A transplantation experiment along climatic gradients suggests limitations of experimental warming manipulations

Transplantation to a warmer site and experimental passive warming are powerful tools for predicting plant responses to climate change. Both techniques are widely applied for the study of plant species and community response to temperature increase. We investigated differ- ences in height increment of Fagus sylvatica seedlings between 2 different techniques: experi- mental warming (passive warming) and transplantation to a warmer site. Additionally, the plants were exposed to an extreme drought to further examine the influence of the different warming techniques in combination with an additional climatic driver. We found significant differences between the 2 warming techniques for height increment, which were mainly attributed to the case with additional drought exposure (significant interaction between warming and drought). Surpris- ingly, when subjected to drought, experimental warming had no negative effect on height incre- ment of seedlings, while transplantation decreased height increment by 32% when subjected to drought. Growth did not show a linear dependence on the magnitude of warming. Differences between the warming techniques can therefore not be explained by differences in realized tem- perature increases. The results of this study emphasize the complexity of simulating global warm- ing, as required for accurate prediction of shifts in plant performance. The role of co-varying parameters, such as evapotranspiration, photosynthetically active radiation, and wind speed, in addition to experimental temperature increases should be acknowledged when ana lyzing ecolog- ical responses to climate warming.