查看更多>>摘要：The degradation of permafrost caused by climate warming accelerates the infiltration and evaporation of water, destroys the hydrological system, and increases the drought stress of boreal forests. However, how climate warming and permafrost degradation affect intrinsic Water Use Efficiency (iWUE) and tree growth is not fully understood. Using tree-ring width and stable carbon isotope composition (delta C-13) data, we analyzed the growth and iWUE of Larix gmelinii on slopes and gullies in three different permafrost degradation areas in Daxing'an Mountains, China. The results showed that the iWUE of L. gmelinii increased significantly in all areas from 1900 to 2015. Radial growth and the ratio of intercellular (C-i) and ambient (Ca) CO2 concentration was significantly negatively correlated with temperature and positively correlated with Palmer Drought Severity Index (PDSI) in severe degradation permafrost areas. Meanwhile, radial growth (basal area increment, BAI) has decreased significantly in recent 40 years, and the decline rate of radial growth in slope topographies was higher than that in gully topographies. Growing season temperature was generally positively correlated with iWUE, but negatively correlated with the radial growth of L. gmelinii. The increase of iWUE was negatively correlated with the radial growth of L. gmelinii. The response of BAI, iWUE, and C-i/C-a to temperature rise and water change was weak in the mild degradation area, and radial growth still increased. In severe and moderate degradation areas, tree physiology (iWUE and C-i/C-a) was driven by temperature and PDSI, indicating that L. gmelinii adopted a conservative water-saving stomatal strategy, and increasing iWUE did not promote tree growth. Although the water use efficiency of L. gmelinii in the southern edge of permafrost area increased with the increase in temperature, it may face more serious drought stress and growth decline in the future.
查看更多>>摘要：Under the influences of climate change and human activities, grassland ecosystems are being invaded by shrubs around the world, especially in temperate semi-arid regions of the Northern Hemisphere. Shrub encroachment can not only affect the properties of grassland ecosystem, but also have an important impact on regional or global climate by affecting biogeochemical and biophysical processes. Although shrub encroachment has become a serious ecological problem in the northern temperate region during the past decades, the possible effects of shrub encroachment on regional climate in northern temperate grasslands remain unclear due to a lack of in situ and long-term environmental records. Based on the satellite-derived land use data, land surface temperature (LST), evapotranspiration, leaf area index, and albedo data, this study quantified, for the first time, the biophysical effects of shrub encroachment on regional climate in semi-arid areas of temperate regions of the Northern Hemisphere. The results indicate that shrub encroachment tends to increase the average annual daytime and mean LST in most of the northern temperate semi-arid region due to increased bare soil fraction. In contrast, shrub encroachment could decrease the daytime LST in relatively humid region of southwest of North America by increasing evapotranspiration (vegetation coverage) and decreasing bare soil fraction. In the arid center region of Central Asia, grassland conversion to shrubland slightly decreases the surface temperature during both daytime and nighttime due to increased albedo caused by decreasing soil water content. Our findings imply that the biophysical effects of shrub encroachment on regional climate should be considered in climate models if they are to accurately simulate climate change in temperate regions of the Northern Hemisphere. More attention should be paid to the climate feedbacks of shrub encroachment, especially considering the distinct effects in different regions and how these climate feedbacks are likely to further impact ecosystem properties.
Shrestha, SurendraWilliams, Christopher A.Rogers, Brendan M.Rogan, John...
15页
查看更多>>摘要：This study examines the post-fire biogeophysical and biochemical dynamics after several high-severity wildfires that occurred in mixed conifer and ponderosa pine forest types in the Sierra Nevada and Klamath Mountains regions between 1986 and 2017. We found a consistent pattern of reduced leaf area index (LAI) in the first year after fire, followed by gradual recovery over the subsequent 25 years. Recovery rate varied between forest types. For example, average summer LAI for 16-25 years post-fire was 88% of the pre-fire average for mixed conifers in the Sierra Nevada, 64% for ponderosa pine in the Sierra Nevada, and 83% for mixed conifers in Klamath Mountains (63, 35, and 64% in winter, respectively). The slower recovery of LAI in ponderosa pine could be due to poor species diversity and drier climate. Summer and winter albedo increased progressively until 12 years post-fire in Sierra Nevada, while it continued to increase until 25 years post-fire in Klamath Mountains. Ponderosa pine had the highest summer (0.148 +/- 0.001) and winter (0.5 +/- 0.0033) albedos. Post-fire changes in evapotranspiration (ET) and gross primary productivity (GPP) were consistent with the changes in LAI. Both summer and winter ET and GPP returned to pre-fire levels by 25 years after fire in mixed conifers of both regions, while the ET and GPP did not recover to pre-fire levels in ponderosa pine. Wildfires increased the land surface temperature (LST) immediately after fire in summer. This effect was significantly higher in mixed conifers of the Sierra Nevada (11 +/- 0.03 degrees C) compared to Klamath Mountains (7 +/- 0.01 degrees C). Our results suggest that reduced ET, consistent with less leaf area and its associated reduced evaporative cooling is the main factor controlling the immediate post-fire warming effect of wildfires in these regions. The findings reported here can be used to understand ecological responses to wildfire in these and nearby ecoregions as they represent mean historical behavior across multiple wildfire events.
查看更多>>摘要：Given their high root plasticity, phreatophytes have flexible water use strategies and can dynamically adjust their rooting depth for the effective uptake of water from soils and shallow aquifers. By this strategy, phreatophytes are strongly ecologically resilient to water stress and thus are commonly grown in drylands. In this study, we used a modified soil-hydrological model, Hydrus-1D, with the implementation of a dynamic root scheme to analyze the role of the distribution and dynamics of roots in simulating evapotranspiration (ET) of phreatophytes with declining groundwater levels (GWLs). The results showed that the root mean square error (RMSE) between simulated and observed ET was reduced by approximately 39% using a model with a more authentic root distribution than the generic root profile. Static root schemes cannot portray the adaptation of phreatophytes to GWL changes well, but their performance can be enhanced by introducing a water compensation scheme, which, however, has a weakness related to the difficulties in parameter calibration. The ET was well retrieved by the model with the dynamic root scheme. Its RMSE was 40% to 70% less than those of the static models, and the Nash-Sutcliffe efficiency reached 0.94, demonstrating the importance of root dynamics in simulating phreatophytic root water uptake. In addition, multiscenario estimations showed nonlinear responses of phreatophyte ET to the rate of GWL decrease (RGWD); that is, as the RGWD accelerated, plant adaptation showed three different stages: no water stress, adaptable and unadaptable. The resistance of plants to water stress decreases with decreasing root growth rate. The identified key ecological thresholds for the RGWD provide a reference for ecological protection in arid areas. We highlight the importance of root processes in the plant response to water stress and suggest that more attention should be given to the root adaptation process in Earth system models.
查看更多>>摘要：The relationships between the daytime CO2 flux density above a wheat crop and both solar irradiance and evapotranspiration are investigated using physical and biophysical measurements made in 1971 in Australia. Well-established flux-gradient relations are used to estimate the hourly CO2 flux density using the measured CO2 concentration at heights of 1 m and 2 m above the crop. Principal component regression identifies four key environmental variables (diffuse and beam shortwave irradiance, leaf-area index, humidity deficit) whose variations together explain almost 80% of the variations in the hourly CO2 flux. For the diffuse ratio Sd/Sg lu 0.9 (generally overcast skies), where Sd and Sg are the observed diffuse and global broadband shortwave irradiances (wavelength range 285 nm to 2800 nm), respectively, both light-use and water-use efficiencies are found to be around double those for Sd/Sg lu 0.1 (sunny skies). For a given value of Sg this doubling in the water-use efficiency is primarily due to the sensitivity of the CO2 flux to the diffuse ratio. Profile measurements of narrowband (350 to 950 nm) irradiance within the canopy confirm that the increase in light-use efficiency is primarily due to the greater interception of radiation in the canopy under an increased diffuse ratio, up to 15 percent in overcast conditions.
查看更多>>摘要：Ecosystem water use efficiency (WUE) is an indicator characterizing carbon-water coupling of the terrestrial ecosystems. The increasing diffuse radiation enhances carbon assimilation. However, the impact of diffuse radiation interacting with biophysical factors on WUE is still not well-understood. Based on carbon and water vapor fluxes measured by the eddy covariance systems during 2019-2020, we explored the effects of biophysical factors on WUE under different sky conditions at 6 plantation ecosystems (2 deciduous broad-leaved and 4 coniferous plantations) in eastern China. WUE on cloudy days was 4.5-21.5% higher than that on clear days at 6 sites, which was attributed to the increase in ecosystem carbon gain and reduction in water loss. Diffuse fraction (DF) exerted significantly positive direct effects on WUE at JZ, MQ, JY and SHB sites (p < 0.05), with the path coefficients of 0.30-0.58. However, it had positive indirect effects on WUE at HS1 and HS2 sites by altering photosynthetically active radiation (PAR), vapor pressure deficit (VPD) and canopy conductance (gc). WUE was primarily regulated by VPD and gc, and the path coefficients of the direct effect were -0.34--0.73 and -0.33- 0.73, respectively. The indirect effect of VPD on WUE through stomatal behavior was remarkable. Compared with the normal year, the regulation of DF on WUE was less in the dry year. For subtropical evergreen coniferous and warm-temperate deciduous broad-leaved plantations, WUE of the dry year was higher than that of the normal year. Warm-temperate deciduous broad-leaved plantations avoided drought by reducing gc to remain conservative water-use strategy, whereas the subtropical evergreen coniferous plantation kept high gc to obtain lower WUE in the dry year. These findings will contribute to understand the influence of diffuse radiation on the coupling between carbon and water vapor fluxes.
查看更多>>摘要：National and global greenhouse gas (GHG) budgets are continually being refined as data become available. Primary sources of the potent GHG nitrous oxide (N2O) include agricultural soil management and burning of fossil fuels, but comprehensive N2O budgets also incorporate less prominent factors such as wetlands. Freshwater wetland GHG flux estimates, however, have high uncertainty, and wetlands have been identified as both sources and sinks. Here, we analyzed a regional database of >26,000 N2O chamber flux measurements sampled across >150 wetlands from the Prairie Pothole Region (PPR) in the Great Plains of North America. Our goal was to identify important land use and hydrologic drivers of N2O flux to help reduce uncertainty in N2O models, and to incorporate these drivers into an upscaled estimate of wetland N2O emissions from the U.S. portion of the PPR. Within individual wetlands, exposed soils with no standing water, such as along wetland edges, were hotspots that accounted for greater than 90% of wetland N2O emissions. In contrast wet (i.e., ponded) areas had minimal or negative N2O flux. N2O flux from wetlands nested within croplands (16.3-17.3 mu g N2O m- 2 hr-1) was, in some instances, nearly double that from wetlands within grasslands (9.2-14.4 mu g N2O m- 2 h-1). We estimated that seasonal N2O flux from PPR wetlands equated to roughly 0.2% (1.04 Tg CO2 equivalents) of the U.S. N2O budget (c. 2019). Overall, even though PPR wetlands are a small net source of N2O to the atmosphere, their emissions are negligible relative to agricultural soil management. Policy and management to restore wetland hydrology and surrounding uplands from cropland to grasslands can reduce landscape N2O fluxes. Future activities focused on wetland N2O flux would benefit from inclusion of adjacent land use and hydrologic factors, as well as from incorporation of temporally dynamic ponded wetland areas.
查看更多>>摘要：Plant growth is controlled by an interplay of internal and external factors. The production of biomass via photosynthesis is dependent on the plant response to environmental variables such as temperature, vapour pressure deficit and light intensity. Short-term responses of plant growth to these variables at fine temporal scales of hours are not well investigated, especially under field conditions. The present study explores the relationship between leaf elongation rate (LER) of young wheat leaves in the field in very high temporal resolution (minutes). Turbulent fluxes of CO2 were measured with the eddy covariance technique and used to derive GPP, and environmental variables such as air and soil temperature, short wave radiation and vapour pressure deficit were simultaneously measured. The analysis revealed the importance of different variables on different temporal scales (hourly, daily). On an hourly scale, GPP and shortwave radiation explain most of the variance of LER, however on a daily scale, air temperature is the main driver. A cross-correlation analysis confirmed that the strongest immediate relationship can be found between LER and GPP and incoming shortwave radiation; variables that are determining photosynthesis. In principal, LER also shows the same diurnal patterns as air temperature and soil temperature, however air and soil temperature lag behind LER. Multivariate growth models show that combinations with GPP or incoming shortwave radiation and air temperature perform best. These results indicate that short term growth processes in young wheat leaves in the field are mainly controlled by incoming shortwave radiation, while the magnitude of growth is controlled by temperature.
NSTL
Elsevier