INTRODUCTION

1. Respiration Response of Mature Sweetgum Stems to Carbon Dioxide Enrichment Nelson T. Edwards and Richard J. Norby Environmental Sciences Division, Oak Ridge National Laboratory Oak Ridge, TN USA 37831-6422 ABSTRACT This research is a part of a Free-Air carbon Dioxide Enrichment (FACE) experiment in a 13-year-old sweetgum (Liquidambar styraciflua) plantation in eastern Tennessee. The Trees are exposed to ambient and 1.4X ambient CO2 concentrations during the growing-season. The objective of this project was to evaluate the effect of CO2 enrichment on stem respiration. Stem respiration was measured by infrared analysis of CO2 concentrations in the air stream pumped through chambers attached to tree stems. An automated system permitted the continuous monitoring of stem respiration rate of 24 trees (six in each of two CO2 enriched plots and six in each of two control plots). Respiration rates in all tree stems followed a diurnal pattern with rates lagging a few minutes behind changes in stem temperature. Rates were higher in trees growing in CO2 enriched plots than in control plots. Differences varied from near zero during parts of the dormant season to 100% during parts of the growing season (typically about 30%). Some trends toward higher Q10 responses were also observed under CO2 enrichment. Some, but not all, of the increased respiration was due to increased stem diameter growth. DISCUSSION Annual total CO2 efflux from stems at breast height was 33% higher in CO2 enriched trees than in control trees during the last half of 2000 and the first half of 2001. A part of this difference can be accounted for by differences in growth rates. However, wood increment based on changes in diameter was only 7% higher in CO2 enriched trees than in control trees during the year 2000 and differences have steadily declined since the experiment started in 1998. Thus, while some of the increased respiration in CO2 enriched trees can be attributed to growth respiration, some of the increase can be attributed to one or more of the following: (1) the energy cost for growing new tissues is higher in CO2 enriched trees than in control trees; (2) the cost for maintenance of living tissues is higher in CO2 enriched trees; (3) some respiration is wasted when substrate concentrations exceed the levels required for growth and maintenance. The increase in stem respiration within hours after turning on the CO2 indicates that stem respiration is driven by photosynthates transported from the leaves and that it takes a few hours for transport from the leaves to the lower stem to occur. This suggests that the quantity of photosynthates transported from the leaves to the stems increases under CO2 enrichment. Photosynthesis rates in the upper canopy at light saturation in the CO2 enriched trees averaged about 46% higher than rates in control trees during the year 2000 (Gunderson et al (submitted). We know from our measurements of stem respiration rates that much of this additional substrate is being metabolized in the stem, but we do not know how all of the energy is utilized. We hypothesize that the increased respiration in CO2 enriched trees, in excess of that from increased growth respiration, is due primarily to an increase in the proportion of living tissues (relative to dead or inactive tissues as heart wood forms) in the CO2 enriched trees. This is supported in part by the fact that respiration is higher in the CO2 enriched trees even during the dormant season when no growth is occurring and when substrates are not being replenished. If new tissues are being produced faster in the CO2 enriched trees than in the controls and if heartwood formation is occurring in both at the same rate then the proportion of living tissues to dead tissues would be higher in the CO2 enriched trees. The importance of the ratio of live tissue to dead in stems in influencing respiration rates has been well documented (Kramer and Kozlowski 1979, 0, Stockfors and Linder 1998) and is demonstrated here by very high respiration rates in the younger portion of the tree stem as compared to the older portion. The study demonstrates the importance of measuring respiration in various aged woody components of the tree if the goal is to scale up to the whole tree or to the whole ecosystem. The study demonstrates the utility of continuously monitored stem respiration as an indicator of phenological events as well as tree responses to environmental variables. INTRODUCTION Stem respiration in deciduous trees occurs continuously resulting in substantial amounts of CO2 released back to the atmosphere after being fixed by photosynthesis. Respiration in plants is generally partitioned into two functional components: construction respiration (Rg) and maintenance respiration (Rm) (McCree 1970). Rg (sometimes referred to as growth respiration) involves O2 consumption and CO2 production resulting from energy production for synthesis of new tissues. Rm, which is regulated primarily by temperature, involves gas exchange during maintenance of cell processes within living cells including protein turnover, regulation of ion and metabolite gradients, and physiological adaptation to a changing environment (Penning de Vries 1975), but may also increase with increases in relative growth rate (Lavigne and Ryan 1997). Partitioning these two components of respiration is important because Rg varies greatly through the year and rates vary with phenology and environmental factors that control growth. Growth rate may increase as temperatures increase, but Rg per unit of stem growth is unaffected by temperature (Penning De Vries et al. 1974). These factors must be taken into consideration when evaluating the response of stem respiration to environmental variables such as CO2 enrichment. Our objective was to quantify the relative amounts of CO2 released from stems of trees exposed to elevated or ambient CO2 and to assess the implications for any differences observed. After CO2 was turned off, there was a gradual decline in stem respiration rates of the high CO2 trees relative to the control trees. After CO2 was turned back on, there was a delay of a few hours before differences in rates began to increase. Stem respiration during the dormant season of 2001 was slightly higher in CO2 enriched trees than in control trees. After leaves were near complete expansion, differences in rates increased. Note the decrease in sub-canopy PAR with leaf emergence. Rates followed the diurnal temperature pattern, but as the growing season progressed respiration rates exceeded rates that would have been predicted from dormant season temperatures probably indicating stem diameter growth. During the growing season respiration rates generally declined during cloudy days and rainfall events. Growth respiration in CO2 enriched trees exceeded growth respiration in control trees by 23%. Maintenance respiration in CO2 enriched trees exceeded maintenance respiration in control trees by 48%. (Note: Calculations of annual maintenance respiration was based solely on temperature responses during the dormant season and may represent an under-estimate.) References Gunderson, C.A., J.D. Sholtis, S.D. Wullschlelger, D.T. Tissue, P.J. Hanson, and R.J. Norby. (Submitted). Environmental and stomatal control of photosynthetic enhancement in the canopy of a sweetgum (Liquidambar styracflua L.) plantation during three years of CO2 enrichment. Kramer, P.J. and T.T. Kozlowski. 1979. Physiology of woody plants. Academic Press, New York, 811 p. Lavigne, M.B. and M.G. Ryan. 1997. Growth and maintenance respiration rates of aspen, black spruce and jack pine stems at northern and southern BOREAS sites. Tree Physiol. 17:543-551. McCree, K.J. 1970. An equation for the rate of respiration of white clover plants grown under controlled conditions. In Prediction and measurement of photosynthetic productivity. Edited by I. Setlik. PUDOC, Wageningen, Netherlands. Pp.  Penning De Vries, F.W.T. , A.H.M. Brunsting and H.H. van Laar. 1974. Products, requirements and efficiency of biosynthesis: a quantitative approach. J. Theor. Biol. 45:339-377. Penning De Vries, F.W.T. 1975. The cost of maintenance processes in plant cells. Ann. Bot. 39: 77-92. Stockfors, J. and S. Linder. 1998. Effect of nitrogen on the seasonal course of growth and maintenance respiration in stems of Norway spruce trees. Tree Physiology 18:155-166. EXPERIMENTAL FACILITIES AND METHODS Stem respiration measurements were made in a planted sweetgum (Liquidambar styraciflua) monoculture on the Oak Ridge National Environmental Research Park in eastern Tennessee. The plantation was established in 1988. When the experiment was started in 1997, there were about 90 trees on each 20-m diameter plot. Exposure to elevated CO2 began in April, 1998 using a free-air CO2 enrichment (FACE) system. In two treatment plots the set-point at the top of the canopy is 565ppmv during the day and 645 ppmv at night, although nighttime exposures were discontinued in 2001. The system is turned off during the dormant season (November - April). Stem respiration was measured at breast height on six trees in each treatment plot and on six trees in each of two control plots. Carbon dioxide concentrations in air pumped through chambers sealed to the sides of the trees were compared to concentrations in air from adjacent empty chambers using an infrared gas analyzer operated in differential mode. An automated switching system allowed us to measure each tree every 112 minutes over a period of 1 year beginning in June of 2000. Q10 values determined during the study were used to estimate rates during power outage periods and when equipment was being serviced. Respiration rate in the upper stem (about 3 cm diameter) was approximately 4x the rate in the stem at breast height (12 to 15 cm diameter). (Note: The differences were minimal when data were expressed per unit of surface area [data not shown].) The growing season Q10 value was higher in control trees than in high CO2 trees, while the dormant season Q10 value was higher in high CO2 trees than in controls. The correlation between stem respiration and stem temperature was stronger during the dormant season than during the growing season. Trees with higher diameter increments tended to have higher respiration rates, but the correlation was not as strong as might be expected if we assume that a high proportion of the respiration in July is due to growth. Acknowledgements This research was supported by the Office of Biological and Environmental Research, US Department of Energy. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the US Department of Energy under contract DE-AC05-00OR22725. This work contributes to the Global Change and Terrestrial Ecosystems Core Project of the International Geosphere-Biosphere Programme.