Carbon Fixation

Carbon fixation and allocation patterns in northern red oak are episodic and closely related to leaf and plant developmental stage (for this discussion, carbon fixation is defined as the carbon exchange rate (CER; ?mol CO₂ m^-2s^-1), or net photosynthetic rate). Unlike most temperate trees, northern red oak has an acropetal pattern of within-leaf differentiation, and physiological leaf maturation continues past full expansion. Leaves of previous flushes are important contributors to the growth of subsequent flushes. Juvenile northern red oak seedlings growing in the field are dependent upon an adequate light environment to maintain a positive carbon balance and improve survival and early growth. Carbon fixation and allocation in northern red oak in field environments are similar to those in a controlled environment.

In one study on changing carbon budgets in response to simulated insect defoliation of 9- to 11-year-old northern red oak saplings, Heichel and Turner (1983) found defoliation significantly decreased carbon fixed from 69 g CO₂ d^-1 in control trees to 24 g CO₂ d^-1 in 75 percent defoliated trees. Even after the defoliated trees had produced new leaves, carbon fixation increased only to 38 g CO₂ d^-1, much less than in undefoliated trees. Sixty percent of the increase in daily carbon fixation was attributed to new foliage, and the remainder was attributed to an increase in carbon assimilation rate of the remaining old leaves (Isebrands et al., 1994).

In another study on northern red oak seedlings at selected stages of growth (QMIs), Hanson et al (1987a) examined the influence of light on carbon budgets. They estimated daily carbon gain of a three-flush northern red oak seedling growing in open conditions to be 1.7 g CO₂ d^-1. Maximum CO2 fixation occurred in full sunlight (1900 ?mol m^-2s^-1), and fixation did not decrease appreciably until there was more than 50 percent shade (500 ?mol m^-2s^-1). In heavy shade (less than 200 ?mol m^-2s^-1), daily carbon gain was always negative. This budget analysis suggested that small northern red oak seedlings growing under a full forest canopy or under dense vegetation do not receive sufficient light for photosynthesis to offset respiratory losses. Thus, mortality is likely. The silvicultural implications of these results are significant- larger seedlings are desirable to decrease competition, and an adequate light environment for the seedlings must be maintained through overstory manipulation and understory control (Isebrands et al., 1994).

Carbon Exchange Rate

CER as a function of QMI

Carbon fixation in northern red oak trees varies widely with stage of leaf development, tree age, and environmental conditions. In northern red oak seedlings growing under optimal conditions in controlled environments, light-saturated CER increased during leaf development (Hanson et al 1988a) and continued beyond full leaf expansion before declining (Hanson et al 1988a, Tomlinson et al 1991). In first-flush leaves, CER increased from the 1-SL stage to the 2-LL stage, decreased during 2-Lg, then increased again during the third flush. The CER of second-flush leaves followed this same pattern, increasing from 2-SL to 3-SL before decreasing during 3-Lg. The maximum CER recorded under controlled environment conditions (i.e., 400 ?mol photosynthetic photon flux density) was between 7 and 8 ?mol m^-2s^-1 (Hanson et al 1988b) (Isebrands et al., 1994).

First-flush leaves of northern red oak seedlings do not attain maximum CER until the plant reaches the leaf linear stage of the second flush (2-LL), well after the first-flush leaves have stopped expanding (see both figures above). This pattern distinguishes northern red oak (and perhaps other oaks) from most temperate trees with simple leaves in which CER usually peaks at or near full leaf expansion. The continued increase in CER beyond full leaf expansion in oak is a result of the continuing development of photosynthetic pigment and enzyme systems rather than leaf anatomical differentiation (Isebrands et al 1988, Tomlinson et al 1991) (Isebrands et al., 1994).  This observation implies that survival and growth of oak seedlings and seedling sprouts is closely tied to the light regime at the forest floor.  Moreover both overstory and understory densities need to be considered when evaluating the adequacy of light reaching the forest floor needed to take advantage the unique carbon exchange rate characteristics of the oaks  .As an example, reduced light levels beneath evergreen understory shrubs in Southern Appalachian hardwood forests limits carbon fixation and suppresses growth of first year oak seedlings {Beier 2005}.

Dark respiration in northern red oak seedlings grown in controlled environments is highest (most negative) during leaf expansion (i.e., SL and LL) and decreases to the lowest (most negative) rate at 100 percent leaf expansion (i.e., Lg). It is stable thereafter through subsequent flushes, averaging -0.5 to -1.5 ?mol m^-2s^-1 (Hanson et al 1987b, 1988a). Other investigators have confirmed these respiration rates in the field (Heichel and Turner 1983. Jurik 1986) (Isebrands et al, 1994).

Carbon fixation rates in northern red oak leaves under field conditions are more variable than in controlled environments; however, the number of reports on this subject is limited. In a drought experiment in which field-grown saplings were transplanted into the greenhouse, Hinckley et al (1978) found that northern red oak exhibited rather low CER with an upper asymptote of 5.7 ?mol m^-2s^-1. CER peaked at 26?C, but dropped dramatically when temperatures approached 40?C. Drought significantly decreased CER to about 1.5 ?mol m^-2s^-1, but rates recovered soon after rewatering. In addition, Heichel and Turner (1983) studied CER of northern red oak saplings in the field before and after simulated insect defoliation. CERs were between 5 and 8 ?mol m^-2s^-1, depending on light conditions. Maximum CER occurred in July and August and then decreased in September and October. CER increased in leaves remaining after defoliation, partially compensating for the adverse effects of defoliation. In a study of CER in mature overstory northern red oaks, Jurik (1986) found that CER varied with light conditions, season, and year. CER of leaves in the top of the canopy reached 14 ?mol m^-2s^-1 during one season, while CER in the understory leaves was only 6.7 ?mol m^-2s^-1. Hanson and others (1987a) concluded from these field studies that leaves of mature northern red oak maintain a rather high level of CER through much of the growing season unless they are subjected to stress such as drought or defoliation (Hanson et al. 1987a) (Isebrands et al., 1994).

In general, there have been few studies of carbon budgets in northern red oak. The paucity of studies is likely due to the lack of fundamental information on carbon fixation and allocation in the species in general and the difficulty in obtaining some of the measurements, such as root respiration, necessary for calculating carbon budgets. More research is needed, and that research should be scaled up from the seedling level to larger trees and from tree crowns to forest canopies. This effort should include studies of carbon allocation during flowering and acorn production (Isebrands et al., 1994).