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Establishment of native grasses is a primary objective of restoration in Pinus ponderosa var. scopulorum (P. & C. Lawson) forests in the southwestern United States. Interactions among native grasses and soil microorganisms generate feedbacks that influence the achievement of this objective. We examined soil chemical properties and communities of plants and soil microorganisms in clear-cuts and P. ponderosa stands thinned and maintained at low and medium tree densities for over 40 years along with high density (unthinned) stands. Phospholipid fatty acids (PLFA) in soils were analyzed to examine arbuscular mycorrhizal (AM) fungi and microbial communities in the three thinning treatments and the unthinned stands with and without a recent broadcast burn. Additionally, two native bunchgrasses, Festuca arizonica and Muhlenbergia wrightii were grown in containers filled with intact soil cores collected from each field plot to more thoroughly compare the abundance of AM fungi and microbial communities across different stand densities and burn treatments. Tree thinning decreased litter cover and increased the abundance and diversity and altered community composition of both herbaceous vegetation and AM fungi. In the mineral soil layer, the pH, total carbon, nitrogen, phosphorus and PLFA profiles did not differ significantly among the four stand density or burn treatments. Mycorrhizal colonization of the container grown grasses did not significantly differ with tree density or burn treatments; however, F. arizonica roots had a strong trend for decreased colonization when grown in soil from high density (unthinned) tree cover. Soil from the containers with F. arizonica had a greater abundance of AM fungal spores. Furthermore, bacterial community composition varied with grass species. Concentration of biomarkers for bacteria were higher in soil that supported F. arizonica compared to soil in which M. wrightii was grown. Our results indicate that the creation of clear-cut openings in forests may increase the abundance and richness of AM fungal propagules and soil bacterial communities were surprisingly resilient to tree thinning and low-intensity fire treatments. These results suggest managing forests to create clear-cut openings generate conditions that favor understory native grasses and AM fungi that are linked to soil bacterial communities.

Mechanized logging equipment causes considerable soil disturbance, but little information is available regarding thresholds for impacts on soil nematodes—critical members of soil foodwebs which perform important ecological functions. We examined responses of nematode communities and soil physical characteristics to the increasing number of passes (one, three, or nine) by a tracked feller buncher during thinning of a xeric mixed conifer forest in New Mexico, USA. Within and between the harvester tracks, we measured soil surface penetration resistance and shear strength, quantified bulk density at four depth increments up to a maximum depth of 27 cm, and characterized nematode assemblages in the upper 10 cm. Eight months after treatment, nematode communities were less impacted than soil physical properties by harvester passes. Total nematode abundance was unaffected by any level of feller buncher disturbance, and sensitive K-selected nematode groups were reduced only at nine passes. Conversely, soil compaction occurred with a single pass and extended deep into the soil profile to at least 23–27 cm. The first pass also decreased surface penetration resistance and shear strength, indicating disruption of soil surface structural integrity. Additional passes did not further increase bulk density or decrease surface structural integrity. Our results indicate that low levels of logging machinery traffic may have negligible effects on nematode communities, but nevertheless emphasize the importance of minimizing areas subjected to disturbance because of impacts on soil physical properties. [ABSTRACT FROM AUTHOR]

There is evidence that the distribution of ecotypes of plants and their symbiotic arbuscular mycorrhizal (AM) fungi and other associated soil biota may be structured by the availability of essential soil nutrients; and that locally adapted partnerships most successfully acquire limiting nutrients. This study tests the hypotheses that plant genotypes are adapted to the water availability of their local environment, and this adaptation involves associations with local soil biota, including AM fungi.We grew semi‐arid Bouteloua gracilis ecotypes from relatively wet and dry sites, with either sympatric or allopatric soil inoculum under moderate and extreme soil drying treatments to examine (a) how varying degrees of water limitation influence grass responses to soil biota and (b) the relationship between AM fungal structures and the responses.Under extreme soil drying, the dry site ecotype tended to perform better than the wet site ecotype. Both ecotypes performed best in either drying treatment when inoculated with their sympatric soil biota. Sympatric pairings produced more AM fungal hyphae, arbuscules and dark septate fungi. Extreme soil drying tended to accentuate these apparent benefits of sympatry to both plants and fungal symbionts, relative to the moderate drying treatment.Our findings support the hypothesis that AM symbioses help Bouteloua gracilis ecotypes adapt to local water availability. This conclusion is based on the observations that as water became increasingly limited, sympatric partnerships produced more AM fungal hyphae and arbuscules and fewer vesicles. The abundances of hyphae and arbuscules were positively correlated with plant growth, suggesting that in sympatric pairs of plants and AM fungi, allocation to fungal structures is optimized to maximize benefits and minimize the costs of the symbioses. This provides strong evidence that co‐adaptation among plants and their associated AM fungi can ameliorate drought stress.Synthesis. Our study documents the role of locally adapted soil borne plant symbionts in ameliorating water stress. We found a relationship between AM fungal structures in roots and plant performance. Generally, plants and fungi from the same site resulted in more positive effects on plant growth. [ABSTRACT FROM AUTHOR]

Plant diversity is critical to the functioning of ecosystems, potentially mediated in part by interactions with soil biota. Here, we characterised multiple groups of soil biota across a plant diversity gradient in a long‐term experiment. We then subjected soil samples taken along this gradient to drought, freezing and a mechanical disturbance to test how plant diversity affects the responses of soil biota and growth of a focal plant to these disturbances. High plant diversity resulted in soils that were dominated by fungi and associated soil biota, including increased arbuscular mycorrhizal fungi and reduced plant‐feeding nematodes. Disturbance effects on the soil biota were reduced when plant diversity was high, resulting in higher growth of the focal plant in all but the frozen soils. These results highlight the importance of plant diversity for soil communities and their resistance to disturbance, with potential feedback effects on plant productivity. [ABSTRACT FROM AUTHOR]

Presents findings from a study of criteria used for teacher evaluation. Includes five tables listing evaluation criteria with the percentage of systems using that specific criteria in their evaluation forms. (MD)

This report reviews important advances in mycorrhizal research that occurred during the past 2 years. We highlight major advancements both within and across levels of biological organization and describe areas where greater integration has led to unique insights. Particularly active areas of research include exploration of the mechanisms underpinning the development of the mycorrhizal symbiosis, the mycorrhizal microbiome, comparisons among types of mycorrhizas from molecular to ecosystem scales, the extent and function of mycorrhizal networks and enhanced understanding of the role of mycorrhizas in carbon dynamics from local to global scales. The top-tier scientific journals have acknowledged mycorrhizas to be complex adaptive systems that play key roles in the development of communities and ecosystem processes. Understanding the mechanisms driving these large-scale effects requires integration of knowledge across scales of biological organization. [ABSTRACT FROM AUTHOR]