Temperature is a fundamental control on tree productivity, yet little is known about the in situ temperature of tree leaves in relation to carbon uptake during the course of the growing season. The aims of this project are to further develop a method to resolve tree-canopy leaf temperature by analyzing the oxygen isotope ratio of tree-rings. Initial results showed a leaf temperature homeostasis during carbon gain in 39 tree species across 50° of latitude. The isotopic method and the homeostasis hypothesis will be further tested in the field at four sites which maximize climatic and species differences, while also maximizing the overall use of publicly-funded meteorological and ecosystem carbon exchange data. At each field site, the isotope-derived leaf temperatures will be compared with completely independent estimates of photosynthesis-weighted canopy temperatures. All of the temperature estimates will be compared on an interannual basis over three growing seasons as well as a multi-year integration. The inclusion of undergraduate students to both learn from and help with the aims of this project is a continuing and primary objective - as is the training of a graduate student and a post-doctoral student and the employment of a technician. The final product of this research will give novel insight into the effects of mean climate and inter-annual weather variation on tree ecology and physiology. These results have the potential to impact our current understanding of many aspects of plant biology - the controls on tree distribution, ecosystem models of forest carbon and water exchange, physiological responses to climate change, climate as a selective pressure - as well as solidify a new use of oxygen isotopes that will be applicable to all plants.
Temperature is a fundamental control on plant productivity and distribution. It is important for us to determine how trees respond to temperature to assess future scenarios of plant productivity, but also to determine the effects of climate as a selective agent for plant evolutionary success through past climatic changes. We developed and examined the hypothesis that the temperature of a plant canopy (all the leaves of a given plant) can be resolved measuring the stable oxygen isotope ratio of cellulose (a primary structural component of leaves and wood). The general hypothesis is as follows: The ratio of heavy 18O to light 16O in the leaf water of plants is largely determined by the evaporative environment, namely relative humidity that a plant experiences during the daytime while it is photosynthesizing. The relative humidity that a plant experiences is determined by the amount of water vapor in the atmosphere vs. the amount of water vapor in the intercellular spaces of leaves. As leaf temperature increases, the relative humidity decreases and the isotope ratio of leaf water (18O/16O) increases. As leaf temperature decreases, the relative humidity increases and the isotope ratio of leaf water decreases. If we have a measure of leaf water 18O/16O and atmospheric water vapor, then we can solve for leaf temperature. The 18O/16O of leaf water equilibrates with carbohydrates that are synthesized during photosynthesis, so that these carbohydrates— and the plant cellulose that is derived from them— contain an isotopic record of leaf water. Therefore, within plant cellulose there is a record of leaf water 18O/16O and, as we hypothesize, a record of leaf temperature. The labeling of photosynthesis-derived carbohydrates by leaf water weights the leaf-temperature reconstruction by carbon gain. Our method, therefore, offers a season-long integrative measure of the interplay between plant carbon uptake and plant energy balance. The initial test of the hypothesis on trees from sub-tropical to boreal sites showed that isotope-resolved canopy temperatures were in a fairly small temperature envelope with a mean of 21 ± 2 °C. This result rested on modeled environmental inputs and we wanted to carefully test it again with measured inputs as well as use independent sources of verification and interspecies variation. Our intensive sampling efforts were at three sites that are part of the continental-scale Ameriflux network. These sites measure continuous fluxes of CO2 and water and therefore give us a site-specific estimate of Gross Primary Productivity (GPP) and meteorological data. We further extended the analysis sampling only cellulose to 16 other Ameriflux sites throughout the country. At our intensively sampled sites we made direct measurements of all isotopic inputs and used the meteorological and flux data to assess how closely isotope-resolved temperatures matched GPP-weighted air temperatures across sites. We also deployed IR canopy temperature sensors at these sites to directly obtain GPP-weighted canopy temperatures of the same trees that we were sampling for isotope analysis. With the GPP-weighted air temperature and canopy temperatures we had two independent means to test our isotope-resolved temperatures. The most significant result is that our method works (see figure). By accounting for species-specific effects (the most significant of these was a persistent difference in carbon turnover time between conifers and angiosperms), we can show that the isotopic resolved canopy temperatures match the GPP weighted air and IR_canopy temperatures across sites and species. To date, we have three publications concerning this method and a fourth published under the auspices of this grant. At least two more manuscripts will be submitted for publication by the end of the year. This work supported the employment of two part-time technicians, one short-term postdoc, a graduate student, three undergraduate technicians and one undergraduate and one high school student who engaged in independent research projects. The PI, postdoc and graduate student presented the outcomes of this work at international meetings and university seminars. All three were also active in communicating, more generally, plant ecophysiology to high school teachers and students through programs on Penn’s campus. Our approach shows the relationship between leaf temperatures, ambient temperatures and carbon gain in trees— and likely all plant types— worldwide. For isotope ecology, we have already made great progress in understanding how isotope signals in leaves are transferred to cellulose as well as how these processes differ between tree species. For plant physiology, we have elucidated pathways of water movement through leaves as transpiration rate changes. Further, our method has been used independently by other labs and we are helping other labs apply it as well. Finally, stable isotopes are increasingly being used in forensic studies. Our work here adds one more tool for forensic experts to extract temperature information from any plant produced material.
