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Thermal Tolerance & Community Change

 

Plant community change in the Biosphere 2 tropical forest mesocosm is associated with long-term warming and enhanced tolerance to high temperatures.

What happens when you build an ecosystem, set the environmental conditions, and then leave it to its own devices?  This experiment has been running for two decades in the form of an artificial tropical forest ecosystem at the University of Arizona’s Biosphere 2 (B2).  Targeted climate conditions are similar to a tropical moist forest, although under the heat-trapping pyramidal glass enclosure, temperatures average several degrees higher than typical Amazon sites. 

Did high temperatures impact the plant community, and can those changes tell us about tropical forest responses to climate change?  In 2010, Ty Taylor completed the fifth survey of the B2 tropical forest flora since 1991.  This 20-yr time series shows an initial precipitous drop in species richness, with the rate of species extinctions decreasing over time (figure 1)

One quarter of the initial 366 plant species remain.  The ferns and gymnosperms were the first to go.  Those species were representatives of cool, moist mountain climates, and probably lacked the high-temperature adaptations required to survive in B2.  As for the other species, an analysis of tree extinctions at the plant family level demonstrated a shift in community composition from one typical of a tropical wet or moist forest to that of a tropical dry forest (figure 2)

This indicates that as a plant community experiences new climate conditions, a filtering process occurs, leaving behind those species that are capable of surviving in the new climate.  The analysis shown in figure 2 demonstrates that the outcome is predictable based on a survey of families alone.  Furthermore, a comparison of whole-forest photosynthesis over a range of temperatures in the B2-TF and a humid tropical forest site in Brazil showed that the B2-TF is substantially more tolerant to high temperatures (figure 3)

We hypothesize that the community shift toward dry-forest representatives has facilitated this enhanced thermal tolerance of the forest. 

These results suggest that tropical forests may be able to maintain function as the climate changes, especially if species are able to move around quickly enough, tracking the climates to which they are best adapted.

 

Students:  Tyeen Taylor, Ph.D. Student, Ecology and Evolutionary Biology, University of Arizona; Marielle Smith, Ph.D. Student, Ecology and Evolutionary Biology, University of Arizona; Simon Stump, Ph.D. Student, Ecology and Evolutionary Biology, University of Arizona.

Collaborators:  Brad Boyle, Research Scientist, Ecology and Evolutionary Biology, University of Arizona; Cyrille Violle, CNRS

Montpellier, France; Joost van Haren, Assistant Research Professor, Biosphere 2, University of Arizona; Rafael Rosolem, Postdoctoral Research Associate, Hydrology and Water Resources, University of Arizona; Brian Enquist, Professor, Ecology and Evolutionary Biology, University of Arizona ; Scott Saleska, Professor, Ecology and Evolutionary Biology, University of Arizona.