Light use efficiency and response to environmental stress: Invasion of shrubs into different communities

Background/Question/Methods

Drivers of woody expansion represent a complex, highly interactive, suite of environmental variables and have typically been attributed to anthropogenic causes including land use and climate change.  Through examining light use characteristics we can provide a better understanding of adaptations and responses to changing climate that enhances growth of expansive woody shrubs.  Research investigating light use and physiological mechanisms during times of stress will facilitate the understanding of how species are able to successfully invade new systems.  Two nitrogen-fixing shrub species that form extensive monotypic thickets were evaluated for light use efficiency (LUE).  Myrica cerifera is a native evergreen dominant in mesic coastal habitats and Elaeagnus umbellata is a deciduous drought resistant invasive shrub.  Laboratory and field studies evaluated responses to drought stress, particularly LUE, chlorophyll responses and mechanisms to dissipate excess light energy.  The Photochemical Reflectance Index (PRI) was measured as an estimate of LUE and thus photosynthetic performance and evaluated at the landscape scale.

Results/Conclusions

Measurements of photosynthesis, chlorophyll fluorescence, pigments and PRI were made following drought stress in the laboratory.  Similar measurements were made in field studies through the growing season.  Both species exhibit high photosynthetic rates and show rapid stomatal response to water stress.  Field observations show high LUE relative to other co-occurring shrub species.   While photosynthetic efficiency is reduced due to water stress, both species exhibit mechanisms to handle excess incident light as stomata close in the absence of chlorophyll degradation.  Enhanced thermal dissipation via xanthophyll pigments is considered to be a main mechanism for handling excess energy; however, both species exhibit other photoprotective mechanisms during water stress.  PRI shows declines in photosynthetic performance only after periods of chronic stress.  These expanding shrubs have high LUE and utilize a combination of stomatal response coupled with protective mechanisms that allows for fast photosynthetic recovery during times of stress, thus enabling success across the landscape.  Climate change should favor species with similar mechanisms for responding to high light under unfavorable conditions.