitrogen‐fixing trees in higher‐latitude compared to lower‐latitude forests is paradoxical because higher‐latitude soils are relatively N poor. Using national‐scale forest inventories from the United States and Mexico, we show that the latitudinal abundance distribution of N‐fixing trees (more than 10 times less abundant poleward of 35° N) coincides with a latitudinal transition in symbiotic N‐fixation type: rhizobial N‐fixing trees (which are typically facultative, regulating fixation to meet nutritional demand) dominate equatorward of 35° N, whereas actinorhizal N‐fixing trees (typically obligate, maintaining fixation regardless of soil nutrition) dominate to the north. We then use theoretical and statistical models to show that a latitudinal shift in N‐fixation strategy (facultative vs. obligate) near 35° N can explain the observed change in N‐fixing tree abundance, even if N availability is lower at higher latitudes, because facultative fixation leads to much higher landscape‐scale N‐fixing tree abundance than obligate fixation.