Long-term interval burning alters fine root and mycorrhizal dynamics in a ponderosa pine forest

Summary

The symbiosis between ectomycorrhizal fungi and roots of pine trees (Pinaceae) influences nutrient uptake and soil structure. Understanding how these fungi respond to prescribed fire and thinning will assist in selecting fuel-reduction treatments that maintain soil integrity. Low-intensity fire and mechanical thinning are two techniques applied to reduce the risk and intensity of forest wildfires in the American North-west. In dry pine-dominated forests, fire suppression and logging of the largest trees are factors contributing to increased tree densities and high fuel loads. Prior to European settlement, frequent burning by native Americans and lightning strikes promoted open stands dominated by large fire-resistant ponderosa pine Pinus ponderosa. In this study, the response of ectomycorrhizal fungi species richness, live fine root biomass and duff levels to thinning and burning regimes was investigated in mixed ponderosa pine and Douglas fir Pseudotsuga menziesii stands in Oregon, northwestern USA.

Study area: Research was conducted in the Wallowa Valley Ranger District in the Blue Mountain Range. Stands were a mix of ponderosa pine and Douglas fir. Stands comprise mainly second-growth trees with stems less than 25 cm diameter at breast height (d.b.h.) and occasional large 100–200-year-old tree. Thinning occurred between July and November 1998. Prescribed burning was delayed until September 2000 because of adverse weather at the time of the scheduled burn in autumn 1999.

Experimental design and sampling procedures: A complete randomized design was used with four replications of each restoration treatment (thinned only, burn only, thinned and burned, and a control) assigned to c. 10 ha treatment units (stands). A sampling grid (c. 2 points/ha) was established within each stand. A soil core (5 cm diameter × 10 cm depth) was collected from three randomly selected grid points/stand in June 1998 (before treatment application) and in June 2001. Post-treatment sampling in burned stands occurred within burned patches. The ponderosa pine (d.b.h. ≥ 20 cm) closest to each grid point was marked with an aluminium tag. Soil cores were taken due east of each at the tree crown edge, and separated into the upper 5 cm and lower 5 cm of the core. Duff depth was measured to the nearest 0.5 cm.

Sorting and processing of ectomycorrhizas: Soil cores were stored at 4°C and processed within 3 weeks of collection. A dissecting microscope was used to sort ectomycorrhizas into morphological types (morphotypes). Viability assessment of the root tips was based on colour and turgidity.  Ectomycorrhizas of each morphotype from a given core sample were placed in individual centrifuge tubes, lyophilized and weighed to the nearest 0.0001 g.

Molecular analyses: DNA extraction, polymerase chain reaction (PCR) amplification and restriction fragment length polymorphism (RFLP) techniques were used for molecular analysis and identification of morphotypyes.

EMF community response: On the pine roots, 178 RFLP species were distinguished. An additional 47 were identified to family or genus and 26 to class. RFLP species identified beyond class level were: Cortinariaceae (15 species), Thelephoraceae (11 species), Atheliaceae (7 species) and Russulaceae (6 species). 138 species (69%) were found in lower soil core samples, 123 (62%) in upper core samples, and 62 (31%) were found in both. All but one recurring species detected in four or more cores (19 species in total) were found in both soil depths. Most species (78%) were detected in only one stand, 11% were detected in three or more stands. The results indicate an EMF community consisting of a large number of species scattered across the site before and after treatment application.

Prior to treatment, there were no significant differences in the number of EMF species among treatments or between upper and lower core samples. After treatment, the control and thinned stands had about 68% more EMF species than the burned and thinned, and burned stands. The number of EMF species did not differ between the upper and lower soil cores except in the thinned and burned treatment (fewer species in the upper 5 cm).

EMF species similarity: 32 species were common to the pre- and post-treatment years (Sorenson index (SI) = 0.29). Most (62%) of recurring species occurred in adjacent cores, typically in the control and thinned stands. Two species, Cenococcum sp. and Wilcoxina rehmii, were each detected in 75% of the stands; Rhizopogon salebrosus and Piloderma sp. were each detected in 50%.

Post-treatment burned (24 spp.) and thinned and burned (23 spp.) treatments contained fewer total species than other time–treatment combinations (31–54 species) and, typically, shared the least number of species (3 to 9, SI = 0.09–0.28) with other time–treatment combinations. Relatively high similarity was seen among most pre-treatment comparisons (8 to 13 species, SI = 0.15–0.27) and between the post-treatment control and thin comparison (10 species, SI = 0.28). Only one species, W.rehmii, was detected in all before and after treatment applications; three, Cenococcum sp., Piloderma and R. salebrosus were detected in at least three treatments before and after application.

EMF root biomass: Fourteen species, each with 2% or more of the total EMF biomass, accounted for 54% of the total biomass. An additional 12 species each contributed around 1% to the total biomass. The distribution within treatments of species indicated that the EMF community consisted of a few dominant species and a relatively large number of infrequent ones. Five of the biomass dominant species (Inocybe sp., Piloderma sp., R.salebrosus, Russula sp. and W.rehmii) were also among the most frequent species, all detected before and after treatment application.

Prior to treatment there were no significant differences in live root biomass among treatments or between upper and lower core samples. Post-application, the control and thinned stands had more live root biomass than the other stands; live root biomass did not differ between the upper and lower soil cores.

Duff depth: Prior to treatment duff depth was similar throughout. After treatment application, it was lower in the thinned and burned stands, and in the burned stands compared to controls. Duff depth was similar between the control and thinned stands.

Conclusions: The results indicate that prescribed burns cause a short-term reduction in EMF species richness and live root biomass. EMF mortality and complete duff reduction after fire have been implicated with poor tree survival and slow forest recovery. This should all be considered prior to introducing prescribed burns.