Hubbard Brook researchers have been studying bird populations for over 50 years. This graph shows the number of birds breeding on a 10-hectare forest plot from 1969-2013. Neotropical migrants show a marked decline early in the record, followed by a stabilization of population more recently. Populations of hort-distance migrants and permanent residents have been stable throughout this period.
The streams at Hubbard Brook are closely interconnected with the forests around them. Ecological theory led us to expect that in the period that we have been measuring the stream chemistry (1963-present), stream nitrate concentrations should have increased because of the continuing deposition of air pollutant nitrogen on the ecosystem, and because the forests have matured and are no longer accumulating nitrogen. In contrast to those expectations, stream nitrate concentrations have declined in the last several decades, leading us to delve deeper into the nitrogen cycle of the forest to explain this surprising finding.
View of the greening-up canopy of the Bartlett Experimental Forest, a sister site to Hubbard Brook, from the top of a tower where carbon dioxide exchange in measured by the eddy covariance technique. The instrument in the foreground is a sonic anemometer. These measurements of the exchange of carbon dioxide between the forest and the atmosphere provide a direct measurement of forest productivity. Year-to-year variation in how much carbon dioxide the forest removes from the atmosphere is strongly related to the timing of the start and end of the growing season, with early spring leaf-out and delayed autumn senescence both tending to increase carbon uptake.
Ecosystems at Hubbard Brook have been subject to acid deposition for over 50 years, leading to the depletion of important nutrients, such as calcium, from the soils. We experimentally replaced the depleted calcium on an entire watershed by spreading a calcium-containing mineral from a helicopter. There was an almost immediate response by the vegetation, particularly sugar maple trees. The increased growth of sugar maple reversed the forest stagnation and decline that had been occurring previously and which continued to occur in a nearby reference watershed.