Abstract
Aim
Plants that host root‐symbiotic nitrogen‐fixing bacteria have an important role in driving terrestrial ecosystem processes, but N‐fixing ability is unequally distributed among plant taxa and ecosystems. Here we explore the large‐scale distribution of N‐fixing plant species worldwide.
Location
Global.
Time period
Present.
Major taxa studied
Vascular plants.
Methods
We estimated root‐symbiotic N‐fixing plant species diversity (as Shannon entropy) and relative richness (log‐ratio of N‐fixing to non‐fixing plant species) for c. 7,800 km2 hexagonal grid cells using the NodDB and Global Biodiversity Information Facility (GBIF) databases. Additionally, we explored the distributions of plant species associated with rhizobia, actinobacteria or cyanobacteria (relative to other plant species), and the relative richness of N‐fixing trees (log‐ratio of N‐fixing to non‐fixing trees). We related N‐fixing plant species distribution to environmental (climate, soil) and biogeographical (biome, realm) variables using multiple linear regression.
Results
N‐fixing plant diversity and relative richness showed unimodal relationships with latitude. Diversity of N‐fixing plants was highest in warm and wet climates, but in dry biomes and in Australasia. The relative richness of N‐fixing plants was highest in warm and dry climates, in tropical and temperate grasslands and in Eurasia. Plants associated with cyanobacteria were more widely distributed near the equator, while those associated with rhizobia were more prevalent at the edge of the tropics, and those associated with actinobacteria at higher latitudes (especially in boreal forests). The relative richness of N‐fixing tree species was highest in cold and dry areas and in boreal forests, with contrasting peaks in the Northern and Southern Hemispheres.
Main conclusions
The distribution of N‐fixing plant species exhibits regional hotspots and coldspots related to both environmental conditions and biogeographical history. Global N‐fixing plant distributions are different for the key root‐symbiotic bacterial groups. Information about N‐fixing plant distribution can improve global models of ecosystem functions and contribute to understanding how plants respond to global change.