Fixed Nitrogen in Shrub Clump Food Webs

Considerable research effort needs to be directed toward understanding the role that other plants, animals, and microorganisms play in the nitrogen cycle of South Texas rangelands (Figure 2). At the moment, one must extrapolate from meager evidence gathered in similar systems or speculate on the basis of observation and general principles. The reader is directed to the US/IBP volume "Nitrogen in Desert Ecosystems" (West and Skujins, 1978) for the most relevant synthesis and to references given in the brief account that follows.

Mesquite grasslands in South Texas are rich in shrub species and mesquite frequently occurs in shrub clumps having diverse composition (Whittaker et al., 1979; see also Yavitt and Smith, 1983). Soon after mesquite colonizes an open spot, other species begin to grow under its canopy. It is generally assumed that the mesquite provides protection from grazing and browsing for seeds which germinate beneath. In any event, such clumps appear to eventually reach a stage at which the mesquite is somehow disadvantaged, possibly by root competition, by the species it attracted. Mesquite-free clumps do exist (Plate 4), but are frequently found to have dead mesquite stumps at their centers, indicating possible negative influence of other shrub species such as desert hackberry (Celtis pallida) and Condalia (Rhamnaceae) on the longevity of mesquite.

Most shrub species that grow around mesquite have thick canopies and further shade the high NO3 zone most subject to denitrification. In addition to reducing the loss of fixed N2 to the atmosphere, these shrubs act as a reservoir of mesquite-fixed nitrogen, which they inadvertently release to the surrounding grasslands in the form of fecal nitrogen deposited by vertebrate and invertebrate leaf feeders and by leaf fall. My students and I have observed numerous instances of widespread defoliation of just one shrub species at a time. For example, major defoliations of Celtis pallida by the snout butterfly occurred at Chaparral Wildlife Management Area in July 1976 and June 1978 (along drainages) and in August 1978 (region-wide) [Gilbert, in prep]. While Celtis pallida defoliation occurs as drought is broken by rainfall (Neck, 1983), other shrubs, including mesquite, are occasionally defoliated by specialist insects, but with different timing.

Such defoliation may contribute to the nitrogen cycle in another way. Branches of repeatedly defoliated Celtis, Condalia, Porlieria, and others typically die and are colonized by lichens (Plate 5) , which not only add to the litter fall, but also can fix nitrogen if bluegreen algae are involved in the symbiosis. Details are not available for this system. Even if lichens (Rychert et al., 1978) are unimportant, the rain of dead bark branches provides a potential carbon source for termite gut floras that do fix nitrogen (see below).

It is also likely that various types of fungi including mycorrhizal groups are favored by the conditions under these shrub clumps, and in turn, reduce the loss of mineralized nitrogen to leaching or volatilization. In addition, such fungi increase a host plant's resistance to drought (Drew, 1979). We can thus view mesquite-based shrub clumps as large, slow-release, nitrogen tablets periodically releasing stored nitrogen as members of the clump are defoliated. Some of that nitrogen must eventually show up in the grass biomass found between clumps. The question is, how much?