查看更多>>摘要：Plant phenology and growth rate are sensitive bio-indicators of climate change and anthropogenic disturbances. Fire is widespread in many ecosystems worldwide. Understanding how plant growth varies in its response to fire disturbance is critical for fire management. Despite the effects of fire on various aspects of plant ecology, such as the composition and growth of vegetation, little is known about the impact of fire on biomass production resulting from changes in functional group composition, plant phenology, and growth rate. Prescribed-burning of three types of grassland along an aridity gradient in Inner Mongolia, China, did not significantly affect species richness or the relative abundance of various functional groups across the three grasslands. Fire-induced advancement (-3 weeks) and elongation (-2 weeks) of growing periods increased perennial grass and community production in the low aridity grassland (aridity index, AI = 0.38). In contrast, marginal changes in phenology did not influence production in the moderate aridity grassland (AI = 0.27). Post-fire delayed and shortened growing period by -5 and 4 weeks respectively and thus reduced community production in the high aridity grassland (AI = 0.20). Post-fire plant growth rate was reduced by -44% in the low aridity grassland but was enhanced by 20-100% in the moderate and high aridity grasslands. The opposing effects of fire on growth rate vs. phenological responses largely negated any overall effect on community production. Our results indicate that phenology and growth rate rather than diversity were the primary contributors to the variation in production after fire. We provide new empirical evidence that prescribed-burning would not be a suitable management tool in extremely arid grasslands because it delays growing period and reduces production, both of which may influence grazing time and intensity. Our findings highlight the compensatory effects of plant phenology and growth rate on the regulation of biomass production.
查看更多>>摘要：To study the effects of heat stress in dairy cattle, animal performance records are merged with weather data from nearby meteorological stations. However, the stations location and distribution may render them less reliable. Alternatively, weather information from several meteorological stations can be interpolated using empirical algorithms to produce gridded estimates of weather parameters at desired spatio-temporal resolutions. The objective of this study was to compare and apply observational weather station data (WSD) and gridded numerical weather prediction (NWP) model data with high spatio-temporal resolution to identify thermal stress thresholds for milk production traits. Weather data were from the German meteorological service and reanalysis was based on the COnsortium for Small-Scale MOdelling (COSMO)-REA6 model. Milk performance data were from over 16 million monthly test-day records for the period 2010 to 2019 in southern Bavaria, Germany. Individual cow records were transformed to herd averages resulting to 797,455 herd test-days from 9,726 herds. These were merged with temperature and temperature humidity index (THI) data from 53 weather stations and corresponding gridded data. There was good agreement between WSD and NWP model data with correlation coefficients of 0.97 for both daily average temperature and THI and 0.84 for relative humidity. However, positive and negative biases were observed in the pre-Alpine regions. The average herd reaction norms were in good agreement and followed similar trends when estimated from WSD and NWP. The heat stress threshold at which milk yield and protein yield started to decline was 16.0 ? for temperature and 60 units for THI. The responses of fat yield, protein and fat contents were generally linear decline with no definite thresholds. Milk urea increased in a non-linear accelerating pattern and there was no undesired effects on SCS. The thresholds obtained in this study may be applied to implement necessary management strategies to mitigate losses.
查看更多>>摘要：The primary focus of this study is to simulate, characterize and validate diurnal patterns of global solar radiation (GSR), temperature, relative humidity and wind speed from daily ground observations from 2017 to 2020 at Obafemi Awolowo University, Ile-Ife, Nigeria. The study also estimates diurnal variations of the meteorological parameters from three daily reanalyses (NASA, NCEP/NCAR and ERA5) and compares the results with hourly station observations (using performance evaluation indices such as mean bias, BIAS; percentage mean bias, PBIAS; root-mean-square-error, RMSE; Nash-Sutcliffe coefficient, NSE; normalised standard deviation, NSD and correlation coefficient, r). Results showed that the empirical models adequately captured the observed diurnal characteristics of the meteorological variables. Good estimates of diurnal patterns of GSR (BIAS = 6.77 Wm(-2); RMSE = 9.20 Wm(-2); PBIAS = 5.68%; NSE = 0.52; NSD = 0.62; r = 0.86), temperature (BIAS = -0.79?; RMSE = 1.90?; PBIAS = -2.43%; NSE = 0.60; NSD = 1.09; r = 0.86), and humidity (BIAS = 2.81%; RMSE = 6.10%; PBIAS = 2.51%; NSE = 0.43; NSD = 0.83; r = 0.73) were obtained. Statistics suggested very strong model fits and close agreements with observations. Large and significant discrepancies (at p <= 0.05), however, were obtained for diurnal simulations of wind speed (BIAS = 0.28 ms(-1); RMSE = 0.40 ms(-1); PBIAS = 9.79%; NSE = -0.10; NSD = 1.36; r = 0.65). Furthermore, the model performances for hourly disaggregation of the parameters varied amongst the reanalyses. The findings provide good basis for generating sub-daily meteorological data resources with wide range of applications in hydrology, climate modelling, and plant growth simulation.
查看更多>>摘要：Plant traits are an expression of strategic tradeoffs in plant performance that determine variation in allocation of finite resources to alternate physiological functions. Climate factors interact with plant traits to mediate tree survival. This study investigated survival dynamics in Norway spruce (Picea abies) in relation to tree-level morphological traits during a prolonged multi-year outbreak of the bark beetle, Ips typographus, in Central Europe. We acquired datasets describing the trait attributes of individual spruce using remote sensing and field surveys. We used nonlinear regression in a hypothesis-driven framework to quantify survival probability as a function of tree size, crown morphology, intraspecific competition and a growing season water balance. Extant spruce trees that persisted through the outbreak were spatially clustered, suggesting that survival was a nonrandom process. Larger diameter trees were more susceptible to bark beetles, reflecting either life history tradeoffs or a dynamic interaction between defense capacity and insect aggregation behavior. Competition had a strong negative effect on survival, presumably through resource limitation. Trees with more extensive crowns were buffered against bark beetles, ostensibly by a more robust photosynthetic capability and greater carbon reserves. The outbreak spanned a warming trend and conditions of anomalous aridity. Sustained water limitation during this period amplified the consequences of other factors, rendering even smaller trees vulnerable to colonization by insects. Our results are in agreement with prior research indicating that climate change has the potential to intensify bark beetle activity. However, forest outcomes will depend on complex cross-scale interactions between global climate trends and tree-level trait factors, as well as feedback effects associated with landscape patterns of stand structural diversity.
查看更多>>摘要：Diffuse radiation is widely known for its higher light use efficiency in ecosystem carbon uptake than direct radiation, which inevitably alters ecosystem evapotranspiration (ET), as plant carbon uptake and water use are closely coupled through leaf stomata. However, although the impact of diffuse radiation on ecosystem carbon uptake has been extensively explored, its impact on ecosystem ET remains unclear on the global scale and across different plant functional types (PFTs). In this study, we explored the impacts of diffuse radiation on ecosystem ET and its coupling to net ecosystem exchange (NEE) based on long-term eddy-covariance observations and the derived diffuse radiation fraction (Kd) at 201 FLUXNET stations. We found that the increase in diffuse radiation results in a net enhancement of ET when Kd was below 0.42-0.48 for most PFTs. Diffuse radiation was more effective in promoting ET than direct radiation because the diffuse fertilization effect (DFE) on NEE tightly regulates ET through stomatal coupling. On average, the efficiency of diffuse radiation (Kd > 0.8) was 1.51 (+/- 0.80) times higher than that of direct radiation (Kd < 0.2) in promoting ET when the PAR level was less than 300 W m- 2, which was lower than the ratio of 2.40 (+/- 0.93) for NEE under the same conditions, because the simultaneous reductions in vapor pressure deficit (VPD) in part offset the enhancement of ET. Consequently, the increase in ET results in a higher evaporative fraction under shaded conditions with more diffuse radiation. We emphasized the importance of considering the DFE on ecosystem ET in assessing aerosol-induced perturbations in the water cycle under the current and future climate.
查看更多>>摘要：Sun-induced chlorophyll fluorescence (SIF) is one of the most promising remote-sensing signals to assess spatiotemporal variation in photosynthesis. Yet, it has been shown that the positive linear relationship of SIF and photosynthesis, often reported from satellite and proximal remote sensing, is mainly driven by the amount of absorbed photosynthetic active radiation (APAR). By normalizing SIF by APAR these structural first-order effects can accounted for and SIF is then reflecting physiological regulation of photosynthetic efficiency. However, because of the confounding contribution of non-photochemical energy dissipation, the relationship between SIF and photosynthetic efficiency is non-linear, and therefore additional measurements have to be included to constrain the predictions of photosynthetic efficiency and photosynthetic electron transport. We grew Zea mays at different phosphorus (P) levels to assess if P-induced reduction in quantum efficiency of PSII (phi PSII), can be estimated by the fluorescence efficiency parameters, APAR normalized fluorescence (Fyield) and the ratio of the two emitted fluorescence peaks (F up arrow ratio), at leaf level. Results were compared to the photochemical reflectance index (PRI), a well-established index related to the activity of the xanthophyll cycle, a protection mechanism which activates under light-stress conditions. We demonstrate that the relationship between phi PSII and Fyield is non-monotonic across a P limitation gradient, rendering the prediction of phi PSII by Fyield alone unfeasible. We show, however, that the pigment corrected PRI (cPRI) and F up arrow ratio (cF up arrow ratio) share a strong linear relationship with phi PSII, thereby enabling the estimation of phi PSII. We demonstrate that a compensation for reabsorption effects improved the estimation of phi PSII by Fratio at foliar level. This may allow improved predictions of photosynthetic light use efficiency parameters without the need of measuring green APAR.
查看更多>>摘要：Turbulence structures are studied for momentum and sensible heat fluxes above different crops, maize and soybean, under neutral and unstable regimes through quadrant analysis. Micrometeorological sensors were installed at the same height (5 m) over productive plots in Buenos Aires province, Argentina. The effect of variations in crop height (in relation to sensor height) throughout the growing season is studied. In the maize experiment, canopy mixing-layer conditions prevail, with strong unstable situations, where a more efficient sensible heat transport relative to that of momentum follows a pattern explained by thermal plume type structures. Ejections are more intense but sweeps dominate in time for sensible heat transport. In addition, sweeps dominate both in intensity and time for momentum flux, due to the closeness of measurements to the canopy top. Above soybean, surface layer behavior is identified with near-neutral conditions, and there is a flux transport pattern associated with hairpin vortex type structures. Sensible heat and momentum fluxes are driven by the same motions, with no clear differences in transport efficiency. In soybean, ejections are more important because of a higher distance of measurement height to the canopy top. For each experiment, turbulence vary mainly due to atmospheric stability, while the effect of variation in the distance between the sensor and the top of the canopy due to plant growth is negligible. In this work, turbulence interactions in the surface layer are identified at heights that are not studied by works based on LES simulations.
NSTL
Elsevier