Abstract
Temperature manipulation experiments are an effective way for testing plant responses to future climate conditions, especially for predicting shifts in plant phenological events. While passive warming techniques are widely used to elevate temperature in low stature plant communities, active warming has been applied less frequently due to the associated resource requirements. In forest ecosystems, however, active warming is crucial to simulate projected air temperature rises of 3–5 K, especially at the warm (i.e., southern and low elevation) range edges of tree species. Moreover, the warming treatment should be applied to the complete height of the experimental plants, e.g., regenerating trees in the understory. Here, we combined open top chambers (OTCs) with active heat sources, an electric heater (OTC-EH) and warming cables (OTC-WC), and tested the effectiveness of these set-ups to maintain constant temperature differences compared to ambient temperature across 18 m2 plots. This chamber size is needed to grow tree saplings in mixture in forest gaps for 3 to 10 years. With passive warming only, an average temperature increase of approx. 0.4 K as compared to ambient conditions was achieved depending on time of the day and weather conditions. In the actively warmed chambers, average warming exceeded ambient temperatures by 2.5 to 2.8 K and was less variable over time. However, active warming also reduced air humidity by about 15%. These results underline the need to complement passive warming with active warming in order to achieve constant temperature differences appropriate for climate change simulations under all weather conditions in large OTCs. Since we observed considerable horizontal and vertical temperature variation within OTCs with temperature differences of up to 16.9 K, it is essential to measure and report within-plot temperature distribution as well as temporal temperature variation. If temperature distributions within large OTCs are well characterized, they may be incorporated in the experimental design helping to identify non-linear or threshold responses to warming.
Highlights
Climate change affects the structure and function of ecosystems (Walther et al, 2002; Thomas et al, 2004; Parmesan, 2006) by altering growth rates, physiology, survival, and distributions of individuals, populations, species, and communities (Dukes and Mooney, 1999; Hobbie et al, 1999; Morin et al, 2008; Reich et al, 2015; Gruner et al, 2017)
The first plot was experimentally warmed with an electric heater, the second plot contained electrical resistance-heating cables laid out on the ground surface, the third plot was passively warmed through the greenhouse effect of the open top chambers (OTCs) and the fourth plot was a full control plot without OTC, in which only the ground was covered with the water-permeable lining and wood chips
Determining passive and active warming effects turned out to be challenging during sunrise and sunset: OTC-EC and OTC-WC were located approximately 10 m to the west of OTC-CTRL and Full-CTRL, which means that the actively heated plots were hit by direct sunlight earlier than the control plots in the morning
Summary
Climate change affects the structure and function of ecosystems (Walther et al, 2002; Thomas et al, 2004; Parmesan, 2006) by altering growth rates, physiology, survival, and distributions of individuals, populations, species, and communities (Dukes and Mooney, 1999; Hobbie et al, 1999; Morin et al, 2008; Reich et al, 2015; Gruner et al, 2017). Temperature manipulation experiments are an effective way of testing and quantifying plant responses to climate change (Ettinger et al, 2019) They are urgently needed to improve and validate models that predict climate driven shifts in phenological events. The passive warming of these chambers was an unintended side effect making them interesting for climate warming research (e.g., Drake et al, 1989) Due to their low infrastructure, maintenance and budget requirements OTCs have been widely used to elevate temperature in open low stature plant communities, such as remote arctic and alpine tundra ecosystems (e.g., Elmendorf et al, 2012), grassland steppe (Klein et al, 2005), temperate grasslands (Carlyle et al, 2011) and saltmarshes (Gedan and Bertness, 2009), but they were rarely used in taller-stature plant communities (Welshofer et al, 2018). OTCs are ineffective without solar irradiance, and have only limited potential for applications in forest ecosystems (De Frenne et al, 2010)
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