by Audrey Verreault | 12 June 2020
Published in Northern Journal of Applied Forestry 14(3): 117-119
Root suckers arising after cutting 10 yr old pin cherry in eastern Québec were studied. Most suckers emerged during the first 2 yr and only a small proportion emerged after 3 or 4 yr. The number of suckers forming a clone was highly variable and was probably influenced by the degree of interspecific competition affecting this shade-intolerant species. No clonal variations were observed in the diameter of parent roots producing suckers, or in the initiation of new roots around the base of suckers up to 5 yr old. Annual growth of sucker basal diameter differed among clones and also within a clone. The oldest suckers had the greatest rates of basal growth. Results from this study indicate that cutting will not prove effective as a mean for controlling pin cherry during cleaning treatments, but will help to maintain dense pin cherry browse for white-tailed deer or moose.
by Marie-Claude Boileau | 11 June 2020
Published in Canadian Journal of Botany 75: 1424-1435
Silvicultural alternatives that differ in the degree of overstory removal may create shady environments that will be problematic for the regeneration of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Gradients of light in the field were used to compare mortality, growth, and leaf morphological acclimation of two conifer species of contrasting shade tolerances: Douglas-fir and western hemlock (Tsuga heterophylla (Raf.) Sarg.). Results after two growing seasons indicated that Douglas-fir mortality occurred mainly at relative light intensity (RLI) below 20%, while western hemlock mortality was evenly distributed along the light gradient. Height, diameter, and biomass of the planted seedlings increased with increasing light for both species but at different rates, and maximum biomass accumulation always occurred in the open. Douglas-fir allocated more resources to stem biomass than western hemlock, which accumulated more foliage biomass. Increases in specific leaf area for Douglas-fir seedlings occurred at RLI £ 0.4 and red / far red (R/FR) ratio £ 0.6, which appear to be the minimal optimum light levels for growth. Conversely, western hemlock seedlings adjusted their leaf morphology in a more regular pattern, and changes were less pronounced at low light levels. These results, along with early mortality results for Douglas-fir, suggest that the most successful way to artificially regenerate this species may be by allowing at least 20% of RLI for ensuring survival and at least 40% RLI for optimum growth.
by Marie-Claude Boileau | 11 June 2020
Published in Plant, Cell and Environment 20: 1462-1472
Experiments were conducted on well watered 1-year-old Douglas fir [Pseudotsuga menziesii (Mirb.) Franco], western hemlock [Tsuga heterophylla (Raf.) Sarg.] and western redcedar (Thuja plicata Donn) seedlings to determine the effects of temperature on whole-plant photosynthetic and stomatal responses to short-term fluctuations in irradiance (Q). Following a step change in Q, time constants (t, the period over which 63% of the total change occurs) for stomatal conductance (gs) and assimilation rate (A) decreased linearly with increasing air temperature (Tair). For example, in western redcedar tA decreased from 30 ± 4 min at 5 °C to 10 ± 1 min at 25 °C. In all cases, tA was within 10-15% of tgs. There was considerable variation in t among individuals within a given species. Differences between species became more pronounced with decreasing temperature. Multiplicative models that included functions for t accounted for 99% of the diurnal variability in A and gs for seedlings exposed to varying air temperature, irradiance and vapour pressure deficit. Estimates of daily A were within 2% of those measured. Intermittent cloud cover and understory shading were approximated by exposing seedlings to 3—4 episodes (³ 1 h) of shade (200 or 500 mmol m-2 s-1 or complete darkness during the day. In such cases, daily A was overestimated by up to 4 and 21%, respectively, if a function for t was excluded from the models. Our results suggest that there is scope for selecting seedling stock for increased carbon assimilation on the basis of reduced time constants. For example, in western redcedar, a 40% reduction in t could lead to increases in daily carbon gains of almost 5% depending on the frequency and degree of shading. If these daily gains were translated into increased dry matter production and compounded, seasonal gains would be even larger.
by Marie-Claude Boileau | 2 June 2020
Published in Forest Ecology and Management 103: 293-305
Three experimental sites in Québec were planted with four different sizes of containerized black spruce seedlings (Picea mariana (Mill.) BSP). We examined the water stress experienced by each stock size of black spruce seedlings in relation to different competing vegetation covers and also the effect of the water stress on spruce growth during the first three growing seasons. The sites consisted of one abandoned agricuItural field and two forest locations. Containers of sizes 45-110, 45-340, 15-700, and 12-1000 were employed to produce the four different sizes of spruce seedlings. At each site, the experimental protocol used a split-plot in a randomized complete block design, in which the presence of a competing vegetation cover (weedy and bare plots) was assigned to the whole plot, while a specific seedling size was assigned to each subplot. At each experimental site both the predawn xylem water potentiaI Yxp and the midday value Yxm were measured three times during each of the first three growing seasons. Data were analysed as a completely randomized split-split-plot design, where selection of seedlings in time was considered as the whole plot. The competing vegetation tended to protect the spruce seedlings from excessive water loss, without depressing the soil-water potential (SWP) to the point of reducing the moisture available to the seedlings. Both Yxp and Yxm were found to decrease significantly with increasing initial seedling size. The increased water stress experienced by the large stock of spruce seedlings had an effect on the absolute growth rate (AGR) in height on only one experimental site. The AGR was impaired by the presence of a competing vegetation cover, but more severely for the smaller stock-size than the larger. The short-term effect of a competition should be based on radial growth; height growth and mortality are not early indicators of such effect. These results emphasize the need to produce a large stock of spruce seedlings with well-developed root systems and root growth capacity, even though only moderate water stress was observed during the first three years of plantation growth.
