Ectomycorrhizal fungi are obligate mutualists with temperate forest trees such as pines, oaks, aspens, and birch. These fungi provide mineral nutrients to the trees in exchange for sugars, and trees require them for growth and survival. Following severe disturbance such as forest fires or logging, trees need to reestablish their relationships with mycorrhizal fungi in order to survive. Prior work has shown that these fungi arrive in an ordered sequence, or "succession", after such disturbances. A new model is proposed to explain this observed pattern through differences in spore dispersal and competitive interactions among mycorrhizal fungi. Four key predictions of this model concerning limitations to aerial spore dispersal and effect of tree root density on fungal competition will be tested through a combination of field sampling and manipulative growth-chamber experiments. The broader impacts of this work are that it will help us understand an essential biotic process necessary for tree establishment, and will build a foundation of basic ecological knowledge for an important group of understudied organisms. Training of graduate and undergraduate students will be an important component of this research. In addition this work takes advantage of its location within Point Reyes National Seashore by feeding back into management decisions within the park and through outreach efforts directed towards public education.
Ectomycorrhizal (EM) fungi are mutualistically associated with fine roots of pines, oaks, and many other forest tree species. The association is necessary for trees to establish and grow. Thus understanding the basic ecology of these fungi is crucial for our understanding of forest regeneration and expansion. The diversity of EM fungi is very high even in relatively simple forest systems. The primary goal of this work was to understand the processes that generate and maintain that diversity. Specifically we examined the process of EM fungal dispersal through space and time and the ways in which dispersal interacted with interspecies competition and ultimately community structure. We did this through a combination of sampling schemes and manipulative experiments, and we analyzed the presence and quantity of various species through DNA-based methods. All aspects of the work outline in the original grant are now completed and 11 papers have been published from this work. One or two more papers are still likely to be produced. Among our key findings are: 1) spores (i.e., the microscopic "seeds") of EM fungi vary significantly in their response to simulated forest fire; 2) spore arrival time (or germination time) largely determines which species of EM fungi colonize pine seedlings and the first arrivals temporarily preempt other potential colonists; 3) spore longevity of a few key EM fungi exceeds 6 years under natural soil conditions; 4) pine trees that invade previously non-forested sites are colonized by a small, predictable subset EM fungi that are the best at spore dispersal and these fungi dominate the root systems of such trees for at least a decade; 5) pines that established beyond forest boarders and that are several decades old, have unpredictable sets of EM fungi associated with them, but the diversity of these fungi declines with the distance from the forest edge; 6) direct measurement of spore dispersal shows that within one kilometer of a forest edge the quantity of most spores can fall below the level necessary for pine seedling colonization. Collectively these results show the spore behavior can explain much of the diversity of EM fungi in young forests and dispersal can be limited within relatively short distances. We also described new species of fungi, developed and tested a new theory of fungal succession, and developed a way to census and quantify fungi through capturing their spores in rainwater traps and sequencing and quantifying their DNA. The main significance of these results is that it adds predictably to events that are critical for pines to establish in previously non-forested or highly disturbed forest settings. For example the longevity of spores in the forest soil and their resistance to heat explains why seedlings are able to establish and find the necessary mutualistic fungi following fire or logging. Furthermore, it predicts which species of fungi will be found in such settings. In contrast, outside of a forest the spore inoculum becoming limiting within a fairly short distance (e.g., 1 km in coastal California) and half or more of the seedlings remain uncolonized and in most years and do not survive. This is true in spite of the fact that spores are produced in astronomic numbers within a forest, and can be detected, although in greatly reduced numbers, even several kilometers from a forest border. During the course of this work one postdoc, four graduate students, and three undergraduates students were trained. The former postdoc is now an assistant professor at Stanford. The graduate students have also made good progress in their careers. One went on to a postdoctoral appointment at the University of California Irvine and has now accepted a faculty position at the University of Hawaii. One accepted a postdoctoral appointment in Lund Sweden. One accepted a postdoctoral position at the University of Minnesota. One is still at Berkeley but is on track to graduate this year. One of undergraduates just co-authored a paper from his work and has been hired at a biotechnology company. Educational outreach to the public was accomplished via multiple talks and articles given by lab members at local area mushroom clubs. We also initiated a mushroom survey of two national parks (Pt Reyes National Seashore, and Yosemite National Park) that directly involved "citizen scientists" from these same local mushroom clubs. This work is still ongoing but has already resulted in the first fungal species lists for both parks, and a vouchered set of collections that have helped us establish a better sequence database for our local EM fungi.
