Mycorrhizal response to the conversion of a sand shinnery oak habitat to a mixed-grass prairie
Through the use of the herbicide, tebuthiuron, portions of a sand shinnery oak (Quercus havardii) ecosystem on the south plains of West Texas were converted to mixed-grass prairie. The vesicular-arbuscular (VA) mycorrhizal status of little bluestem (Schizachyrium scoparium) and purple threeawn (Aristida purpurea) were examined over the 1986 growing season to assess the impact of the conversion on VAM development 4 and 8 years after herbicide treatment as compared to an untreated control site. VA fungal spore densities and species occurrence were also determined for each of these sites. A series of three-way analyses of variance were used to evaluate the effect of species, season, and herbicide treatment on seven measures of VAM status. Potential changes in VA mycorrhizal dependence and VA fungal species occurrence among sites for each plant species as well as selected soil parameters were used as an indicator of the long-term stabilization of the system and success of the conversion.
Percent infection, infected root length, and total root length varied significantly between plant species among sites and between species across seasons, with little bluestem generally having greater total and infected root lengths. Seasonal patterns in total root length varied between the two grasses. Little bluestem produced the greatest amount of root during the summer, whereas purple threeawn produced the most root material during the late fall and early spring. Also, purple threeawn formed little or no infected root containing arbuscules during the growing season which suggests that purple threeawn is less dependent on VAM than is little bluestem. Seasonal patterns of percent root moisture varied significantly among sites as well as between species. Changes in root moisture could have a profound affect on VAM development. Nonetheless, even when variation in percent root moisture was controlled via analysis of covariance, the complex interaction among species, site, or season remained for infected and total root lengths.
Seasonal patterns in VAM development, regardless of plant species, were altered following the conversion, particularly in the 1982 site. These differences in patterns of VAM development may reflect changes occurring in nutrient availability or VAM species composition. The disturbance, however, is not long-lasting in that seasonal patterns of VAM infection in the 1978 site were similar to that observed In the control.
VA fungal spore densities were not significantly different among sites suggesting that total spore production was not altered by the conversion. Nonetheless, differences In particular species densities and their occurrence across sites were apparent. Whereas Scutellospora and Glomus species occurred primarily in the control and oldest treated site (1978), Acaulospora species predominated in the youngest herbicide-treated area. Overall, spores of Acaulospora species occurred in the highest numbers in all sites. The observed changes in VAM fungal species occurrence may account for the altered seasonal pattern in VAM development in the 1982 site.
Nitrogen, phosphorus, and soil pH were significantly different across sites. Nitrogen (NO3- NH^) levels rose as time from herbicide application increased; in contrast, phosphorus levels were the highest in the 1978 treated site. The soils of the 1978 site were the most alkaline and those of the 1982 site the most acidic.
The lack of a significant difference in spore densities before and after habitat conversion suggests that VAM dependency has not been altered drastically as a result of herbicide application. Differences were detected in VAM fungal spore occurrence and may have resulted from changes in soil characters among the three sites. These changes In fungal occurrence may account for some of the differences in plant species abundance, thus affecting the subsequent succession. In addition, lower infection levels for P3A and its decline in the 1978 site indicate that the direction of the succession is toward obligately mycotrophic species.