Effects of Foliar Nitrogen and Water Deficit on the Carbon Economy of Shorea smithiana Sym. Seedlings

NmRI t:NT AND WATER STATUS generally affect assimilation ability, which is an important factor for growth and survival of seedlings. Both stomatal conductance and net assimilation rate (A) decline as water stress progresses. When A was depressed by decreasing stomatal conductance, leaf intercellular space C02 concentration (Ci) either remained unchanged (e.g., Wong et al. 1985, Raschke & Resemann 1986) or increased (e.g., Wong et al. 1985, Sharp & Boyer 1986, Ben etal. 1987, Vos & Oyarzun 1987, Bjorkman & Schafer 1989). These results indicated that not only stomatal but also non-stomata1 factors affect assimilation during water stress. In addition to assimilation, respiration is an important determinant of net carbon gain in seedlings at the whole-plant level. Givnish (1988) showed that the respiration of support tissues raises the compensation point at the whole-plant level. The respiration rate of leaves is generally higher than that of support tissues such as stems and roots, and thus dry matter partitioning among plant parts is important for the daily carbon gain of whole plants. Ambient air C02 concentrations (C02) are spatially and temporally variable in forests. Air C02 decreases with increasing height in forests (e.g., Aoki et al. 1975), meaning that C02 experienced by canopy leaves decreases as seedlings grow. Bazzaz and Williams (1991) and Bazzaz (1996) suggested that daytime depression of C02 may limit the growth and survival of plants. Nutrient and water limitations can be easily mitigated in the nursery environments, but limitations due to reduction of C02 are more problematic. If C02 is below the whole-plant compensation point, then the plant will experience a net loss of carbon. Thus, the C02 compensation point is a useful measure for understanding growth limitations due to C02 levels, and indirectly, water and nutrient availability as well. Despite the importance of carbon balance at the whole-plant level, few studies have examined carbon gain and C02 compensation point at the whole-plant level (Medrano et al. 1995, Lloyd & Farquhar 1996). Most studies have focused on carbon gain and gas exchange characteristics at the single-leaf level as affected by foliar nitrogen (N) or water status. Here, we present the results of a nursery experiment on the effects of foliar N content and short-term soil moisture deficit on the carbon economy at the single-leaf and whole-plant level of Shorea smithiana Sym. seedlings grown in pots in East Kalimantan, Indonesia. The instantaneous growth C02 compensation point was estimated from the intercept of two lines, a daily net carbon gain against ambient C02 (A-Ca curve) and a daily carbon loss due to respiration in plant organs (stems and roots daily respiration rate + leaf nocturnal respiration rate). The actual C02 compensation point at the whole-plant level, however, will be higher than the instantaneous growth C02 compensation point because of the construction, and possibly reproductive costs, of the plant (Givnish 1988). Thus, this adjusted intercept will be considered as the instantaneous C02 compensation point for growth. An increase in the C02 compensation point above the ambient C02 level means that the growth, and possibly survival, of a plant will be limited. In this study, the effects of decreases in nitrogen and water availability on the growth of tropical tree seedlings were examined from a viewpoint of the instantaneous growth C02 compensation point. The experiment was conducted at Bukit Soeharto Education Forest (BSEF) in East Kalimantan, Indonesia (0°52'S, 117'01'E; 60 m elev.). In this area, vegetation is dipterocarp forest, mean annual rainfall is ca 2300 mm, mean annual air temperature is 27.3"C, and soils are Ultisols (Ohta & Effendi 1992). Fallen fruits from a S. smithiana (Dipterocarpaceae) tree were collected from BSEF in early April 1995. The fruits were sown immediately in ca 50 plastic pots, 9 cm in diameter and 14 cm in depth, and filled with mineral soil taken from the site where the fruits were collected. Soil in half of the pots was fertilized with an organic fertilizer containing 1.18 . +. 0.24 percent (2 ? SD, N = 6) nitrogen to induce differences in foliar nitrogen (N) content (mineral soi1:fertilizer; 9:l [v:v]). The pots were placed under a shading frame (ca 45% full sunlight) in a nursery at BSEF. Daily maximum photosynthetic