Temperature Response and Adjustment in Trees:
Physiological and Ecological Basis for Forest Responses to a Warmer Climate
Goal and Objectives
The primary goals of the project are to:
- Improve our ability to predict and understand forest responses to atmospheric warming by testing assumptions about the distribution of forest species, adaptation, and physiological plasticity
- provide a physiological basis for predicting long-term changes in forest composition and function (above and below ground ecosystem functions).
The ecophysiological assumptions being tested are:
These assumptions are used in empirical predictions of forest responses, for lack of a mechanistic explanation of species distribution or ecosystem functional responses to climatic warming.
- Species-specific differences in temperature sensitivity determine range limits.
- Climatic factors such as temperature and precipitation are the major determinants of plant distribution
- Species and ecotypes are adapted to their thermal environments.
- Atmospheric warming will result in the loss of species not adapted to warmer climates (i.e., species whose current ranges are cooler than predicted for the future).
- Warming will result in faster plant growth rates of well-adapted species, loss of poorly-adapted species, and higher respiration rates in all plants and soil ecosystems.
Current ranges (yellow-green) of the four study species and predicted ranges (green-blue) under future climatic conditions of 2x current [CO2] based on predictions of the occurrence of suitable habitats in climatic change scenarios.
(click to enlarge)
Maps are taken from:
Prasad, A.M. & L.R. Iverson. 1999-ongoing. A Climate Change Atlas for 80 Forest Tee Species of the Eastern United States [database]. , Northeastern Research Station, USDA Forest Service, Delaware, Ohio.
Several observations, however, suggest that the conventional assumptions are inadequate for predicting forest succession and forest function in a warming climate:
Our research goal is to fill the knowledge gap limiting our ability to predict changes in forest ecosystem function and structure as climates warm.
- There is substantial uncertainty about the determinants of equatorial (warmer) range boundaries— water availability might override temperature sensitivity.
- Little is known about the physiological basis for species differences in temperature responses.
- Physiological temperature adjustments are known to occur in some species, including changes in temperature optima, homeostatic rate adjustments in enzymatic processes, altered membrane fluidity, etc.
- Homeostatic adjustments in CO2 exchange could alter predictions of plant carbon balance, hence growth and, ultimately, survival in warmer temperatures.
- Soil ecosystems could respond by either physiological adjustments, or changes in species composition better adapted to warmer temperatures.
- Temperature acclimation of soil ecosystem CO2 exchange rates would change assumptions used in modeling biological carbon sequestration and the long-term ability of ecosystems to buffer rising atmospheric CO2 concentrations.
Our two-fold approach involves:
- An open-top chamber study will directly test the assumptions relating changes in species distribution to warmer temperatures. Species from geographic ranges encompassing a range of climates will be grown in experimentally manipulated temperature treatments.
- A large-tree physiology task will take measurements to verify that physiological adjustment mechanisms are valid in established forest trees. This task will measure foliar temperature responses in of multiple species during cool and warm parts of the season.
The open-top chamber experiment consists of nine chambers located at the Global Change Field Research Facility on the Oak Ridge National Environmental Research Park
The experimental design includes:
- Seedlings of four deciduous temperate tree species chosen because their native ranges represent a continuum of thermal environments. Empirical methods also predict that some will be more common, less common, or will disappear from this portion of their range in response to atmospheric warming. Species are:
- Plants are exposed year-round to three experimental warming treatments:
- ambient temperature tracking
- ambient +2.5°C
- ambient +5°C
- Assessments of temperature sensitivity (responses) and acclimation (adjustment) potential in the open-top experiment include comparisons of
- Seedling growth, survival, and phenological patterns in response to warming, and
- Acclimation of physiological CO2 exchange (photosynthesis and respiration) to seasonal and treatment-induced differences in air temperatures.
- Ecosystem level responses and adjustments are being assessed in the below-ground portion of the experimental chambers. Soil respiration (microbial and root zone components) is evaluated for responsiveness and adjustment potential at several time steps, including diurnal, seasonal and long-term (treatment) differences in temperature.
The large-tree physiology task expands the scope and significance of the seedling-based measurements. Temperature response measurements in established forest trees will demonstrate whether mature tree foliage is capable of the adjustments observed in seedlings.
In both tasks, physiological measurements will be used to establish response predictors based on physiological mechanisms which can be used to influence model structures.
- Physiological assessments generated throughout the season will encompass cooler and warmer parts of the growing season to determine the extent of adjustment to inter-annual temperature variation and the potential for acclimation to long-term climatic warming or extreme events.
- Observations on multiple species (beginning with, but not limited to, those in the temperature manipulation experiment) will ascertain the general applicability of adjustment processes across deciduous species.
Identifying the ecophysiological bases underlying fundamental species differences will provide a theoretical framework for improving the temperature responsiveness of both process-based and empirical models of forest growth, succession, productivity, and water use, and will result in more robust predictive models.
- Installation of chambers and heating and cooling systems began in late summer 2001 and was completed by spring 2002.
- Planting and experimental treatments in the open-top chambers were initiated in the spring of 2002, and the first season of measurements (physiology, height and diameter growth and autumn phenology) have been completed.
- Temperature targets of the experimental treatments are being successfully maintained summer and winter.
- Soil moisture content is being monitored, and supplemental soil moisture provided to alleviate any soil moisture differences caused by secondary effects of increased evapotranspiration or larger plants in elevated temperature chambers.
- Assessments of temperature sensitivity (responses) and acclimation (adjustment) potential in these species have begun. Treatment differences in senscence have been observed. Initial measurements of foliar physiology have demonstrated the potential for acclimation of CO2 exchange to treatment temperatures.
- Measurements to assess soil ecosystem responses are ongoing, including:
- Response of soil respiration to warming
- Responsiveness of soil respiration in the different treatments to diurnal and seasonal temperature fluctuations
- Comparison of total belowground respiration of soil + root zone with soil respiration in root exclusion zones.
- Data from the chamber experiment are being supplemented by physiological measurements in the large-tree verification task. Physiological temperature response curves generated in established trees throughout the growing season have captured a range of prevailing temperatures. In the first species completed, seasonal acclimation was demonstrated in mature tree foliage, and is being compared with seedling physiological responses.
Carla A. Gunderson -- PI, project management, aboveground measurements, large-tree field task coordination and measurements
Nelson T. Edwards --Co-PI, belowground ecosystem respiration, coordination of facility operation and analysis of system performance data analysis.
Jeffery S. Riggs - Instrumentation design and control, system data collection.
Student / intern participants
Keiran O’Hara - site preparation, seedling planting, growth measurements, leaf physiology, large tree physiology, leaf litter sampling and analysis
Rebekah Hutton - soil moisture measurements, seedling growth measurements, data management, leaf physiology, leaf litter sampling and analysis
Susan Geist - soil respiration measurements and data analysis
Sara Jawdy - soil respiration and soil moisture
Carolyn Reilly - soil respiration and leaf litter data
Carrie Pendley - large-tree temperature response
Emily Greer - large-tree temperature response
We are grateful to Don Zak and Matt Tomlinson of the School of Natural Resources & Environment, University of Michigan for providing us with dormant Populus grandidentata roots and advice on propagation.