Tag Archives: forest

The history beneath your feet

I think about dirt a lot more than most people. Probably this is the result of my background in ecosystem science, the study of how nutrients and energy flow throughout the biosphere. The soil represents a huge sink for all major nutrients that sustain life on earth, and an important source of nutrients such as carbon and nitrogen that are naturally recycled to the atmosphere.

But it’s not economic, or really even environmental value that makes soil so fascinating to me. It’s history. In the forests of northeastern Puerto Rico where I’m currently conducting research, the historic record deposited in soil is up to 8, 10 or 15 meters thick and stretches back 300,000 years into the past, to a time when the island itself was shooting up out of the Gulf of Mexico due to rising magmatic rock deep beneath the ocean floor.

If you start looking closely at the composition of soil, you will quickly discover a wealth of information recorded within it. Tiny grains of minerals, produced from thousands to millions of years of water dissolving rock, form the physical matrix on which life has developed. As these clean, crystalline minerals slowly rot, they are chemically transformed into new compounds such as clays. Clays and other secondary minerals add stickiness to the soil, allowing decomposing organic materials to adhere. Slowly, a strange collection of organic materials that were once living and the inorganic ingredients that supported their existence begins to accumulate. As this assortment of the dead and rotting grows, so does the living biomass that it sustains. Most of the soil microfauna is involved in feeding off dead (or other living) organic materials and ultimately recycling nutrients that would otherwise be locked away forever. Embracing death is a way of life in the world’s most biologically diverse ecosystem.

But wait- I was talking about history. Yes, living organisms, dead organic materials, clays and minerals are all important components of the soil, but how do we piece together a history (and of what? The geology that the soil formed over? The forest that once stood atop it? The long-dead animals whose traces still linger within it?) from such a complex and dynamic system?

The answer is not entirely clear. But I am convinced that history is sitting in the dirt, rotting away, waiting patiently for someone to find a way to unwrap the stories contained within.

 

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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.

Fossil forests reveal a subtropical Antarctica

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Antarctica today is buried under a sheet of ice up to 5 miles thick, but this wasn’t always the case.  Fossilized forest stands of the now-extinct tree known as Glossopteris have been found in northeastern Antarctica. These trees existed in stands as thick as 20,000 per acre. These fossils have been found at 20-25 degrees from the South Pole; a latitude which today receives no sunlight for half of the year.

Glossopteris fossils provide important evidence for currently accepted distribution of continental plates in the Permian period that ended 250 million years ago. Fossils have been found in regions as distant as Patagonia, India and southern Australia. In the Permian, these landmasses were joined into a southern supercontinent known as Gondwana. The mass extinction that marks the end of the Permian period is believed to have led to the disappearance of Glossopteris.

The distribution of several extinct species in Gondwana. Glossopteris distribution shown in green.

These ancient forests tell more than just continental distribution, however- they provide insight into ancient climates, and possibly even into a major event in plant evolutionary history.

Paleobotanists have reconstructed Glossopteris as a tree that tapers upward like an evergreen. However, the leaves of this tree were broad and lance-shaped, and are thought to have fallen at the end of the growing season. A much warmer climate would have had to exist for such a tree to flourish. This corroborates paleoclimate data, which places Antarctica in a subtropical climate zone during the Permian.

A specimen of Glossopteris with well-preserved reproductive structures was found in Queensland, Australia. Dating to 250 million years ago, the structures found in this specimen indicate a very simple form of pollination . The pollen tubes and ovule examined from this in Glossopteris imply a close relationship with extant seed plants such as conifers and angiosperms. Angiosperms, the widespread group of flowering plants that dominate many terrestrial ecosystems today, are not thought to have evolved until over a hundred million years later. Glossopteris may therefore represent a missing link in the early evolution of pollination biology.

An artist’s rendition of a Glossopteris tree

For anyone interested in the evolutionary history of plants, this is a great interactive timeline developed by plant biologists at Cambridge University.

http://www.ensemble.ac.uk/projects/plantsci/timeline/timeline.php

Citations
1.    Nishida, H., Pigg, K.B. & Rigby, J.F. Palaeobotany: Swimming sperm in an extinct Gondwanan plant. Nature 422, 396-397 (2003).
2.    Pigg, K.B. & McLoughlin, S. Anatomically preserved Glossopteris leaves from the Bowen and Sydney basins, Australia. Review of Palaeobotany and Palynology 97, 339-359 (1997).
3. Peter Jupp on “Ancienct Destructions”
http://www.ancientdestructions.com/site/destructions/antarctic-fossil-forests.php