BOREAS is a regional-scale experiment to study the high-latitude boreal ecosystem.The objective of BOREAS is to improve our understanding of the interaction between the earth’s climate system and the boreal forest at short and intermediate time scales, in order to clarify its role in global change.
In the six years following the 1990 initiation of early planning for BOREAS, the science team has participated in seven major field campaigns; one in late 1993, four in 1994, and three during 1996. Each IFC, and the BOREAS experiment as a whole were conducted according to detailed experiment plans and all experimental and operational objectives as stated in those plans have been achieved. 1995 was a year of intense data analysis that led to the redeployment of 1996 to fill critical scientific gaps in the data record.
Analysis of the data began in mid-summer of 1994 as the data were being collected and is being continued through 1999 in the BOREAS Followon program. The findings so far have been significant in terms of their implications for global change and are summarized here.
The boreal ecosystem, occupying roughly 17 percent of the vegetated land surface and thus an important driver of global weather and climate, absorbs much more solar energy than previously believed. Albedo measurements in BOREAS shows that this forest absorbs nearly 91 percent of the sun’s incident radiation. These results have already been incorporated into operational weather forecast models and are making significant improvement in operational forecasts. For example, the 1996 spring BOREAS campaign highlighted large cold biases (-10 degrees C) in the 2-m forecast temperature over the boreal forest in both the US and European operational weather models. Analysis revealed the cause to be unrealistic model snow albedos that did not include the shading effect of the overlying forest canopy. Actual snow albedos in the forest were measured in BOREAS to be 0.2 while the forecast models carried values of 0.8. Incorporation of the more realistic snow albedos reduced the surface bias and in turn, the more realistic surface temperature predictions had large effects downstream; lower tropospheric temperature bias, and greatly improved forecast scores over the North Pacific and North Atlantic. As of December 1996, the European model has incorporated these changes into its operational forecasts. It is also planned to eventually use satellite data to track snow-related albedo dynamics during the spring and fall.
BOREAS is also revealing a fascinating new picture of the boreal ecosystem function. While it is known that much of the boreal ecosystem consists of wetlands, numerous lakes, and water-logged peat beds on which much of the forest grows, BOREAS tower and aircraft measurements show that the atmosphere above is extremely dry; low humidity and deep boundary layer convection often mimic conditions found only over deserts. Ground based measurements of the tree physiological rates in BOREAS show that this atmospheric desiccation is a result of the forest’s strong biological control limiting surface evaporation. The data further show this tight control is linked to the low soil temperatures and low below ground nutrient turnover rates that subsequently limit above ground photosynthetic capacity.
Very recent results from BOREAS measurements are also beginning to shed additional light on the problem of the missing global carbon.Could the boreal ecosystem with its low productivities really be sequestering the 1 to 3 gigatons of carbon per year indicated by other studies independent of BOREAS. This amount is half the anthropogenic input to the Earth’s atmosphere. The boreal ecosystem by conventional estimates occupies 12 million km2 or from our satellite estimates as much as 20 million km2 (sum of high-latitude deciduous and coniferous forests from the ISLSCP I Land Cover product). Given this large area globally, the boreal forest needs only to sequester between 50 to 80 g Cm-2yr-1 to explain a 1 gigaton annual global sink. 50 to 80 g Cm-2yr-1 is a very small rate in comparison to the estimated uncertainty of about ± 50 g Cm-2yr-1 in tower measurements of the annual net ecosystem exchange (NEE) of carbon. The tower-measured carbon NEE for mature boreal dominants during 1994, 1995 and 1996 showed that they ranged from being a net annual carbon source of 50 g Cm-2yr-1 to a carbon sink of 50 g Cm-2yr-1 . Thus while these direct measurements place some limits on the role of mature boreal dominants in global carbon sequestration we cannot infer with useful certainty the source/sink strengths of the global boreal ecosystem. Rather, a more reliable estimate must await the incorporation of the processes driving boreal carbon dynamics into carbon and ecosystem process models and associated scaling studies in BOREAS. Activities are currently underway in BOREAS Followon to incorporate improved process understanding into ecosystem process models driven by remote sensing to scale carbon fluxes from the tower level to the study area level, where they can be compared to aircraft flux measurements, then ultimately to the global scale using satellite-derived forcing variables. These studies must also take into account the role of the fire and disturbance-driven land cover change important in governing boreal landscape dynamics. BOREAS Investigators , analyzing AVHRR visible and near infrared images, showed that about 30% of the study region has undergone fire in the last 25 years, much larger than previously believed and that 3% of the ecosystem burned during 1994 alone.
Tower, chamber and other measurements did reveal much new information and confirmed previous findings of underlying processes governing carbon dynamics. For example, BOREAS investigations showed that for mature black spruce sites the small annual NEE values of ± 50 g Cm-2yr-1 result from a small difference between two very large carbon flux rates: photosynthetic uptake and respiration loss of carbon, each of the order of 1500 g Cm-2yr-1 . Seasonal dynamics are also important; photosynthetic uptake dominates respiration in the summer producing a net carbon accumulation while respiration from deep soils dominates in the winter producing a net carbon loss nearly equal to summertime accumulation. Summer vegetation growth accumulates carbon in roughly equal amounts in production of wood, leaves, roots and moss cover; decomposition of previous year’s vegetation litter within the soil is responsible for carbon loss. These gain/loss processes may be driven in different directions by climate variation. Carbon accumulation below ground appears to be driven primarily by fine root production, which is in turn correlated with high above ground productivity. Above ground productivity is greatest in years with long, cool growing seasons, i.e. an early spring thaw followed by cool summers, and late frost. Decomposition rates are highest in warm, dry soils, thus soil carbon loss should be smallest in cold wet years when soil temperatures remain low and saturated with water. Modeling both photosynthesis and respiration is thus crucial to understanding ecosystem NEE and its dependence on climate.
Remote sensing research and development during BOREAS produced a number of results that are key to regional scale understanding of carbon, water and energy exchange. In this regard, improved radiative transfer models developed during BOREAS should provide more reliable components in numerical snow melt models, a key to forecasting the effects of climate change on the timing of snow melt which is critical to determining interannual variation and longer term trends in carbon sequestration. In addition BOREAS investigations showed how radar can be used to monitor soil and tree thaw, also critical in turning on photosynthesis in the models. Importantly, many remote sensing efforts in BOREAS showed that satellite data can be used to reliably map the major land cover types, making possible for the first time a careful assessment of the entire global boreal biome. Furthermore, the AVHRR data record now stretches back more than 15 years. Studies by BOREAS investigators also showed that this record can be used to monitor the response of the biome to climate warming. Finally, remote sensing was used during BOREAS to develop regional scale parameter maps for all the major driving variables governing carbon/water and energy processes. These maps should prove to be invaluable in followon scaling studies scaling models and hypotheses from the plot level to the region