More and more puzzle pieces fell into place by the end of each summer’s fieldwork. A picture formed that showed the control forest close to a steady state. The forest had only slow gains of energy and biomass. The forest bank account was still increasing biomass the way your dollars left in the bank gather interest. The dead components of the forest were broken down and released to the soil at about the same rate that plants made new biomass. Nutrients cycled at the same rate that decomposing biomass gave chemical building blocks back to the soil. New rock released about the same amount of chemicals to the soil as forest streams carried out of the ecosystem. As the endless chain of nutrients revolved, dust and rain added significant amounts of new materials to this ecosystem.
A low amount of materials ran out of the ecosystem in the streams compared to runoff in other forests. But enough organic matter escaped to aid aquatic food webs that formed downstream in the black shadows of trees and bushes.
So the control forest held its own and slowly grew more biomass. More nitrogen was found to enter than to leave this undisturbed drainage basin.
On the other hand, the experimental clear-cut community, sprayed to prevent trees and shrubs from growing, was not holding its own. The researchers learned that cutting increased the removal of nitrogen. Over five times as much nitrate was found in the experimental clear-cut streams as had been in the same streams before cutting. With the loss of living root systems to anchor the soil and with the pull of gravity, some soil crept down the slopes. Soil and nutrients were lost in solution as rain and stream waters flowed down the hillsides. There was much more nutrient loss after clear-cutting. The energy-holding organic matter and the nutrient chemical account went down. One experimental area had been cut and sprayed to prevent regrowth immediately, while the other had been cut and allowed to regrow naturally. Though the control forest gained nitrogen, both experimental cut areas lost nitrogen from their ecosystems.
Perhaps the nitrogen-fixing microbes were more active and fixed more nitrogen in a cut-over area. Or, root exudates from living plants might have controlled the number of bacteria or the amount of nitrogen the microbes fixed in the soil. Maybe the plants in the control forest consumed more and held more nitrogen than in a cut-over area. Some, part, or all of these ideas might be true – maybe none. Why were the cut-over areas losing more nitrogen by way of the streams? More data and information were needed. Results often raised new problems to be solved.
In experimental areas where the natural regrowth had not been delayed after a clear-cut, there was an increase in the biomass when the second succession of plants grew. The bank account got big deposits of “the green.” Nutrients and energy increased at a rapid rate. Here the ecosystem was in an unstable, rapidly increasing state.
The clear-cut was the “real” experiment, but descriptive base lines had to be discovered before the changes made by cutting showed up. You couldn’t know how much more or less it rained at your house this year than last by taking only this year’s measurements. You have to know last year’s rainfall measurement too. That measurement would be your base line for comparing this year’s measurement.
By each summer’s end the investigators had made more base-line measurements. They had gotten numbers that could be put into formulas and translated into computer language. They had changed their species counting numbers into biomass, energy, and nutrient measurements. Now such items as amphibians, mammals, birds, and plants – unlike things – could be compared.
Suppose you and a friend wanted to know who had the most groceries. How would you compare? You might change the groceries into dollars and compare their cost. Or if you wanted to compare the quantity, you could change unlike things like apples, milk, and cereal into pounds and ounces. Or each of you might burn your groceries and measure the heat in calories. Then you could see who had had the most potential energy in his shopping bag. Changing unlike things into a common measurement is what many of the researchers did.
The scientists figured out the weight, or biomass, in grams per hectare of the organisms they studied. They figured out the number of calories in each organism’s biomass and the number of calories used by the organism to live. Then they compared their unlike organisms in grams per hectare and calories per square meter.
Birds had less biomass than salamanders or mammals in the forest. But more energy, measured as calories, passed through the bird community. The birders had measured the amount of energy the birds used by measuring their respiration in the laboratory and their activity in the forest. Birds burned a lot of calories to live.
The researchers had a lot of information. They figured out the amount of energy that flowed through certain individual populations of plants and animals. They measured much of the potential energy tied up as biomass in the ecosystem. They measured the new nutrients that entered the ecosystem with the rainfall or that were released from the existing rocks.
Many of the steps in the nutrient cycle and energy flow had numbers that showed how much of the forest’s energy and nutrients were in or used by each member of the Hubbard Brook ecosystem. Scientists had studied the nutrients as they cycled from the ground into plants then into animals. Nutrients moved along food chains to dead animals and detritus that was broken down by small microscopic organisms. Nutrients finally returned to the soil to be recycled into plants, and so the endless chain went round and round.
Some of the nutrients moved out of the ecosystem by way of streams or the territorial wanderings of the animals. But nutrients and energy taken from the ecosystem by departing animals such as the birds and bats were balanced by those brought in with animals entering the ecosystem boundary. And so territorial wandering and migration losses were regained seasonally. The control forest neared a steady state.
