Legumes on Tejon Ranch: how are they different from other plants and what ecological role do they have?
Ellery Mayence, Tejon Ranch Conservancy Senior Ecologist
Legumes, or plants in the Fabaceae family, are notable in that most have symbiotic (i.e., mutually beneficial) relationships with soil-borne nitrogen-fixing microorganisms or bacteria. The way this works is that bacteria such as Rhizobium or Bradyrhizobium colonize the root systems of a host plant, causing it to form nodules or structures to house the bacteria (Figure 1). Over time the plant provides photosynthetically-sourced carbohydrate to the bacteria (i.e., energy), which in turn, reciprocates this offering by providing the plant with nitrogen in a readily-usable form (keeping in mind that despite being a dominant constituent of the Earth’s atmosphere, nitrogen gas is not directly available for plant use). Rather, plants depend on fixed forms of nitrogen such as ammonia or nitrate (the two most common forms of nitrogen in commercial fertilizer). Being that nitrogen is crucial for even the most basic of plant functions, the benefit of these bacterial associations cannot be overstated, as non-leguminous species depend largely on whatever nutrients just happen to be available in the soil.
Figure 1. Five leguminous plant species commonly found in grassland habitat on Tejon Ranch. Top images show aboveground portions of each plant, while bottom images show each plant’s respective roots and their associated bacterial nodules.
Though most are aware of the use of legume cover crops in agricultural landscapes as a means for boosting soil fertility, legumes are also beneficial constituents in non-agricultural, native plant communities. These benefits are perhaps most pronounced in nutrient impoverished settings such as extremely weathered, leached and/or old soils. The decomposed granitic soils found on both the San Joaquin and Antelope Valley sides of Tejon Ranch would be an example of such soils. So, with respect to the more than 100,000 acres of grassland habitat on the ranch, many of which contain leguminous species (at least 68 species of legumes are known to exist on the ranch), there are several ecological questions that could be asked regarding how legumes in these ecosystems function:
(1) What ecological niche do nitrogen-fixing legumes fill and has their role (and even their need to fix nitrogen) changed as a result of increased atmospheric nitrogen deposition (due to industrialization and intensive agricultural activity), and how does soil type affect their distribution?
(2) How long does nitrogen fixed by leguminous species remain in the soil matrix, and to what extent do hyper-competitive, fast growing nonnative annual grasses that co-occur with legumes in Tejon's grassland habitat benefit from their presence?
The intent of this blog post is not to answer these questions, but rather to bring to light the subtle nuances of different vegetation communities, how ecosystem processes differ across these communities, and how they function at present relative to historically when Central Valley and adjacent foothill habitats were not dominated by nonnative annual grasses (and when atmospheric conditions differed). Ongoing research carried out by the University of California Berkeley Range Ecology Laboratory along with the Tejon Ranch Conservancy is investigating grassland plant communities as influenced by landscape position, soil properties, grazing intensity, and fluctuations in annual precipitation patterns.