The effect of grass transpiration on the air temperature (original) (raw)
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Recent Micrometeorological Studies of Sensible Heat Flux in the Plant-atmosphere System
Procedia Environmental Sciences, 2013
For many years scientists working in fields related to micrometeorology have used the "Eddy Covariance (EC)" technique to study the transfer of water vapour, carbon dioxide and other greenhouse gases between plants, soils, bodies of water and the atmosphere at the boundary layer. This complex statistical technique uses high frequency measurements of the movement of air in the three dimensions along with the analysis of an air sample taken from the same position at the same time to determine the net exchange, or flux, of carbon dioxide, water vapour and sensible heat. Monitoring stations are typically installed above a canopy, field of crop or grassland, where some of the prerequisites of meaningful readings such as homogeneity of terrain can be attained. Acquisition and maintenance of the instrumentation required are expensive. Therefore, alternative methods are of interest and, if proven reliable, they may also be implemented to overcome routinely problems in direct measurements obtained by EC, such as gap filling. On the basis of recent literature, this paper reports the results of experiments carried out to evaluate the reliability of two alternative methods based on surface renewal analysis to estimate sensible heat flux.
Global Change Biology, 2006
The response of soil respiration (R s) to temperature depends largely on the temporal and spatial scales of interest and how other environmental factors interact with this response. They are often represented by empirical exponential equations in many ecosystem analyses because of the difficulties in separating covarying environmental responses and in observing below ground processes. The objective of this study was to quantify a soil temperature-independent component in R s by examining the diel variation of an R s time series measured in a temperate deciduous forest located at Oak Ridge, TN, USA between March and December 2003. By fitting 2 hourly, continuous automatic chamber measurements of CO 2 efflux at the soil surface to a Q 10 function to obtain the temperature-dependent respiration (R t) and plotting the diel cycles of R t , R s , and their difference (R i), we found that an obvious temperature-independent component exists in R s during the growing season. The diel cycle of this component has a distinct day/night pattern and agrees well with diel variations in photosynthetically active radiation (PAR) and air temperature. Elevated canopy CO 2 concentration resulted in similar patterns in the diel cycle of the temperature-independent component but with different daily average rates in different stages of growing season. We speculate that photosynthesis of the stand is one of the main contributors to this temperature-independent respiration component although more experiments are needed to draw a firm conclusion. We also found that despite its relatively small magnitude compared with the temperaturedependent component, the diel variation in the temperature-independent component can lead to significantly different estimates of the temperature sensitivity of soil respiration in the study forest. As a result, the common practice of using fitted temperature-dependent function from night-time measurements to extrapolate soil respiration during the daytime may underestimate daytime soil respiration.
Biogeosciences, 2008
CO 2 efflux at the soil surface is the result of respiration in different depths that are subjected to variable temperatures at the same time. Therefore, the temperature measurement depth affects the apparent temperature sensitivity of field-measured soil respiration. We summarize existing literature evidence on the importance of this effect, and 5 describe a simple model to understand and estimate the magnitude of this potential error source for heterotrophic respiration. The model is tested against field measurements. We discuss the influence of climate (annual and daily temperature amplitude), soil properties (vertical distribution of CO 2 sources, thermal and gas diffusivity), and measurement schedule (frequency, study duration, and time averaging). Q 10 as a 10 commonly used parameter describing the temperature sensitivity of soil respiration is taken as an example and computed for different combinations of the above conditions. We define conditions and data acquisition and analysis strategies that lead to lower errors in field-based Q 10 determination. It was found that commonly used temperature measurement depths are likely to result in an underestimation of temperature sensitiv-15 ity in field experiments. Our results also apply to activation energy as an alternative temperature sensitivity parameter. Abstract Introduction Conclusions References Tables Figures Back Close Full Screen / Esc Printer-friendly Version Interactive Discussion . Here, we examine an additional restriction which has received remarkably little attention in literature. In most field studies, column-integrated soil respiration and its sensitivity are quantified by a single temperature measurement, while the total flux is a sum of source terms from various depths, which are exposed to different temperature regimes. Because of the attenuation and phase shift of tem-5 perature fluctuations with increasing depth, the apparent Q 10 will depend on the temperature measurement depth. This possibility was mentioned first by , but without quantification. predicted that Q 10 values would increase with temperature measurement depth, and recognized that this complicates comparisons between studies. Recently, several field studies with multiple 10 temperature measurement depths have been published (;. All of them show an increase of apparent Q 10 with depth. The same effect has also been identified in model simulations by . To our knowledge, no explanations of the varying shape of these relationships have 15 been provided so far. In addition, it is unclear which Q 10 value, if any, is most appropriate when temperature measurements at multiple depths are available. , and use the temperature measurement depth yielding the highest R 2 . suggest that the temperature -efflux curve with the lowest hysteresis indicates the most appropriate temperature 20 measurement depth. Since most studies use a single, more or less arbitrary, temperature measurement depth, the effect of varying temperature measurement depth is often not considered. The aim of this study is to quantify the error in Q 10 determination caused by different temperature measurement depths as a function of soil properties, climate, and mea-25 surement schedule. To this end, we present a simple model and validate it against field measurements of heterotrophic respiration. We consider this model as a tool that helps with the design of field studies with meaningful temperature measurement depths, and with a more appropriate interpretation of existing datasets.
Wind-induced Leaf Transpiration
Wind-induced Leaf Transpiration
While the significance of leaf transpiration (f e ) on carbon and water cycling is rarely disputed, conflicting evidence has been reported on how increasing mean wind speed (U) impacts f e from leaves. Here, conditions promoting enhancement or suppression of f e with increasing U for a wide range of environmental conditions are explored numerically using leaf-level gas exchange theories that combine a stomatal conductance model based on optimal water use strategies (maximizing the 'net' carbon gain at a given f e ), energy balance considerations, and biochemical demand for CO 2 . The analysis showed monotonic increases in f e with increasing U at low light levels. However, a decline in modeled f e with increasing U were predicted at high light levels but only in certain instances. The dominant mechanism explaining this decline in modeled f e with increasing U is a shift from evaporative cooling to surface heating at high light levels. New and published sap flow measurements for potted Pachira macrocarpa and Messerschmidia argentea plants conducted in a wind tunnel across a wide range of U (2 − 8 m s −1 ) and two different soil moisture conditions were also employed to assess how f e varies with increasing U. The radiative forcing imposed in the wind tunnel was only restricted to the lower end of expected field conditions. At this low light regime, the findings from the wind tunnel experiments were consistent with the predicted trends. (G.G. Katul). mental conditions, conflicting empirical results on the sign of ∂f e /∂U emerged . Positive, negative or week dependency of f e on U for numerous forested canopies has been highlighted and discussed elsewhere . This is perhaps not surprising and has been foreshadowed by Monteith [52] who pointed out that wind effects on f e are a vexing problem because of their non-monotonic effects. The thickness of the laminar boundary layer pinned to a leaf surface, which monotonically depends on U, determines the diffusive path length for the exchanges of gases between the leaf surface and the turbulent atmosphere above the laminar boundary layer. However, U also dictates the heat exchange between leaves and the overlying atmosphere as well as the degree of evaporative cooling experienced at the leaf surface depending on the radiation load.
Acclimatization of soil respiration to warming in a tall grass prairie
2001
Abstract The latest report by the Intergovernmental Panel on Climate Change (IPCC) predicts a 1.4–5.8 C average increase in the global surface temperature over the period 1990 to 2100 (ref. 1). These estimates of future warming are greater than earlier projections, which is partly due to incorporation of a positive feedback. This feedback results from further release of greenhouse gases from terrestrial ecosystems in response to climatic warming 2, 3, 4.
1] In this paper, we analyzed 3 years of carbon flux data from continuous eddy covariance measurements to investigate how soil moisture, rain pulses, and growth alter the response of ecosystem respiration to temperature. The data were acquired over an annual grassland and from the grass understory of an oak/grass savanna ecosystem in California. We observed that ecosystem respiration was an exponential function of soil temperature during the winter wet season and a jump in ecosystem respiration occurred, at comparable temperatures, during the spring growth period. The depletion of the moisture from the soil reservoir, during spring, limited ecosystem respiration after its volumetric water content dropped below a threshold of 0.15 m 3 m À3 . The senescence of grass during the summer switched the source of ecosystem respiration to heterotrophic bacteria and fungi. During the summer, respiration proceeded at a low basal rate (about 0.10 to 0.3 g C m À2 d À1 ), except when summer rain events stimulated large dynamic pulses in heterotrophic respiration. Peak respiratory pulses were on the order of 60-80 times baseline and could not be explained by functions that depend on mean soil moisture and temperature. We found that the magnitude of the respiratory pulses was inversely related to its prerain value and that the time constant, describing the exponential decay of the respiratory pulses after the rain event, was a function of the amount of rainfall. The amount of carbon lost, in association with a few summer rain events, was greater at the site with higher primary productivity and soil carbon content.
Plant and Microclimate: A Quantitative Approach to Environmental Plant Physiology (2 Ed.)
This rigorous yet accessible text introduces the key physical and biochemical processes involved in plant interactions with the aerial environment. It is designed to make the more numerical aspects of the subject accessible to plant and environmental science students, and will also provide a valuable reference source to practitioners and researchers in the field.
Plant Biology, 2008
Productivity and climate models often use a constant Q 10 for plant respiration, assuming tight control of respiration by temperature. We studied the temperature response of leaf respiration of two cold climate species (the Australian tree Eucalyptus pauciflora and the subantarctic megaherb Pringlea antiscorbutica, both measured in a field setting) on a short timescale (minutes) during different times within a diel course, and on a longer timescale, using diel variations in ambient temperature. There were great variations in Q 10 depending on measuring day, measuring time and measuring method. When Q 10 was calculated from short-term (15 min) manipulations of leaf temperature, the resulting values were usually markedly smaller than when Q 10 was calculated from measurements at ambient leaf temperatures spread over a day. While for E. pauciflora, Q 10 estimates decreased with rising temperature (corroborating the concept of a temperature-dependent Q 10 ), the opposite was the case for P. antiscorbutica. Clearly, factors other than temperature co-regulate both leaf respiration rates and temperature sensitivity and contribute to diel and seasonal variation of respiration.