Introduction Page 11A |
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As you have seen in previous pages of this Introduction, we know that, on average, rainfall is about the same every month of the year, and streamflow levels are highest in March, April and May. As the graph to the left shows, we also know if a year is drier or wetter than average. We know these things because scientists have been monitoring precipitation at over 20 different locations in the HBEF for over 40 years. According to this graph, what are the three wettest years? The two driest? Was the year you were born wetter or drier than normal? What does this graph indicate about the annual variability of precipitation at the HBEF? Do you see a trend in these data? Can you think of reasons why it might be important to know the Valley has received more (or less) precipitation in one year than it does in an "average" year? Scientists have also monitored snow depths at several locations throughout the HBEF. The graph to the right shows the snow depth in the forest immediately adjacent to Rain Gauge 2 (near the bottom of Watershed 1; see map) on the sampling date closest to 1 February every year from 1956-2001. What can you tell from this graph? Is early February snowpack variable in the HBEF, or is it the same every year?
Can you tell from this graph alone if more snow falls now than in 1956 - or would you need more information? What are some reasons for long-term snowpack monitoring? Monitoring environmental variables is useful because it helps scientists learn more about the functioning of ecosystems. For example, we now know that annual precipitation and snowpack levels vary from year to year. This variability helps to explain many other patterns in the HBEF ecosystem, such as tree growth and streamwater chemistry. Long-term monitoring can also make us aware of trends that might be influenced by humans. For example, as you saw in previous pages in this Introduction, we have been able to learn about trends in acid precipitation because we have monitored and analyzed precipitation samples for over 40 years. Soil & Lead HBEF scientists have also been studying soil since the beginning of the HBES. They have studied the amount of organic matter (from living material) in the soil, depths of different horizons, temperature, water content, soil solution (water in the soil), and different elements and nutrients in the soil, including lead. The graph below shows how lead levels in the soil in Watershed 6, the reference watershed, have declined over time.
Why do you think it might be important to know how much lead is present in soil? Can you think of anything that might explain this apparent decline in lead? As you may know, before 1973 gasoline used in automobiles contained lead. Lead is especially harmful to the brain, kidneys and reproductive systems of humans, and can accumulate in our bodies as we breathe it in. It also has negative impacts on our environment. As a result, only unleaded gasoline is now sold and used in our cars. When leaded gasoline was being used, lead would leave cars in exhaust, and eventually get deposited on the Earth's surface - including the HBEF. This graph shows that after leaded gasoline began to be phased out, soil lead levels decreased in the HBEF. Because scientists were monitoring soil lead levels in the 1970s, and continued to monitor these levels for several decades, we can hypothesize that the ban on leaded gasoline may have been responsible for this downward trend. In order to track changes in a forest community over time or space, or to compare forest communities in different regions, it is essential to be able to quantify the forest in some meaningful, consistent way. At the HBEF we do this in two ways. First, we quantify forest composition in terms of total basal area and density for each tree species. Basal area is the cross-sectional area of a tree trunk at breast height (1.37 m above ground level), and is therefore a measure of the size of the trees. Density is strictly the number of stems over a given area. Second, we measure biomass of the forest. Biomass is simply the mass (or weight) of the trees, and can be broken down into different plant parts. Biomass, when divided into size classes and followed over time, gives us an idea of the maturity and productivity of a forest stand. Mature forests have most of their biomass in larger trees, while younger forests have more of their biomass in saplings or even shrubs and herbs. The graph below shows biomass data for Watershed 6.
* Johnson, C. E., T. G. Siccama, C. T. Driscoll, G. E. Likens and R. E. Moeller. 1995. Changes in lead biogeochemistry in response to decreasing atmospheric inputs. Ecol. Appl. 5(3):813-822. |
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