CO2 elevation, canopy photosynthesis, and optimal leaf area index

Ecology, Dec, 1997 by T. Hirose, D.D. Ackerly, M.B. Traw, D. Ramseier, F.A. Bazzaz

INTRODUCTION

Elevation in atmospheric C[O.sub.2] concentration is believed to increase dry mass productivity of [C.sub.3] plants (Bazzaz 1990). Increase in productivity of a plant canopy may result from increasing leaf area index (LAI, total leaf area per unit ground area) as well as from increasing photosynthetic activity of leaves. Conflicting results, however, have been reported on the effect of C[O.sub.2] elevation on leaf area development, e.g., LAI increased with C[O.sub.2] elevation in the canopy of perennial ryegrass (Nijs et al. 1988), soybean (Cambell et al. 1990), and rice (Rowland-Bamford et al. 1991), while it remained the same in artificial tropical forest eco-systems (Korner and Arnone 1992). Studying the effect of C[O.sub.2] elevation on canopy development in two co-occurring annuals, we showed a strong correlation between LAI and aboveground plant nitrogen (Hirose et al. 1996b). Since there was no significant difference between C[O.sub.2] levels in the regression of LAI against aboveground N, we hypothesized that leaf area development is controlled primarily by the amount of N available for plant growth. This hypothesis suggests that the LAI would increase with C[O.sub.2] elevation if and only if N uptake from the soil is simultaneously increased, e.g., due to increased substrate supply for root growth. In order to further investigate this hypothesis, in this paper we present a model of canopy photosynthesis, based on the distribution of leaf N and photosynthetic photon flux density (PPFD) through the canopy, and we analyze the effects of C[O.sub.2] elevation and N availability on canopy photosynthesis and LAI. The gradient of leaf N that develops in parallel with a gradient of light climate is incorporated into the model, because it strongly influences canopy photosynthesis (Hirose and Werger 1987b).

In this paper we first show leaf N distribution in the canopy of monospecific and mixed stands of two co-occurring annual species (Abutilon theophrasti and Ambrosia artemisiifolia) established under ambient (360 [[micro]liter]/L) and elevated (700 [[micro]liter]/L) C[O.sub.2] conditions. Second, we model leaf photosynthesis as a function of leaf N and incident PPFD, taking account of the effect of C[O.sub.2] elevation on the light-saturated rate, dark respiration, and the initial slope of light-response curve. Third, canopy photosynthesis is calculated for monospecific and mixed stands of the two species established under ambient and elevated C[O.sub.2] conditions. These different combinations provide a variety of canopy structures that are used to validate our canopy photosynthesis model. And finally, we determine optimal LAI that maximizes daily canopy photosynthesis with a given amount of N for leaf growth, where N was distributed optimally in the canopy. We show that the optimal LAI does not increase with C[O.sub.2] elevation when the availability of N for leaf growth is limited.

MATERIALS AND METHODS

Determination of canopy structure

The experiment to determine canopy structure was conducted at the Harvard University OEB Glasshouse in March-May 1994, using seeds of Abutilon and Ambrosia from an Illinois seed supply company (Hirose et al. 1996b). Seedlings were germinated and grown in round pots (0.1 m in diameter and 0.1 m deep) in either an ambient (360 [[micro]liter]/L) or elevated (700 [[micro]liter]/L) C[O.sub.2] atmosphere. Two-hundred and forty pots were placed contiguously in a square arrangement giving a density of 100 pots/[m.sup.2] in each of six C[O.sub.2] and temperature-controlled zones. Plants were grown in pots to exclude root competition between individuals, allowing us to focus on interference in light interception and to reduce size variation among individuals. Adjacent zones were assigned to different C[O.sub.2] atmosphere treatments and were blocked together, for a total of three blocks. Temperature was maintained at 25 [degrees] C day and 20 [degrees] C night. Lighting was supplemented by halogen bulbs to extend day length (14 h). The photosynthetically active photon flux density (400-700 nm, PPFD) was up to 1200 [[micro]mol] [multiplied by] [m.sup.-2] [multiplied by] [s.sup.-1] on clear days.

During the 2nd and 3rd wks following germination, plants were thinned to one per pot. Plants were fertilized on days 24, 27, and 32 with 55 mL of full-strength nutrient solution (0.2 g/L for N, P, and K). Thus, each plant received 82.5 mg each of N, P, and K in total. Each pot had a saucer, so that nutrients that leached from the soil remained in the water available to the pot, and so that pot water content could be maintained at field capacity. When the plants were 3 wk old, they were arranged to form either monospecific or mixed (1:1) stands of Abutilon and Ambrosia. In each stand, there were 18 plants arranged in a rectangle (3 x 6 pots). In the mixed stand, there were nine plants of each species distributed in a checkerboard pattern. Surrounding each stand there was a single row of border plants in the same arrangement. The three stands were arranged east to west with the narrow sides adjacent. The resulting rectangle (5 x 24 pots) was bordered by shade cloth during the 6th wk to eliminate edge effects. The shade cloth wall, a black layer sandwiched between two white layers, was set level with the top of the canopy and adjusted as the canopy height increased.