The Effects of Atmospheric Carbon Dioxide Enrichment, Irradiance and Water Stress on Seedling Growth and Physiology of Liquidambar Styraciflua and Pinus Taeda

In order to predict possible effects of increasing atmospheric carbon dioxide on woody plant succession and community species composition, more detailed information is needed on the individual response of successional species to this changing environmental factor. The objective of this study was to investigate the growth and physiological response of two important successional woody species, Liquidambar styraciflua L. (sweetgum) and Pinus taeda L. (loblolly pine) to long term CO₂ enrichment. Carbon dioxide effects were studied under different irradiance and soil moisture conditions since these environmental factors, as well as nutrient availability, influence their pattern of invasion into abandoned fields and establishment under forest canopies in the North Carolina piedmont. Growth and physiological experiments were conducted in controlled environment facilities at the Duke University Phytotron. Elevated atmospheric CO₂ concentration enhanced growth with the greatest increase in height, leaf area, basal stem diameter and total dry weight seen for sweetgum seedlings grown under high irradiance. Increases in dry matter accumulation were associated with early CO₂ enhancement of net assimilation rate, with increases in amount of leaf surface area contributing more towards maintenance of larger size as seedlings aged. For sweetgum seedlings in particular reduction of growth by low irradiance and soil drought under normal atmospheric CO₂ was compensated for by growing plants under elevated CO₂. Photosynthetic acclimation of sweetgum and loblolly seedlings was not significantly altered by increasing atmospheric CO₂. However, CO₂ enrichment had a substantial differential effect on drought tolerance of these two species. Sweetgum seedlings grown at elevated CO₂ maintained higher plant water potentials and net photosynthetic rates during a period of drought than when grown at present day CO₂ level. In addition water use efficiency in these seedlings was also increased. In contrast CO₂ enrichment had much smaller effects on total plant water potential, net photosynthesis, transpiration and water use efficiency of loblolly seedlings. Physiological and growth response data in this study strongly suggest that the continued increase in atmospheric CO₂ may influence distribution patterns of these two species primarily in open fields.