Expert Q&A: Dr. Peter Groffman

How did you become interested in ecology, and in leaf litter in particular?

I was interested in environmental science, but I didn’t know specifically what I wanted to study. I enrolled at the University of Virginia in 1976, and they had a relatively new environmental science department consisting of geology, hydrology, meteorology, and ecology. You had to take a core course in each of those areas, and when I took a course in ecology, I knew that was it for me. I was interested in ecosystems ecology: the study of whole systems as the fundamental units of nature that capture energy from the sun, send that energy through food chains to support biodiversity, and cycle water, carbon, and nutrients. I became interested in studying ecosystem approaches to agriculture and went on to pursue a PhD doing that, and it was all about litter. My dissertation research was on the ecological effects of no-till agriculture, which is where you leave surface litter on the soil.

Ecosystems do three things: production, consumption, and decomposition. Litter becomes very fundamental to that decomposition piece. As an ecosystem ecologist, I’m a sucker for a discussion about leaf litter! I started laughing when I got your request to talk about leaf litter, because litter is one of the first things I started thinking about when I got to graduate school.

One of the things you do studying in ecosystem approaches to agriculture is ask, “Can we use insights from natural ecosystems to improve the functioning of agricultural systems?” In no-till agriculture, a litter layer begins to accumulate when you remove the tillage. I studied this litter layer in forests and realized it is super important for the resilience of forest ecosystems, particularly to the stress of rainfall and soil erosion.

A raindrop falling from the sky has a tremendous amount of energy. I don’t know if you’ve ever seen those pictures of a raindrop striking the soil, but it looks like an explosion. There’s a huge amount of destructive power in that raindrop, but in a forest, the soil never sees that destructive power. For one thing, many of the raindrops hit the trees and leaves, and that bounces some of the energy off. But even in the winter and spring, when there are few or no leaves on the trees, leaf litter protects the soil from erosion. Over the long term, soil erosion is the greatest threat to natural ecosystems because soil resources take a long time to recover. If a forest is disturbed and needs to regrow rapidly, the litter also decomposes quickly, releasing nutrients and facilitating regrowth.

Can we incorporate that into agricultural systems? That’s essentially what no-till systems do. No-till systems have 90% less erosion than conventionally tilled systems and that is mostly because of the presence of litter. That caused me to reckon at an early stage how incredibly important the litter layer was as a structural and functional component of forest ecosystems.

How does that importance change in different types of forests or climates?

There is a lot of variation in litter. If you become a litter nerd and start wandering around the woods looking at litter, you see it differs quite a bit. The easiest difference to see is between conifer and broadleaf forests. Nobody wants to eat pine needles, so they accumulate over long periods of time. If you go back into the old forestry literature from the 1800s, they make a distinction between “mor” and “mull” forest soils. Mull forest soils are associated with deciduous forests and they’re softer. They have a whole series of characteristics that you can predict. Mor forest soils are associated with coniferous forests. They are thicker and have different kinds of layers and associated biology. Different types of vegetation give you different types of forest floors, which then give you a whole different suite of biodiversity and functions associated with litter.

Leaf litter is controlled by how much litter is produced, and then the decomposition rate. In tropical forests, leaves fall down in the morning, and they’re gone by the afternoon. They decompose really fast, so climate has a big effect on this.

Another thing that has a big effect on leaf litter is earthworms. In general, the litter contains soil fauna like beetles, mites, springtails, etc., and they kind of get the first crack at the litter. A couple of things have come in recently to disrupt that.

There are virtually no native earthworms in North America. When the glaciers came down [as far south as where New Jersey is today], they disturbed the soil and killed all the earthworms, and they have been slow to recolonize. Even if you go south of the glacial boundary, like into Virginia or North Carolina, you still don’t find a lot of native earthworms. European and Asian species have displaced the native species. A couple hundred years of studying northern soils taught us that they have really thick litter layers, because there are no earthworms. But starting around the 1980s, people began noticing that the thick litter was disappearing, and that European species of earthworms were colonizing in places like Canada, New York, and Vermont and causing the litter to disappear in just three to five years.

People like earthworms. I try to tell people that earthworms are bad and invasive, and people look at me like I have three heads. Darwin wrote about how great earthworms were. You can’t even convince the scientists. European species of earthworms have been here for a long time. I believe they were introduced by English colonists in Jamestown. People grew up with them and they have the sense that they are good, and a symptom of healthy soil. If someone turns over a shovel full of dirt in their garden and they see earthworms, they say, “Oh good. The soil must be healthy.” But the earthworms are there because they’ve been adding compost and taking good care of the soil. Earthworms are not making the soil healthy.

The European species of earthworms have a big effect on the amount of litter in our forests. More recently, there have been Asian species of earthworms from the genus Amynthas. People call them jumping worms because they wiggle like crazy. They can also get very big.

While Europeans earthworms are active in March, these Asian worms are much more active late in the summer. They live right on the surface of the soil in the litter. People are now quite concerned about that. [They rapidly consume leaf litter, which strips the forest floor of organic matter, reduces nutrient availability, and alters the food web]. People who used to argue with me about whether or not earthworms were a problem are now calling me up saying, “I’ve got these earthworms in my forest, and the litter is changing quite dramatically.”

Are invasive earthworms a problem in other continents or is it just a North American problem?

That is an interesting question. In England, they have a problem with a parasite of earthworms. Non-native New Zealand flatworms are killing the native earthworms. I do not know if the Amynthas worms have spread from Asia to Europe, but I suspect they are a problem. The thing about invasive species is that our own beloved, native species are often a problem someplace else. Spartina, for example, is a problematic, invasive species in China. I think it’s the same thing with earthworms. I suspect the European species are causing problems in Asia and the Asian species are causing problems in Europe.

In places where there are still native earthworms in North America, are they beneficial in leaf litter?

When we study ecosystems, we like to classify things. Classification allows us to come up with expectations for what a system is going to do. I think that we’re developing different expectations for forests that have earthworms. Are they going to sequester less carbon? Probably. Are they going to cycle nutrients less efficiently? Probably. But it is hard to say whether they’re “bad” or “good.” They’re just different.

We’re developing a typology or a classification to be able to say, “Here’s what our expectations are.” Sites with earthworms are more susceptible to erosion. What makes people really upset is when these earthworms come in and the litter layer disappears; you start to see signs of soil erosion. You start to see signs that the little plants that live in the litter layer disappear. Other plants come in and then there seems to be what they call an “invasional meltdown,” where the earthworms change the soil, and then you get these invasive plants that live with the earthworms, and then you get invasive insects, then you get invasive birds. There is this whole sequence of events.

I used to give a talk called “Snow is good. Worms are bad.” People would fuss at me saying, “You can’t say worms are bad.” I can say worms are bad if I’m interested in a specific ecosystem service, like carbon sequestration, and there is less carbon sequestration when earthworms come in. But really, the systems with the worms are different than the ones without the worms.

Even so, have you seen any successful efforts to sort of manage invasive earthworms to better protect leaf litter in North America? If so, what are best practices?

What we’ve learned about invasive species in general is that once you’ve got them, you’ve got them. There are five billion zebra mussels in the Hudson. Are you going to pick up every rock and scrape the zebra mussels off? I don’t think so. In many cases, the focus with invasive species is minimizing their spread. There is a wonderful campaign in Minnesota called “Contain Those Crawlers,” and they have this really cool poster in every bait shop along the South Shore of Lake Superior because they think that’s how the worms spread.

So, if you are working on, say, a forest management plan, and you notice the presence of invasive worms, you just have to accept that they are there and understand what that tells you about the forest and it’s future, rather than trying to “manage” the invasive species?

One way to get rid of them is to make the soil really acidic, which humans have done unintentionally in many parts of the world. We had an acid rain problem that was particularly marked in Central Europe, in the mountains of Slovakia and the Czech Republic. The acid rain was so severe that it killed the earthworms and all the other soil fauna. One of the things they noticed in the 60s, 70s, and 80s was that the litter started piling up in very thick layers because everything [that would otherwise consume the litter] was dead.

The acidification ended very abruptly in Eastern Europe when the Berlin Wall fell, and many factories closed down. Just a couple of years after the Berlin Wall fell, the litter started disappearing because a bunch of organisms came back to the litter. We work in the Northeastern U.S., and our soils are also deacidifying, so we’re expecting to lose some litter and some forest floor. It is something we’re predicting is going to happen over the next 50 years or so. But acidifying soils is probably not a good way to “manage” invasive earthworms because it has many negative effects on soils and plants.

How important is leaf litter in downstream, aquatic ecosystems?

Think about a little stream in the mountains of the Northeastern U.S. It’s a little stream in a big forest, so it is completely fed by the litter. All of the carbon that drives that little stream ecosystem is allochthonous, it comes from the outside. But as the stream gets bigger, it starts to get some sunlight, so then there’s some autochthonous production as algae grows. Then it becomes a mixture of terrestrial and aquatic carbon. As the stream gets big, the terrestrial carbon kind of fades, but still, a lot of the carbon that goes into the headwaters of the stream gets transmitted all the way down to the ocean. Along the way it is a food source for microorganisms, and ultimately larger organisms.

Some colleagues of mine at the Cary Institute wrote a paper called “Are fish made of maple leaves?” They had a little lake in the forest of Wisconsin and although algae was growing in the lakes, they suspected that the carbon that was feeding the food chain in the lake was probably coming from the forest. They got a carbon isotope of the leaves, a carbon isotope of the fish, and a carbon isotope of the algae. What did the fish look like? They looked like the leaves, not the algae. Litter provides some habit in aquatic ecosystems, but mostly it provides food chain support.

Do you have a favorite tidbit or little known fact about leaf litter that is often overlooked by most people?

The erosion piece—that leaf litter is protecting the forest soil from erosion, which is the greatest threat to the integrity of these forest ecosystems. If the forest loses soil, it’s going to go downhill and never come back.

The other thing is that the litter layer creates heterogeneity that allows different species to germinate. I don’t know if you’ve spent time wandering around in European forests, but they’re really different from American forests. They have been managed very intensively. In some cases, they rake all of the litter out of the forest and use it as animal bedding, or they collect fallen branches and burn them. The population in Europe was always high, and for a very long time, wood was the only building material and source of fuel. By the time we got serious about cutting down trees in America, we discovered fossil fuels, so the demand for wood products went down. Another thing you’ll see in European forests are vast areas of one species. They have these beech forests, for example. They are quite beautiful, but also spooky because there is only one species of tree. This is because they have been harvesting all of the litter and dead branches, so the only thing that can germinate there is this beech species. The view of litter varies quite a bit culturally and in Europe. They think it’s wasteful that we leave the litter and downed wood. Even though they now have fossil fuels in Europe, they still manage forests pretty intensely. There’s a little bit of a natural forest movement there, but not much.

When you talk about leaf litter supporting heterogeneity, is that because birds eat the insects in the leaf litter, and when those birds poop, the poop may contain seeds of other tree species?

Yes. The litter will pile up in certain spots and then you’re going to get a lot of bugs underneath there. Then you’re going to get a lot of birds. This creates a little hotspot of biodiversity.

Karin Burghart, a professor at the University of Maryland who works in urban systems, has done fantastic work demonstrating that if we want to have some biodiversity in urban neighborhoods, we’re going to have to pile up litter in [residential] yards. But it becomes a perception problem because people like to rake their leaves. I go back to my start in no-till agriculture. Farmers used to have clean tilled fields and at first there was a huge aesthetic, perceptual problem where farmers were like, “I can’t leave all that trash on my field!” Now, that perception has switched completely. Today, when farmers look at their fields and there’s no residue on them, they’re like, “Oh, no; it looks naked and vulnerable.”

People’s perceptions and values influence their behaviors, but they can be changed.

Who is leading the way, in terms of changing public behavior and perception related to leaf litter?

In Germany and the Netherlands, they no longer manage their parks as intensively as they used to. If you wander in Berlin or some of the Dutch cities, you’ll see this. You might see a street tree where they’ve just let everything grow underneath it, or a park where Ailanthus germinates and they just let it grow. Their attitude is, “This is the native, urban vegetation and we’re going to let it grow.” To my eye it looks messy and unappealing, but I have the old aesthetic. Both the Germans and the Dutch have a lot of very highly manicured gardens, and this “let stuff grow” attitude may be a reaction to that. This attitude may not be adopted everywhere though . . .

What do you consider to be the most severe threat to leaf litter?

Earthworms are certainly reducing the amount of litter in northern forests within the glaciated regions. In managed areas, the threat is humans. In agricultural fields, which cover a lot of area, there is much more litter than there used to be—and that is a positive change. But in urban areas, human behavior is going to have to change. I think there is more litter in urban areas now than there was when my father was asking me to rake the leaves in suburban New Jersey in the 60s, but we still have a long way to go. To me the greatest threats to litter are people in agriculture and yards, and earthworms in the native ecosystems. And then there’s climate change, which is a difficult threat to sort out.

Is that just beginning to be studied?

Yes. It is a huge question because people really want to know about the amount of carbon that ecosystems are holding onto. We know that if it’s warmer and wetter, decomposition is faster. But we also know that if it’s warmer and wetter, you get more production. And so how those two are going to play out is an open question.

Many of our readers are practitioners of ecological restoration, which often requires construction. In areas where the native soil has been degraded, compacted, or removed, what best practices can you recommend to quickly restore the microbial and biological functions of leaf litter?

Construction soils can be really heavily compacted. There are a bunch of techniques for loosening up that soil, mixing in some compost, and then covering it with litter. That’s not my area of research, but practitioners of ecological restoration know that they need to minimize the use of heavy equipment, and they need to know that they’re going to have to do some kind of subsoiling. Stuart Schwartz at the University of Maryland, Baltimore County, has done fantastic work on big subdivisions where they compacted the soil when they built the subdivisions. [See this site, as well as the Resources section of this issue, for information about Dr. Stuart Schwartz’ work on suburban subsoiling]

You mentioned the raking of litter to use for animal bedding in Europe earlier. I read a study that revealed that in Swiss forests, where that was done from the Middle Ages until end of the 19th century, the soil organic carbon concentration is still 17% lower than it is in forests that were not raked. How important is time when it comes to restoring the functions of leaf litter in areas where it’s been removed or degraded?

Some functions are going to come back really fast. The protection from erosion is going to come back very quickly. Soil carbon takes a long time to build up. But some things like protection from erosion and providing habitat for beetles and ground nesting birds and small mammals, that’s going to come back pretty quickly.

Do you think knowledge about leaf litter could be better applied to conservation and ecological restoration? If so, how?

I think tidal wetland people get it right. There is so much wrack, so much litter, in tidal wetlands, and people who are restoring tidal wetlands realize that it’s important.

Practitioners ask, “What are the benefits we’re going to get?” I focus on erosion. Karin Burghart’s work focuses on soil insects which then support the food chain. European examples suggest that it’s important for the system’s self-sustainability, and what plants are going to germinate over time. Germination is a big deal. Seeds are picky, and they’ll only germinate in certain places at certain times. We know that litter is important for seed germination.

Scientists have done a decent job articulating the fact that if you have litter, you’re going to get erosion control, greater biodiversity, more carbon sequestration, and you’re going to get a better, more self-sustaining ecosystem. Those things matter to practitioners, and I think they realize these facts.

You have been deeply involved in the Baltimore Ecosystem Study and the Hubbard Brook Ecosystem Study. Can you talk about the significance of both of those studies and what, if anything, they’ve revealed about the importance of leaf litter? [Note: The Hubbard Brook Ecosystem Study is a decades-long research program at the 7,800-acre northern hardwood forest in the White Mountains of New Hampshire. The Baltimore Ecosystem Study, which began in 1998, is a long-term study that aims to advance understanding of urban areas as an ecosystem type. More information about the National Science Foundation’s Long-Term Ecological Research (LTER) network can be found on their website.]

At Hubbard Brook, the thickness of leaf litter is among the most fundamental, long-term data that we have. We have been tracking litter there for decades. There is a book about the work at Hubbard Brook called Pattern and Process in a Forested Ecosystem (Bormann and Likens, 1979, 253 p.). I read that book in graduate school, and that’s where I got all of these ideas about erosion and how important litter was. [The work at Hubbard Brook has included] collecting and tracking litter, looking at things like the impacts of all kinds of things, including acidification and deacidification. Manipulating litter has been very fundamental at Hubbard Brook and in Baltimore. There are some things we know about how natural systems work. Just as I asked in graduate school, “Can we incorporate this knowledge into agricultural systems,” the Baltimore study asks, “Can we incorporate this into residential and urban systems?”

Again, I point to Karin Burghart’s work, which focuses on what happens when you add piles of litter in the backyard. LTER studies have been fundamental for understanding the benefits of leaf litter, and there’s a whole network of LTER sites. Earlier, you asked about the role of litter in other kinds of ecosystems. I also did work at the Konza Prairie LTER site in Kansas. There, the big treatment is burning versus non-burning. I got out there and the unburned sites had a lot of detritus piling up. I was like, “this is good!” and they said, “No, it’s bad because it shades the soil.” In the prairies you get fire coming through every year that burns away the litter and then the new grasses come roaring out. They’re adapted to that. Where litter accumulates, it makes it harder for these plants to grow. That was a mindblower to me because it was a different perspective. The LTER sites are so useful.

The Baltimore Ecosystem Study still exists. The long-term ecological research funding is gone, but we still have long-term studies there where, just like at Hubbard Brook, we’re tracking the condition of the forest floor and forest litter. We really would like to do more experiments where people try different management strategies in their yard.

We have [a study involving] National Wildlife Federation-certified yards, high-intensive yards, low-intensive yards, and yards with rain gardens in them. But it really does work. You can see the effects on soil arthropods and then it goes up to insects and birds. In the National Wildlife Federation-certified yards, it was a lot of extra work for the laboratory technicians because there were so many soil insects they needed to count. You know, it’s not rocket science. If you manage your yard using native plants versus nothing but grass, you’re going to get big improvements in biodiversity.

I read an article about you which said, “Uncovering the basic truths about how ecosystems function across urban and non-urban areas and how humans interact with this ecosystem is what spurs Groffman’s interest.” Is there anything related to the functions of leaf litter and the ways in which people interact with it that you’re still curious about? If a considerable sum of money were to fall in your lap for research, what would you study?

One thing we’ve learned from urban work is to do community-driven, participatory research. In Baltimore, I study a lot of water quality issues, and when we go into neighborhoods, we have gotten better at saying, “What do YOU think is the problem with water in this neighborhood?” A lot of community members are so concerned about public drinking water that they won’t drink it. They are also concerned about dirty puddles their children play in. You learn a lot when you ask people what they think. Then you say, “What data would we have to collect together?” This, to me, is a frontier in science: community-driven, participatory action research, where we work with communities to say, “What are the questions?” and then collect and interpret the data together. Whenever we’ve done that, and we’ve done it both at Hubbard Brook and in Baltimore, we’ve learned a lot. The people on the ground have really interesting insights into these systems. When they participate in the research, they’re much more amenable to the results. If I would like people to manage their yards differently, I’ve got to talk to them and find out what the question is. We need to do this participatory research to ask, “What would convince you to leave litter in your yard?”

At Hubbard Brook, we’re currently obsessed with how deacidification is playing out along with climate. The forest floor is this really important component, I want to know what’s happening to it over the next 30-50 years. Deacidification is going to cause one effect. Climate change is going to cause another. The increase in the productivity of the trees is going to cause yet another effect. There are a lot of factors so it’s hard to study. I’m very interested in continuing to monitor changes in the litter layer at Hubbard Brook and other places.

How can people follow LTER studies related to leaf litter?

The LTER sites and data are all publicly available. We also publish scientific papers which, can be a bit detailed (boring) for general readers. There was recently a fantastic NPR story about songbird mapping at Hubbard Brook, and it featured one of our graduate students [Miranda Zammarelli, a Ph.D. candidate at Dartmouth College and a behavioral ecologist], who was great. Today’s students are doing a much better job communicating about science.