by Audrey Verreault | 28 May 2020
Published in The Canadian Entomologist 133: 333-341
Studies were conducted to document the oviposition biology of female white pine weevil, Pissodes strobi Peck. Data were recorded on female fecundity and daily feeding and oviposition activities. In addition, we compared fecundity of 1- and 2-year-old females. Female white pine weevils laid a mean ± SE total number of 132.3 ± 7.5 eggs (range 40-344 eggs) during an average oviposition period of 5.8 ± 0.4 weeks. It is during the first 5 weeks that white pine weevils lay the most eggs, with an egg-laying peak in the second week of the oviposition period. For a few females, egg production did not cease until the 22nd week. There was no difference between the number of eggs laid in the photophase and in the scotophase, but higher numbers of feeding punctures were observed during the night than during the day. The oviposition and feeding results for 2-year-old female white pine weevils were similar to those for 1-year-old females. This information contributes to the management of white pine weevils by providing a better understanding of some of the mechanisms of its population dynamics.
by Marie-Claude Boileau | 26 May 2020
Published in Oecologia 133: 1-9
The long-term responses of forests to atmospheric CO2 enrichment have been difficult to determine experimentally given the large scale and complex structure of their canopy. We have developed a CO2 exposure system that uses the free-air CO2 enrichment (FACE) approach but was designed for tall canopy trees. The system consists of a CO2 -release system installed within the crown of adult trees using a 45-m tower crane, a CO2 monitoring system and an automated regulation system. Pure CO2 gas is released from a network of small tubes woven into the forest canopy (web-FACE), and CO2 is emitted from small laser-punched holes. The set point CO2 concentration ([CO2 ]) of 500 mmol mol-1 is controlled by a pulse-width modulation routine that adjusts the rate of CO2 injection as a function of measured [CO2 ] in the canopy. CO2 consumption for the enrichment of 14 tall canopy trees was about 2 tons per day over the whole growing season. The seasonal daytime mean CO2 concentration was 520 mmol mol-1. One-minute averages of CO2 measurements conducted at canopy height in the center of the CO2-enriched zone were within ±20% and ±10% of the target concentration for 76% and 47% of the exposure time, respectively. Despite the size of the canopy and the windy site conditions, performance values correspond to about 75% of that reported for conventional forest FACE with the added advantage of a much simpler and less intrusive infrastructure. Stable carbon isotope signals captured by 80 Bermuda grass (Cynodon dactylon) seedlings distributed within the canopy of treated and control tree districts showed a clearly delineated area, with some nearby individuals having been exposed to a gradiant of [CO2], which is seen as added value. Time-integrated values of [CO2] derived from the C isotope composition of C. dactylon leaves indicated a mean (±SD) concentration of 513±63 mmol mol-1 in the web-FACE canopy area. In view of the size of the forest and the rough natural canopy, web-FACE is a most promising avenue towards natural forest experiments, which are greatly needed.
by Marie-Claude Boileau | 26 May 2020
Published in Forest Ecology and Management 168: 177-186
A precommercial thinning trial, conducted in 1978 in a 20-year-old balsam fir (Abies balsamea [L.] Mill.) stand in the province of Québec, Canada, produced a broad range of stand densities over 60 permanent plots. Plots were measured at 5-year intervals until 1998 and the data were analyzed to verify the principal effects that are expected from this treatment in terms of individual tree growth and production per hectare. The diameter growth during the 20-year period following thinning was inversely related to the residual density. The increase in diameter of trees in thinned plots accumulated mainly during the first 10-year period. One explanation why thinning did not increase the diameter throughout the entire 20-year inventory period involves a spruce budworm (Choristoneura fumiferana [Clem.]) outbreak that occurred during this period. Total volume per hectare after 20 years was proportional to residual stand density, while merchantable volume per hectare was similar through the range of densities. If trees larger than 15 cm in diameter at breast height are considered exclusively, then thinning significantly increased 20-year stand yield. Therefore, the most important effect of precommercial thinning is to increase the volume per hectare of large diameter trees. Any conclusions on the impact of this treatment on stand yield must take into account the minimum usable diameter that is considered.
by Marie-Claude Boileau | 26 May 2020
Published in Tree Physiology 22: 363-371
Experiments were conducted on 1-year-old western red cedar (Thuja plicata Donn.) seedlings to determine the response of illuminated foliage to reversible changes in total photosynthetic foliage area (La). Reductions in La were brought about by either shading the lower foliage or by reducing the ambient CO2 concentration (ca) of the air surrounding the lower part of the seedling. ln the latter case, the vapor pressure was also changed so that transpiration rates (E) could be manipulated independently of photosynthetic rates (A). We hypothesized that following such treatments, short-term compensatory changes would occur in stomatal conductance (gs) and A of the remaining foliage. These changes would occur in response to hydraulic signals generated by changes in the water potential gradient rather than changes in the distribution of sources and sinks of carbon within the seedling. When a portion of the foliage was shaded, there was an immediate reduction in whole-seedling E and a concomitant increase in gs, A and E in the remaining illuminated foliage. However, the inter-cellular CO2 concentration did not change. These compensatory effects were fully reversed after the shade was removed. When the lower foliage A was reduced to < 0 mmol m-2 s-I, by shading or lowering ca, and E was either unchanged or increased (by adjusting the vapor pressure deficit), there was no significant increase in gs and A in the remaining foliage. We conclude that compensatory responses in illuminated foliage occur only when reductions in LA are accompanied by a reduction in whole-plant E. The relationship between the reduction in whole-seedling E and the increase in A is highly linear (r2 = 0.68) and confirms our hypothesis of the strong regulation of gs by hydraulic signals generated within the seedling. We suggest that the mechanism of the compensatory effects is a combination of both increased CO2 supply, resulting from increased gs, and a response of the rate of carboxylation, possibly related to the activity of Rubisco.
by Marie-Claude Boileau | 26 May 2020
Published in Wetlands 22(2): 225-233
Low water tables typically found in peatlands during dry summer periods or in the vicinity of drainage ditches may lead to moisture deficiency in porous surface peats. Episodes of drought stress might compromise the growth benefits brought about by lower ground-water levels. We examined the water relations of black spruce (Picea mariana) trees on a natural peatland during relatively wet (1990) and relatively dry (1991) summers. Seasonal patterns of pre-dawn and mid-day shoot water potentials and stomatal conductance were not related to peat water content or to water-table depth. There was no evidence of water stress or osmotic adjustment in sampled trees during wet and dry growing seasons. Our soil moisture data showed that although water-table levels were as low as —66 cm in 1991, water availability in the root zone remained high. Even with the absence of mid-day water stress during the summer of 1991, a 50% reduction in stomatal conductance as compared with the previous year was found. We suggest that signals from the bulk of the roots located in dry peat top layer contributed to the regulation of stomatal conductance.
