stumps or trees were S; isolates from pine stumps were 80% S and 20% J. 1996. Use of taxon-specific competitive-priming PCR to study host P; isolates from pine, incense cedar, and western juniper trees were 23% specificity, hybridization, and intergroup gene flow in intersterility groups S and 77% l? The recovery of a well-established hybrid "SP" genet in a of Phytopathology 86543-551. pine center was confirmed by isozyme analysis. The PCR amplification of the mitochondrial Mu-ML6 region also was diagnostic for the two 'lXvo intersterility groups (IX&) of the forest pathogen Heterobasidion IS&, but in areas where both fir and pine mortality centers were present, annosum are found in California: S and I? We devised a polymerase chain about 7% of S isolates yielded the P-specific fragment. These results reaction (PCR) method called taxon-specific competitive-priming (TSCP) indicate the possibility of gene flow in nature between the two ISGs. The PCR to differentiate the two IS&. Using TSCP-PCR. we typed 537 live presence of S isolates on trees previously regarded as exclusive P hosts isolates and dry basidiocarps from 204 trees and 114 stumps from 60 broadens the potential host range of this ISG. sites in eight California national forests. All isolates from fir and sequoia The basidiomycete Heterobusidion annosum (Fr.) Bref. is regarded as a complex species containing at least three intersterility groups (ISGs): F, P, and S (3.19). ISGs generally represent biological species among which gene flow is virtually nil (2). In the case of H. annosum, however, intersterility barriers are not complete and display a marked regional variation. In Europe, 4 to 10% interfertility between the S and the p ISGs has been reported (19, 30). and results from in vitro tests with North American isolates indicate that up to'18% between-ISG interfertility is possible (5,13). High levels of interfertility between two ISGs were recorded when pairing small numbers of isolates of the F ISG from Italy and S isolates from northern Europe (37% interfertility) and North America (100% interfertility) (3,30). This high interfertility is consistent with the hypothesis that in allopatric speciation abrupt intersterility barriers between conspecific populations are not present (1). Intersterility barriers are often associated with the development of host specificity or preference (2). In the H. annosum complex, the P ISG is mostly associated with mortality of trees in the genera Pinus, Juniptvus, and Calocednts. The S ISG host range includes the genera Picea, Abies, Pseudotsuga, Tsuga, and Sequoiadendron (20,25). To date, the F ISG has only been recorded on A. albu in some regions of southern and central Europe (3.22). Different host associations also are correlated with different pathologies caused by H. annosum. For instance, in North America and Europe, P isolates kill the roots of living pine trees, and S isolates either cause butt rot of spruce trees and hemlocks or sapwood and heartwood decay of roots and boles of true firs (25,26).

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The 12-site National Fire and Fire Surrogate study (FFS) was a multivariate experiment that evaluated ecological consequences of alternative fuel-reduction treatments in seasonally dry forests of the US. Each site was a replicated experiment with a common design that compared an un-manipulated control, prescribed fire, mechanical and mechanical + fire treatments. Variables within the vegetation, fuelbed, forest floor and soil, bark beetles, tree diseases and wildlife were measured in 10-ha stands, and ecological response was compared among treatments at the site level, and across sites, to better understand the influence of differential site conditions. For most sites, treated stands were predicted to be more resilient to wildfire if it occurred shortly after treatment, but for most ecological variables, short-term response to treatments was subtle and transient. Strong site-specificity was observed in the response of most ecosystem variables, suggesting that practitioners employ adaptive management at the local scale. Because ecosystem components were tightly linked, adaptive management would need to include monitoring of a carefully chosen set of key variables. Mechanical treatments did not serve as surrogates for fire for most variables, suggesting that fire be maintained whenever possible. Restoration to pre-settlement conditions will require repeated treatments over time, with eastern forests requiring more frequent applications.

Isolates of Heterobasidion annosum (Fr.) Bref. representing North American S and P and European S, P, and F intersterility groups were subjected to isozyme analysis. European S, P, and F groups had more variability than the North American S and P groups in expected hterozygosity, number of alleles per locus, and percent polymorphic loci. In contrast with the North American S and P groups, the European intersterility groups could not be distinguished from each other on the basis of individual isozyme loci, although significant differences in allele frequencies exist between European S and P groups. This suggests that evolution proceeded at different rates in the intersterility groups, or intersterility barriers appeared later in the European populations relative to the North American populations of H. annosum. Changes in climate and host species associations during the Tertiary may have been a major factor in evolution of H. annosum intersterility groups. Key words: allozymes, forest tree hosts, playnological events, evolutionary relationships, Hymenomycetes, root disease.