Multiple friends with benefits? The belowground negotiations of trees and fungi

“Give me an example of how your training in mathematics led you in a new or surprising direction as a biologist,” said Mike. (As part of my preparation for job interviews, Mike has been inserting questions and prompts like these into our conversations at random. He takes his advising role very seriously!)

“Well, once upon a time in a far off land where the ocean is too cold to swim in,” I began (it was late September on Cape Cod, and the water was still luxuriously warm), “I was thinking a lot about how trees invest in their fungal partners.”

I was in my first year of Ph.D. work at Stanford when inspiration struck. My thesis was starting to take shape: I’d be studying the processes that maintained diversity in ectomycorrhizal fungi, the belowground partners that help trees gather nutrients and water from the soil. The host trees obviously play a major role: it’s their “payments” (in terms of carbon food) that support those fungi. How much the trees invested, in whom, and when, were — and largely remain — open and important questions.

Just a year before, I’d been working with Mike on a masters thesis that used a branch of mathematics called “optimal control theory.” This is the kind of math we turn to when we want to answer questions like, “How should I invest my fixed amount of money to control the spread of a disease?” (We’d been using it to ask “How should I distribute my fishing fleet to catch the most fish?”)

With partial differential equations and Hamiltonians and greek letters dancing in my head, I thought: “Why can’t we use the same tools to understand tree investments?” Basically, we just need to adopt the tree’s perspective to ask the question, “How should I pay my fungal partners so that I wind up having grown the most at the end of the day?” Taking a traditionally bioeconomic tool, and applying it to a purely biological system, made perfect sense to me — but was also a new approach.

And so, the idea for a new paper, out this month in The American Naturalist, was born.

The road from inspiration, in 2011, to publication, in 2016, has been long, inventive, and exciting. The paper we published is not really the one I intended to write. Instead, it describes an unexpected finding — which makes me very happy, because, like my coauthor Mike Neubert, I believe that mathematical models are most  valuable when they surprise us. We’ve also had a lot of support along the way — from interested colleagues to my thesis committee members to the journal’s very enthusiastic editorial staff.

And, five years later, I have to say: I’m really proud of this one.

The official press release is online here; since we wrote it, I’ve also included it below.


Sometimes, even low-quality partners are worth maintaining

A fruiting body of the ectomycorrhizal fungus Amanita muscaria pokes through a carpet of pine needles shed by its mutualistic partners. A. muscaria coexists on tree root systems with dozens of other ectomycorrhizal fungal species. Photographed in Cora Lynn, New Zealand, by H. V. Moeller (January 2012).

A major challenge of ecology is explaining species diversity in natural communities. For example, why do some species interactions—like the mutualistic relationship between plants and the below-ground fungi that help them gather nutrients from the soil—involve dozens of partners, especially when some of those partners don’t pull their weight in the relationship? Although such ‘low-quality’ partners die quicker, grow more slowly, and/or move nutrients less efficiently, they still remain part of the mutualistic community

In a recent paper, mathematical ecologists Holly Moeller and Michael Neubert of the Woods Hole Oceanographic Institution present a new explanation for the persistence of low-quality community members: They may be actively maintained by their partners as part of an “optimum” investment strategy designed to maximize their partner’s growth.

Using a mathematical model of the exchange of carbon and nutrients by trees and fungi, Moeller and Neubert show that a tree’s investment in its fungal partners depends on environmental conditions. In their model, trees invest only in high-quality fungal partners when the environment is constant, but, when the environment is variable (e.g., if nutrient supply fluctuates over time), trees may also nurture low-quality partners to meet their nutrient demand. Because long-lived organisms like trees and fungi are almost certain to experience environmental variation during their lifetimes, such active partner investment may be an important mechanism that maintains species diversity.

Find our article, at last “officially” published online, on the journal’s website. I’ve also posted a PDF here.

New paper on New Zealand Douglas-fir invasion published by Ecology

P. menziesii under native N. solandri canopy.

A Douglas-fir (Pseudotsuga menziesii) seedling pops through the mossy groundcover in a native southern beech (Nothofagus solandri) forest. Unlike other invasive pines, Douglas-fir is shade tolerant, which makes it an especial threat to New Zealand’s forests.

“Homogocene.”

That’s the term some of my scientist colleagues are using for the current ecological epoch, during which humans have moved different species all around the planet.

We can’t help it. We move some of these species deliberately — because they’re key crops, because they make good companions, or because they remind us of home. We move some accidentally, when they tag along on airplanes or boats.

Unfortunately, our global exchange program has had some nasty side effects. Introduced species can become invasive, in some cases taking over landscapes and displacing native species. Certain places, like island ecosystems whose delicate balance has evolved in isolation for millennia, are especially vulnerable.

New Zealand is one such place.

Today, numerous North American tree species, originally brought to the country by foresters seeking the ideal species for their timber industry, are invading across both North and South islands. Douglas-fir (Pseudotsuga menziesii), the giant of the Pacific Northwest, is one such tree.

In a new study published this month in the journal Ecology, my Ph.D. advisor and I worked with collaborators in New Zealand to understand the belowground community at the Douglas-fir invasion front. We were particularly interested in a group of fungi called ectomycorrhizae: These mushroom-formers travel through the soil collecting nutrients and water, which they then deliver to their host trees in exchange for sugars (carbon that the tree fixes through photosynthesis). Without these mutualistic fungi, Douglas-fir (and other invasive pines) can’t grow and expand their range. So identifying the types of fungi that are available is one way to figure out why Douglas-fir is becoming invasive.

We show that Douglas-fir’s complement of fungi is context-dependent: That is, where you are matters. In Douglas-fir plantations, invading seedlings can find lots of familiar faces, fungi that were brought to New Zealand (either deliberately or accidentally) from Douglas-fir’s home range in North America and therefore make ideal partners. Outside plantations, community composition is more variable — and might explain why Douglas-fir can invade in some places, and not others. Troublingly, however, the Douglas-fir invasion into native forests doesn’t seem to be fungus-limited, possibly because Douglas-fir, apparently uniquely among invasive trees around the world, can form associations with native New Zealand fungi.

Read more at the journal’s website.

Download the pdf.

Latest paper on marine reserve design published by NRM

Schooling fish

Schooling fish (I’d appreciate an ID assist!), Kua Bay, Hawaii, June 2015

In today’s fast-paced, increasingly crowded world, we often view our conservation goals as directly in conflict with our economic ones. Save a forest, or cut it down for lumber? Preserve a wetland, or drain it for human habitation?

Marine reserves, parts of the ocean where commercial fishing is prohibited, are usually seen as embodying that conflict. On the one hand, they provide great conservation benefits by acting as safe zones for overfished species. On the other hand, they remove a portion of the fish stock from harvest and, in so doing, seem to remove money from fishermen’s pockets.

Fortunately, a growing body of literature is showing that marine reserves may be part of an economic best-case management strategy. That is, sometimes, you want to protect a portion of your fish stock, so that you can make more money when you fish the rest.

In a new paper out this month in Natural Resource Modeling, Mike Neubert and I highlight the economic benefits of marine reserves when fishing damages habitat. There are many cases in which habitat damage occurs: here on Cape Cod, the long history of bottom trawling for cod and haddock is just one example.

Using a very simple model in which we divide habitat into two portions, we show that profits are maximized when one of those two portions is closed to fishing. The reason for this is that the closed “patch” provides undamaged habitat that houses a healthy fish stock. The spillover from that patch (i.e., the fish that leave the reserve and swim into the fished habitat) can then be caught and brought to market.

For more information, contact Mike or me. You can also read the full paper online. [Access via journal website] [pdf]

Want to know even more? Check out our 2013 paper, which uses a more complicated model (partial differential equations to model continuous space) to show the emergence of marine reserve networks protecting >80% of habitat as part of the economically optimal management strategy. [pdf]