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.
by Audrey Verreault | 28 May 2020
Published in Proceedings of the National Academy of Science 97(21): 113311-11336
Black spruce (Picea mariana [B.S.P.] Mill.) and red spruce (Picea rubens Sarg.) are two conifer species known to hybridize naturally in northeastern North America. We hypothesized that there is a progenitor-derivative relationship between these two taxa and conducted a genetic investigation by using sequence-tagged-site markers of expressed genes. Based on the 26 sequence-tagged-site loci assayed in this study, the unbiased genetic identity between the two taxa was quite high with a value of 0.920. The mean number of polymorphic loci, the mean number of alleles per polymorphic locus, and the average observed heterozygosity were lower in red spruce (P = 35%, AP = 2.1, Ho = 0.069) than in black spruce (P = 54%, AP = 2.9, Ho = 0.103). No unique alleles were found in red spruce, and the observed patterns of allele distribution indicated that the genetic diversity of red spruce was essentially a subset of that found in black spruce. When considered in combination with ecological evidence and simulation results, these observations clearly support the existence of a progenitor-derivative relationship and suggest that the reduced level of genetic diversity in red spruce may result from allopatric speciation through glaciation-induced isolation of a preexisting black spruce population during the Pleistocene era. Our observations signal a need for a thorough reexamination of several conifer species complexes in which natural hybridization is known to occur.
by Marie-Claude Boileau | 28 May 2020
Published in Canadian Journal of Forest Research 30: 59-66
Regeneration of first-cut strips in a two-cut system of strip clear-cutting was compared to that of large clear-cutting in four different areas representative of the black spruce (Picea mariana (Mill.) BSP) stands of the boreal forest of Québec. Seedlings were more evenly distributed in clearcut strips than in large clearcuts. Differences of about 10 000 black spruce seedlings per hectare and 20% of stocking were observed in favour of clearcut strips compared to large clearcuts. Black spruce stocking was about 14% larger on lowland than on upland sites but height growth was better on upland sites. A regeneration problem similar to that of large clearcuts was observed when the second strips were cut. One year after cutting these second strips, winter harvesting resulted in a 23% gain in black spruce stocking as compared to summer harvesting. Even if black spruce stocking marginally increased during the years following winter harvesting, the height advantage of the preserved advance growth justifies the application of this harvesting method. The strip clear-cutting system effectively improved the stocking of former black spruce stands but if the stocking level of advance growth is adequate, careful harvesting to preserve advance regeneration should be the preferred method since it would be more cost-efficient.
