Individual study: Response of arbuscular mycorrhizal propagule densities to prescribed burning and thinning of ponderosa pine Pinus ponderosa in the Fort Valley Experimental Forest and Mt Trumbull Resource Conservation Area, Arizona, USA
Korb J.E., Johnson N.C. & Covington W.W. (2003) Arbuscular mycorrhizal propagule densities respond rapidly to ponderosa pine restoration treatments. Journal of Applied Ecology, 40, 101-110
A major objective of ponderosa pine Pinus ponderosa forest restoration is to increase herbaceous understorey diversity and production levels that emulate reference conditions. Fungal mycorrhizae play an important role in plant nutrition, nutrient cycling and the development of soil structure. The inoculum potential for arbuscular mycorrhizae (AM) and ectomycorrhizal (EM) fungi was investigated in thinned-only, thinned and prescribed burned (both restoration treatments) and unthinned and unburned control stands in northern Arizona ponderosa pine forests. The relationships between mycorrhizal fungal propagule densities and plant community and soil properties were quantified.
Study sites: This research was conducted at the Fort Valley Experimental Forest (and adjacent areas) near Flagstaff, and the Mt Trumbull Resource Conservation Area north-west of Grand Canyon National Park, Arizona, USA.
Experimental design: Two experimental blocks were established in summer 1998 at Fort Valley and four in summer of 1999 at Mt Trumbull. Woodland treatment stands (c.16 ha each), were randomly assigned. Treatments at Fort Valley were: i) no thinning or burning (control); and ii) thinning to a low level of replacement trees based upon pre-settlement (historical) tree densities. Thinning treatments at Fort Valley received prescribed burning treatments following the sampling for this study during the summer of 2001. Treatments at Mt Trumbull included these two treatments with the addition of prescribed burning and thinning.
Twenty systematically located plots were established in each stand within which herbaceous understorey and overstorey tree composition and abundance, fuel loads and tree canopy cover were sampled.
Soil sampling: At Fort Valley, soil samples were randomly taken from 10 of the 20 plots along a 50 m transect within each of the four stands in late May 1999 (6 months after thinning) and in May 2000 (18 months after thinning). At Mt Trumbull soil samples were taken similarly within each of the eight stands in late June 2000 (5 months after thinning and 3 months after burning). Two samples, 20 m along each transect (one for the AM bioassay and one for the EM bioassay) were also taken.
Vegetation sampling: At Fort Valley, pre-treatment data for the herbaceous understorey, overstorey tree composition and abundance, fuel loads and tree canopy cover was recorded in 1998. Post-thinning treatment data were collected in 1999 and 2000. At Mt Trumbull, pre-treatment data was collected in 1999 and post-treatment data in 2000.
Herbaceous and shrub understorey community were recorded along the same 50 m transects where soil samples were collected. Every 30 cm along the transect (a total of 166 points), substrate (plant, litter, soil, wood, rock) was recorded. If a plant, the species was identified and its height recorded. Plant cover was estimated by dividing the number of plant hits by 166 points for each transect. At the same points, tree canopy cover and fire severity were also recorded as: lightly burned - charred litter, duff (humus layers) and wood; moderately burned - litter mostly consumed, duff deeply burned and wood recognizable; severely burned - with litter and duff consumed leaving white ash and soil often reddish.
Overstorey tree data were collected in 400-m² circular plots centred in each plot. Tree species and diameter at breast height (d.b.h., 1.37 m) were recorded for all live and dead trees.
Laboratory analysis: Bait-plant bioassays quantify total mycorrhizal fungi more accurately than direct counts of sporocarps, spores or colonized root lengths. In this experiment maize Zea mays bioassays were used to determine the relative amount of infective propagules of AM fungi and a ponderosa pine bioassay to determine the relative amount of infective propagules of EM fungi. Root tips were classified either as a living or dead EM tip or a living or dead non-mycorrhizal tip. Different morphological types and colours were also recorded.
Soil analysis: Soil samples from each transect were analysed for pH, total N, total P and organic C at the Bilby Research Soil Analysis Laboratory, Flagstaff.
The relative amount of infective propagules of AM fungi was significantly higher in samples collected from both restoration treatments (thinned and burned) than their paired controls (unthinned and unburned stands). The restoration treatments had no significant effect on the relative amount of infective propagules of EM fungi.
During the first 2 years, restoration treatment had no significant effect on the overall cover of herbaceous AM plants, even though plant cover did increase in the restoration treatment areas. AM abundance was highly correlated with grass cover and therefore areas with higher grass cover (restoration treatments) also had higher propagule densities of AM fungi. Contrary to expectations, restoration treatments did not change EM propagule densities, illustrating that even though their host-plant density was significantly reduced, EM fungi are able to maintain viable propagules for at least 2 years following thinning and 1 year following thinning and prescribed burning.
Conclusions: These results indicate that population densities of AM fungi can rapidly increase following thinning and burning treatments in northern Arizona ponderosa pine forests. This has implications for restoring the herbaceous understorey of these forests because most understorey plants depend on AM associations for normal growth.
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