A forest on glass beads

Defying all expectations,  a forest has grown on the beach. Just east of Philadelphia is  the New Jersey Pine Barrens- a vast wilderness almost completely ignored outside of a small group of park rangers, volunteer fire fighters and hardy locals whose families have eked out a subsistence living here for generations. And yet is is considered the largest undisturbed wilderness in the northeastern corridor with a incredibly unique ecology and a set of rare endemic species.

The Pine Barrens region spans most of southeastern New Jersey from the Atlantic coast inland across the mid-Atlantic coastal Plain. The coastal plain is essentially a huge wedge of sand that has accumulated over 5 million years of sedimentary deposition from the Applalachian mountains and southerly flowing rivers such as the Hudson. A long history of sedimentary accumulation has created a flat, unvarying topography. 18 feet of height divide the highest point in the Pine Barrens “uplands” region and the lowest point in the “lowlands”, and yet this seemingly homogeneous landscape has produced broad environmental gradients over incredibly short distances.

The reason for these gradients seems to relate to the unique nature of the sandy, nutrient depauperate soil. Sand can act as either a sieve or a water trap depending on its spatial position. At higher elevations,  rain drains freely through the sandy soils, leaching away any nutrients that accumulate and producing highly acid conditions that few plants can survive on. The uplands forests are dominated by drought-resistant, fire-tolerant pitch pine trees with a smattering of oaks. A few scrubby, low-nutrient requiring members of the Ericaceae family, such as huckleberry and highbush blueberry, dominate the understory. At low elevations, rain accumulates and has nowhere to go. The water table is generally high, producing soils that are saturated year-round. In the most saturated places, one finds peat bogs and white cedar swamps more reminiscent of the deep south than the mid-Atlantic. Swamps grade into tall, shady oak-dominated forests dotted with an occasional pine in the drier lowlands.

The stark contrast between uplands and lowlands vegetation is not just a product of  the soil conditions. The uplands and lowlands communities produce feedbacks on the environment that maintain the land in precisely the same condition for generations, such that nothing new can manage to gain a competitive edge.

In the uplands,  pine trees exude organic acids into the soils, maintaining their soils in a state of poor nutrient quality that nothing else can survive on. Every few years pines drop their needles, but not before sucking nearly all the nutrients out and back into the branches, ensuring that few nutrients are added back to the soil. The low nutrient-quality of this leaf litter slows microbial decomposition, causing years of litter to accumulate on the surface. This litter serves as kindle that enhances the spread of forest fires the pine trees require to sprout. These forest fires keep the less fire-resistant oaks at bay.

In the lowlands, wet conditions prevent forest fires from scorching the landscape with regular ferocity seen in the uplands. Oak trees are able to gain a firmer footing here, and once established, produce a shady understory that pine saplings cannot survive in. The oak trees drop their leaves annually, adding more nutrients to the ground and producing a soil richer in organic matter than enhances the development of a herbaceous understory, which helps crowd out pine saplings.

I stood over a pit we had just dug in the ground, staring down into what resembled a layer cake of dark chocolate, vanilla and red velvet. Distinct stratification in the soil profile  generally indicates a long history of mobilization processes. Organic matter leached down through the chocolately topsoil will sometimes produce a white, organic-free layer beneath, known to soil scientists as an E-horizon. Deeper still, weathering products from the underlying bedrock will accumulate in the subsoil and form complexes with the organic matter that has been transported down. The reddish layer I was seeing in the deep soil was the result of iron accumulation and subsequent oxidization by microbes in need of an energy source. In fact, the entire soil profile, from brown to white to red, is very typical of a class of soils known as Spodosols that dominate the New England and Canadian boreal forests, where low temperatures cause decomposition and other soil-forming processes to occur slowly, resulting in a surface buildup of organic matter and eventually the formation of distinct, colorful stratified layers. What, then, was such a soil doing in the Pine Barrens, a much warmer climate than New England, and a region with few soil nutrients and barely any organic matter inputs?

It turns out that the Pine Barrens soils which so closely resemble Spodosols may in fact be a relic from a much earlier time and different climate. At the height of the Last Glacial Maximum approximately 18,000 years ago, a large continental ice sheet known as the Wisconsan Glacier ended a mere 40 miles north of the Pine Barrens.  The New Jersey climate probably resembled those seen in the high Canadian boreal today, and there is little doubt that the soils that formed were some version of Spodosols. It is entirely possible that, given the state of extreme stasis that the Pine Barrens have existed in since the beginning of the Holocene, not much has occurred to alter the soils from their former state.  A forest that grows today on glass beads has thrived because of its ability to maintain stasis. Peeling back the layers of that forest reveals this stasis to be true, but only for a fleeting moment in the geologic record.

Tree girdling reveals that photosynthates drives soil activity late in the growing season

The role of trees in nourishing the soil has long been understood by ecologists, but the extent to which trees control belowground processes remains unclear. This is due in large part to the difficulties associated with measuring root activity and the dynamic interactions between roots, mycorrhizal fungi, and the soil environment.

To develop a better understanding of how photosynthates, the sugars produced by photosynthesis, influence patterns of growth and metabolic activity in the soil, a group of ecologists in Sweden decided to conduct a large scale girdling experiment. Tree girlding involves stripping tree bark all the way to the depth of the tree’s xylem, the long tubes that transport water from the soil to the leaves using negative pressure and the difference in water potential between the soil and the atmosphere. Xylem is located well inside the living tissue of a tree, but most importantly for a girdling experiment, it is located directly inside the phloem. Phloem is the piping that runs down the tree and transports photosynthates produced in the leaves via gravity into the rest of the plant. Thus by stripping a tree down to its xylem, the researchers are removing phloem and cutting off the supply of photosynthates to the roots and the soil. This allows them to study the effect of photosynthate removal on the soil without influencing roots, water transport or any other soil parameters.

In this large-scale experiment conducted in a boreal Scots pine forest in northern Sweden, six 900m² plots were chosen and rouglhy 120 trees per plot were girdled.  In each plot, half of the trees were girdled early in June and half in late August. Early and late girdling were used to detect whether phenological (seasonal) differences affected root photosynthate production.

The results of this experiment were striking. In the early-girdled plots, soil respiration declined by 27% compared to control plots within the first five days of girdling. (Remember, “soil respiration” refers to the amount of CO₂ released by the soil. It is a proxy for total soil metabolic activity, including the respiration of microbes, fungi, nematodes and other small soil invertebrates, and even roots themselves! Thus more respiration = healthier, more metabolically active soil.) By the end of the growing season, roughly 50% less respiration had occurred in the early-girdled plots. The occurrence of ectomychorizal fungi, an essential nutrient-acquiring symbiote for most plants, was also dramatically reduced. The late-girdled plots responded even faster, with respiration declining almost 40% within the first 5 days of girdling. Interestingly, the researchers found less response to girdling toward the edge of the girdled plots, indicating that soil organisms here were acquiring some photosynthates from neighboring, ungirdled trees.

The more rapid declines of soil respiration in the late girdling plots fit with our current understanding of how trees use their photosynthates. Early in the growing season, and especially in conifer forests, trees allocate most new photosynthates towards shoot, bud and needle production. Simultaneously, trees begin tapping their root carbon stores to enhance above ground growth. Later in the growing season more photosynthates are sent belowground to nourish the soil community. By this point root carbon stores are also diminished.  Girdling trees later in the growing season cuts off carbon supply to the soil at precisely the moment when it is being ramped up.

The flux of photosynthates to the roots has a big impact on belowground processes, with a clear seasonal component. In moving forward in our understanding of whole ecosystem carbon balances, understanding that trees are conduit, connecting the earth to the atmosphere and transforming both in the process, will, I believe, be an essential paradigm to adopt.

 

Reference:

Ho¨gberg P, Nordgren A, Buchmann N et al. (2001) Large-scale forest girdling shows that photosynthesis drives soil respiration. Nature, 411, 789–792.

 

 

 

Whats up with nitrogen at Hubbard Brook?

Buried deep in the White Mountains of New Hampshire, the Hubbard Brook experimental forest has provided ecologists with questions and answers for over four decades.  And more questions.

Hubbard Brook was founded on the principle that it was the ideal location for “small watershed” experiments: it is a forest ecosystem whose energetic inputs and outputs could be fully accounted for and directly measured. Ecologists have been able to ask many broad and profound questions at Hubbard Brook, and the forest has told them a story from which many fundamental ecological principles have been distilled. What causes forests to grow? Where do they get their nutrients? What sort of energy enters and leaves an ecosystem? How are human activities impacting forests? Are they resistant to change, and are they resilient in the wake of disturbance?

Before tackling any of these big questions, you may well wonder about my first statement: that the energetic inputs and outputs of the forest can be fully accounted for and measured. How can one possibly measure all the energy that enters and leaves a forest? In the case of Hubbard Brook, there is a deceptively simple answer: it’s all in the streams. Keep a record of what enters and leaves the streams, and you’ll have a good idea of what is entering and leaving the forest as a whole.

Plants need three things to grow: sunlight, nutrients and water. Of the plant essential nutrients, nitrogen is the most important in temperate deciduous forests, but phosphorous, calcium, magnesium and sulfur, among others, are also essential.  In describing where  nutrients and energy come from, ecologists like to talk about autochthonous and allochthonous inputs. Autochthonous inputs come from within the system. Allochthonous inputs come from elsewhere. In the case of nitrogen, an autochthonous source would be a leguminous plant, or a plant that can convert atmospheric nitrogen into a plant useable form through the biochemical process known as nitrogen fixation. An allochthonous input might be a nitrogen fertilizer sprayed on a field.

Hubbard Brook has plentiful rain and sunlight to support a healthy forest. The first unknown that ecologists tackled was nitrogen, and I’ll focus on it from here on out because understanding nitrogen at Hubbard Brook has proved more challenging than anyone ever imagined.

Where are the plants getting their nitrogen? It turns out that for the last 14,000 years (since the retreat of the North American glacier that allowed the northeastern US to re-vegetate), all nitrogen that has entered and fed Hubbard Brook has come from the atmosphere. From the rain. No nitrogen fixing plants, no weatherable rock-derived nutrients. All that plant nourishment has entered the forest silently, dissolved in the summer rains or the winter snow.

It also turns out that all nitrogen eventually leaves  Hubbard Brook by water as well, in a readily soluble form known as nitrate, or NO3-. Streams cut across the forest, forming discrete, independent watersheds over a relatively small space. (A watershed simply refers to a region whose hydrologic inputs and outputs are well defined and accounted for). Streams are an integral part of the Hubbard Brook forest. Much as your bloodstream carries essential nutrients to your tissue and allows your body to flush out toxins, streams transport nutrients to different parts of the forest and flush away excess chemicals that the system doesn’t use. Through long term monitoring of the stream chemistry at Hubbard Brook, including measurements of headwater inputs and lowland outputs, ecologists have developed an accurate record of how much nitrogen enters and leaves the system annually.

In an unpolluted forest, how much nitrogen enters, and how much nitrate leaves, tells ecologists a lot about the forest’s growth and nutrient requirements. At Hubbard Brook, scientists have found an annual cycle of stream nitrate that reflects a strategy the trees have evolved in response to a predictable environment.

When leaves  drop in autumn, nitrate export in streams peaks. Leaves fall both directly into the stream and onto the surrounding soil, where they are decomposed and their nutrients can be leached away. Over time, this process would lead to a dramatic reduction in the nitrogen held in the system, if not for the annual winter replenishment of nitrogen through snowfall. Snow contains nitrogen just like rain does, and as it builds up on the forest floor during the winter, a stockpile is created for the next growing season. Trees have timed their first buds to break in the spring just as this snow is melting and releasing a pulse of nitrogen into the earth. This is essentially how things work at Hubbard Brook, in the absence of human influence.

However, Hubbard Brook, situated just north of the Washington-New York-Boston urban corridor, is by no means isolated from human pollution. Nitrous oxide gases emitted from fossil fuel combustion dissolve in the atmosphere and are rained out across the world, often at great distances from the source. Since at least the 1960’s, excess nitrogen has been entering Hubbard Brook as nitrate through this process. As a negatively charged anion, nitrate often picks up a hydrogen cation, H+, to balance its charge. “Hydrogen cation” is basically a fancy term for acid (remember ocean acidification?) The greater the concentration of hydrogen in solution, the more acid a solution is. Nitrogen pollution, ecologists realized, was doing something very strange to rainwater chemistry at Hubbard Brook. The concept of acid rain was born.

Thus is should not be surprising that another more powerful trend overlies the annual nitrogen cycle at Hubbard Brook. From first measurements in the late 1950’s until 1970, scientists observed a steady, annual rise in stream water nitrogen export. Since 1970, however, nitrate concentrations have been dropping. This has been attributed in large part to the passage of the Clean Air Act and the subsequent reduction in nitrous oxide emissions from automobiles.

Here’s what is causing ecologists to scratch their heads. Nitrogen levels in Hubbard Brook streams have now been dropping steadily for forty years. They are approximately at their 1960 level, and this downward trend shows no indication of leveling off. This reduction is dramatic, unexpected, and not accounted for by the emissions reductions enforced through the Clean Air Act. It is becoming increasingly clear that something else about the ecosystem has fundamentally changed.

What exactly is happening to cause reduced nitrogen outflow from Hubbard Brook forests concerns ecologists for a variety of reasons. The main reason ecologists are concerned is because the forest has stopped growing. In fact, it is now thought that Hubbard Brook may be losing biomass.

Could reduced nitrogen loss reflect increased nutrient limitation in the forest, and a need for trees to hold tightly to the nutrients they have? Could it mean rainfall is no longer supplying the nitrogen the forest needs? Could the soil microbial community no longer be decomposing and transforming nitrogen in the manner that the forest has depended on for thousands of years? Could it possibly reflect some other fundamental hydrologic shift, perhaps induced by climate change?

The most compelling theories in my view are that the phenomenon of “missing nitrate” is climate change driven, or driven by changes in soil chemistry and loss of important soil cations. I’ll explain both of these ideas briefly and leave you to ponder.

Climate change is certainly causing other weird stuff to happen at Hubbard Brook. The forest typically experiences sub-freezing temperatures all winter and builds up a healthy snowpack. Snow insulates the soil and streams alike, preventing them from freezing. This allows frogs, salamanders and some fish to hibernate over winter without freezing. Recently, warmer and more varied winter weather patterns have lead to reduced snow cover, with the consequence that the soil and stream water are now freezing instead. (The number of frozen salamander bodies discovered in the winter of 2006 was apparently unprecendented). Loss of snow cover is also disrupting the annual nitrogen cycle, which could well be leading to a springtime nitrogen limitation for growing trees.

Calcium, which typically takes the form Ca2+ in soils, turns out have a high affinity for nitrate. The two ions balance each other’s charge, and when nitrate leaches out of soil, it often brings a calcium ion along with it. Actually, it often requires the charge-balancing association provided by calcium to leave the soil in the first place. Well, calcium turns out to be another very important plant nutrient, and unfortunately is not as quickly replenished through rain as nitrogen. Declining soil calcium levels will, over time, produce soil that leaches nitrate less readily. Nitrate loss in streams could thus be indicative of a serious calcium deficiency. Incidentally, sugar maple, a common species in New Hampshire forests and a calcium-accumulating tree, is dying out and may be entirely lost these forests within the next several decades.

What’s up with nitrogen at Hubbard Brook? No one really knows yet, but we’d sure like to figure it out.

In trying to paint a picture of the dynamic nature of nitrogen at Hubbard Brook, I’ve left quite a bit of detail out and haven’t discussed some of the fascinating experimental manipulations that have been conducted at the forest. Hopefully I’ve touched on enough of the important ideas that this ramble provides some sense of the many layers of complexity that ecologists must piece through to understand a system or solve a problem.

 

The title and much of the content of this article I’m crediting to Dr. Gene Likens, a senior ecologist at the Hubbard Brook experimental forest who provided fascinating information on the forest during a conversation and a guest lecture at the University of Pennsylvania.

In an unpredictable environment, trees network for stability

In the highly variable ridge, slope and valley mosaic that forms the Luquillo Mountains of northeastern Puerto Rico, Dacroydes excelsa, commonly known as Tabnuco, dominates the landscape. Though tropical forests are generally quite diverse and seen as ideal environments for plant growth, life in this rainforest can actually be quite challenging. Powerful hurricanes pass through the Caribbean annually and hit the Puerto Rican mainland every few years. Large swatches of the Luquillo forest were flattened several years back when hurricane Hugo struck in 1989.  Aside from directly damaging or wiping out forest stands, hurricanes cause landslides that severly erode the already shallow, nutrient depauperate soils.

On steep, harsh slopes that experience such frequent disturbance, what allows one tree species to gain a competitive advantage over the hundreds of others struggling to survive? Rather than compete fiercely for limited resources only to be at the mercy of the next devastating hurricane, Tabunuco trees have adopted an alternative strategy- cooperation and resource sharing through root grafting.

Root grafting, the joining of neighboring tree roots to produce a network, is a phenomenon that scientists have been aware of for decades, though the extent of its occurrence and the benefits that it provides trees are largely unknown. In Tabunuco forests, however, root grafting is widespread and many of its benefits obvious.

Tabunuco trees grow in dense stands and will graft roots with neighboring trees as they mature, forming unions that comprise anywhere from two to over a dozen trees. A clear advantage of this strategy in an environment that experiences powerful storms is structural stability. Trees that have entered unions increase their base of support and are less likely to be uprooted during a wind event or landslide. In increasing their wind-firmness, individual trees boost their survival chances during a storm. Fewer uprooting events also reduces the probability of a major landslide and helps ensure the retention of the surface organic matter that contains most of the forest’s available nutrients.

Root networks can also improve soil conditions during the off-season. Densely packed surface roots form “organic benches” which trap leaves and other decaying plant matter rather than allowing these important nutrient sources be washed downslope. Roots aerate the soil, facilitating decomposition and nutrient flow. They also “prime” the surrounding soil for productivity by releasing sugary compounds that stimulate beneficial microbial activity (the interaction between plants and microbes in the root zone known as the “rhizosphere” is another fascinating topic entirely, which I will do attempt to do justice to in the future).

Scientists are now discovering previously undetectable advantages of Tabunuco grafting that underscore the high degree of sophistication and evolutionary purpose in the development of these networks. It is now known that root networks can actually serve as conduits for the transfer of carbon and essential nutrients between trees. This can provide an immense competitive advantage over non-networked trees. Tabunuco trees that receive the most sunlight and produce the most carbon through photosynthesis can transfer carbon to neighboring Tabunucos to ensure the long-term health and survival of the community. Individuals of less common species, such as the Caribbean palm and Colorado tree are excluded from Tabunuco networks and must compete for growth given only the resources available in the vicinity of their roots.

Though in Tabunucos root grafting precludes the need for inter-tree competition, it is theoretically possible that trees could use grafting for more selfish purposes. Ecologists have speculated whether trees can gain a competitive advantage over their neighbors by leeching a neighbor’s nutrients, much as the fungal organisms that associate symbiotically with plant roots can become greedy and actually sap nutrients from their host under stressful conditions. Root networks may even serve as a conduit for disease or herbicide transfer, allowing trees that produce or tolerate a harmful compound to efficiently clear out their competitors.

Citation-
Basnet, K., F.N. Scatena, G.E. Likens, and A.E. Lugo. 1992. Ecological consequences of root grafting in tabonuco (Dacryodes excelsa) trees in the Luquillo Experimental Forest, Puerto Rico. Biotropica 25:28-35.

Maybe we should reconsider raking our leaves

I recently learned a fascinating fact about leaf raking that should be painfully obvious to a forest ecologist- it’s bad for trees! Every spring, deciduous trees produce leaves that they use throughout the growing season for photosynthesis and sugar production.  Plants concentrate essential nutrients such as nitrogen, potassium, calcium and magnesium in their leaves, as these nutrients are all required in relatively high amounts to perform photosynthesis.

As winter approaches and the growing season ends, trees withdraw many of the proteins and nutrients they have stockpiled in leaves back into their woody tissue, so that these nutrients can be recycled to make new leaves the following year. However, most trees are able to do even better than this- after their leaves have fallen, the nutrients that couldn’t be recaptured in time are decomposed into the surface soil surrounding the tree, and will be available for uptake through the roots several years later. This regular flux of plant essential nutrients back to the soil through leaf litter means that plants depend on those same nutrients, year after year, to grow new leaves.

In fact, if you look at the typical architecture of a deciduous tree, it is no accident that probably appears like two umbrellas attached together at their handles. The top umbrella is the above ground parts of a tree from the base of the trunk to its canopy. The bottom umbrella is inverted and planted into the ground. It is composed of a main taproot that drives straight down into the earth, and lateral roots that branch out horizontally. Of these lateral roots are branching networks of finer and finer “root hairs” and associated fungi that are able, through their enormous surface area, to mine the soil underneath a tree for nutrients. Everything that is dropped from the top umbrella should theoretically be recoverable by this root system.

I’d imagine most of you can already see where this is going, but I find that sometimes simple truths are quite elusive. When we rake our leaves in the fall to maintain our clean, grassy lawns, we are removing loads of nutrients that our trees are expecting to get back! We are creating an artificially open, leaky system, that trees have spent millions of evolutionary years refining. A recent paper in a relatively esoteric research journal, “Nutrient Cycling in Agroecosystems” (who reads that??) attempted to quantify the impact of historic leaf raking on old agricultural towns in central Europe. The fascinating bit of historical information in this paper is that centuries ago, medieval farmers actually knew that leaves were a great nutrient source- farmers removed leaves from nearby forests specifically to use as fertilizer on their fields. This paper claims that the result of historic leaf raking is that the “majority of central European forests were severely depleted of nutrients…when modern long-term rotation forestry became the dominant form of forest land use”.

So next fall, when you’re pulling out your rakes or enlisting your kids to do so for a few dollars, think carefully about your trees. In all likelihood, the average patch of suburban lawn is already so nutrient depauperate from numerous land use changes (deforestation, asphalt paving, over-fertilization, the cultivation of a monoculture of non-native grasses, to name a few) that removing a few leaves isn’t going to make a big difference. But if I’ve learned anything from Malcom Gladwell, it’s that little changes that add up to produce big effects, and if medieval Europeans were knowingly removing nutrients from their forests, I figured modern suburbanites should at least be aware.