María Valdés

Drought effects on fine-root and ectomycorrhizal-root biomass in managed Pinus oaxacana Mirov stands in Oaxaca, Mexico

The effects of a severe drought on fine-root and ectomycorrhizal biomass were investigated in a forest ecosystem dominated by Pinus oaxacana located in Oaxaca, Mexico. Root cores were collected during both the wet and dry seasons of 1998 and 1999 from three sites subjected to different forest management treatments in 1990 and assessed for total fine-root biomass and ectomycorrhizal-root biomass. Additionally, a bioassay experiment with P. oaxacana seedlings was conducted to assess the ectomycorrhizal inoculum potential of the soil for each of the three stands. Results indicated that biomasses of both fine roots and ectomycorrhizal roots were reduced by almost 60% in the drought year compared to the nondrought year. There were no significant differences in ectomycorrhizal and fine-root biomass between the wet and dry seasons. Further, the proportion of total root biomass consisting of ectomycorrhizal roots did not vary between years or seasons. These results suggest that both total fine-root biomass and ectomycorrhizal-root biomass are strongly affected by severe drought in these high-elevation tropical pine forests, and that these responses outweigh seasonal effects. Forest management practices in these tropical pine forests should consider the effects of drought on the capacity of P. oaxacana to maintain sufficient levels of ectomycorrhizae especially when there is a potential for synergistic interactions between multiple disturbances that may lead to more severe stress in the host plant and subsequent reductions in ectomycorrhizal colonization.

Isolation and symbiotic characteristics of Mexican Frankia strains associated with Casuarina

In the absence of available symbiotic nitrogen-fixing Frankia strains associated with Casuarina trees in Mexico for reforestation purposes, isolation was undertaken using root nodules from trees growing in different habitats in Mexico, from the coast of the Gulf of Mexico up to 2550 m above the sea level. A total of 24 strains were isolated and clonal cultures were obtained from one filament of each strain. The use of acetate as the sole carbon source was essential for the isolation of the endosymbiont from the nodules due to the fact that other contaminant actinomycetes utilize propionate. Clonal cultures were obtained, and cultural and symbiotic characteristics of pure cultures were assessed. All strains grew well in stirred DPM (defined propionate medium) with no mineral nitrogen. Isolates showed hyphae, multilocular sporangia and characteristic vesicles. The presence of the gene nifH was also demonstrated, with all strains being able to nodulate Casuarina equisetifolia. Nitrogenase activity (acetylene reduction) of the formed root nodules varied among the different associations depending on the isolate used to inoculate the plants. Several of the isolates can be used as inoculants for the propagation of Casuarina trees.