Effect of climatic conditions on tuber yield (Solanum tuberosum L.) in the European 'CHIP' experiments

The main objective of the CHIP project was to perform `standardised' investigations of potato (Solanum tuberosum L. cv Bintje) responses to increased O3 and CO2 concentrations by means of open-top chambers (OTC) and free air carbon dioxide enrichment (FACE) systems. The experimental sites are located across Europe representing a broad range of different climatic conditions. In 1998 and 1999 a total number of 12 OTC experiments and four FACE experiments were conducted. According to the specific needs for subsequent modelling purposes, environmental data were collected during experiments, i.e. air temperature, global radiation, air humidity (vapour pressure deficit (VPD)), soil moisture and trace gas concentrations. In the present paper, the results of these measurements are summarised. It was shown that the experiments covered a considerable range of growing season mean air temperatures (13.8–19.9 °C) and global irradiances (12.0–21.3 MJ m-2 per day), the most important driving variables for crop growth simulation models. Analysis of the soils used during the experiments demonstrated that in most cases sufficient nutrient elements were available to guarantee an undisturbed growth. Mean concentrations of CO2 and O3 in ambient air and in different treatments illustrate the observed variability of trace gas exposures between different sites and experiments. However, the effects of these parameters on growth and yield are subject of separate papers. The general climatic conditions across Europe are also causing important growth and yield effects. Comparison of marketable tuber yields revealed an increase at higher latitudes. This result was associated with lower temperatures and VPD and longer day lengths at the higher latitudes, which in turn were associated with longer growing seasons.