Nitrogen Fixing Lichen Roles from Desert to Alpine in the Sangre de Cristo Mountains, New Mexico

Lichens containing blue-green algae were measured by transects and reconnaissance in six major temperate vegetation types to estimate possible significance in annual nitrogen budgets and patch dynamics within ecosystems, and to estimate how a functional ecosystem attribute, i.e. nitrogen fixation, correlates with vegetation structure. Lichen cover was negligible in sagebrush desert, pinyon-juniper woodland, ponderosa pine forest and alpine tundra, but averaged 72 m2ha-1 in Douglas fir forest and 33 m2ha-1 in sprucefir forest. North slopes of Douglas fir forest had the highest average cover, 130 m2ha-'. All macrolichens were terrestrial species of Peltigera with P. canina the overwhelming dominant. Based on published laboratory nitrogen fixation rates, lichens probably provide a significant input to the total annual nitrogen budget in the Douglas fir north slope forests and possibly in the spruce-fir forests. A feedback loop relating the role of nitrogen fixing lichens to patchiness within ecosystems is hypothesized. Nitrogen fixing lichens correlate poorly with vegetation structure. Excluding the cold alpine tundra, moisture is considered the primary determinant of nitrogen fixing lichen abundance. Nitrogen fixation by lichens containing blue-green algae is believed widespread in many terrestrial ecosystems (Rogers et al., 1966; Fogg & Stewart, 1968; Denison, 1973; Henriksson & Simu, 1971; Hitch & Stewart, 1973; Alexander & Schell, 1973; Crittenden, 1975; Forman, 1975). The importance of the fixed nitrogen and its roles in the ecosystem are at present largely uncertain due to the paucity of quantitative studies on the distribution of nitrogen-fixing lichens. Nitrogen from lichens may play a significant role in the annual nitrogen budget of an ecosystem by affecting primary productivity and nutrient recycling rates, compartment sizes and outputs. Denison (1973) and Pike et al. (1972) measured the biomass of epiphytic lichens in a tall, wet Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] forest in Oregon and, using two methods, estimated a nitrogen input of 1.8 or 10 kg N ha-1 yr-1. 1 We greatly appreciate Eustaquio Castro-Mendoza for transportation and sampling aid in the field, and James R. Gosz for comments on the manuscript. We also are grateful to William F. Mahler and Ronald K. Wetherington for countless courtesies and stimulation while the research was in progress at the Ft. Burgwin Research Center, Southern Methodist University, Taos, New Mexico 87557. 2 Department of Botany, Rutgers University, New Brunswick, New Jersey 08903. 3 Department of Biology, Southern Methodist University, Dallas, Texas 75275. This content downloaded from 157.55.39.102 on Mon, 03 Oct 2016 05:17:12 UTC All use subject to http://about.jstor.org/terms 562 THE BRYOLOGIST [Volume 80 Forman (1975) measured lichen cover and biomass in a tropical montane rainforest in Colombia and using published laboratory nitrogen fixation rates crudely estimated an input of 1.5-8 kg N ha-1 yr-1. Becker et al. (1977) found nitrogen fixing lichen biomass in a 2.7 ha North Carolina oak forest to average 0.02 kg ha-1, with a maximum 0.18 kg ha-1 for a moist section, and concluded that lichens cannot be significant contributors of nitrogen to the forest as a whole. Other studies have measured or estimated nitrogen fixation of lichen patches or microsites within an ecosystem. For example, some samples within an ecosystem were reported to have high nitrogen fixation rates in a moist mild tundra in Iceland (Crittenden, 1975), a tundra at Point Barrow, Alaska (Alexander & Schell, 1973), Antarctic tundras (Fogg & Stewart, 1968; Horne, 1971), several temperate sites in Great Britain (Hitch & Stewart, 1973), a New Mexican desert (Shields et al., 1957), an Arizona desert (Cameron & Fuller, 1960) and an Australian arid zone (Rogers et al., 1966). A second role of nitrogen produced by lichens, unlike atmospheric nitrogen input, may be in producing environmental heterogeneity within the ecosystem and the consequent patchiness in distribution of organisms. In the American tropics the abundance of nitrogen fixing lichens appears to drop off sharply with decreasing moisture (Forman, 1975). Hitch and Stewart (1973) documented experimentally the overriding importance of moisture deficits in limiting nitrogen fixation by lichens. To better understand which ecosystems may depend on biological nitrogen input, the abundance of lichens in a broad spectrum of ecosystems needs study, especially with respect to moisture gradients. In the broader context of relating ecosystem structure and function, such a study might permit comparison of this functional attribute (nitrogen fixation) with a range of different ecosystem or vegetation structure. Gosz (1975) studied the nutrient and hydrologic dynamics in a spectrum of ecosystem types in the Sangre de Cristo Mountains of New Mexico and found atmospheric nitrogen input varied with elevation as a result of large quantities of precipitation at high elevations and of dust at low elevations. Nitrogen output in stream water varied elevationally as a result of a complex of six factors. Nutrient loss was generally minimal in the intermediate elevation, highly productive Douglas fir forest. The objective of this study was 1) to determine the amount of nitrogen fixing lichens in major vegetation types of the Sangre de Cristo Mountains of New Mexico and 2) to estimate whether or not lichens may be significant in the nitrogen budget or patchiness of terrestrial ecosystems of southwestern North America, and whether or not this functional ecosystem attribute, nitrogen fixation, is related to vegetation structure or climate.