by Audrey Verreault | 2 June 2020
Published in Agricultural and Forest Entomology 1: 189-194
1) Larval performance of Dioryctria abietivorella (Grote) (Lepidoptera: Pyralidae) was compared when insects were reared in white spruce, black spruce and Jack pine seed orchards. For each species, half of the insects developed in the presence of cones, while the other half was maintained on branches without cones.
2) Significantly faster development rates were observed on spruce species when compared to Jack pine. Significant differences in survival were also recorded between the different feeding treatments.
3) The presence of seed cones on spruce species significantly increased insect mean weight compared with branches without cones, but no significant differences were noticed between the different tree species with cones.
4) White spruce is the most suitable host tree for fir coneworm feeding and the availability of seed cones plays an important role in determining D. abietivorella larval performance.
by Audrey Verreault | 2 June 2020
Published in Ecosystems 18: 629-642
Soils are a major component of the global C cycle, and considerable effort has been dedicated to improve our understanding of factors controlling soil organic C (SOC) turnover and stabilization in the last decades. Carbon stable isotopes are useful in this respect as they represent an integrative indicator of SOC biogeochemical processing. In the present study, C concentration and d13C were measured in soil horizons of 21 forest sites located at the transition zone between the hardwood and the conifer forest in Québec, Canada, and related to 13 biophysical variables to identify the main drivers of SOC storage and turnover. Carbon concentrations in the forest floor (FF) and the B- and C-horizons were, respectively, strongly correlated with percentage of clay (Pclay), the mean annual precipitation: potential evapotranspiration ratio (MAP:PET), and percentage of hardwoods (Phwd). In FF, d13C was poorly correlated with the studied variables, whereas in mineral horizons, it was significantly correlated with mean annual air temperature (MAAT) and the percentage of conifers (Pc) and Pclay. Across the studied area, d13C increased on average by 2.00/00 from the FF to the C-horizon. The isotopic enrichment with soil depth (b) was strongly negatively correlated with Pc, which explained 55% of its variability among sites. This suggests that the vegetation type is an important driver of soil C long-term turnover rate in forest ecosystems. Overall, our data suggest that hardwood forest expansion in response to climate change might reduce the stability and the storage of SOC in the future.
by Marie-Claude Boileau | 29 May 2020
Published in Forest Ecology and Management 130: 77-97. 10.1016/S0378-1127(99)00176-0
The effects of competition from red raspberry (Rubus idaeus L.) and northern hardwood tree species on white spruce (Picea glauca (Moench) Voss) seedlings were examined on a clearcut site of the boreal mixedwood forest of the Bas-Saint-Laurent region of Québec, Canada. A controlled experiment involving a gradient of five vegetation densities on the basis of the leaf area index (LAI) was established in a completely randomized plot design with six replications. Each of the five levels of vegetation cover (including vegetation-free plots) were examined to evaluate how they affected environmental factors (quantity and quality of light reaching the spruce seedlings, and soil temperature), spruce growth (height, basal diameter, volume index, and above-ground biomass), spruce mortality, browsing damage, spruce foliar mineraI nutrition, as well as the stand structural development, during the first 5 years after seedling planting.
Each spruce growth variable analyzed in this study, according to a RMANOVA procedure, followed a negative hyperbolic form of density dependence of competitive effects. Loss of growth in young white spruce plantations in competition with northern hardwoods is likely to occur with the first few competitors. ln cases where higher levels of competing vegetation were maintained over time, loss of spruce growth was extremely severe, to an extent where the exponential growth character of the young trees has been lost. At the end of the fifth year, spruce growing with no interference were larger in mean total above-ground biomass by a factor of 9.7 than those growing with the highest level of vegetation cover. Spruce did not develop a strategy of shade avoidance by increasing tree height, on the contrary. Spruce mortality differed among treatments only in the fifth year, indicating that early evaluation of spruce survival is not a strong indicator of competitive effects, when compared to diameter growth. Spruce foliar N and Ca contents were significantly reduced by the first level of competing vegetation cover, while K increased with the density of the vegetation cover, and P and Mg were not affected. Nitrogen nutrition of young white spruce planted on recently disturbed sites is discussed in relation to the potential root discrimination of this species against soil nitrate, a reaction observed by Kronzucker et al. [Kronzucker, H.J., Siddiqi, M.Y., Glass, A.D.M., 1997. Conifer root discrimination against soil nitrate and the ecology of forest succession. Nature London 385, 59-61]. The effects of hardwood competition indicate a prevalence of competition for light over a competition for nutrients, as revealed by the substantial increase in the hld ratio of white spruce. Two indicators, hld ratio and the quantity of light received at the tree seedling level, are suggested as a basis for the management of hardwood competition in a white spruce plantation.
Analysis of the stand structural development indicates that spruce height distribution was affected only by moderate or dense cover of vegetation, while diameter distribution, when compared to competing vegetation-free plots, was affected by the lowest level of vegetation cover. This study shows that competition influenced the stand structural development in the same way as genetic and micro-site factors by aggravating the amplitude of size inequality. The impact of hardwood competition is discussed in view of reaching an equilibrium between optimal spruce plantation growth and benefits from further silvicultural treatments, and maintaining hardwood species known to improve long term site quality, within a white spruce plantation.
by Marie-Claude Boileau | 28 May 2020
Published in Tree Physiology 20: 229-238
We investigated effects of nutrient addition on several physiological characteristics of 60-cm-tall black spruce (Picea mariana Mill. B.S.P.) layers (i.e., rooted branches of overstory trees) and 20-cm-tall planted seedlings on a clear-cut, N-limited boreal site. After two growing seasons, current-year and one-year-old needles of fertilized trees (layers and seedlings combined) had higher net photosynthetic rates (An) and maximum capacity of Rubisco for CO2 fixation (Vmax) than unfertilized trees. One-year-old needles of fertilized trees had higher stomatal conductance (gs), higher water-use efficiency, and lower intercellular to ambient CO2 ratio than unfertilized trees. Additionally, fertilized trees had higher predawn and midday shoot water potentials than unfertilized trees.
Stomatal conductance of 1-year-old needles was 23% higher in seedlings than in layers, but there were no significant differences in gs of current-year needles between the regeneration types. For both needle age-classes, An and Vmax of layers were 25 and 40% higher, respectively, than the corresponding values for seedlings. The higher values of An, Vmax and foliar N concentration of layers compared with seedlings after two growing seasons may be associated with the larger root systems of the layers compared with the transplanted seedlings.
