The Biosphere 2 rainforest is diverse (~90 species in 0.2 ha) ecosystem that has been evolving for the past 25 years in a glass and metal enclosure. Part of the evolution has been species loss, which appears to mainly driven by species exposure to high temperatures that are the norm in greenhouse enclosures. The enclosure allows us to experiment with environmental conditions likely to become more common in tropical forests in Biosphere 1. These include experimentation with temperature, precipitation and CO2 levels. During the experiments we monitor the environmental conditions and response of the whole system and main tree species for changes in carbon and water cycling.
Over the past year we have initiated a series of drought experiments to understand better what sources trees use to get their water. Where trees get there water from has been a long-standing debate in the ecohydrological community. The rainforest inside Biosphere 2 allows us to explore how plants might adjust their water uptake strategies through a controlled drought. In summer of 2014 we conducted a drought and sampled tree species for the water composition within their stems, at different soil depths, and from the subsoil drainage. We then added a δ2H label to the initial rainfall events after the drought, to determine where this water is stored in the soil (the samples are still being processes at the Global Institute for Water Security, University of Saskatchewan). In the fall of 2015 we will inject δ2H labeled water at the base of the soil profile and then determine which trees take up this deep water and when, if at all, that water is released to the “stream” draining the rainforest. We will use the unique testing features of the B2 rainforest to answer several key technical questions: how different are the δ18O and δ2H from soil water sampled from with suction lysimetry (mobile water) and cryogenic extraction (strongly held water, only available to plant roots) and how do they compare to plant water and sub-soil drainage water? Is the plant water isotope composition different across the 7 canopy species, or do different plant species use different water sources? Do these differences change (or not) through a 6-week drought, or does the water use strategy change with drought for different tree species?
Creating a Drought in the Rainforest
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