. 1999. Fall cutting management affects yield and persistence of alfalfa in Atlantic Canada. Can. J. Plant Sci. 79: 57-63. The existing recommendation to avoid harvesting alfalfa during a critical fall rest period, based on calendar dates, is under review in Canada and adjacent areas of the United States. The effect on yield and persistence of four fall cutting management treatments (based on cumulative growing degree-days between harvests) was investigated for two cultivars of alfalfa (Medicago sativa L. 'Apica' and 'Oneida VR') at five sites in Atlantic Canada. The average seasonal DM yield for the five sites in the first production year was 1.6 to 2.3 t ha -1 greater with an additional harvest as compared with the two-harvest system (Truro, Nappan, Fredericton, and Charlottetown) and the one-harvest system in St. John's. Seasonal DM yield benefits were reduced over time; by the third production year, there was no increase in seasonal DM yield with an additional harvest. The seasonal DM yield increased with increasing the interval between the final harvest and the previous one. Taking an additional harvest resulted in higher winter plant mortality between the first and second production years in Truro and Nappan, and the third and fourth production years in St. John's. At the other two sites (Charlottetown and Fredericton), and for the first three production years in St. John's, winter plant mortality was unchanged with an additional harvest but regrowth potential in the following year was reduced. At the two sites that experienced a severe winter (Nappan and Truro), the cultivar Oneida VR was more adversely affected by the additional harvest than Apica. Our results indicate that in Atlantic Canada, taking an additional harvest increases the risk of winterkill. If an additional harvest is to be taken, increasing the interval between the final harvest and the previous one to at least 500 growing degree-days will decrease the risk of winterkill and might have a positive effect on next year's regrowth.

Tree improvement programs usually consist of multiple breeding populations that target different climatic or ecological regions. Parent breeding material normally originates from and is deployed within the same breeding region, assuming optimal local adaptation of populations. Given the climate trends observed over the last several decades in western Canada, this assumption is unlikely to still be valid. This problem needs to be addressed either by delineating new deployment areas for improved planting stock or by selecting genotypes suitable for changed climatic environments. In a case study for white spruce, we analyzed height data from 135,000 trees grown in 44 genetic tests established and evaluated over a period of 35 years by industry and government agencies in Alberta. We show how the risk of planting maladapted trees can be minimized by moving planting stock to new areas, or by eliminating genotypes from breeding programs that are sensitive to anticipated future climate environments. Transfers that outperformed local sources consistently originated from locations with higher temperatures, suggesting north or northwest transfers. However, adaptation to cold appears to be a prevalent driver for genetic population differentiation in spruce, thus limiting how far material may be moved in current reforestation efforts to address future climate change.

The effect of land use change from agricultural to short rotation woody crops on soil respiration and its temperature sensitivity is not fully understood. We studied the effect of land use change on soil respiration, in a factorial experiment, 4 years after converting an agricultural field to a Walker poplar (Populus deltoides × Populus × petrowskyana var. Walker) plantation in the boreal region of northeastern Alberta. Overall, total soil respiration was greater in the agricultural plots (planted to alfalfa, Medicago sativa L.) than in the poplar plots. Soil respiration and soil temperature at the 10-cm depth in both land uses had similar seasonal and diurnal variations. The season-long temperature sensitivity (Q 10 ) of daily and nighttime soil respiration in the alfalfa plots was greater than that in the poplar plots, with Q 10 values of 5.4 vs. 4.9, respectively. Our data also show that, 4 years after land use conversion, the heterotrophic respiration was smaller in the poplar plots than in the adjacent agricultural plots, indicating potential benefits of plantation establishment in reducing heterotrophic respiration. However, the temperature sensitivity of soil respiration based on monthly nighttime rates, which minimizes the plant phenological influences, was greater in the poplar plantation, suggesting that soil respiration could become greater in the poplar plantation under a future warmer climate. We conclude that establishment of poplar plantations, which are known to have a fast rate of biomass production for long-term carbon storage, may help mitigate climate change by reducing heterotrophic and total soil respiration in the Canadian boreal region, but the long-term implications (e.g., changes in the temperature sensitivity of soil respiration over time) need to be further studied.

No abstract

. 2008. Autotrophic and heterotrophic respiration rates across a chronosequence of hybrid poplar plantations in northern Alberta. Can. J. Soil Sci. 88: 261Á272. Distinction of autotrophic (root, R r ) versus heterotrophic (R h ) soil respiration is important to understanding C balance in forest ecosystems and the contribution of both respiration components to total soil respiration (R s ) may change as stands mature. We studied R r and R h in a chronosequence of hybrid poplar plantations that were 4, 6, 8, and 13 yr old in summer 2005. We hypothesized that R r will become increasingly larger and will also represent a greater proportion of R s as plantation age increases. Our results showed that both R s and R h showed significant seasonal variations; however, R r were much more variable. No significant differences among plantation ages were observed for any of the respiration parameters (R r , R h and R s ) measured throughout the summer. No significant differences were found for the contribution of R h to R s between plantation ages or between sampling dates. The average proportion of R h to R s across all sites and sampling dates was 0.6390.026 (mean9standard error), indicating that heterotrophic respiration dominated total soil respiration in the studied hybrid poplar plantations. The power of the statistical tests in this study was likely quite low due to the inclusion of only two replicates for each plantation age.Key words: Autotrophic, heterotrophic respiration, root exclusion, trenching, hybrid poplar, chronosequence Saurette, D. D., Chang, S. X. et Thomas, B. R. 2008. Taux de respiration autotrophe et he´te´rotrophe dans une chronose´quence de peupliers hybrides dans le nord de l'Alberta. Can. J. Soil Sci. 88: 261Á272. Pour mieux comprendre le bilan du carbone dans les e´cosyste`mes forestiers, il importe de faire la distinction entre la respiration autotrophe (racines, R r ) et he´te´rotrophe (R h ) du sol, deux composantes dont l'apport a`la respiration totale (R s ) peut changer avec l'e´volution du peuplement. Les auteurs ont examine´la R r et la R h dans une chronose´quence de peupliers hybrides de 4, 6, 8 et 13 ans durant l'e´te´2005. Ils ont e´mis l'hypothe`se que la R r ne cesserait d'augmenter et repre´senterait une part de plus en plus grande de la R s avec le vieillissement du peuplement. Les re´sultats indiquent que la R s et la R h varient sensiblement avec la saison; mais que la R r le fait davantage. Aucun des parame`tres de la respiration (R r , R h et R s ) quantifie´s durant l'e´te´ne fluctue sensiblement avec l'aˆge du peuplement. La contribution de la R h a`la R s ne varie pas e´norme´ment avec l'aˆge du peuplement ni la date d'e´chantillonnage. La R h repre´sente une proportion de 0,6390,026 (moyenne9erreur-type) de la R s pour l'ensemble des sites et des dates d'e´chantillonnage, signe que la respiration he´te´rotrophe dominait dans les plantations de peupliers hybrides examine´es. Les tests statistiques employe´s dans le cadre de cette e´tude n'ont qu'une valeur relativement faibl...