Responses of some conifers to light availability: survival, growth, morphological characteristics, and physiological behavior

To improve our understanding of shade tolerance, I studied responses in survival, growth, morphological characteristics, and physiological behavior to light availability of naturally regenerated saplings and/or planted seedlings of five tree species [ponderosa pine (Pinus ponderosa), western larch (Larix occidentalis), lodgepole pine (Pinus contortd), Douglasfir (Pseudotsuga menziesii), and Engelmann spruce (Picea engelmannii)] in central and southern interior British Columbia. Lodgepole pine saplings had a greater terminal and lateral increment than Douglas-fir saplings in high light environments. With decreasing light availability, (i) terminal increment, mean lateral increment, and total lateral increment decreased in both species and lodgepole pine had a greater decrease in lateral growth than Douglas-fir; (ii) the ratio of mean lateral to terminal increment in lodgepole pine did not change but significantly increased in Douglas-fir; (iii) the ratio of total lateral to terminal increment decreased in lodgepole pine but increased in Douglas-fir; and (iv) specific leaf area in both species increased, with a marginally greater increase in Douglas-fir; however, Douglas-fir always had a greater specific leaf area than lodgepole pine. Comparison of dynamic and steady-state photosynthesis between understory- and open-grown Douglas-fir seedlings indicated that when a PPFD of 200 or 500 µmol m⁻² s⁻¹ was applied (following a 10-min period at a PPFD of 50 µmol m⁻² s⁻¹) induction times required to reach 50 and 90% of steady-state net photosynthetic rates in the understory - grown branches were shorter than those in the open-grown branches. On a per unit leaf area basis, the understory-grown branches had a lower dark respiration rate and higher net photosynthetic rates at low PPFDs, but lower net photosynthetic rates at high PPFDs than the open-grown branches. However, on a dry leaf mass basis, there were no differences in dark respiration rates between the understory-grown and open-grown branches, but net photosynthetic rates of the understory-grown branches were equal to or higher than those of the open-grown branches at all PPFDs. After three growing seasons, survival of planted ponderosa pine and western larch seedlings decreased with decreasing light availability from intermediate to low light environments, whereas survival of Douglas-fir and Engelmann spruce seedlings did not vary with varying light environments. In low light environments, survival increased in order from western larch to ponderosa pine to Douglas-fir to Engelmann spruce. With decreasing light availability, 1) all study species except western larch had decreased height growth and Engelmann spruce had the greatest decrease among all the species; 2) diameter growth decreased for all study species and Douglas-fir had the greatest decrease; and 3) total biomass growth decreased for all study species and there were no difference in the extent of decreases in both absolute and relative biomass growth rates among the three evergreen conifers; however, ponderosa pine had the highest absolute biomass growth rate over three growing seasons in any given light environment. The ratio of absolute above- to below-ground biomass and specific leaf area varied with light environments in Douglas-fir and Engelmann spruce but not in ponderosa pine and western larch seedlings. These findings suggest the occurrence of interspecific differences in survival and growth, morphological, and physiological characteristics in response to light availability. This study finds that shade-tolerant species (i) have higher survival in low light environments but generally a lower growth rate (height, diameter, and biomass) in both high and low light environments, and (ii) are more efficient in capturing limiting light resources in low light environments due to plasticity in specific leaf area, lateral to terminal growth ratio, and above- to below-ground biomass growth ratio. However, more work is needed to determine which characteristics are the best measures of shade tolerance, and more importantly, to determine the causes of shade tolerance.Forestry, Faculty ofGraduat