The growth and photosynthetic response of under-planted conifer seedlings to changes in understory light environment
Creators: name: Karakatsoulis, John
Publisher: name: University of British Columbia
Year: 2009

description: The rapid invasion of non-crop vegetation following disturbance (i.e. clearcut forest harvesting) in southwestern British Columbia was studied in conjunction with the growth and physiological response of planted evergreen conifer seedlings. Primarily, this thesis investigated the changes in understory light (photosynthetic active radiation — PAR) environment and the ability of Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir), Thuja plicata Donn. (western redcedar), Tsuga heterophylla (Raf.) Sarge. (western hemlock), and Abies grandis (Dougl.) Forbes (grand fir) to photosynthesize and grow under low and changing light environments. The study was undertaken at the Malcolm Knapp Research Forest which is located in the southwestern portion of British Columbia (49° 15' N and 122° 31' W) at an elevation of 140 m. The climate is described as wet, cool mesothermal with mild winters and dry, warm summers. Vegetation growth and invasion following clearcut logging was rapid. Within two years of harvesting the average vegetation height was 0.39 m and covered 48 % of the area. Within six years, vegetation height averaged 2.75 m and covered the entire site (100 %). Mid-summer light (PAR) levels within the vegetation plots (30 cm above the ground) declined from an average of 66 % to less than 4 % within four years. Net photosynthetic rates of planted understory Douglas-fir, grand fir, and western hemlock seedlings were generally less than 2.0 µmol m⁻²s⁻¹ in mid-summer. In contrast, planted seedlings of the same species growing in the open had mid-day photosynthetic rates at approximately 8.0 µmol m⁻²s⁻¹. However, these static mid-summer measurements of understory PAR and the resultant photosynthetic response of conifer seedlings failed to account for the seasonality of light availability. Further studies of the seasonality of understory light availability within deciduous stands showed that PAR levels in early spring and late fall (period when the overstory canopy is leafless) went as high as 75 % of light levels in the open. The mid-day photosynthetic rates of the underplanted conifer seedlings were found to track the seasonality of understory light availability. Two distinctive photosynthetic peaks by the conifer species were shown to correspond with the fall and spring periods when understory light availability beneath deciduous vegetation was high due to leaf fall (in autumn) and prior to leaf flush in the spring. Conversely, the lowest photosynthetic rates were recorded during the summer months when the deciduous canopies were fully foliated, resulting in low levels of understory PAR, and during the winter months when air and/or soil temperatures were less than 5 °C. A further study was conducted to determine and quantify the contribution of understory sunflecks to overall photosynthetic photon flux density (PPFD) and to the CO₂ uptake rates of understory western redcedar growing within stands of Alnus rubra Bong, (red alder) and Betula papyrifera Marsh, (paper birch). Western redcedar was capable of responding to changing understory PAR by closely tracking sunfleck activity. Sunfleck peaks between 200 and 445 µmol m⁻²s⁻¹ were closely correlated with photosynthetic peaks between 2 and 5 µmol m⁻²s⁻¹. Conversely, net photosynthesis of western redcedar declined rapidly with the passing of sunflecks and remained very low (&lt;0.5 µmol m⁻²s⁻¹) during periods of diffuse light (PAR ≈ 20 - 25 µmol m⁻²s⁻¹). It was estimated that between 69 and 83 % of CO₂ fixed by understory western redcedar can be attributed to photosynthesis during periods of sunflecks and this may help to explain its presence in understory environments within west coast ecosystems. The results presented in this thesis have implications for forest management as it applies to silviculture systems and the ability to utilize certain conifer species in the understory of managed and unmanaged stands.