Ecosystem Analysis -- Hydrology -- Chemistry -- Input-Output Budgets -- Weathering -- Nutrient Cycles and Mass Balances -- The Northern Hardwood Ecosystem in the Hubbard Brook Valley in Relation to Other Forest Ecosystems -- Summary Discussion and Conclusions.

Gene E. Likens's work on the biogeochemistry of a forested ecosystem, primarily centered on the long-term
Hubbard Brook Ecosystem Study (HBES), is a seminal contribution to ecology, Hubbard Brook Experimental Forest
. He employed a watershed approach, meticulously tracking the inputs and outputs of water and chemicals, to reveal fundamental ecological processes. A major finding was the discovery of acid rain in North America and its link to fossil fuel combustion, highlighting the critical role of long-term studies in uncovering such environmental problems. His research further demonstrated how acid rain leaches vital nutrients, particularly calcium, from forest soils, impacting forest health and leading to a significant depletion of base cations in both soils and surface waters. Additionally, his work explored the effects of disturbances like deforestation on nutrient cycling and water quality, showing that disruptions to forest ecosystems can lead to increased nutrient export and erosion. Ultimately, Likens's long-term research has significantly influenced environmental policy, including the Clean Air Act Amendments of 1990, and continues to underscore the value of sustained ecological investigations for understanding and addressing complex environmental challenges.