(3a) Plants and Soils: "Biophysical" measurements and models

Need and Objectives for Enhanced Measurements and Models

Regional and continental scale carbon fluxes between the soil-plant system, the atmosphere and the ocean are the product of diverse ecosystems responding to the interactive effects of climatic and edaphic factors, natural disturbance, air pollution, land use and management. Soil- and plant-associated sources and sinks of carbon are intimately coupled, but on short and medium time scales they may respond in different ways, and even in different directions, to climate and/or human perturbations. Land/atmosphere carbon exchange is highly variable because of these factors, both temporally and spatially. In this document, we use the term "biophysical" to refer to the ensemble of physiological, ecological, climatic, and other environmental factors that regulate fluxes of major carbon gases from terrestrial ecosystems, including soils.

There is a relatively good knowledge base of the size and distribution of soil and vegetation carbon stocks of the major terrestrial ecosystem types in the U.S. However, there are still very limited data on changes of soil and vegetation carbon stocks (i.e. whether ecosystems are functioning as net sinks or sources for CO2). In non-forested ecosystems (e.g.. grasslands, croplands, wetlands), comprising 2/3 of the U.S. land surface, variation and change in soil carbon is the overriding control on net ecosystem C flux. Therefore understanding and quantifying soil C fluxes and C stock changes are imperative for understanding the continental-scale C balances. Changes in live and dead above-ground biomass is important in forests, and in some non-forest systems. Change in soil C may be important over the longer term for forests.

Ecosystem C flux is poorly documented for some ecosystems. For example, “woody encroachment”, which refers to the expansion of sparse woodlands into grasslands as a result of grazing and fire suppression practices, has been identified as a likely significant carbon sink, yet there are few estimates available. Comprehensive, systematic measurements of carbon stocks and fluxes in soils and vegetation will be a major product of the NACP.

Peatlands are particularly important among currently non-inventoried ecosystems. Though peatlands cover only 12% of the surface of North America, and total ecosystem productivity rates are low, stocks of soil carbon are huge. [Harden et al. 1992]. An amount of C (~455 Pg) equal to about 60% of the C pool in the atmosphere is stored within meters of the surface. Peatlands and non-forested wetlands are also significant sources of atmospheric CH4 [e.g. Crill et al., 1999]. Peatlands are especially vulnerable to climatic warming. Peat remains stable only while frozen or saturated with water and changes in temperature, precipitation or surface hydrology can quickly change a peatland from a small sink for CO2 and a source of CH4, to a strong source of CO2 and a small sink for CH4.

Recent studies point to the large role that disturbance regimes (recovery, frequency) play in the long-term pattern of net carbon uptake over North America. The current C sink in U.S. forests appears to be primarily the consequence of land management activities of the 19th and 20th centuries, bringing into focus the important role of historical legacies in regulating current balances in major ecosystems. These studies are based on forest and agricultural inventory data, historical rates of agricultural clearing and abandonment, historical rates of wood harvest, wild fire statistics, and growth and decay rates derived from the ecological literature. Because of inconsistent and incomplete sampling through the historical period, and lack of process understanding, the uncertainty in attributing the estimated C sink to past disturbance is high. Natural disturbances, particularly fire and insect epidemics, may be playing an increasingly important role in regional C fluxes. The impacts of these disturbances on ecosystem processes and C transfers among different C pools need to be understood and quantified.

The past and current role of other factors affecting terrestrial C, such as increasing atmospheric CO2 and N deposition, is poorly understood at large spatial scales. Better quantification of the relative contribution of different factors is critical for monitoring and verifying the effect of C sequestration activities, and attributing observed changes to the appropriate cause. Carbon sequestration is likely to become part of a suite of policy measures to reduce greenhouse gases in the atmosphere, and verification of the effectiveness of these measures will be needed.

A variety of data collected at different temporal and spatial scales, coupled with a rigorous estimation process, will lead to improved regional- and continental-scale estimates of land/atmosphere carbon exchange. Data sources include reconstruction of land use/land cover history from statistical records, compilation of past and ongoing resource inventories, a variety of remote sensors, and many different kinds of intensive ecosystem monitoring and process studies such as those conducted at Long-Term Ecological Research (LTER) sites and a network of direct flux measurement sites (AmeriFlux). Temporal resolution of land use records and inventory data is low (5-10 years), but spatial resolution can be very high (county-level to individual tracts of land). Data from CO2 flux towers, remote sensing, and enhanced atmospheric sampling can provide information with much higher temporal resolution, complementing the high spatial resolution of more traditional land-based data. Observations at intensive ecosystem monitoring sites provide validation for more extensive measures, and process-level understanding for interpreting larger-scale phenomena.

The various data sources are input to a variety of modeling approaches, from bookkeeping to biophysical/biochemical process models. Advances in modeling, such as newly emerging dynamic global vegetation models (DGVMs) and high resolution biophysical models, will play a major role in integrating the land data with atmospheric monitoring. Models are also required to integrate the biophysical estimates with socioeconomic models to provide the critical link between the NACP and the needs of policy makers and land managers.

Despite the variety of available land-based and satellite data, and continuing efforts to improve modeling capabilities, estimates of land/atmosphere C exchange are unacceptably imprecise, and not uniformly available for the land surface of North America (or anywhere else in the world). A critical concern is that, with the exception of measurements at CO2 flux towers, none of the land surface measurements are designed to monitor changes in C stocks or fluxes. The observations therefore lack features needed to attain higher precision and reliability. Key deficiencies include lack of complete ecosystem C measurements (particularly below-ground C), gaps in spatial coverage, inconsistent procedures with time and location, and lack of sufficient temporal resolution (re-measurement intervals as long as 15 years in important areas). Model interpolations have been used to fill in the missing information, but evidently rigorous field sampling, traceable to established long-term benchmarks, is needed.

The goals for NACP development of plant/soil ("biophysical") measurements and models are to:

  • reduce the uncertainty in land-based monitoring of changes in carbon stocks;
  • fully integrate land-based measurements with atmospheric measurements; and
  • provide the mechanistic foundation for inverse modeling and data assimilation.

Several research objectives will support these goals:

  • To reduce the uncertainty of ongoing inventory and monitoring of national greenhouse gas emissions from land, and improve ability to attribute observed changes to natural and human disturbances;
  • To develop well-quantified large-scale estimates of C exchange with the atmosphere, for independent validation of estimates derived primarily from atmospheric measurements; and
  • To provide the information on ecosystem-level soil and plant carbon fluxes necessary to understand and interpret larger-scale regional and continental flux estimates that will be obtained in the NACP.

A long-term observational strategy should include several key elements:

  • Identification of gaps in current sampling strategies (land classes, locations, potential importance in terms of C pool sizes and fluxes)
  • Enhance established networks or begin new activities to fill gaps, including in situ sampling and remote sensing.
  • More complete and comprehensive, longer-term soil-plant-atmosphere exchange data for representative ecosystems.
  • Full exploitation and efficient management of existing data (in situ and satellite): better acquisition, assimilation, analysis, and dissemination.
  • Assemble and distribution ancillary data sets needed for interpretation: soil characteristics, hydrology, meteorology/climatology, etc.
  • Analysis and modeling activities leading to resolution of discrepancies between predictions of biogeochemical models and atmospheric inverse models.
  • Development and testing of new plant-soil-atmosphere models for integrating land and atmospheric measurements.

Hierarchical Approach to Integrating Biophysical Measurements

The integrating strategy for biophysical measurements and models includes a number of key features:

  • Linking observations across space and time using a nested design
  • Linking observations with understanding from process studies
  • Careful selection and definition of parameters for integration
  • Representative of major land cover and land-use types
  • Estimation of critical variables for understanding and quantifying C fluxes
  • Measurement of common variables across tiers using standard protocols
  • QA/QC at all sample phases, and quantification of estimation errors
  • Dynamic coupling of atmospheric, biospheric, and human systems
  • Advances in integrated modeling technology and analysis

Large-scale land monitoring programs can be implemented efficiently using a tiered (multi-phase) sampling strategy. Some data elements are identically defined and collected at each tier to provide links among tiers, while other variables may be unique to one or a subset of tiers. Spatial and temporal resolution of data collection will be unique and complementary among tiers. The combination of remote sensing, extensive inventory, medium intensity sampling, and intensive observations comprise a powerful, flexible, and potentially efficient data collection system. The sample tiers can be linked statistically so that inferences about the entire population within cover classes can be made. The observation system should have the capability to integrate with atmospheric monitoring, but should also stand alone to provide independent estimates of C fluxes for validation and as a contribution to ecosystem science.

Multi-tiered sampling and analysis systems have been designed and implemented in the U.S. for land inventories and more recently for linking new remote sensors with field measurements. For the NACP, the first tier involves wall-to-wall mapping and remote sensing of cover class at the continental scale. Middle tiers include (1) existing extensive land inventories composed of a large number of sample plots, and (2) a proposed new set of approximately 1000 medium intensity plots with more resolved direct measurements of C flux than land inventories, also selected to represent typical conditions across the landscape. The final tier includes the existing and potentially new intensive observation sites where the most detailed observation are made, such as LTER and AmeriFlux sites.

The following table illustrates the multi-tier concept with a listing of few of the variables likely to be central to the land observation system:

Example
data elements / Mapping and remote sensing / Extensive inventory (FIA and NRI) / Medium-intensity sample (new) / Intensive observation sites (LTER and Ameriflux)
Land cover class / X / X / X / X
Leaf area index / X / X / X / X
Live
Biomass / X / X / X / X
Land cover change / X / X / X
Wildfire disturbance / X / X / X
Climate variability / X / X
Soil CO2 flux / X / X
Methane flux / X / X
Dissolved organic C / X
Ecosystem CO2 flux / X

A complete and well-defined set of variables will be developed as this plan becomes implemented. The role of modeling and analysis is central to the evolution of an efficient data collection system. Appropriate estimators will be defined through modeling and analytical studies, and recommendations made for enhancing current observations for North America to produce an efficient, continuing multi-tier network that is optimized for estimating C flux at multiple temporal and spatial scales. An efficient way to integrate across scales is not apparent for some critical variables such as soil CO2 and methane flux. Preliminary studies and pilot implementation tests will be undertaken to develop a strategy for these variables.

Enhancements to Ongoing Terrestrial Monitoring Networks

Remote Sensing

Current land inventory systems in the U.S. use a combination of high-altitude aerial photography and Landsat Thematic Mapper (TM) data for land classification and for area change detection. A global observation system using the EOS-MODIS sensor and a network of ground observations has been deployed for estimating productivity and land cover change. LANDSAT-TM and EOS-MODIS are already acquiring data that provide land cover estimates but in the case of Landsat, processing of continental-scale observations into useable land cover change products is not currently routine or systematic. The MODIS land cover and land cover change products are providing systematic but coarse observations at continental scales, but the linkages between MODIS land cover change estimates and carbon stock changes have not yet been sufficiently developed or tested to evaluate capability to provide carbon flux estimates.

Additional remote sensors are becoming operational to provide more direct estimates of above ground biomass stocks (e.g. LIDAR and RADAR from both airborne and space platforms). Hyper-spectral airborne measurements may be useful for distinguishing between living and dead biomass. These technologies hold promise for estimates of carbon inventories at continental scales, but the resources to support systematic observations are not currently available.

Several specific needs have been identified for improving estimates of carbon stocks with the help of remote sensing products: (1) timely systematic and routine processing of satellite data from the North American Continent into land cover and land cover change products, including both natural and human disturbances(2) integration of satellite observations with in situ measurements of carbon stocks and existing inventories (3) augmentation of satellite and in situ estimates of carbon stocks with airborne and surface measurements (4)development appropriate estimation models.

Additional details about remote sensing contributions to the NACP are included in the Appendix.

Extensive Inventories

Current large-scale land inventories conducted primarily by USDA (FIA and NRI) employ multi-tier sampling strategies involving remote sensing and ground measurements. These continuous inventories provide baseline information about land cover, management intensity, productivity, and disturbance that can be used to estimate carbon stock changes over 5-10 year periods. A very high sampling intensity allows detailed description of some of the causes of observed carbon stock changes, such as the effects of vegetation growth, mortality, and harvesting. Historical data are available to trace land use and management history.

The ability of current land inventories to provide true monitoring of C stock changes and estimates of C flux is limited in several key ways: lack of complete ecosystem C measurements; lack of sufficient temporal resolution; and lack of easily available and usable historical data. Models based on data from ecosystem process studies and intensive monitoring sites are used to fill in data gaps, but uncertainty is high.

Gaps in spatial coverage-

Major gaps in the U.S. include some “reserved” areas of the U.S.; lightly sampled areas of the Intermountain West, the Pacific Coast, and Alaska; developed lands especially urban and suburban areas; and large areas of public nonforest land (mostly grazing land in the U.S. West). Large areas of Canada and Mexico, all cover types, have been hardly sampled at all with field plots, or if sampled, the data is currently inaccessible. Although enhancements to ongoing inventories are filling some of these gaps, especially in forested areas, it is unlikely that these improvements in coverage will be fully implemented with repeated measurements during the early stages of the NACP. Therefore an interim strategy is needed to increase the use of current and historical remote sensing data to identify land cover status and changes, coupled with selected new field measurements to estimate biomass and other ecosystem C stocks and rates of change for undersampled areas.

Sparsely measured C pools-

Enhancements to measurements in all major cover classes include stumps, live and dead roots, mineral soil, litter, and coarse woody debris. Limited measurements of complete ecosystem C stocks and fluxes are available from intensive sites, and some pilot efforts are underway to modify extensive inventories, but representative spatial coverage is spotty. An aggressive field campaign early in the NACP is required to collect data on these poorly measured C pools particularly in areas that have been poorly represented in past data collection efforts. This new information will facilitate the development of ecosystem carbon budgets that represent the major conditions on the landscape, both disturbed and undisturbed. Some new direct measurements of C fluxes will be needed during the intensive field program and at a new medium-intensity sampling network (discussed below). These new measurements may include litter production and soil CO2 and CH4 fluxes, which can be combined with remote sensing estimates of land cover and land cover change to provide validation data for ecosystem flux estimates from atmospheric measurements and for input to model estimates of the North American C budget. The methods to convert relatively simple allometric field measurements to estimates of mass need improvement. Estimation methods (mathematical models) will be developed to relate the quantity of C is different ecosystem C pools to the measurements typically taken with the extensive inventory system, such as tree diameter, height, and size of dead wood. The applicability of biomass equations will be reviewed and supplemental equations derived from new field studies if necessary. Special studies will be required to improve estimation of some C pools, for example, the density of decaying wood for various species and regions. Improved estimates of the translocation of harvested agricultural and forest products are needed both at the national scale (exports) and for regional studies in order to match the land accounting with the atmospheric accounting for the same regions.