Long-term effects of burning slash on plant communities and arbuscular mycorrhizae in a semi-arid woodland

Published source details Kristin E. Haskins & Catherine A. Gehring (2004) Long-term effects of burning slash on plant communities and arbuscular mycorrhizae in a semi-arid woodland. Journal of Applied Ecology, 41.
Share Tweet Email

Summary

Burning of woody debris wood is a common management technique to reduce fuel loads in order to reduce wildfire risk and fire intensity, and increase establishment of understorey vegetation in many semi-arid woodlands in the USA. However, the consequences of such burning on plant communities and below-ground mycorrhizal fungal are poorly understood. This study examined the responses of understorey plants and arbuscular mycorrhizal (AM) fungi to slash burning following fuel wood harvest in a pinyon–juniper woodland in northern Arizona, 5-years after harvesting.

Study site: This study was conducted in pinyon–juniper woodland in the Coconino National Forest (1585 m a.s.l.), replicated at two sites 2 km apart, approximately 80 km south of Flagstaff, Arizona, USA. Dominant vegetation consists of pinyon pine Pinus edulis and Utah juniper Juniperus osteosperma. Both sites are similar in plant community composition, soil characteristics and grazing intensity (grazed/browsed by native ungulates).

In 1995, the sites were opened to fuel wood harvest. Remaining slash was piled up around tree stumps and burned by USA Forest Service personnel. No heavy equipment was used so effects of soil compaction were minimal. The data presented in this study were collected in 2000–01, approximately 5 years after the slash had been burnt.

Understorey plant communities and arbuscular mycorrhizal inoculum potential: The overall understorey plant communities of treated sites next to non-burned sites (i.e. controls) was compared. Secondly, the plant community of burned areas and interspace areas in treated sites with canopy areas and interspace areas in controls was compared. The control sites were located adjacent to the treated sites. In both study areas, four 50 m transects were established in burned and control sites (total of 16 transects). Each transect contained interspace areas and either slash areas (burned) or canopy areas (control). Using the line intersect method, plants were identified along the transect every 0.5 m (100 data points/transect). Species richness, evenness and diversity (Simpson's diversity index) from these data points was calculated.

Plant communities and arbuscular mycorrhizal (AM) inoculum potential (through bioassay using intact soil cores collected in the field) was analysed along 16 transects in:

1) burned slash pile sites

2) interspaces between burned slash piles

3) non-burned canopy sites

4) non-burned interspaces

AM colonization of dominant native and exotic plants in burned and non-burned areas was quantified.

Understory plants: A total of 50 plant species were recorded, but only 44% were common to both burned and control sites. Burned sites had significantly more plant cover (61-67%) than controls (28-47%), but had significantly fewer understorey plant species than non-burned areas.

Exotic species were four times more abundant at burned slash sites than at other sites. There were no significant differences in species richness, evenness or Simpson's diversity with respect to either burning or canopy presence, and there was no significant interaction

There were very few pinyon or juniper seedlings present in any areas. The highest mean percentage of seedlings was 2% and was found in the control canopy treatment. There was no burn, canopy or interaction effect.

Arbuscular mycorrhizal fungi Exotic plants from burned and non-burned areas exhibited levels of AM colonization 50% greater than native plants. Bioassay and field-collected plants exhibited similar levels of AM colonization and there were no significant treatment differences.

The total biomass of bioassay plants grown in soil from burned slash sites averaged 19% greater than plant biomass from other sites, and had 27% more phosphorus in their shoot tissue; however, tissue nitrogen contents were similar.

Conclusions: These results suggest that either fire did not reduce AM inoculum potential, or that AM fungal populations had recovered during the interim 5 years after the slash was burned. Changes in understorey plant communities and increases in exotic species as a consequence of burning could result from soil changes (e.g. species of AM fungi present), reduction of native species in the seed bank or greater dispersal ability of exotic plants compared with native plant species. The authors suggest that burning slash piles in pinyon–juniper woodlands can result in plant communities that are persistently dominated by exotic species. Management approaches that utilize fuel wood harvest without burning remaining woody debris, or incorporation of seeding of native plants may achieve the desired results in terms of conservation objectives.

Note: If using or referring to this published study, please read and quote the original paper. The original paper can be viewed at: http://blackwellpublishing.com/submit.asp?ref=0021-8901