Showing papers by "Yufang Jin published in 2014"

Vegetation controls on northern high latitude snow‐albedo feedback: observations and CMIP5 model simulations

Abstract: The snow-masking effect of vegetation exerts strong control on albedo in northern high latitude ecosystems. Large-scale changes in the distribution and stature of vegetation in this region will thus have important feedbacks to climate. The snow-albedo feedback is controlled largely by the contrast between snow-covered and snow-free albedo (Δα), which influences predictions of future warming in coupled climate models, despite being poorly constrained at seasonal and century time scales. Here, we compare satellite observations and coupled climate model representations of albedo and tree cover for the boreal and Arctic region. Our analyses reveal consistent declines in albedo with increasing tree cover, occurring south of latitudinal tree line, that are poorly represented in coupled climate models. Observed relationships between albedo and tree cover differ substantially between snow-covered and snow-free periods, and among plant functional type. Tree cover in models varies widely but surprisingly does not correlate well with model albedo. Furthermore, our results demonstrate a relationship between tree cover and snow-albedo feedback that may be used to accurately constrain high latitude albedo feedbacks in coupled climate models under current and future vegetation distributions.

Contrasting controls on wildland fires in Southern California during periods with and without Santa Ana winds

Abstract: Wildland fires in Southern California can be divided into two categories: fall fires, which are typically driven by strong offshore Santa Ana winds, and summer fires, which occur with comparatively weak onshore winds and hot and dry weather. Both types of fire contribute significantly to annual burned area and economic loss. An improved understanding of the relationship between Southern California's meteorology and fire is needed to improve predictions of how fire will change in the future and to anticipate management needs. We used output from a regional climate model constrained by reanalysis observations to identify Santa Ana events and partition fires into those occurring during periods with and without Santa Ana conditions during 1959–2009. We then developed separate empirical regression models for Santa Ana and non-Santa Ana fires to quantify the effects of meteorology on fire number and size. These models explained approximately 58% of the seasonal and interannual variation in the number of Santa Ana fires and 36% of the variation in non-Santa Ana fires. The number of Santa Ana fires increased during years when relative humidity during Santa Ana events and fall precipitation were below average, indicating that fuel moisture is a key controller of ignition. Relative humidity strongly affected Santa Ana fire size. Cumulative precipitation during the previous three winters was significantly correlated with the number of non-Santa Ana fires, presumably through increased fine fuel density and connectivity between infrastructure and nearby vegetation. Both relative humidity and the preceding wet season precipitation influenced non-Santa Ana fire size. Regression models driven by meteorology explained 57% of the temporal variation in Santa Ana burned area and 22% of the variation in non-Santa Ana burned area. The area burned by non-Santa Ana fires has increased steadily by 1.7% year−1 since 1959 (p < 0.006); the occurrence of extremely large Santa Ana fires has increased abruptly since 2003. Our results underscore the need to separately consider the fuel and meteorological controls on Santa Ana and non-Santa Ana fires when projecting climate change impacts on regional fire.

Mapping the Daily Progression of Large Wildland Fires Using MODIS Active Fire Data

Abstract: High temporal resolution information on burnt area is needed to improve fire behaviour and emissions models. We used the Moderate Resolution Imaging Spectroradiometer (MODIS) thermal anomaly and active fire product (MO(Y)D14)asinputtoakriginginterpolationtoderivecontinuousmapsofthetimingofburntareafor16largewildland fires. For each fire, parameters for the kriging model were defined using variogram analysis. The optimal number of observations used to estimate a pixel's time of burning varied between four and six among the fires studied. The median standarderrorfromkrigingrangedbetween0.80and3.56daysandthemedianstandarderrorfromgeolocationuncertainty was between 0.34 and 2.72 days. For nine fires in the south-western US, the accuracy of the kriging model was assessed using high spatial resolution daily fire perimeter data available from the US Forest Service. For these nine fires, we also assessed the temporal reporting accuracy of the MODIS burnt area products (MCD45A1 and MCD64A1). Averaged over the nine fires, the kriging method correctly mapped 73% of the pixels within the accuracy of a single day, compared with 33% for MCD45A1 and 53% for MCD64A1. Systematic application of this algorithm to wildland fires in the future may lead to new information about vegetation, climate and topographic controls on fire behaviour. Additional keywords: carbon emissions, fire growth, fire propagation, fire spread.

Controls on the spatial pattern of wildfire ignitions in Southern California

Abstract: Wildfireignitionrequiresacombinationofanopenspark,andsuitableweatherandfuelconditions.Modelsof fire occurrence and burned area provide a good understanding of the physical and climatic factors that constrain and promote fire spread and recurrence, but information on how humans influence ignition patterns is still lacking at a scale compatible with integrated fire management.We investigated the relative importance of the physical, climatic andhuman factors regulating ignition probability across Southern California's National Forests. A 30-year exploratory analysis of one-way relationships indicated that distance to a road, distance to housing and topographic slope were the major determinants of ignition frequency. We used logistic and Poisson regression analyses to model ignition occurrence and frequency as a function of the dominant covariates. The resulting models explained ,70% of the spatial variability in ignition likelihood and 45% of the variability in ignition frequency. In turn, predicted ignition probability contributed to some of the spatial variability in burned area, particularly for summer fires. These models may enable estimates of fire ignition risk for the broader domain of Southern California and how this risk may change with future population and housing development. Our spatially explicit predictions may also be useful for strategic fire management in the region. Additional keywords: biophysical drivers, fire frequency, fire ignition, human influence, Mediterranean ecosystems, spatial regression model, wildland fire risk.

Ecological consequences of variation in precipitation: Separating short- versus long-term effects using satellite data

Abstract: Aim: Precipitation controls the production of semi-arid plants through various mechanisms that operate at a range of time-scales. Short-term variation in precipitation affects vegetation through adjustments in plant physiology and leaf phenology, whereas long-term effects are mediated by plant establishment and mortality, community composition and disturbance regimes. Our goal is to use remote sensing observations to separate the short- and long-term effects of variation in precipitation on ecosystem production. Location: California, USA. Methods: We used time series of gridded absorbed photosynthetically active radiation (APAR) to quantify the short- and long-term responses of diverse ecosystems to variation in precipitation across large productivity and precipitation gradients. We investigated the relationships between temporal sensitivity of APAR to interannual variation in precipitation and mean annual precipitation (MAP), ecosystem properties and disturbance. Results: APAR increased with precipitation both interannually within locations and across locations with MAP. The slope of the interannual relationship, which reflects the sensitivity of APAR to short-term fluctuations in precipitation, varied with climate, vegetation type and structure, and time since disturbance. The interannual APAR sensitivity decreased from c. 0.5 MJ m-2mm-1 at a MAP of 300mm year-1 to less than 0.05 MJ m-2mm-1 at 1000mm year-1. The slope of the spatial relationship, which reflects the long-term sensitivity of APAR to climate, decreased from c. 2.5 MJ m-2mm-1 at 300mm year-1 MAP to c. 0.6 MJ m-2mm-1 at 1000mm year-1. The initial physiological and leaf area effects of a precipitation shift were amplified five-fold over time by gradual changes in population density and species composition. Main conclusion: The impact of a hydroclimatic shift on the primary production, structure and function of California's terrestrial ecosystems depends heavily on time-scale and how rapidly changes in plant population density and community composition can occur. © 2013 John Wiley & Sons Ltd.

Management and climate contributions to satellite-derived active fire trends in the contiguous United States

Abstract: Fires in croplands, plantations, and rangelands contribute significantly to fire emissions in the United States, yet are often overshadowed by wildland fires in efforts to develop inventories or estimate responses to climate change. Here we quantified decadal trends, interannual variability, and seasonality of Terra Moderate Resolution Imaging Spectroradiometer (MODIS) observations of active fires (thermal anomalies) as a function of management type in the contiguous U.S. during 2001–2010. We used the Monitoring Trends in Burn Severity database to identify active fires within the perimeter of large wildland fires and land cover maps to identify active fires in croplands. A third class of fires defined as prescribed/other included all residual satellite active fire detections. Large wildland fires were the most variable of all three fire types and had no significant annual trend in the contiguous U.S. during 2001–2010. Active fires in croplands, in contrast, increased at a rate of 3.4% per year. Cropland and prescribed/other fire types combined were responsible for 77% of the total active fire detections within the U.S and were most abundant in the south and southeast. In the west, cropland active fires decreased at a rate of 5.9% per year, likely in response to intensive air quality policies. Potential evaporation was a dominant regulator of the interannual variability of large wildland fires, but had a weaker influence on the other two fire types. Our analysis suggests it may be possible to modify landscape fire emissions within the U.S. by influencing the way fires are used in managed ecosystems.

Erratum: Long-term trends and interannual variability of forest, savanna and agricultural fires in South America (Carbon Management (2013) 4:6 (617-638))

Abstract: Corrigendum Following the publication of the Research Article by Yang Chen, Douglas C Morton, Yufang Jin, G James Collatz, Prasad S Kasibhatla, Guido R van der Werf, Ruth S DeFries & James T Randerson titled ‘Long-term trends and interannual variability of forest, savanna and agricultural fires in South America’ in the December 2013 issue of Carbon Management (Carbon Management 4[6], 617–638 [2013]), it has been brought to our attention that one author surname was incorrectly printed and should have appeared as: Yang Chen* 1 , Douglas C Morton 2 , Yufang Jin 1 , G James Collatz 2 , Prasad S Kasibhatla 3 , Guido R van der Werf 4 , Ruth S DeFries 5 & James T Randerson 1 The authors and editors of Carbon Management would like to sincerely apologize for any inconvenience or confusion this may have caused our readers. Carbon Management (2014) 5(1) future science group