Broad-scale forest die-off associated with drought and heat has now been reported from every forested continent, posing a global-scale challenge to forest management. Climate-driven die-off is frequently compounded with other drivers of tree mortality, such as altered land use, wildfire, and invasive species, making forest management increasingly complex. Facing similar challenges, rangeland managers have widely adopted the approach of developing conceptual models that identify key ecosystem states and major types of transitions between those states, known as "state-and-transition models" (S&T models). Using expert opinion and available research, the development of such conceptual S&T models has proven useful in anticipating ecosystem changes and identifying management actions to undertake or to avoid. In cases where detailed data are available, S&T models can be developed into probabilistic predictions, but even where data are insufficient to predict transition probabilities, conceptual S&T models can provide valuable insights for managing a given ecosystem and for comparing and contrasting different ecosystem dynamics. We assembled a synthesis of 14 forest die-off case studies from around the globe, each with sufficient information to infer impacts on forest dynamics and to inform management options following a forest die-off event. For each, we developed a conceptual S&T model to identify alternative ecosystem states, pathways of ecosystem change, and points where management interventions have been, or may be, successful in arresting or reversing undesirable changes. We found that our diverse set of mortality case studies fit into three broad classes of ecosystem trajectories: (1) single-state transition shifts, (2) ecological cascading responses and feedbacks, and (3) complex dynamics where multiple interactions, mortality drivers, and impacts create a range of possible state transition responses. We integrate monitoring and management goals in a framework aimed to facilitate development of conceptual S&T models for other forest die-off events. Our results highlight that although forest die-off events across the globe encompass many different underlying drivers and pathways of ecosystem change, there are commonalities in opportunities for successful management intervention.

Widespread, high levels of tree mortality, termed forest die-off, associated with drought and rising temperatures, are disrupting forests worldwide. Drought will likely become more frequent with climate change, but even without more frequent drought, higher temperatures can exacerbate tree water stress. The temperature sensitivity of drought-induced mortality of tree species has been evaluated experimentally for only single-step changes in temperature (ambient compared to ambient + increase) rather than as a response surface (multiple levels of temperature increase), which constrains our ability to relate changes in the driver with the biological response. Here we show that time-to-mortality during drought for seedlings of two western United States tree species, Pinus edulis (Engelm.) and Pinus ponderosa (Douglas ex C. Lawson), declined in continuous proportion with increasing temperature spanning a 7.7 degrees C increase. Although P. edulis outlived P. ponderosa at all temperatures, both species had similar relative declines in time-to-mortality as temperature increased (5.2% per degrees C for P. edulis; 5.8% per. C for P. ponderosa). When combined with the non-linear frequency distribution of drought duration-many more short droughts than long droughts-these findings point to a progressive increase in mortality events with global change due to warming alone and independent of additional changes in future drought frequency distributions. As such, dire future forest recruitment patterns are projected assuming the calculated 7-9 seedling mortality events per species by 2100 under business-as-usual warming occur, congruent with additional vulnerability predicted for adult trees from stressors like pathogens and pests. Our progressive projection for increased mortality events was driven primarily by the non-linear shape of the drought duration frequency distribution, a common climate feature of drought-affected regions. These results illustrate profound benefits for reducing emissions of carbon to the atmosphere from anthropogenic sources and slowing warming as rapidly as possible to maximize forest persistence.

Microbial contamination of drinking water poses an important health risk which causes severe illnesses and epidemics. In order to improve surface and drinking water quality, the understanding of fecal pathogen contamination processes including their prevention and control needs to be enhanced. The watershed model soil water assessment tool (SWAT) is commonly used to simulate the complex hydrological, meteorological, erosion, land management and pollution processes within river basins. In recent years, it has been increasingly applied to simulate microbial contamination transport at the watershed scale. SWAT is used in this study to simulate Escherichia coli (E.coli) and fecal coliform densities for the agriculturally dominated Payne River Basin in Ontario, Canada. Unprecedented extensive monitoring data that consist of 30 years of daily hydrological data and 5 years of bi-weekly nutrient data have been used to calibrate and validate the presented model here. The calibration and validation of the streamflow and nutrients indicate that the model represent these processes well. The model performs well for periods of lower E. coli and fecal coliform loadings. On the other hand, frequency and magnitude of higher microbial loads are not always accurately represented by the model.

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20752号 / 理博第4328号 / 新制||理||1622(附属図書館) / 京都大学大学院理学研究科生物科学専攻 / (主査)教授 石田 厚, 教授 工藤 洋, 教授 田村 実 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Observations of Bromus tectorum L. (cheatgrass or downy brome) monocultures have shown that populations are susceptible to stand die-off or replacement failures. Die-offs, where the seed bank from the previous year fails to emerge, occurs in cheatgrass stands and it is unclear the trigger or cause. The fungus Fusarium has been identified in plant and seed samples from die-offs and may drive die-off activity through pathogenicity. Die-off recovery may take several years but cheatgrass populations eventually reestablish. The purpose of our study was to determine whether Fusarium is a potential player in a die-off, and understand how die-offs recover after multiple years of stand failure. Our objectives were to determine: 1- litter and water effects on die-off activity; 2- if fungal pathogens, such as Fusarium, decrease the proportion of cheatgrass emergence in a die-off; and 3- whether direct or broadcast seeding, water, and litter treatments increase establishment in recovering die-offs. Litter absent plots had significantly (P < 0.0001 and P < 0.001) more emergence at 49.2% and 41% compared to litter present plots 21.3% and 23.7%. The litter absent plots significantly (P = 0.0003 and P = 0.001) increased survival (82% and 52%) compared to litter present plots (70% and 41%). Direct planted versus broadcast seeding had significantly (P < 0.0001) more emergence, 36% to 11.9%. The addition of Fusarium inoculum to field plots did not effectively replicate anticipated disease levels. The fungicide treatment did not have a significant influence at either site. The results from the study indicate that nothing inhibits cheatgrass from establishing following a persistent die-off disturbance. A unique window may be available for land managers to revegetate natives in invasive populations as large quantities of cheatgrass seeds fail to emerge during die-off events.

The genus Fusarium has a wide host range and causes many different forms of plant disease. These include seed rot and seedling blight diseases of cultivated plants. The Fusarium-caused diseases of wild plants are less well-known. In this study we examined Fusarium sp. n-caused disease development on non-dormant seeds of the important rangeland weed Bromus tectorum as part of broader studies of the phenomenon of stand failure or ‘die-off’ in this annual grass. We previously isolated an undescribed species in the Fusarium tricinctum species complex from die-off soils and showed that it is pathogenic on seeds. It can cause high mortality of non-dormant B. tectorum seeds, especially under conditions of water stress, but rarely attacks dormant seeds. In this study, we used scanning electron microscopy (SEM) to investigate the mode of attack used by this pathogen. Non-dormant B. tectorum seeds (i.e., florets containing caryopses) were inoculated with isolate Skull C1 macroconidia. Seeds were then exposed to water stress conditions (-1.5MPa) for 7 d, then transferred to free water. Time lapse SEM photographs of healthy vs. infected seeds revealed that hyphae under water stress conditions grew toward and culminated their attack at the abscission layer of the floret attachment scar. A prominent infection cushion, apparent macroscopically as a white tuft of mycelium at the radicle end of the seed, developed within 48 hours after inoculation. Seeds which lacked an infection cushion completed germination upon transfer to free water, whereas seeds with an infection cushion were almost always killed. In addition, hyphae on seeds that did not initiate germination lacked directional growth and did not develop the infection cushion. This strongly suggests that the fungal attack is triggered by seed exudates released through the floret attachment scar at the initiation of germination. Images of cross-sections of infected seeds showed that the fungal hyphae first penetrated the caryposis wall, then entered the embryo, and later ramified throughout the endosperm, completely destroying the seed.