After a more than a century of fighting to keep fire out of forests, reintroducing it is now an important management goal. Yet changes over the past century have left prescribed burning with a big job to do. Development, wildfire suppression, rising global temperatures, extended droughts, exotic species invasions, and longer fire seasons add complexity to using this practice.
Managers must consider how often, how intensely, and what time of year to burn; for insights they often look to how and when fires burned historically. However, attempting to mimic historical wildfires that burned in hot, dry conditions is risky. Burning in fall or spring when temperature and humidity are low reduces the risk of prescribed fires becoming uncontrollable, but does it have the intended effects? How do forest ecosystems that historically were adapted to fire respond when fire is reintroduced after so much time without it?
Forest Service researchers Becky Kerns and Michelle Day conducted a long-term experiment in the Malheur National Forest, Oregon, to assess how season and time between prescribed burns affect understory plant communities in ponderosa pine forests. They found that some native plants persisted and recovered from fire but didn’t respond vigorously, while invasive species tended to spread. These findings may help forest managers design more effective prescribed-fire treatments and avoid unintended consequences.
Soil microbial communities occupy the most biologically diverse habitats in the world. A single gram of soil can support more than several thousand fungal taxa near the root rhizosphere (Buée et al., 2009). As mentioned in other chapters in this book, many factors can influence the microbial communities associated with tree leaves, stems, and roots. Differences in host species (Prescott and Grayston, 2013), cultivar type within a species, soil type, physiological status of host, and pathogen presence can influence variation in microbial communities (Costa et al., 2007; Aira et al., 2010; Chaparro et al., 2013; Yuan et al., 2015). Ecological balance within the associated microbial community is critical for plant health, especially in the rhizosphere, and disturbances can cause imbalances within the microbial communities. Previous studies have documented that beneficial microbial relationships can enhance seedling vigor, seed germination, plant development, and plant growth that lead to higher plant productivity, whereas attacks by plant pathogens can alter the microbiome structure, functionality, and activity (Trivedi et al., 2012). Beneficial microbial interactions can lead to improved host resistance against pathogenic bacteria and fungi. For example, beneficial microbial taxa can secrete various allelopathic chemicals and toxins that provide the plant with protective barriers that impede plant pathogens. The rhizosphere has been shown to contain diverse and complex biological communities that encompass bacteria, fungi, oomycetes, and many other microorganisms, such as archaea, nematodes, and viruses (Raaijmakers et al., 2009). Other tree organs, including leaves, branches, and stems, are also known to contain a diverse suite of microbial taxa, but overall diversities are typically lower than those found in soils (Baldrian, 2017). Although microbial diversity can vary greatly, pathogens can greatly affect microbial communities. This chapter will briefly review the concept of pathobiome, how microbial communities protect against plant disease, and various changes that can occur within microbial communities in the presence of plant pathogens. Because these research topics are recently developing in forest sciences, examples will be derived from cropping systems as diverse as wheat, apples, and forests. As expected, microbial communities can be vastly different within annual vs. perennial cropping systems; however, the influence of plant pathogens on microbial communities and their ecological roles have been documented primarily in diverse cropping systems.
Desarmillaria caespitosa, a North American vicariant species of European D. tabescens, is redescribed in detail based on recent collections from the USA and Mexico. This species is characterized by morphological features and multilocus phylogenetic analyses using portions of nuc rDNA 28S (28S), translation elongation factor 1-alpha (tef1), the second largest subunit of RNA polymerase II (rpb2), actin (act), and glyceraldehyde-3-phosphate dehydrogenase (gpd). A neotype of D. caespitosa is designated here. Morphological and genetic differences between D. caespitosa and D. tabescens were identified. Morphologically, D. caespitosa differs from D. tabescens by having wider basidiospores, narrower cheilocystidia, which are often irregular or mixed (regular, irregular, or coralloid), and narrower caulocystidia. Phylogenetic analyses of five independent gene regions show that D. caespitosa and D. tabescens are separated by nodes with strong support. The new combination, D. caespitosa, is proposed.
Armillaria root and butt diseases, which are a global issue, can be influenced by changing environmental conditions. Armillaria gallica is a well-known pathogen of diverse trees worldwide (Brazee and Wick 2009). Besides A. gallica causing root rot of Hemerocallis sp. and Cornus sp. in South Carolina (Schnabel et al. 2005), little is reported on the distribution and host range of A. gallica in the southeastern United States. In July 2017, three Armillaria isolates were obtained from two naturally occurring hosts in Georgia, U.S.A., and cultured on malt extract medium (3% malt extract, 3% dextrose, 1% peptone, and 1.5% agar). One isolate (GA3) was obtained in Unicoi State Park near Helen, Georgia (latitude 34.712275, longitude -83.727765, elevation 498 m) from the basal portion of Rhododendron sp. with extensive root/butt decay, but no crown symptoms were evident. GA4 and GA5 (latitude 33.902433, longitude -83.382453, elevation 215 m) were isolated from wind-felled Quercus rubra (red oak) with root disease at the State Botanical Gardens in Athens, Georgia. GA4 was associated with a large root ball (∼4-m diameter), and GA5 was obtained from a mature tree with infected roots, with characteristic spongy rot of Armillaria root disease. Crown symptoms could not be evaluated because the crowns had been removed before the collections. Several other oaks with Armillaria root disease were noted throughout the State Botanical Gardens. Pairing tests reduced these three isolates (whiteish mycelia with a dark, brownish crust and rhizomorphs) to two genets with GA4 = GA5. Both genets (GA3 and GA4) were identified as A. gallica using translation elongation factor 1α (tef1) sequences (GenBank nos. MT761697 and MT761698, respectively) that showed ≥97% identity (≥98% coverage) with A. gallica sequences (KF156772, KF156775). Also, nine replications of somatic pairing tests showed 33 to 67% compatibility with A. gallica (occurs in southeastern United States), compared with 0 to 22% for A. mexicana, A. mellea (occurs in southeastern United States), A. solidipes, and Desarmillaria tabescens (occurs in southeastern United States). To our knowledge, this note represents the first report of A. gallica on Rhododendron and Q. rubra in Georgia, U.S.A., which has experienced severe drought in recent decades (e.g., Williams et al. 2017) that could predispose trees to Armillaria infection (e.g., Wargo 1996). Q. rubra was previously reported as a host of A. gallica in Arkansas (Kelley et al. 2009) and Massachusetts (Brazee and Wick 2009), U.S.A. In Missouri, U.S.A., A. gallica has been reported as a weak pathogen with potential biological control against A. mellea (Bruhn et al. 2000). Other reports from several regions on various hosts suggest pathogenicity of A. gallica is associated with changing climate (Kim et al. 2017; Kubiak et al. 2017; Nelson et al. 2013). Wide genetic variation and/or cryptic speciation within A. gallica may account for differences in ecological behavior (Klopfenstein et al. 2017), but this is difficult to evaluate because Armillaria pathogenicity tests cannot be used on most forest tree seedlings. This study suggests that A. gallica is more widespread than previously known, and its adverse impacts on woody plants may intensify over time, depending on the environmental conditions. Further studies are needed to determine environmental influences on A. gallica, the full distribution of A. gallica, and its effects in forests of the southeastern United States.
Brown root rot (caused by Phellinus noxius) and myrtle rust (caused by Austropuccinia psidii) are natural disturbances in their native tropical and subtropical forest ecosystems. A tree infected with either fungal pathogen becomes unhealthy and likely dies, sometimes within 3 months. These pathogens are threatening forest ecosystems around the world as they spread through international trade or other means, such as by wind or through the soil. Climate change also is creating environmental conditions that will allow these pathogens to survive in novel forest ecosystems where they haven’t been found historically.
An international team headed by researchers with the USDA Forest Service and Colorado State University analyzed the genetics of the two pathogens and mapped their likely spread based on the current locations of the various subgroups of each pathogen and contemporary and projected future climates. They found that distinct genetic subgroups of each pathogen occupied different ecological niches and caused varying damage to host trees.
The genetic diversity of these pathogens creates a potent threat, and this information is critical for agencies that regulate trade. The Hawaii Department of Agriculture, for example, is working with the USDA Animal Plant Health Inspection Service to prohibit the importation of plants in the myrtle family from locations where myrtle rust pathogens of a specific genetic subgroup are known to occur.
Insects are essential components of forest ecosystems, representing most of the biological diversity and affecting virtually all ecological processes (Schowalter 1994). Most species are beneficial (Coulson and Witter 1984, Haack and Byler 1993), yet others periodically become so abundant that they threaten ecological, economic, social or aesthetic values at local to regional scales (tables 6.1 through 6.3). Insects influence forest ecosystem structure and function in complex and dynamic ways, for example, by regulating certain aspects of primary production; nutrient cycling; ecological succession; and the size, distribution and abundance of plants and other animals (Mattson 1977, Mattson and Addy 1975). Effects on forest vegetation range from being undetectable, to short-term reductions in crown cover, to modest increases in background levels of tree mortality, to extensive amounts of tree mortality observed at regional scales.
This report assesses recent forest disturbance in the Western United States and discusses implications for sustainability. Individual chapters focus on fire, drought, insects, disease, invasive plants, and socioeconomic impacts. Disturbance data came from a variety of sources, including the Forest Inventory and Analysis program, Forest Health Protection, and the National Interagency Fire Center. Disturbance trends with the potential to affect forest sustainability include alterations in fire regimes, periods of drought in some parts of the region, and increases in invasive plants, insects, and disease. Climate affects most disturbance processes, particularly drought, fire, and biotic disturbances, and climate change is expected to continue to affect disturbance processes in various ways and degrees.
The Douglas-fir tussock moth (DFTM) is a forest defoliating insect that is subject to periodic population outbreaks. These outbreaks are sometimes spatially synchronized across hundreds of kilometers. The DFTM’s complex population dynamics are thought to result primarily from two regimes of population control: at outbreak-level population densities, DFTM populations are subject to control by viral infection, whereas endemic-level insect densities are maintained by generalist predators. Generalist predation delays the onset of insect outbreaks. In this paper, we show that variation in generalist predation can be modeled as a result of variability between locations and that predation is reduced when the accessibility of larvae to flying predators is restricted. However, protecting larvae from flying predators did not increase larval survival by the same amount from site to site. Thus, local effects of predation show considerable spatial variation, even within a 10-km area. The effects of this variability on the spatial synchrony of population outbreaks remain unclear.
In July-August 2019, seven Armillaria isolates (derived from rhizomorphs and mycelial fans of infected roots) were collected in association with woody hosts in the central Mexico: states of Guanajuato (MEX204), Jalisco (MEX206, MEX208, MEX209), and Michoac´an (MEX211, MEX214, MEX216). All seven isolates were identified as Armillaria gallica based on translation elongation factor 1a (tef1) gene sequences (GenBank accession nos. MN839636 to MN839642 for MEX204, MEX206, MEX208, MEX209, MEX211, MEX214, and MEX216) and somatic pairing tests against known tester isolates.