Understanding the mechanisms that maintain plant biodiversity in forests and other natural ecosystems is a fundamental challenge in ecology. While the impact of invasive pathogens and pests on ecosystems can be dramatic, the functions of pathogens and pests that are native members of ecosystems are less well understood. For example, do pathogens and pests promote biodiversity by limiting the most dominant species? Alternatively, do species achieve dominance through evolution of higher levels of resistance or tolerance to endemic pathogens. There is growing evidence that in biodiverse tropical forests, biodiversity is promoted by pests and pathogens that target the most dominant species. However, much less is known about the mechanisms operating in less diverse cool, temperate forests such as the conifer forests of the Pacific Northwest.

This project has focused on the role of native oomycete pathogens in an old growth forest in the Pacific Northwest, specifically the Wind River Forest Dynamics Plot in Washington state, which is affiliated with the Smithsonian ForestGEO network. Oomycetes, or water molds, are notorious agricultural pathogens that also damage many forest ecosystems, for example causing the Sudden Oak Death disease.

The project conducted very detailed surveys of oomycetes in the soils, waterways, roots and leaves of trees and shrubs, and drip-water from tree canopies at the Wind River site, and some sampling at a similar site in Oregon, the H.J. Andrews Experimental Forest. The surveys included culturing and DNA bar-coding experiments. In parallel, the project conducted very detailed measurements of the growth and survival all trees greater than 1 cm in diameter, as well as genetic fingerprinting of over 700 trees belonging to the 8 most abundant species at the site. A third survey examined the annual abundance of seeds and survival of seedlings at numerous sites around the forest that were treated with fungicides or left untreated.

The results revealed that oomycetes were exceptionally rare, compared to similar survey results from managed forests and younger natural forests. The few oomycete pathogens found were from broadleaf understory shrubs such as Vanilla Leaf (Achlys triphylla). Two new oomycete species (Pythium rhizoterrae and Pythium aquasilvae) were found. The two most common species that were found (Pythium rhizoterrae and Pythium macrosporum) did not appear to kill any of the conifer species tested, but did kill some non-forest plants. Overall, these data tend to support the hypothesis that a major factor in the stability of old growth conifer forests may be resistance to the endemic pathogens. At the same time, the ability of the most abundant species to kill non-forest plants raises the question of whether these pathogens help the forest resist the establishment of invasive plant species. Preliminary data from spraying of seedling plots with fungicides suggested that oomycetes and fungi might kill some seedlings survival, but further data analysis will be required to confirm this finding. Surveys of oomycetes and fungi in the H.J. Andrews Experimental Forest indicated that trees of the same species were more widely spaced than expected (negative density dependence) at locations where the greatest numbers of fungal and oomycete species were found, suggesting that pathogens may have a role in promoting biodiversity in this forest. The tree growth and mortality surveys at the Wind River site suggested that the principal cause of tree death was wind damage, but the role of root rotting pathogens in making trees more vulnerable to wind damage remains to be explored. Overall therefore, the project results suggest that resistance to endemic pathogens may be an important factor in the stability of old growth forests, but that

further studies of other classes of pathogens such as fungi, and of insect pests, will be required to test this hypothesis more completely. Contributing non-biotic factors such as wind and drought will also need to be examined.

This project has trained a new generation of molecular ecologists. Undergraduate researchers conducted much of the project research. By partnering five institutions on this project, two of them smaller undergraduate institutions, undergraduates had outstanding opportunities to participate in cutting edge research. Under the guidance of graduate students and a postdoctoral fellow, a total of 41 individual undergraduates contributed to the tree growth and mortality surveys, analysis of seed production and seedling survival, and development of new methods for characterizing disease resistance genes in conifers. Many of these undergraduates participated in multiple years of project work.

The Wind River Experimental Forest is an NSF-funded National Ecological Observatory Network (NEON) site and part of the CTFS-ForestGEO global network of large forest dynamics plots. Thus, the knowledge and data from our study will strengthen an already large set of ecological data at this site and set the stage for comparative studies in other forests. The development of improved molecular barcoding for oomycete species will be valuable for oomycete ecologists around the world.

Last Modified: 11/30/2021
Modified by: Brett M Tyler