I've been focusing on plant secretions and such that affect sward composition. It's part of a comprehensive effort to make my swards happy places for the species I value as forage. After working on moisture, PH, tilth, organic matter and nutrients I shifted to bacteria and fungi. For the most part if the other things are good then the bacteria and fungi will thrive but there are exceptions.

One well known one is tannins such as are found in oak leaves. They harm many forage species in part by being toxic to nitrifying bacteria such as nitrobacter. There's not a lot you can do on a large scale. You can't realistically rake acres of semi-forested pasture, and the trees have other value. You just live with that problem.

Another known issue is juglone. "Juglone is found naturally in the leaves, roots and bark of plants in the Juglandaceae family, particularly the black walnut." It too retards nitrifying bacteria, though as with tannins it is most recognized for its alleopathic effects on seedlings of other species.

A new plant secretion has been discovered that works obliquely to harm competitors.

An invasive weed that has spread across much of the U.S. harms native maples, ashes, and other hardwood trees by releasing chemicals harmful to a soil fungus the trees depend on for growth and survival, scientists report this week in the Public Library of Science. The tree-stifling alien, garlic mustard (Alliaria petiolata), first introduced into the U.S. in the 1860s, has since spread to Canada and 30 states in the East and Midwest, with recent sightings as far west as Oregon. . .

"While vanishing habitat caused by human activity is the number one threat to biodiversity, there is great concern over the impact of accidental and intentional dispersal of alien invasive species across the globe," says Kristina A. Stinson, a plant population biologist at the Harvard Forest, Harvard's ecology and conservation center in Petersham, Mass. "In North America, thousands of nonnative plants and animals have become established since European settlement and many more continue to be introduced. Some alien species cause little harm, while others can become very aggressive and radically transfigure their new habitat.

"The mechanisms for this phenomenon and its potential long term impacts remain poorly understood," Stinson adds, "but one possibility is that invasive species may disrupt fragile ecological relationships that evolved over millions of years."

Stinson and her colleagues found that garlic mustard targets arbuscular mycorrhizal fungi (AMF), which form mutually beneficial relationships with many forest trees. These fungi have long filaments that penetrate the roots of plants, forming an intricate interwoven network that effectively extends the plant's root system. AMF depend on plants for energy and plants depend on the fungi for nutrients. When tree seedlings, which depend strongly on AMF, began to decline in the presence of garlic mustard, the researchers suspected that the invasive plant might thwart this symbiotic relationship.

Several earlier posts have extolled the virtues of AMF and the value of glomalin, the durable carbon compound created by AMF hyphae which so improves soil. It is worth noting that AMF isn't only of value to trees. It also establishes a barter relationship with many other species in swards and forests, and even some crops. They serve the community but not "ruderals", plants that don't fit in a cooperative and symbiotic way with a co-evolved community of species.

One of the species I've been planning to interseed in my diverse swards is annual Lezpedeza. It's a warm weather legume that should fix a little nitrogen and provide some good quality forage when most other legumes have given up for the dog days of summer. There's a perennial Lezpedeza too but it is considered an invasive species since it has alleopathic characteristics; it has tannins in its leaves which slowly make the soil around it inhospitable for other species. Annual Lezpedeza has some tannins, but it isn't as bad and isn't invasive.

But, there may be a larger problem. Many community plants reportedly disengage from their network of symbionts and nutrient exchange partners when conditions are good. Their mutual poverty is the glue of community, and it all falls apart if they become wealthy. That may mean that all the work I have done to make them happy and healthy threatens their health. It isn't just that my reward for good work is that I get to keep doing it forever since they adapt and become dependent, it may make them vulnerable to bad bugs. Not all soil bacteria and fungi are beneficial, and it seems that some of them exploit the now unused community connections of wealthy plants, attaching to and invading plants that have dropped their connections. It may be that the only kind of plants that will thrive in my swards are ruderals that never participate in community behaviors.

I'd hate that since the native trees, some of them hundreds of years old, are a significant part of the charm of these swards. I do take pains to tailor management to their needs, but I wonder if I'm doing it well enough? For example, I route irrigation water around stands of blue oaks since water in the dry season is known to harm them by inducing them to detach from their soil fungi partners. I need more information.