Seasonal photosynthetic responses to light and temperature in white spruce (Picea glauca) seedlings planted under an aspen (Populus tremuloides) canopy and in the open

Abstract

Photosynthetic light and temperature response curves were measured seasonally in seedlings of white spruce (Picea glauca (Moench.) Voss) grown for two years in the understory of aspen (Populus tremuloides Michx.) or in the open in central Alberta. Light-saturated rate of net photosynthesis, the optimum temperature for net photosynthesis, transpiration rate, photochemical efficiency, and stomatal and mesophyll conductances increased from spring to summer and declined thereafter, whereas dark respiration rate and compensation and saturation points were highest in spring. Depression of photosynthetic parameters was greater in open-grown seedlings than in understory seedlings during the periods in spring and autumn when night frosts were common. Net photosynthetic rates were similar in understory and open-grown seedlings in summer, but they were significantly lower in open-grown seedlings in spring and autumn. Significantly lower transpiration rates and stomatal conductances in open-grown seedlings than in understory seedlings were also observed at 15 and 25 degrees C in the autumn. Shoot and needle growth were less in open-grown seedlings than in understory seedlings. In summer, when irradiances were low in the aspen understory, understory white spruce seedlings maintained a positive carbon balance by decreasing their compensation and saturation points and increasing their photochemical efficiency compared to spring and autumn.

Describing condition-specific determinants of competition in boreal and sub-boreal mixedwood stands

“…Studies have shown a relatively flat photosynthetic response to air temperatures between 5 and 15 o C for lodgepole pine, subalpine fir and Engelmann spruce (Huxman et al 2003) and between 10 and 25 o C for Engelmann spruce (DeLucia and Smith 1987). Man and Lieffers (1997b) found a similar photosynthetic insensitivity for white spruce between 5 and 25 o C in the spring and fall, when light is most abundant in understory. During the summer, photosynthesis at saturating irradiance increased with temperature, but there was little difference at lower light levels (e.g., below 150 mol PAR, which would be common in mixedwood understories at moderate broadleaf densities -see Constabel and Lieffers 1996).…”

“…Light interception in the overstory increases with broadleaf stem density (or basal area), while light interception in the understory decreases (Comeau 2001, Comeau andHeineman 2003). Consequently, conifer survival, height growth and stemvolume increments in the understory have been shown to be negatively correlated with broadleaf stem density or basal area (Lieffers and Stadt 1994, Man and Lieffers 1997b, Jobidon 2000, Comeau 2001, Lieffers et al 2002. Minimum understory light thresholds for conifer survival may be exceeded in dense broadleaf stands (Pinno et al 2001) with light levels observed as low as 2% full light (Lieffers et al 2002), which can result in high mortality among the most shade-tolerant conifers.…”

“…Observations of photosynthetic productivity of understory conifer trees during the spring (and to a lesser degree autumn) broadleaf leafless periods suggest that these high-light windows may constitute an important ecological strategy to enhance the survival, growth and annual carbon balance of understory conifer trees (Baldocchi et al 1984;Constabel and Lieffers 1996;Mielikainen 1996;Man and Lieffers 1997a, 1997bGill et al 1998).…”

“…The reduced demand on edaphic resources by broadleaf trees during their leafless periods should help to sustain the potentially high photosynthetic rates of understory conifers given the enhanced light availability at these times (particularly in the spring, due to the longer days and higher solar angles relative to fall leafless periods) (see Man and Lieffers 1997b). The importance of temporal separation in leafing habits on the availability of edaphic resource for understory conifers may vary with broadleaf density in a manner similar to that described for light.…”

“…During the summer, photosynthesis at saturating irradiance increased with temperature, but there was little difference at lower light levels (e.g., below 150 mol PAR, which would be common in mixedwood understories at moderate broadleaf densities -see Constabel and Lieffers 1996). Subfreezing air temperatures during the growing season are known to impede photosynthesis in trees (Öquist 1983, Man and Lieffers 1997b, Lamontagne et al 1998, Krasowski and Simpson 2001, but again, some evidence suggests that conifers may be less sensitive than deciduous species. At sub-freezing air temperatures, Lamontagne et al (1998) found that black spruce and jack pine (Pinus banksiana Lamb.)…”