Last Modified: 4/21/03
by John Campbell

Justification and Problem Selection...

A major goal of forest management and environmental policy is the protection of long-term productivity of forest and aquatic ecosystems. Biogeochemical cycling of essential plant nutrients is an important ecological process that has direct bearing on productivity and sustainability. In forests with limited nutrient capital, such as much of the forest landscape in New England, the efficient cycling of nutrients between soil, plant and forest floor is critical to maintaining healthy and productive forests. Over the past 40 years, New England forest ecosystems have received significant amounts of atmospheric deposition which has altered biogeochemical cycling by leaching base cations from the soil and changing nitrogen dynamics. Additionally, the cumulative effects of natural disturbances, past landuse (logging and/or farming), current intensive harvesting practices, and projected future logging can impact available nutrients through removal of nutrients in the harvested biomass and short-term increases in leaching.

Aquatic ecosystems are also impacted by acidic deposition, primarily through changes in their terrestrial source areas. As base cations become depleted from soils, there is less buffering capacity and increased amounts of aluminum are leached into streams and lakes. The prospect of nitrogen saturation will further acidify streamwaters and will affect other aspects of chemical and biological quality. Understanding the linkages between terrestrial and aquatic ecosystems is critical in providing better information on the long term impacts of acidic deposition.

Potential changes in future forest productivity and stream quality due to the interactive effects of acidic deposition and timber harvesting are determined by inherent site factors that influence base cation availability, aluminum mobilization, and nitrogen dynamics. Base cations are primarily supplied by weathering of minerals from either bedrock or glacial till. Our understanding of this process is limited and it is difficult to adequately quantify inputs. Although we can identify soil characteristics that make a site susceptible to significant reductions in nutrient capitals, we do not know the distribution of sensitive sites throughout the region. Recent studies have indicated that New England ecosystems are more prone to nitrogen saturation than previously thought. It is important to gain a better understanding of the processes that control nitrogen retention and find efficient methods to evaluate the degree of nitrogen saturation at a given site. Changes in nutrient availability, including calcium depletion and nitrogen retention, will have subtle, long-term effects on forest growth and composition.

To address these concerns we have selected five problems:

  • How do the cumulative effects of disturbance and atmospheric deposition affect biogeochemistry of nutrient base cations, forest health, and ecosystem function in northern hardwood forests? What is the regional extent of cation depletion and what additional tools are needed to document the change?
  • What is the range of mineral weathering contributions to nutrient capital in northeastern forests?
  • What processes and conditions control the extent to which forest and aquatic ecosystems respond to nitrogen deposition? What factors control nitrogen retention in New England ecosystems?
  • What are the dynamics of community structure, biomass accumulation, and nutrient uptake of northeastern forest ecosystems, and how are they effected by disturbances?
  • Synthesize research knowledge and long-term data bases to develop guidelines for natural resource managers to protect stream quality, wetlands, and riparian areas.

Much of the research on these problems will be conducted at Hubbard Brook Experimental Forest in conjunction with many cooperators. This Research Work Unit is responsible for administration of Hubbard Brook Experimental Forest, which is a National Science Foundation Long Term Ecological Research site and a Biosphere Reserve.

Approach to Problem Solution...

Problem 1...

How do the cumulative effects of disturbance and atmospheric deposition affect biogeochemistry of nutrient base cations, forest health, and ecosystem function in northern hardwood forests? What is the regional extent of cation depletion and what additional tools are needed to document the change?

Previous work by this Research Work Unit has shown that both acidic deposition and timber harvesting contribute to removal of base cations, such as potassium, magnesium, and especially calcium, from forest ecosystems. New England forests in particular have been subject to heavy acid deposition loads and repeated timber harvest. Continuing losses of base cations due to these disturbances will lead to nutrient deficiencies and imbalances and subsequent declines in forest health and productivity.

We will use 3 approaches to study this problem: (1) use calcium addition studies to better understand the processes and impacts involved in both calcium depletion and recovery, (2) use a regional survey to document the extent of calcium depletion and attempt to find either a soil or vegetative index for calcium levels, and (3) continue using watershed ecosystem analysis to refine cation budgets for New England forests and determine impacts of disturbance.

Calcium addition studies. There is increasing concern that ecosystem productivity of northern hardwood forests in the White Mountains may be limited by calcium availability. This concern is based on information collected at Hubbard Brook Experimental Forest and the surrounding region. First, periodic remeasurement of the long-term reference watershed (W6) at HBEF shows that the forest is not growing at the rate expected and in some areas tree mortality is high. The second source of information is the significant reduction in the amount of Ca in the forest floor of W6 over a 22 year period. Although some of the reduction of Ca in the forest floor is due to plant uptake, leaching losses due to acidic deposition and recent reductions in calcium deposition have occurred over the past 30 years. Concern about the availability of Ca and possibly other base cation nutrients has been reinforced by projections of Research Work Unit scientists regarding the status of ecosystem base cation capitals over the next 100 to 120 years.

Calcium availability to plants may also be affected by increased mobility of Al that accompanies soil acidification. Elevated concentrations of Al inhibit the uptake of Ca by some plants. Sugar maple has been shown to be sensitive, with inhibition of base cation uptake occurring at soil water Al concentrations of 80 micromoles per liter.

We will investigate how changes in nutrient availability affect northern hardwood ecosystems by using forest plots to study the response of vegetation and soils to additions of both Ca and Al. This study will be conducted at HBEF in stands dominated by sugar maple. In addition to providing important information on the response of trees, stands and soils to added Ca and Al, results and methods development from this study will be used in the design and implementation of a watershed scale, chemical manipulation experiment under consideration for the Hubbard Brook Ecosystem Study.

Regional survey. A regional survey to (1) determine if cation depletion is a problem in low-elevation, commercial forests in New England, (2) develop criteria for evaluating whether an ecosystem is likely to be deficient in cations, and (3) find a few simple indices that managers could use to determine if a site is depleted in base cations was initiated in 1992. This study utilizes plots along a gradient from upstate New York eastward to southern Maine. The plots were purposely selected to represent worst-case scenarios in terms of nutrient depletion. Criteria included deep, sandy soils with low cation availability, a past history of intensive land use, and either white pine or red oak types. The study is scheduled for completion in 1996-97. A second phase built upon results of this regional survey will involve analyzing the Northeast Station's Forest Inventory and Analysis data base to identify sites in New England where cation depletion would be most probable. Such information could be helpful in developing guidelines on harvest intensities and other management options.

Watershed ecosystem analysis. The Work Unit has a long history of utilizing small, gaged watersheds to analyze biogeochemistry and nutrient cycling in forest ecosystems. In the past several years, this approach has been applied at sites away from Hubbard Brook, including the Cone Pond and Sleeper's River research watersheds, in order to obtain regional representation needed to scale research results to other areas. Cone Pond watershed is an acidified watershed that has experienced natural distubances including fire and extensive blowdown during hurricanes. Sleeper's River is an alkaline site that had been intensively harvested prior to 1930. The watershed studies will continue to address processes and impacts related to regional issues including cumulative effects of past, present, and future land uses, cation depletion, and nitrogen saturation.

Research within this problem will be tailored to produce guidelines for protecting nutrient capitals of New England forests. Anticipated users are managers of industrial and government-owned forests and also small private woodland owners such as members of New Hampshire Timberland Owners Association. Findings that relate to streamwater concentrations of cations will be of value to aquatic managers, particularly those that must assess fish habitat. Although most of this research will be conducted in New England, results will be useful throughout the Appalachian Mountains in the East. Results from this problem, and from problems 2, and 3, will aid in understanding processes and mechanisms related to impacts of acidic deposition on forests. Results will be useful in development and application of models of how soils and associated forest ecosystems respond to loss of calcium and/or increased cycling of nitrogen. Methods developed as part of this research will assist researchers working in other parts of the Northeast.

The response of forests and aquatic ecosystems to nutrient deficiencies and imbalances is subtle and complex. The plot level study of forest response to calcium additions will provide valuable results over the next five years, but the results will only be specific to one particular set of site factors. We plan to expand this study to a full watershed manipulation, but will need external funds for this long term study which will extend past this current Work Unit description. We expect a high level of success from our regional survey, but like the calcium addition study, it is specific to a limited set of site factors. Thirty years of experience using watershed ecosystem analysis will ensure a high level of success over the next five years comparing the processes in ecosystems that span a wide range of sensitivity to acidification.

Problem 2...

What is the range of mineral weathering contributions to nutrient inputs and buffering capacity in northeastern forests?

Mineral weathering, a basic soil forming process, is the major source of several nutrient elements (Ca, Mg, K) in forested ecosystems, and is responsible for the majority of H-ion neutralization in forest soils. Variation in weathering rates over short temporal and spatial scales may be great, but is difficult to predict due to limitations in methodology for measuring and modeling this process. Watershed mass balance studies often provide the best framework for estimating weathering rates in the field. However, production of base cations by weathering may be difficult to separate from changes in the forest floor or soil exchangeable cation pools, and from soil losses due to storage in aggrading biomass pools.

Our approach is to develop field-based methodologies for detecting spatial and temporal variations in weathering rates. Isotopic tracers have recently been developed which allow estimation of weathering rates independent of changes in biomass storage or soil depletion. Isotopic analysis coupled with studies of mineral depletion across soil profiles will be pursued in order to develop estimates of long-term temporal variation in weathering rate. Use of these methods with archived samples will allow estimation of shorter-term variations. Temporal variations could then be evaluated for correlations with soil mineral depletion, air temperature, precipitation quantity, and acidity.

Bedrock mapping shows that the schist bedrock at the Hubbard Brook Experimental Forest is quite variable in its mineralogic and chemical composition. Our research will correlate variation in bedrock composition with variations in streamwater chemistry and nutrient fluxes across the experimental watersheds.

Understanding of spatial variation in weathering is complicated by redistribution of various rock types by glacial erosion and deposition. We have devised a model which predicts distribution of bedrock sources in glacial till, the soil parent material for most of the northeast. Further development of this model will include expansion to predict soil parent material mineral and bulk chemical composition, validation at forested sites across the region, and automation with GIS to allow analysis of regional patterns. These features will allow development of regional predictions of sensitivity to acidification and base cation depletion that are much more accurate than those based solely on bedrock geology. This model can be combined with other spatially explicit data bases on soil properties, forest cover, and deposition to provide a better understanding of potential impacts of atmospheric deposition on ecosystems. Additionally, the ability to extrapolate research results from experimental watersheds to unmonitored sites and to the regional level will be enhanced.

Studies of weathering induced by lichens and bryophytes on bare rock surfaces will be pursued to quantify weathering on exposed outcrops. These communities may contribute to watershed fluxes far in excess of their aerial proportion due to production of lichen acids, and the mechanical influence of lichens in increasing effective surface area through frost-wedging.

Research on this problem involves new and unique approaches. We face limitations in data collection due to the labor intensity of sample preparation and constraints on isotopic analysis (low sample volume). However, we expect a moderate to high chance of success in using this novel approach for calculating weathering rates. Till source area modeling using a GIS will require cooperative efforts in data base development, but should not limit our ability to accomplish this research.

Problem 3...

What processes and conditions control the extent to which forest and aquatic ecosystems respond to nitrogen deposition? What factors control nitrogen retention in New England ecosystems?

Elevated nitrate loss from forest ecosystems is an indicator of ecosystem damage and disruption of the nitrogen cycle. Harmful effects of elevated nitrate in terrestrial and aquatic ecosystems include acidification of soil and surface waters, plant nutrient imbalances, forest decline, and eutrophication of surface waters. Natural and anthropogenic disturbances, such as pest outbreaks, atmospheric nitrogen deposition and clearcutting, can cause elevated nitrate levels in streamwater. Recent research has suggested that the relationship between nitrate loss and factors such as nitrogen deposition is not simple. Better understanding of the effects of nitrogen deposition is critical for developing air pollution policy.

Our long-term goal is to determine which factors control nitrogen retention in forest and related aquatic ecosystems in New England. We will begin addressing this problem with a combination of field and laboratory research. Field work will include regional measurements of N in surface water. This regional component is important both in order to determine how similar the Hubbard Brook Experimental Forest is to other ecosystems in the region, and also to provide results that are relevant for regional forest management. We will evaluate the importance of dissolved organic nitrogen (DON) as a form of nitrogen loss from forest ecosystems. In addition, we will examine linkages between the carbon and nitrogen cycle including the relationships between nitrate, DON, and dissolved organic carbon (DOC). Finally, we will evaluate the potential of natural abundance of 15N in forest soils as a tool for assessing nitrate loss patterns in streamwater.

We expect a high level of success of accomplishing this research. The Work Unit efforts on nitrogen retention compliment research being conducted by scientists from several universities, a private research institute, and other federal agencies. Our research will be integrated with that of cooperators in the Hubbard Brook Ecosystem Study and the HB-LTER Program. Cooperators are investigating deposition and export of N, biogeochemical cycling, and microbial mediated processes at Hubbard Brook Experimental Forest. A new area of cooperation that we will specifically develop is with the microbial ecologist in RWU NE-4505.

Problem 4...

What are the dynamics of community structure, biomass accumulation, and nutrient uptake of northeastern forest ecosystems, and how are they effected by disturbances?

Responsible sustainable ecosystem management requires the maintenance of long-term forest productivity. This concept specifies that management activities not degrade the ability of a site to support a healthy, vigorous forest. The forests of the Northeast have a history of major disturbances including clearing for agriculture, forest fires, hurricanes, extensive logging at the turn of the century, recent intensive harvests, and acidic deposition. How have the forests of the Northeast responded to these disturbances over the decades? How have these disturbances affected the regeneration potential of these forests? Will future stands be similar in composition, biomass, and nutrient content to those of the past?

These questions will be answered by using a series of permanent research plots. At the Hubbard Brook Experimental Forest in central New Hampshire there are a series of watersheds that have been harvested. Watershed 5 (WS 5) was whole-tree clearcut in 1983-84; WS 101 was conventionally clearcut in 1970; WS 4 was clearcut in progressive strips in 1970-74; WS 2 was clearfelled in 1965 and sprayed with herbicides for 3 years; WS 1, 3, 6, and 7 were logged very heavily in 1907-9. Little information is available concerning changes in natural forest stands during the first 30 years after harvest. Understanding the dynamics of this early succession over time and across the landscape provides essential information to forest managers. Permanent vegetation analysis plots on WS 2, 4, 5, and 101 are providing this information, including the dynamics of the herb, shrub, and tree communities. Currently data are available from repeated measurements of the 85 year old stand on WS 6. An intensive survey of WS 1 was conducted in 1965 when the stand was about 55 years old. We propose to duplicate that survey to assess changes over the 30 year period.

Ecologists frequently uses the 80 year old forests at HBEF as reference areas. But, since they developed following logging, how do they compare with unlogged areas? The Bowl Research Natural Area is near Hubbard Brook and, as far as we can tell, has not been logged. We conducted a major survey of the vegetation of the Bowl in 1974 and again in 1994 with permanent plots established in 1994. These data will be used to compare the old-growth stand with mature stands, and to assess changes over the 20 years. These comparisons should give us insight into natural changes that occur as forests mature, and possibly the reactions to 20 years of acidic deposition. Likewise we will continue present studies of old-growth vegetation on the Cone Pond watershed located 10 km east of Hubbard Brook. Studies of tree rings and soil charcoal show that this 33 ha forest experienced hurricane damage and severe fire in the early 1800s, and hurricane damage again in 1938, but that there has been no land clearing or logging. This provides an opportunity to obtain baseline data on forest development after natural disturbances to contrast with stands that have had past human disturbances.

We also have permanent plots established on 3 contiguous watersheds in the central hardwood forest of south-central Connecticut. The stands include a mature reference watershed, a stand that was thinned, and a stand that was whole-tree clearcut in 1981. These data are providing good initial stand development information that is otherwise unavailable in the northeast region of the oak-hickory forest which has very low base cation reserves.

Results of this research will be provided to audiences through several outlets. For the scientific community, we will be able to address questions of forest development and diversity from clearcuts to old-growth conditions through scientific journal articles. For landowners and land managers we will be able to predict future stand conditions following intensive harvest and thinnings through papers in applied journals and technical reports and presentations at professional meetings. For the environmentally concerned public, we will be able to provide information on the recovery of forests following disturbances through publications and tours of the research sites. We expect a high level of success in accomplishing this research.

Problem 5...

Synthesize research knowledge and long-term data bases for forest and aquatic managers and policymakers to protect stream quality, wetlands, and riparian areas.

Technology transfer is an important responsibility of the Work Unit. Past efforts have included incorporating Work Unit findings into Best Management Practices to control non-point pollution from forests, providing information to National Forest planners, and preparing handbooks on ecology and management of New England forests. Use of the ecosystem approach, pioneered at Hubbard Brook, and findings on acid rain have been widely used by policymakers to develop legislation and management approaches. The Forest Service Rensselaerville Roundtable in 1994 emphasized continuing and expanding efforts to integrate science and policymaking.

We will continue to devote efforts to technology transfer, including the area of Best Management Practices. In recent years, the Hubbard Brook Ecosystem Study has obtained findings relevant to Best Management Practices for protecting riparian areas, wetlands, and fish habitat. These findings will be synthesized and transferred to forest and aquatic managers and state regulatory agencies. National Forests in Region 9 are also an important audience, particularly in regard to information that will assist their efforts to reintroduce Atlantic salmon to upland, forest streams in New England.

The Northeast Station is deeply committed to developing the Northeast Decision Model, a PC based model designed to aid forest managers in selecting appropriate management practices. The model utilizes knowledge about a variety of forest amenites including water, wildlife, and aesthetics. Long-term data and knowledge about water resources obtained from the Hubbard Brook Ecosystem Study is critical to the decision model, and will be incorporated by Work Unit scientists. Also, work will continue on a stand-alone decision model dedicated to water resources and drawing information from all Northeast Station watershed units. This model will allow forest and aquatic managers to choose from a variety of water yield and water quality goals, then provide them with management recommendations. Area of applicability will be the 20 states administered by Northeastern Area State and Private Forestry.

The White Mountain and Green Mountain National Forests rely on research findings from Hubbard Brook to develop Forest Plans, comply with the National Forest Management and Clean Air Acts, and address public challenges to management activities. Our Work Unit will continue to synthesize research findings and data to help National Forests meet these needs. Stream chemistry and nutrient cycling in Class I Wilderness Areas in GMNF and WMNF will be studied for 3-5 years to provide Region 9 with important baseline data for screening Applications for Prevention of Significant Damage. This research will also be useful in ongoing efforts to scale-up Hubbard Brook research to regional levels.

Mission Problem. The Hubbard Brook Experimental Forest will be maintained:
(a) for forest ecosystem research;
(b) as a Biosphere Reserve; and
(c) as one of 18 or more national sites for Long-Term Ecological Research (LTER).

The Hubbard Brook Experimental Forest has been operated for 40 years by the U.S. Forest Service to gather hydrometeorologic information. For the past 32 years we have cooperated with many educational institutions, federal and state agencies, private foundations, and foreign groups in the collaborative Hubbard Brook Ecosystem Study. This long-term monitoring of baseline parameters of the forest will continue and include: precipitation quantity and chemistry, streamflow quantity and chemistry, physical and chemical characteristics of soil and soil solution, meteorological parameters, and vegetative cover. The small watershed approach using experimentally treated watersheds for examining biogeochemical processes in forest ecosystems, pioneered at Hubbard Brook, will continue to be used to test new hypotheses. Coordination of the large number of scientists and their assistants working at Hubbard Brook will continue to be the responsibility of this work unit.

The work unit is also responsible for archiving and retrieval of long-term data sets for the Hubbard Brook Ecosysem Study. The unit's data manager collects and stores data files and documentation from cooperators, makes them available to other Hubbard Brook scientists, and maintains an Internet Gopher and microcomputer bulletin board for data files that are available to the general public.

Considerable effort is spent in presenting information about research results from the Hubbard Brook Experimental Forest to many visitors (about 600/yr) including scientists (foreign and domestic), administrators, legislators, managers, and practicing foresters, conservation groups, students (college, primary schools) and the general public.


This work unit cooperates with many educational institutions, private foundations, and federal and state agencies. The major cooperators and their contributions include:

Cooperator Scientists Graduate Students Technical Assistance Land for Study Sites Funds
Cornell University X X      
Dartmouth College X X      
University of New Hampshire X X      
State Univ. NY, Syracuse X X X    
Brown University X        
Syracuse University X X      
University of Vermont X X      
Yale University X X      
Institute of Ecosystem Studies X        
USFS NE RWU-4505 X        
USFS Green Mt. Nat. For.       X  
USFS White Mt. Nat. For.       X  
U.S. Geological Survey X X X X  
Natural Resource Conservation Service     X    
National Science Found.         X
Connecticut State Dept. Environmental Cons.     X X  
Appalachian Mountain Club X     X  
Cooperative Forestry Research Unit (Univ. Maine) X     X