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.
