Rain pulses - how different data streams provide insight in response-times and help to attribute biophysical properties to ecosystem processes.

Rain pulses play a major role for the carbon cycle in semiarid ecosystems, as they can release large amounts of stored carbon. Physical and biological processes, triggered by the availability of water start to develop on various time scales and are dependent on the amount of available water. Especially, in savanna type ecosystems with an herbaceous understory and sparsely distributed trees the response time of the two plant functional types to rain pulses might be different. We present results from an ongoing large-scale nutrient manipulation experiment (MANIP) in a Mediterranean savanna type ecosystem and its response to rain pulses. Within MANIP the footprint areas from two out of three ecosystem eddy co-variance (EC) sites were fertilized with nitrogen (NT) and nitrogen plus phosphorous (NPT), the third served as the control (CT). The analysis combines EC data to determine the net ecosystem exchange, PhenoCam data to define the senescence and re-greening period, SAP-flow measurements to evaluate the response of trees to rain pulses, high frequency (1 Hz) CO2-concentration measurements to estimate the response time to of the ecosystem to rain pulses, and meteorological measurements to quantify the intensity of the rain pulses. Additionally, at NT canopy reflectance and SIF are measured continuously for trees and grasses. The combination of SIF and SAP-flow measurements allows to separate the contribution of trees to ecosystem fluxes and can be utilized to partition NEE into ecosystem respiration and gross primary productivity during the senescence period. The analyses focus on three topics; (i) utilizing high frequency dynamics of CO2 concentration to disentangle physical and biological responses to water availability; (ii) fertilization effect of respiration pulses on ecosystem fluxes; (iii) response of tree transpiration to rain pulses. CO2 concentrations show an instantaneous reaction to rain fall. Within minutes concentrations increase strongly and follow distinct patterns. The fertilization effect is not clear in terms of respiration magnitude triggered by the rain pulse but the fertilized areas show slightly more carbon uptake during daytime after a precipitation pulse. Sap-flow measurements indicate a response of the trees to the rain pulses which effect nocturnal and daytime sap velocities.