We have developed new ways to measure carbon dioxide in tree stems. Using these techniques we found that high concentrations of CO₂ (3-15%, compared to 0.04% in the atmopshere) are transported in xylem sap. We also developed a new method for measuring stem respiration that accounts for sap-transported CO₂. questions we are addressing include the contribution of internal CO₂ to measurements of the respiration rate of woody tissues and whether the transported CO₂ is recycled through corticular and leaf photosynthesis. For more information, see:

We are studying the effects of elevated CO₂, elevated air temperature and drought stress on loblolly pine seedlings at three sites in Georgia.  The purpose of this project is to test a conceptual model designed to predict the response of a tree species to climate change across its geographic range.  We will also be examining the potential interactions of temperature, CO₂ concentration and water availability on growth, net photosynthesis, respiration and other physiological processes.

We are examining the relationship between nutrient uptake and transpiration rate in a controlled environment study in growth chambers.   Transpiration has been modified by high or low vapor pressure deficits and plants have been grown in one of three levels of nutrient availability.  Our objective is to determine if plants that are nutrient stressed have greater overall water use or higher rates of transpiration in order to take up more nutrients.

Applications of physiologically based models for forest management decisions

Models of forest productivity based on physiological processes, often called process models, are usually too complicated and data-intensive to be useful to forest managers. We are currently modifying a hybrid or simplified process model called 3-PG (Landsberg, J. and Waring, R. 1997. Forest Ecology & Management 95:209-228) to try to fill this gap. Our objective is to make tree physiological information more useful for predicting growth and yield of forest plantations. Our target species for model testing purposes is loblolly pine (Pinus taeda L.) which is grown extensively in the southeastern United States for wood and fiber uses. We are developing an easy-to-use version of the model and have also modified the model to allow users to incorporate silvicultural and management options into the scenarios. The current version of the model (3-PG+) can be found here.

We investigated the amounts and sources of water used by pines, scrub oaks and the wiregrass understory in the longleaf pine/wiregrass savannas of south Georgia. We also studied the effect of different levels of nitrogen availability on water use by these ecosystems. This project was conducted at the Jones Ecological Research Center. Stable isotope analyses and tree and wiregrass water use measurements provided a picture of the sources and quantities of water used by the plants in these ecosystems. For more information, see:

Climate change effects on loblolly pine (Pinus taeda, L.)

One of the most important issues concerning global climate change is that air temperatures may substantially increase, altering the climate and affecting ecosystem functioning. In this study we looked at how various physiological factors that contribute to growth (such as photosynthesis, respiration, leaf area development and carbon allocation) are altered under different climatic regimes. Loblolly pine seedlings were placed in five different environments from Coweeta, NC to Gainesville, FL. Across these sites there is an 8°C change in mean annual temperature. Soil conditions were optimized for growth by daily fertigation and genetic variation was controlled. The results of this project should help us understand how this tree species will respond to changing climatic conditions predicted for the near future.

Net primary productivity and intercepted radiation in hardwood and pine forests

In this study we compared the net primary productivity (NPP) of diverse natural mixed hardwood stands and planted pine monoculture stands on adjacent sites at Coweeta LTER, NC in the southern Appalachian Mountains. We measured intercepted radiation throughout 2005 and estimated net primary productivity in paired stands in different environmental conditions to determine if they differ in NPP and if this difference can be attributed to the amount of radiation intercepted by the stands.

Water storage in heartwood of trees

In this study we used continuous Time Domain Reflectometry measurements to estimate seasonal and diurnal changes in water content in the heartwood of trees. The study was designed to quantify water storage in the heartwood of large oak trees and determine how much this stored water contributes to transpirational water use.

Assessing temporal variability in the radial distribution of stem flow and predicting total stem flow

Measurements of sap flow are widely used to estimate water use and storage in forests, yet there are challenges in scaling point-measurements of sap flux to whole tree water use. In this series of studies we measured many aspects of sap movement in the xylem of pine trees. Among our findings was that the profile of sap flux across the sapwood radius varied diurnally with the rate of transpiration. We also provided methods to mathematically describe the profile, and to estimate the time lag between changes in environmental conditions at the canopy and sap flux in the stem close to the ground. For more information, see:

Carbon sequestration

While it might seem obvious that forests have the potential to sequester a large amount of carbon, it is less obvious when the forest in question is a managed plantation. Plantations are becoming increasingly common around the world as the demand for wood and paper products continues to increase at a rapid rate. In managed forests there are two main potential sinks for carbon storage: wood products and soil. We investigated the change in carbon soil stocks in forests that have received various levels of management intensity and estimated the amount of carbon that could be sequestered in various products, such as wood in houses. From these estimates we will be able to determine if the high productivity, and therefore high carbon gain, of plantation forests can compensate for the carbon losses from the soil after harvesting and planting and from the unused carbon residues from harvested trees. For more information, see:

Light interception and forest growth

We examined the relationship between intercepted solar radiation and forest productivity in a series of studies in loblolly and slash pine plantations that differ greatly in resource availability and growth rates. Results demonstrated that intercepted solar radiation is very well correlated with forest growth. We looked at the underlying causes of this relationship, such as differences in nitrogen uptake, canopy architecture, patterns of carbon allocation and rates of physiological processes. For more information, see: