PROJECT SUMMARY

Atmosphere-biosphere interactions are a key component of the Earth’s physical system. Understanding this interaction is a crucial part of efforts to improve global change simulation models, and monitor variations in the carbon budget. While atmospheric data generally are available for most land areas, no such network exists to collect comparable information about biosphere activity. Recent satellite information can contribute to the development of global biospheric databases. In order to realize this potential, these remotely sensed data must be carefully calibrated with surface information. This approach is the most straightforward way of deducing the physical basis of, and developing a historical context for, the satellite’s observations. This study will continue to combine models and techniques produced in previously funded projects with surface vegetation data sets, surface energy balance and carbon flux data, and new remote sensing products, to further development of a physical interpretation of the first appearance of spring foliage, commonly called the “green wave,” in mid-latitudes. The green wave is particularly important because it is one of the crucial biospheric variables necessary for accurate modeling of processes such as water balance and net primary productivity. This project will first use extensive phenological, satellite-derived, meteorological, and mass exchange information from selected sites to facilitate a detailed examination of lower atmospheric energy and carbon flux in the context of surface and satellite green wave measures. Next, existing empirical models will be used to explore 20th century green wave geographic and temporal variability across mid-latitudes. These results will allow a time line to be developed which interconnects the various atmospheric and biospheric changes associated with the green wave phenomenon, forming the conceptual basis for a future mid-latitude green wave model. Further, the baseline (30 year normals) and 20th century change assessments of the timing of the onset of spring across mid-latitude locations produced, will serve as references for future research. Therefore, this project will demonstrate the critical roles that surface phenological and meteorological data and models can play within atmosphere-biosphere simulations and for global change monitoring, when used in concert with satellite-derived bioclimatological data.

RESEARCH FINDINGS

In the first phase of this project extensive phenological, satellite-derived, meteorological, and mass exchange information from selected sites were used to facilitate a detailed examination of lower atmospheric energy and carbon flux in the context of surface and satellite green wave measures. Data came from the Harvard Forest, MA, Lamont ARM, OK, Morgan-Monroe, IN, Oak Ridge, TN, Park Falls, WI, and the UW-Milwaukee Field Station sites. All stations are eddy covariance towers expect Lamont and UW-M which are Bowen-ratio systems.

The results show that net ecosystem exchange (NEE, carbon dioxide flux), latent minus sensible heat flux (best reflecting the relative change in these two variables), and net radiation all display distinct changes relative to Spring Indices (SI) first bloom date (measure of phenological development). Two aspects are most important: 1) that the patterns of change are quite similar, despite the different land cover types and geographical locations of the sites, and 2) existing differences between the sites can be logically explained by the differences in land cover type. For example, when arranged in phenological time, using SI values, the northern deciduous forest and grassland sites show the earliest response (Park Falls, Harvard Forest, and Lamont), the “middle” station (Morgan-Monroe) shows an intermediate response timing, and the southern site (Oak Ridge) shows the latest response. Thus, phenology exhibits considerable power in organizing the aggregated carbon and energy flux responses of diverse stations in the spring, and is even able to reflect different rates of spring NEE (carbon flux) change on an annual basis at the different sites.

In the second phase of the project, existing empirical models were used to explore green wave geographic and temporal variability across mid-latitudes, over the 1955-2002 period. Daily maximum-minimum temperature data from stations covering all temperate portions of the Northern Hemisphere (NH), over the longest possible time were gathered for this effort, with the National Climatic Data Center (United States) providing most of these data.

Overall results are consistent with prior studies examining smaller areas, confirming a nearly universal quicker onset of early spring warmth (SI first leaf date, -1.2 days/decade), late spring warmth (SI first bloom date, -1.0 days/decade), and last spring freeze date (-1.5 days/decade), as well as a lengthening growing season (1.6 days/decade) across most temperate Northern Hemisphere land regions over the 1955-2002 period. However, dynamics differ among major continental areas with North American first leaf and last freeze date changes displaying a complex spatial relationship. Europe presents a spatial pattern of change, with western continental areas showing last freeze dates getting earlier faster, some central areas having last freeze and first leaf dates progressing at about the same pace, while in portions of Northern and Eastern Europe first leaf dates are getting earlier faster than last freeze dates. Across East Asia last freeze dates are getting earlier faster than first leaf dates. These findings demonstrate that a comprehensive suite of measures linking plant development (phenology) with its basic climatic drivers can be used to monitor general growing season impacts of global warming.

This work provides a common set of measures related to plant development evaluated across the entire NH over a standard period. These results do not replace regional phenology studies, but rather provide a consistent and convenient framework within which to compare their results. By showing the basic climatic drivers of change in various regions, the reported measures will allow a first approximation of basic impacts on plant growth, potential for ripple/species competition effects in selected ecosystems, and agricultural impacts caused by global warming.

Indeed, if just Europe is examined (15 degrees W to 30 degrees E longitude), average change in SI first leaf onset (-2.2 days/decade) compares well with the -2.3 days/decade advancement rate of spring phenological events reported by previous studies. This close match in one continent supports the probable accuracy of the -1.2 days/decade trend reported for the entire NH. Further, other previously reported features such as earlier spring events changing faster than later events, growing season length extending about seven days since the 1960s, and last spring 0 degrees C freeze date in the USA changing by -1.3 days per decade, are consistent with the results of this project.

Lacking a global phenology network, basic impacts of global warming on the growing season in different regions and the hemisphere as a whole can be effectively monitored by using measures such as those reported here. Yet more phenological monitoring is needed. There is strong need to keep current ground phenology observation networks in operation and expand them whenever possible, given their low cost/high value, the coarse nature of satellite-derived data, and the difficulties in comparing ground and satellite measures of phenology. Additional research is needed to understand how these two complimentary sources of phenological data can be compared more effectively and realize their synergistic benefits.