A new method was developed, evaluated, and applied to generate a global dataset of growing-season chlorophyll-a (chl) concentrations in 2011 for freshwater lakes. Chl observations from freshwater lakes are valuable for estimating lake productivity as well as assessing the role that these lakes play in carbon budgets. The standard 4 km NASA OceanColor L3 chlorophyll concentration products generated from MODIS and MERIS sensor data are not sufficiently representative of global chl values because these can only resolve larger lakes, which generally have lower chl concentrations than lakes of smaller surface area. Our new methodology utilizes the 300 m-resolution MERIS full-resolution full-swath (FRS) global dataset as input and does not rely on the land mask used to generate standard NASA products, which masks many lakes that are otherwise resolvable in MERIS imagery. The new method produced chl concentration values for 78,938 and 1,074 lakes in the northern and southern hemispheres, respectively. The mean chl for lakes visible in the MERIS composite was 19.2 ± 19.2, the median was 13.3, and the interquartile range was 3.90-28.6 mg m −3 . The accuracy of the MERIS-derived values was assessed by comparison with temporally nearcoincident and globally distributed in situ measurements from the literature (n = 185, RMSE = 9.39, R 2 = 0.72). This represents the first global-scale dataset of satellitederived chl estimates for medium to large lakes.
A multidecadal analysis of fire in Alaskan Arctic tundra was completed using records from the Alaska Large Fire Database. Tundra vegetation fires are defined by the Circumpolar Arctic Vegetation Map and divided into five tundra ecoregions of Alaska. A detailed review of fire records in these regions is presented, and an analysis of future fire potential was performed based on future climate scenarios. The average size of tundra fire based on the data record is 22 km2 (5454 acres). Fires show a mean size of 10 km2 (2452 acres) and median of 0.064 km2 (16 acres), indicating small fires are common. Although uncommon, 16 fires larger than 300 km2 (74 132 acres) have been recorded across four ecoregions and all five decades. Warmer summers with extended periods of drying are expected to increase fire activity as indicated by fire weather index. The implications of the current fire regime and potential changes in fire regime are discussed in the context of land management and ecosystem services. Current fire management practices and land-use planning in Alaska should be specifically tailored to the tundra region based on the current fire regime and in anticipation of the expected change in fire regime projected with climate change.
The study uses satellite Moderate Resolution Imaging Spectroradiometer albedo products (MCD43A3) to assess changes in albedo at two sites in the treeless tundra region of Alaska, both within the foothills region of the Brooks Range, the 2007 Anaktuvuk River Fire (ARF) and 2012 Kucher Creek Fire (KCF).Results are compared to each other and other studies to assess the magnitude of albedo change and the longevity of impact of fire on land surface albedo. In both sites there was a marked decrease of albedo in the year following the fire. In the ARF, albedo slowly increased until 4 years after the fire, when it returned to albedo values prior to the fire. For the year immediately after the fire, a threefold difference in the shortwave albedo decrease was found between the two sites. ARF showed a 45.3% decrease, while the KCF showed a 14.1% decrease in shortwave albedo, and albedo is more variable in the KCF site than ARF site 1 year after the fire. These differences are possibly the result of differences in burn severity of the two fires, wherein the ARF burned more completely with more contiguous patches of complete burn than KCF. The impact of fire on average growing season (April-September) surface shortwave forcing in the year following fire is estimated to be 13.24 ± 6.52 W m À2 at the ARF site, a forcing comparable to studies in other treeless ecosystems. Comparison to boreal studies and the implications to energy flux are discussed in the context of future increases in fire occurrence and severity in a warming climate.
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