Temporal and spatial patterns of fungal diversity along an elevation gradient in an arid ecosystem with implications to ecosystem functioning



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Fungal degradation of plant residues is the primary mechanism of plant litter decomposition in most systems. Although decomposition is an essenfial ecological process, it is unclear whether decomposition rates are controlled by fungal taxonomic diversity, fungal functional diversity or some combination of both. Understanding such controls may help to predict immediate and lasting effects of climate change and anthropogenic disturbances on ecosystem health and stability. To evaluate the linkage between taxonomic and funcfional diversity of fungi, and litter decomposition, the decomposition of leaves oi Agave lechuguilla was followed over a 2-year period in a Chihuahuan Desert watershed. Lechuguilla leaves were air-dried, weighed, tagged, and placed in 5 vegetation zones of the watershed in June 97. Vegetation zones included a lowland scrub, creosotebush bajada, sotol grassland, oak forest, and oak-pine forest. The 5 zones differed in moisture, soil nitrogen concentration, and mean annual temperature. Every 6 months, 10 tagged leaves per zone were collected until June 99. Decomposition rates were measured as mass loss per unit time. Changes in soluble carbon, holocellulose, and lignin fractions were measured during decomposition. Taxonomic diversity was obtained by plating leaf particles on a general medium (MEA+) and on a xerophilic and xerotolerant medium (DG18). To determine fungal functional diversity, a novel method was developed (FungiLog) that examines the ability of fungi to use a variety of carbon substrates for growth. Mass loss from decomposing lechuguilla leaves after 2 years ranged from 38.5% in the oak forest to 24.5% in all other vegetation zones. Mass loss was correlated positively with litter moisture content along the elevational gradient. The soluble carbon fraction of the litter decreased from 39.1% to less than 1% of the initial mass in all vegetafion zones after 24 months in the field. The soluble carbon fraction represents readily decomposed carbon sources for decomposers as well as readily leached compounds. There was a perceived increase in the holocellulose fracfion over the 24-month period, most likely due to an increase in fungal biomass. The lignin fraction decreased from an inifial 5.8% to a final 0.3%. Lowland desert zones were dominated by slow growing, dark pigmented species of fungi such as Coleophoma sp. The high elevation zones were dominated by fast growing, prolific spore-producers such as Microdochium, Penicillium, and Fusarium spp. Fungal species richness only on DG18 was correlated positively to mass loss. Fungal isolation frequencies were higher when litter moisture content was higher, but isolation frequency was not found to be correlated with mass loss. The lower vegetation zones exhibited higher isolation frequencies than litter from the upper vegetation zones. The oak-forest fungi had the greatest functional diversity. The two low elevation zones had the lowest functional diversity. Mass loss was correlated with substrate richness, diversity, and evenness, as well as with CO2 evolufion. Spatial and temporal patterns in temperature and water availability appear to be the major regulating factors that control decomposition rates in this watershed of the Chihuahuan Desert. Compared to fungal taxonomic diversity, functional diversity was a better predictor of decomposition rates in this landscape for all vegetation types.



Fungi -- Ecology -- Chihuahuan Desert, Biodiversity -- Chihuahuan Desert, Species diversity -- Chihuahuan Desert, Biodegradation -- Chihuahuan Desert, Watersheds -- Chihuahuan Desert