Data from: Nitrogen-fixing tree abundance in higher-latitude North America is not constrained by diversity

The rarity of nitrogen (N)-fixing trees in frequently N-limited higher-latitude (here, > 35°) forests is a central biogeochemical paradox. One hypothesis for their rarity is that evolutionary constraints limit N-fixing tree diversity, preventing N-fixing species from filling available niches in higher-latitude forests. Here, we test this hypothesis using data from the USA and Mexico. N-fixing trees comprise only a slightly smaller fraction of taxa at higher vs. lower latitudes (8% vs. 11% of genera), despite 11-fold lower abundance (1.2% vs. 12.7% of basal area). Furthermore, N-fixing trees are abundant but belong to few species on tropical islands, suggesting that low absolute diversity does not limit their abundance. Rhizobial taxa dominate N-fixing tree richness at lower latitudes, whereas actinorhizal species do at higher latitudes. Our results suggest that low diversity does not explain N-fixing trees' rarity in higher-latitude forests. Therefore, N limitation in higher-latitude forests likely results from ecological constraints on N fixation.

长期受氮限制的高纬度（此处指纬度>35°）森林中，固氮树木（nitrogen-fixing trees）的稀缺性是一个核心生物地球化学悖论。针对这类树木稀缺性的一项假说认为，演化约束限制了固氮树木的多样性，使得固氮物种无法占据高纬度森林中的可利用生态位。本研究利用美国与墨西哥的森林调查数据对该假说进行验证。尽管固氮树木的多度仅为低纬度地区的1/11（分别占森林基面积的1.2%与12.7%），但其在分类群中的占比在高纬度与低纬度地区仅存在小幅差异（属水平占比分别为8%与11%）。此外，热带岛屿上的固氮树木多度较高，但仅隶属于少数物种，这表明绝对多样性较低并不会限制其多度。低纬度地区的固氮树木丰富度以根瘤菌类群为主，而高纬度地区则以放线菌根瘤型物种为主。本研究结果表明，多样性不足并非高纬度森林中固氮树木稀缺的原因。因此，高纬度森林的氮限制很可能源于固氮作用受到的生态约束。