Ecological effects of experimental drought and prescribed fire in a southern California coastal grassland

How drought and fire disturbance influence different levels of biological organization is poorly understood but essential for robust predictions of the effects of environmental change. During a year of severe drought, we conducted a prescribed fire in a Mediterranean-type coastal grassland near Irvine, California. In the weeks following the fire we experimentally manipulated rainfall in burned and unburned portions of the grassland to determine how fire and drought interact to influence leaf physiological performance, community composition, aboveground net primary productivity (ANPP) and component fluxes of ecosystem CO2 exchange and evapotranspiration (ET). Fire increased leaf photosynthesis (A net) and transpiration (T) of the native perennial bunchgrass, Nassella pulchra and the non-native annual grass, Bromus diandrus but did not influence ANPP or net ecosystem CO2 exchange (NEE). Surprisingly, drought only weakly influenced A net and T of both species but strongly influenced ANPP and NEE. We conclude that despite increasing experimental drought severity, prescribed fire influenced leaf CO2 and H2O exchange but had little effect on the component fluxes of ecosystem CO2 exchange. The differential effects of prescribed fire on leaf and ecosystem processes with increasingly severe drought highlight the challenge of predicting the responses of biological systems to disturbance and resource limitation

Measurements of CO2 exchange over a woodland savanna (Cerrado Sensu stricto) in southeast Brasil

The technique of eddy correlation was used to measure the net ecosystem exchange over a woodland savanna (Cerrado Sensu stricto) site (Gleba Pé de Gigante) in southeast Brazil. The data set included measurements of climatological variables and soil respiration using static soil chambers. Data were collected during the period from 10 October 2000 to 30 March 2002. Measured soil respiration showed average values of 4.8 molCO2 m-2s-1 year round. Its seasonal differences varied from 2 to 8 molCO2 m-2s-1 (Q10 = 4.9) during the dry (April to August) and wet season, respectively, and was concurrent with soil temperature and moisture variability. The net ecosystem CO2 flux (NEE) variability is controlled by solar radiation, temperature and air humidity on diel course. Seasonally, soil moisture plays a strong role by inducing litterfall, reducing canopy photosynthetic activity and soil respiration. The net sign of NEE is negative (sink) in the wet season and early dry season, with rates around -25 kgC ha-1day-1, and values as low as 40 kgC ha-1day-1. NEE was positive (source) during most of the dry season, and changed into negative at the onset of rainy season. At critical times of soil moisture stress during the late dry season, the ecosystem experienced photosynthesis during daytime, although the net sign is positive (emission). Concurrent with dry season, the values appeared progressively positive from 5 to as much as 50 kgC ha-1day-1. The annual NEE sum appeared to be nearly in balance, or more exactly a small sink, equal to 0.1 0.3 tC ha-1yr-1, which we regard possibly as a realistic one, giving the constraining conditions imposed to the turbulent flux calculation, and favourable hypothesis of succession stages, climatic variability and CO2 fertilization

Daily canopy photosynthesis model through temporal and spatial scaling for remote sensing applications

Because Farquhar’s photosynthesis model is only directly applicable to individual leaves instantaneously, considerable skill is needed to use this model for regional plant growth and carbon budget estimations. In many published models, Farquhar’s equations were applied directly to plant canopies by assuming a plant canopy to function like a big-leaf. This big-leaf approximation is found to be acceptable for estimating seasonal trends of canopy photosynthesis but inadequate for simulating its day-to-day variations, when compared with eddy-covariance and gas-exchange chamber measurements from two boreal forests. The daily variation is greatly dampened in big-leaf simulations because the original leaf-level model is partially modified through replacing stomatal conductance with canopy conductance. Alternative approaches such as separating the canopy into sunlit and shaded leaf groups or stratifying the canopy into multiple layers can avoid the problem. Because of non-linear response of leaf photosynthesis to meteorological variables (radiation, temperature and humidity), considerable errors exist in photosynthesis calculation at daily steps without considering the diurnal variability of the variables. To avoid these non-linear effects, we have developed an analytical solution to a simplified daily integral of Farquhar’s model by considering the general diurnal patterns of meteorological variables. This daily model not only captures the main effects of diurnal variations on photosynthesis but is also computationally efficient for large area applications. Its application is then not restricted by availability of sub-daily meteorological data. This scheme has been tested using measured CO2 data from the Boreal Ecosystem–Atmosphere Study (BOREAS), which took place in Manitoba and Saskatchewan in 1994 and 199

Ecological effects of experimental drought and prescribed fire in a southern California coastal grassland

Howdroughtandfiredisturbance influence different levelsofbiological organization is poorly understood but essential for robust predictionsoftheeffectsofenvironmental change. Duringayearofseveredrought, we conductedaprescribedfireinaMediterranean-typecoastalgrasslandnear Irvine,California.Inthe weeks following thefirewe experimentally manipulated rainfallinburned and unburned portionsofthegrasslandto determine howfireanddroughtinteract to influence leaf physiological performance, community composition, aboveground net primary productivity (ANPP) and component fluxesofecosystem CO2 exchange and evapotranspiration (ET).Fireincreased leaf photosynthesis (A(net)) and transpiration (T)ofthe native perennial bunchgrass, Nassella pulchra and the non-native annual grass, Bromus diandrus but did not influence ANPP or net ecosystem CO2 exchange (NEE). Surprisingly,droughtonly weakly influencedA(net) and Tofboth species but strongly influenced ANPP and NEE. We conclude that despite increasingexperimentaldroughtseverity,prescribedfireinfluenced leaf CO2 and H2O exchange but had little effect on the component fluxesofecosystem CO2 exchange. The differentialeffectsofprescribedfireon leaf and ecosystem processes with increasingly severedroughthighlight the challengeofpredicting the responsesofbiological systems to disturbance and resource limitation