Dryland legume shrub persists through succession but is excluded by the absence of its dinitrogen-fixing mutualist

Background/Question/Methods

• Understanding the factors influencing the distribution of legumes is critical to reliably model large-scale inputs of symbiotically fixed nitrogen into terrestrial ecosystems. However, the distribution of legumes in drylands, which represent a significant proportion of the earth’s surface, is still poorly understood. • We used a combination of a large-scale field survey and controlled experiment to determine the abiotic and biotic factors influencing the distribution of Calicotome villosa, an abundant and widespread Mediterranean legume shrub.

Results/Conclusions

• We found that C. villosa was abundant in early- and mid-successional sites, and persisted well into late-successional sites. A major abiotic factor affecting C. villosa was soil type, with shrub abundance and seedlings recruitment significantly lower in calcareous soils. Furthermore, we found C. villosa was absent from large regions mainly due to the absence of its dinitrogen-fixing mutualist. Inoculation of seedlings grown in soils from sites where C. villosa was absent, greatly increased performance and survival rates. • We present strong evidence for the exclusion of a perennial legume by a combination of soil type and the absence of a compatible mutualist. This result highlights the need to take into account both the environment and mutualist distribution when attempting to present and future geographical distributions of legumes.