Nurse plants and desert communities

The nurse shrub, Mimosa luisana. The Spanish common name is 'madre de los tetechos' or 'mother of the tetechos'. Tetechos, Neobuxbaumia tetetzo, are a type of columnar cactus. Photo by naturalia.

Deserts can be some of the harshest environments on earth, particularly for young plants. The presence of some plant species (called 'nurse plants'), however, can make it easier for young plants to establish. Mimosa luisana is a legume that fixes nitrogen. Other plants can take up this nitrogen that would not otherwise be available to them. This week's paper is lead by Alicia Montesinos-Navarro at the Universidad Nacional Autónoma de México, and is in Ecology. Montesinos-Navarro and her colleagues investigated which plants took up the nitrogen fixed by M. luisana in the Valley of Zapotitlán, Mexico. M. luisana is a nurse plant because it provides shade and nitrogen enabling the establishment of young plants under it, especially tetecho cacti, Neobuxbaumia tetetzo, earning it the Spanish common name 'madre de los tetechos'. 

Montesinos-Navarro and her colleagues investigated which plants take up the nitrogen fixed by M. luisana because they were interested in a classic question in ecology, which is how is diversity maintained in communities? Why are plant communities not monocultures of only one species but rather diverse assemblages? The traditional explanation has been that closely related species (or individuals of the same species) compete too much to coexist in high numbers. They occupy the same 'niche' in terms of their needs for certain soil, light, or water conditions. Different species in contrast can coexist more peacefully because they have slightly different niches and compete less. Montesinos-Navarro and her colleagues looked at the coexistence question from a different perspective and asked whether rather than competition, facilitation could be a driving force in community interactions. In particular, whether plants might facilitate the establishment of more distantly related species more than closely related species, generating community diversity. 

What they did

Montesinos-Navarro and her colleagues selected 14 plant species that co-occur with and are facilitated by M. luisana. They calculated the phylogenetic distance (distance of relatedness) between M. luisana and each of these species. Additionally they examined the total nitrogen content of the leaves of each of these species in comparison to M. luisana. They then soaked the leaves of the M. luisana plants in a stable isotope of nitrogen that is very rare in nature (15N). The M. luisana plants took up this nitrogen and it was transferred into the soil and into neighboring plants through shared fungal interactions. Finally they measured the amount of 15N in the neighboring plants.

What they found

Plants that were more distantly related from M. luisana were more different in the total nitrogen content of their leaves (not the 15N here, just naturally occurring leaf nitrogen). It makes intuitive sense that nitrogen fixing plants such as M. luisana have lots of nitrogen in their leaves in comparison to their non-nitrogen fixing distant relatives. This creates a source-sink gradient, where more distantly related plants may have more demand for nitrogen than plants more closely related to M. luisana. Correspondingly, these more distantly related plants took up more of the 15N added to the M. luisana than closely related plants did. 

The takeaway

The nitrogen fixed by the nurse plant, M. luisana, is taken up more by distantly related plants than closely related plants. This means that nitrogen fixing could facilitate the coexistence of distantly related plant species in the community. So rather than co-occurring because they don't compete as much with M. luisana, they could co-occur because they benefit more from the presence of M. luisana.