Earlier springs decrease peak summer productivity in North American boreal forests
TL;DR: In this paper, the authors analyzed nearly three decades (1982?2008) of observational records and derived products, including satellite microwave and optical imagery as well as upscaled ecosystem flux observations, to better understand how shifts in seasonality impact hydrology and productivity in the North American boreal forests.
Abstract: In the northern high latitudes, alternative hypotheses with regards to how warming-related shifts in seasonality influence ecosystem productivity exist. Increased plant growth associated with a longer growing season may enhance ecosystem productivity, but shifts to earlier springs may also negatively influence soil moisture status and productivity during the peak of the growing season. Here, we analyzed nearly three decades (1982?2008) of observational records and derived products, including satellite microwave and optical imagery as well as upscaled ecosystem flux observations, to better understand how shifts in seasonality impact hydrology and productivity in the North American boreal forests. We identified a dominant adverse influence of earlier springs on peak summer forest greenness, actual evapotranspiration and productivity at interannual time scales across the drier western and central sections of the North American boreal forests. In the vast regions where this spring onset mechanism operates, ecosystem productivity gains from earlier springs during the early portion of the growing season are effectively cancelled through corresponding losses in the later portion. Our results also indicate that recent decadal shifts towards earlier springs and associated drying in the midst of the growing season over western North American boreal forests may have contributed to the reported declines in summer productivity and increases in tree mortality and fire activity. With projections of accelerated northern high-latitude warming and associated shifts to earlier springs, persistent soil moisture deficits in peak summer may be an effective mechanism for regional-scale boreal forest dieback through their strong influence on productivity, tree mortality and disturbance dynamics.
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01 Dec 2012
Abstract: We upscaled FLUXNET observations of carbon dioxide, water, and energy fluxes to the global scale using the machine learning technique, model tree ensembles (MTE). We trained MTE to predict site-level gross primary productivity (GPP), terrestrial ecosystem respiration (TER), net ecosystem exchange (NEE), latent energy (LE), and sensible heat (H) based on remote sensing indices, climate and meteorological data, and information on land use. We applied the trained MTEs to generate global flux fields at a 0.5 degrees x 0.5 degrees spatial resolution and a monthly temporal resolution from 1982 to 2008. Cross-validation analyses revealed good performance of MTE in predicting among-site flux variability with modeling efficiencies (MEf) between 0.64 and 0.84, except for NEE (MEf = 0.32). Performance was also good for predicting seasonal patterns (MEf between 0.84 and 0.89, except for NEE (0.64)). By comparison, predictions of monthly anomalies were not as strong (MEf between 0.29 and 0.52). Improved accounting of disturbance and lagged environmental effects, along with improved characterization of errors in the training data set, would contribute most to further reducing uncertainties. Our global estimates of LE (158 +/- 7 J x 10(18) yr(-1)), H (164 +/- 15 J x 10(18) yr(-1)), and GPP (119 +/- 6 Pg C yr(-1)) were similar to independent estimates. Our global TER estimate (96 +/- 6 Pg C yr(-1)) was likely underestimated by 5-10%. Hot spot regions of interannual variability in carbon fluxes occurred in semiarid to semihumid regions and were controlled by moisture supply. Overall, GPP was more important to interannual variability in NEE than TER. Our empirically derived fluxes may be used for calibration and evaluation of land surface process models and for exploratory and diagnostic assessments of the biosphere.
948 citations
TL;DR: It is suggested that future studies should primarily focus on using new observation tools to improve the understanding of tropical plant phenology, on improving process-based phenology modeling, and on the scaling of phenology from species to landscape-level.
Abstract: Plant phenology, the annually recurring sequence of plant developmental stages, is important for plant functioning and ecosystem services and their biophysical and biogeochemical feedbacks to the climate system. Plant phenology depends on temperature, and the current rapid climate change has revived interest in understanding and modeling the responses of plant phenology to the warming trend and the consequences thereof for ecosystems. Here, we review recent progresses in plant phenology and its interactions with climate change. Focusing on the start (leaf unfolding) and end (leaf coloring) of plant growing seasons, we show that the recent rapid expansion in ground- and remote sensing- based phenology data acquisition has been highly beneficial and has supported major advances in plant phenology research. Studies using multiple data sources and methods generally agree on the trends of advanced leaf unfolding and delayed leaf coloring due to climate change, yet these trends appear to have decelerated or even reversed in recent years. Our understanding of the mechanisms underlying the plant phenology responses to climate warming is still limited. The interactions between multiple drivers complicate the modeling and prediction of plant phenology changes. Furthermore, changes in plant phenology have important implications for ecosystem carbon cycles and ecosystem feedbacks to climate, yet the quantification of such impacts remains challenging. We suggest that future studies should primarily focus on using new observation tools to improve the understanding of tropical plant phenology, on improving process-based phenology modeling, and on the scaling of phenology from species to landscape-level.
750 citations
TL;DR: In the case of an earlier spring and a later autumn, carbon uptake (photosynthesis) increases considerably more than carbon release (respiration) in temperate forests in the eastern US as mentioned in this paper.
Abstract: The timing of life-history events has a strong impact on ecosystems. Now, analysis of the phenology of temperate forests in the eastern US indicates that in the case of an earlier spring and a later autumn, carbon uptake (photosynthesis) increases considerably more than carbon release (respiration).
592 citations
17 Dec 2014
513 citations
TL;DR: In this paper, the temporal correlations between EOS and environmental factors (i.e., temperature, precipitation and insolation), as well as the correlation between spring and autumn phenology, using partial correlation analyses were determined.
Abstract: The timing of the end of the vegetation growing season (EOS) plays a key role in terrestrial ecosystem carbon and nutrient cycles. Autumn phenology is, however, still poorly understood, and previous studies generally focused on few species or were very limited in scale. In this study, we applied four methods to extract EOS dates from NDVI records between 1982 and 2011 for the Northern Hemisphere, and determined the temporal correlations between EOS and environmental factors (i.e., temperature, precipitation and insolation), as well as the correlation between spring and autumn phenology, using partial correlation analyses. Overall, we observed a trend toward later EOS in ~70% of the pixels in Northern Hemisphere, with a mean rate of 0.18 ± 0.38 days yr-1 . Warming preseason temperature was positively associated with the rate of EOS in most of our study area, except for arid/semi-arid regions, where the precipitation sum played a dominant positive role. Interestingly, increased preseason insolation sum might also lead to a later date of EOS. In addition to the climatic effects on EOS, we found an influence of spring vegetation green-up dates on EOS, albeit biome dependent. Our study, therefore, suggests that both environmental factors and spring phenology should be included in the modeling of EOS to improve the predictions of autumn phenology as well as our understanding of the global carbon and nutrient balances.
284 citations
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TL;DR: In this article, the authors characterize the interannual variability in the spatial distribution of snow and energy fluxes of snowmelt in watersheds of a maritime setting, Tokopah Basin (TOK) in California's southern Sierra Nevada, and a continental setting, Green Lake 4 Valley (GLV4) in Colorado's Front Range, using a 12 year database (1996-2007) of hydrometeorological observations and satellite-derived snow cover.
Abstract: [1] The distribution of snow and the energy flux components of snowmelt are intrinsic characteristics of the alpine water cycle controlling the location of source waters and the effect of climate on streamflow. Interannual variability of these characteristics is relevant to the effect of climate change on alpine hydrology. Our objective is to characterize the interannual variability in the spatial distribution of snow and energy fluxes of snowmelt in watersheds of a maritime setting, Tokopah Basin (TOK) in California's southern Sierra Nevada, and a continental setting, Green Lake 4 Valley (GLV4) in Colorado's Front Range, using a 12 year database (1996–2007) of hydrometeorological observations and satellite-derived snow cover. Snowpacks observed in GLV4 exhibit substantially greater spatial variability than in TOK (0.75 versus 0.28 spatial coefficient of variation). In addition, modeling results indicate that the net turbulent energy flux contribution to snowmelt in GLV4 is, on average, 3 times greater in magnitude (mean 29% versus 10%) and interannual variability (standard deviation 17% versus 6%) than in TOK. These energy flux values exhibit strong seasonality, increasing as the melt season progresses to times later in the year (R2 = 0.54–0.77). This seasonality of energy flux appears to be associated with snowmelt rates that generally increase with onset date of melt (0.02 cm d−2). This seasonality in snowmelt rate, coupled to differences in hydrogeology, may account for the observed differences in correspondence between the timing of snowmelt and timing of streamflow in these watersheds.
78 citations
TL;DR: In this paper, the authors investigated changes in freeze-thaw (FT) seasons and evapotranspiration (ET) from 1983 to 2006 and their connections in the northern cryosphere by analyzing independent satellite remote sensing derived FT and ET records.
Abstract: Seasonal frozen states in the northern terrestrial cryosphere limit vegetation photosynthetic activities and evapotranspiration (ET) through cold temperature constraints to biological processes and chemical unavailability of water as a result of being frozen Seasonal transitions of the landscape between predominantly frozen and thawed conditions are analogous to a biospheric and hydrological on/off switch, with marked differences in ET, vegetation productivity and other biological activity between largely dormant winter and active summer conditions We investigated changes in freeze–thaw (FT) seasons and ET from 1983 to 2006 and their connections in the northern cryosphere by analyzing independent satellite remote sensing derived FT and ET records Our findings show that the northern cryosphere (≥ 40°N) has experienced advancing (−25 days/decade; P = 0005) and lengthening (35 days/decade; P = 0007) non-frozen season trends over the 24-year period, coinciding with an upward trend (64 mm/year/decade; P = 0014) in regional mean annual ET over the same period Regional average timing of spring primary thaw and the annual non-frozen period are highly correlated with regional annual ET (|r| ≥ 075; P < 0001), with corresponding impacts to annual ET of approximately 06 and 05% per day, respectively The impact of primary fall freeze timing on ET is relatively minor compared with primary spring thaw timing Earlier onset of the non-frozen season generally promotes annual ET in colder areas but appears to suppress summer ET by increasing drought stress in the southernmost parts of the domain where water supply is the leading constraint to ET The cumulative effect of future freeze-thaw changes on ET in the region will largely depend on future changes of large-scale atmosphere circulations and rates of vegetation disturbance and adaptation to continued warming Copyright © 2011 John Wiley & Sons, Ltd
64 citations
"Earlier springs decrease peak summe..." refers methods in this paper
...A similar relationship was found earlier over western NA boreal regions utilizing an AET data set that was generated through a more physically based approach (Zhang et al 2011)....
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TL;DR: In this article, the authors compared the timing of spring thaw determined from approximately 25-km resolution daily radar backscatter data with observed daily river discharge time series and model simulated snow water content data for 52 basins (5000-10,000 km 2 ) across Canada and Alaska for the spring of 2000.
56 citations
TL;DR: In this article, the community land model version 4 (CLM4) is applied to explore the spatial-temporal patterns of spring vegetation growth trends over the northern mid-high latitudes (NMH) (>25 N) between 1982 and 2004 during the spring season through the 23 yr period.
Abstract: The Community Land Model version 4 (CLM4) is applied to explore the spatial‐temporal patterns of spring (April‐May) vegetation growth trends over the northern mid‐high latitudes (NMH) (>25 N) between 1982 and 2004 During the spring season through the 23 yr period, both the satellite-derived and simulated normalized difference vegetation index (NDVI) anomalies show a statistically significant correlation and an overall greening trend within the study area Consistently with the observed NDVI‐temperature relation, the CLM4 NDVI shows a significant positive association with the spring temperature anomaly for the NMH, North America and Eurasia Large study areas experience temperature discontinuity associated with contrasting NDVI trends Before and after the turning point (TP) of the temperature trends, climatic variability plays a dominant role, while the other environmental factors exert minor effects on the NDVI tendencies Simulated vegetation growth is broadly stimulated by the increasing atmospheric CO2 Trends show that nitrogen deposition increases NDVI mostly in southeastern China, and decreases NDVI mainly in western Russia after the temperature TP Furthermore, land use-induced NDVI trends vary roughly with the respective changes in land management practices (crop areas and forest coverage) Our results highlight how non-climatic factors mitigate or exacerbate the impact of temperature on spring vegetation growth, particularly across regions with intensive human activity
54 citations
TL;DR: In this paper, the authors used the 30-year normalized-difference vegetation index (NDVI) time series from AVHRR/MODIS satellite sensors to assess the regional vegetation dynamic changes in the Tao River Basin, which cuts across the Eastern Tibetan Plateau (ETP) and the Southwestern Loess plateau (SLP).
Abstract: The 30-year normalized-difference vegetation index (NDVI) time series from AVHRR/MODIS satellite sensors was used in this study to assess the regional vegetation dynamic changes in the Tao River Basin, which cuts across the Eastern Tibetan Plateau (ETP) and the Southwestern Loess Plateau (SLP). First, principal component and correlation analyses were carried out to determine the key climatic variables driving ecological change in the region. Then, regression models were tested to correlate NDVI with the selected climatic variables to determine their predictive power. Finally, Sen's slope method was used to determine how terrestrial vegetation has responded to regional climate change in the region. The results indicated an average winter season NDVI value of 0.14 in the ETP but only 0.04 in the SLP. Primarily driven by increasing temperature, vegetation growth has generally been enhanced since 1981; spring NDVI increased by 0.03 every 10 years in the ETP and 0.02 in the SLP. Further, results from trend analyses suggest vegetation growth in the ETP shifted to earlier-start and earlier-end dates, however in the SLP, the growing season has been extended with an earlier-start and later-end date. The precipitation threshold for vegetation germination, measured by the cumulative spring rainfall, was found to be 44 mm for both the ETP and SLP.
44 citations