Gene E. Likens

Seasonal patterns in acidity of precipitation and their implications for forest stream ecosystems

Data collected since 1965 at a network of nine stations in the northeastern United States show that precipitation is most acid in the growing season (May-September) and least acid in winter (December-February). For the Hubbard Brook station in New Hampshire, where the mean hydrogen ion content of precipitation ranges between 46 peq 1−1 in winter and 102 peq 1−1 in summer, the seasonal pattern in acidity correlates closely with seasonal differences in S deposition from the atmosphere. As summer precipitation passes through the forest canopy, H ion concentrations are lowered by an average of 90%, primarily as a result of exchange with other cations. In winter the H ion content of incident precipitation is lowered from a mean of 50 peq 1−1 to a mean of 25 peq l−1 during storage in the snowpack.

Chemical Concentrations in Cloud Water from Four Sites in the Eastern United States

Event samples of cloud and fog water were collected in 1984 and 1985 as part of the Cloud Water Project, a large-scale network designed to chemically analyse cloud and rain water from ten sites in North America. The data presented here are from four sites in the eastern United States that ranged in elevation from 5 m to 1534 m, and in geographic location from Virginia to Maine.

The Steady State as a Component of the Landscape

Models of ecosystem development usually portray autogenic succession as an orderly progression of biologic changes (e.g., Odum, 1969; Woodwell, 1974). The macroenvironment within which development occurs is presumed to be more or less constant throughout the autogenic sequence. Yet every terrestrial ecosystem is subjected to a range of disturbances varying from those that barely alter the structure, metabolism, or biogeochemistry of the ecosystem to those that wholly or dramatically change the system. Defining “disturbance” is itself a considerable problem, because it is difficult to draw a line between biological and physical-chemical events that may be considered within the scope of autogenic development and other events that might be considered to seriously deflect the autogenic pattern. In developing the Hubbard Brook Biomass Accumulation Model (Chapter 1) of ecosystem development, we followed the procedures of Odum (1969), Botkin et al. (1972a,b), and Woodwell (1974) and emphasized autogenic development, while deemphasizing exogenous disturbance. This was a necessary decision if our model was to reflect an uninterrupted sequence from the initiation of secondary development to the establishment of the steady state.

Introduction to Ecosystem Science

This introduction briefly describes the book’s content. The book defines the ecosystem, describes the chief characteristics of ecosystems and the major tools used to analyze them, and presents major discoveries that scientists have made about ecosystems. It also lays out important questions for the future. And although the book is not specifically about ecosystem management, some management implications of ecosystem science are described.

Development of Vegetation after Clear-Cutting: Species Strategies and Plant Community Dynamics

The focus of this chapter is the regrowth of vegetation during the Reorganization and Aggradation Phases. Our emphasis will be on plant community dynamics and species strategies, but it must not be forgotten that ecosystem development involves an array of organisms—plants, animals, and microorganisms. Parasites, predators, symbionts, and decomposers may speed or direct changes in plant populations, or they themselves may decline or disappear as a result of changes in plant-community dynamics. Integrating our knowledge of these relationships represents a major challenge for future ecosystem studies.

Ecosystem Development and the Steady State

The biomass accumulation model of ecosystem development which we propose (Chapter 1) has four phases after clear-cutting: Reorganization, Aggradation, Transition, and Steady State (Figure 1–2). In discussing this model, our strategy has been to move from the most verified to the least verified aspects. Many elements of the first two phases, Reorganization and Aggradation (Chapters 1–5), are based on observation and measurement of, and experiment with, actual stands; and the conclusions now can be tested and evaluated. We now consider the remaining phases of the model, Transition and Steady State.

Reorganization: Loss of Biotic Regulation

The Reorganization Phase in the northern hardwood developmental sequence is characterized by drastic changes in hydrologic, energetic, ecological, and biogeochemical processes that in the Aggradation Phase were fairly constant and predictable. Rates of net primary production, transpiration, and nutrient uptake registered by plant growth during the first growing season after cutting are far below levels in the uncut forest. There are also rapid and marked increases in internal ecosystem parameters like decomposition, nitrification, available soil moisture, and soil temperature and export parameters like summertime streamflow, nutrient concentration in stream water, and erosion. Cutting also imposes immediate and significant shifts in stores of nutrients and organic matter in the living (loss) and dead (gain) biomass compartments of the ecosystem.

The Northern Hardwood Forest: A Model for Ecosystem Development

A. S. Watt in his classic paper, “Pattern and process in the plant community” (1947), isolated a central dilemma of modern ecology. n…clearly it is one thing to study the plant community and assess the effects of factors which obviously and directly influence it, and another to study the interrelations of all components of the ecosystem with an equal equipment in all branches of knowledge concerned. With a limited objective, whether it be climate, soil, animals or plants [populations] which are elevated into the central prejudiced position, much of interest and importance to the subordinate studies and… to the central study itself is set aside. To have the ultimate even if idealistic objective of fusing the shattered fragments into the original unity is of great scientific and practical importance; practical because so many problems in nature are problems of the ecosystem rather than of soil, animals or plants [populations], and scientific because it is our primary business to understand…. [As] T. S. Eliot said of Shakespeare’s work: we must know all of it in order to know any of it.

Forest Harvest and Landscape Management

When designing a plan to harvest forest products, forest managers face a variety of questions: Will the plan yield the most satisfactory profit commensurate with short- and long-term expectations? Does the plan promote or allow for the establishment of an adequate crop of new individuals of desirable species? Is the environmental impact acceptable? How can a high level of productivity of the forested ecosystem be sustained? What is the maximum or optimum rate of harvest?

Reorganization: Recovery of Biotic Regulation

The aggrading northern hardwood ecosystem has a considerable capacity to exercise control over hydrology and biogeochemistry and to regulate the flow and use of solar energy (Chapter 2). When this control is at its maximum, the ecosystem is most stable, with highly predictable and low net losses of nutrients and a fairly constant annual evapotranspiration rate. Regulation is rooted in internal ecosystem processes such as transpiration, nutrient uptake, decomposition, mineralization, and nitrification. Clear-cutting results in marked changes in internal ecosystem processes and a distinct loss of biotic regulation over energy flow, biogeochemistry, and hydrology (Chapter 3).