Influence of Shoot Structure on Light Interception and Photosynthesis in Conifers

Abstract

The influence of shoot structure on net photosynthesis was evaluated under field conditions for the central Rocky Mountain (United States) conifers Picca engelmannii (Parry ex Engelm.), Abies lasiocarpa (IHooki Nutt.), and Pinus contorta (Engelm.). In all species, the greater number of needles per unit stem length on sun shoots correlated with a smaller silhouette leaf area to total leaf area ratio (STAR). Decreased The importance of intercepting direct-beam sunlight in plants is evidenced by a large variety of behavioral and structural adaptations in individual leaves. Relatively few studies, however, have evaluated the implications of shoot structure on physiological processes. Recent data on conifer photosynthesis in shoots oriented at different angles to the irradiating beam have illustrated the potential importance of a complex shoot morphology in influencing shoot-level photosynthesis (6, 9). In fact, illuminating conifer shoots from all angles using an integrating sphere resulted in higher photosynthetic flux densities that were more typical or broadleaf species (12). Other workers have reported that individual conifer needles also had higher photosynthetic flux densities than for entire shoots exposed to direct beam light (5, 7).The purpose of the present study was to evaluate the influence of the complex shoot structure found in conifers on photosynthetic flux densities under field conditions. Measurements of net CO2 assimilation of individual shoots were combined with morphological measurements that estimated the influence of needle orientation and mutual shading on sunlight interception. Similar measurements were made for shoots which were experimentally modified to minimize these structural effects. Photosynthesis was measured for relatively short-needled spruce and fir species, and a longer-needled pine species, that are dominant members ofthe subalpine forests of the Central Rocky Mountains.Leaf Area and Photosynthesis. Field measurements of plant photosynthetic flux densities are now commonly made by enclosing a shoot, leaf, or leaf portion into a transparent chamber exposed to solar radiation and then determining CO2 flux into the leaf material. For laminar leaves, photosynthetic flux densities are then usually calculated on the basis ofthe actual amount of leaf area intercepting direct beam radiation (11). When the direct radiation is perpendicular to the plane ofthe leaf, the area of the leaf shadow cast on a surface beneath and parallel to the leaf plane is termed PLA'. The PLA for thin, flat leaves closely approximates one-halfTLA, and its use in calculating photosynthesis leads to consistent and comparable measurement units. However, an equally justifiable area for use in quantifying photosynthesis for species with complex leaf arrangements, such as conifers, has not been comprehensively evaluated. Previous researchers have often defined a PLA for conifers as equal to onehalf TLA, or as the total projection area measured when all needles were removed from a shoot and spread on a fl...

Effects of overstory paper birch (Betula Papyrifera March) on physiology, growth, and biomass responses of under-planted conifer seedlings

“…In order to minimize light interception in the clear-cut, western redcedar changed the orientation of the entire branch rather than individual needles (data not presented). These structural adaptations are consistent with other studies (Carter and Smith 1985;Leverenz and Hinckley 1990;Stenberg 1996;Niinemets 1997) and may suggest that, under different light conditions, these conifer species alter needle and branch level morphology to maximize the interception of light.…”

“…Quantity of light was greater in the 1200 stems ha· 1 compared to the 800 stems ha· The light interception efficiency of conifers has been quantified by the ratio of shoot silhouette area to total needle surface area (STAR) (Carter and Smith 1985;Leverenz and Hinckley 1990;Stenberg 1996). The ratio varies with shoot geometry and orientation ; shade shoots generally having a larger STAR than sun shoots (Stenberg 1996).…”

Influence of light and competition on crown and shoot morphological parameters of Norway spruce and silver fir saplings

“…Crown, shoot and needle morphology have a critical influence on the light use efficiency and thus on the growth and competitive ability of understory saplings [4,5,12,19,21]. Therefore, understanding how these traits vary in relation to interacting factors is important in order to promote and assure effectively natural regeneration [1].…”

Effects of water supply on gas exchange in Pinus pinaster Ait. provenances during their first growing season

“…Therefore, under water shortage, transpiration rate (E) or the ratio photosynthetic rate to transpiration rate (instantaneous water use efficiency, A/E) are important factors to consider. The ratio A/E has been used as a distinguishing criterion for drought tolerance, both between species [6,21] and intraspecifically [39,49,56]. Nevertheless A/E does not give an integrated value through time and some contradictory results have been found [30], since A/E based selection depends on competence and intensity and duration of water stress period [8,11,45].…”