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.