by Svetlana Savin | 30 January 2019
Published in New Forests 43(5): 849-868. https://doi.org/10.1007/s11056-012-9324-x
In the Canadian boreal forest, conifer plantations are mainly used to overcome poor natural regeneration following harvesting or wildfires. However, competitive interactions can impair the successful establishment of forest plantations, especially in the presence of ericaceous species, such as Kalmia angustifolia, that are good competitors for soil resources. We used data from a silvicultural trial established in northwestern Québec (Canada) to test the hypothesis that Kalmia’s effect on seedling growth is an asymptotic nonlinear function of proximity to Kalmia at time of planting. Our main objective was to derive species-specific thresholds of Kalmia-proximity tolerance at the time of planting for boreal sites prone to Kalmia invasion following harvesting. Using nonlinear quantile regressions (90th percentile), height, diameter and growth of planted Picea mariana and Pinus banksiana seedlings over 8 years was regressed against the initial distance to the nearest Kalmia stem at time of planting. We also investigated the relation between annual growth and the distance to nearest Kalmia stem during any given year between age 2 and 8-years. Finally, we evaluated how mechanical soil scarification influenced Kalmia recovery over time by looking at its rate of spread, estimated from mean distance to planted seedlings over 8 years. Silvicultural treatments increased the intercepts and asymptotes of all variables for Pinus. However, asymptotic values for Picea could not be identified, thus precluding threshold assessment for initial distance to Kalmia at planting for this species. For Pinus planted on scarified sites, a Kalmia-free radius of 60 cm would lead to ~80 to 85 % of the 90th percentile of maximum potential height at age 8 years. Mechanical scarification created Kalmia-free microsites that were rapidly re-invaded by spread of Kalmia; planting immediately following soil preparation of these sites is therefore important for ensuring successful plantation establishment.
by Claire Morin | 30 January 2019
Published in Ecological Society of America 23(1): 21-35. https://doi.org/10.1890/12-0425.1
There is general consensus that wildfires in boreal forests will increase throughout this century in response to more severe and frequent drought conditions induced by climate change. However, prediction models generally assume that the vegetation component will remain static over the next few decades. As deciduous species are less flammable than conifer species, it is reasonable to believe that a potential expansion of deciduous species in boreal forests, either occurring naturally or through landscape management, could offset some of the impacts of climate change on the occurrence of boreal wildfires. The objective of this study was to determine the potential of this offsetting effect through a simulation experiment conducted in eastern boreal North America. Predictions of future fire activity were made using Multivariate Adaptive Regression Splines (MARS) with fire behavior indices and ecological niche models as predictor variables so as to take into account the effects of changing climate and tree distribution on fire activity. A regional climate model (RCM) was used for predictions of future fire risk conditions. The experiment was conducted under two tree dispersal scenarios: the status quo scenario, in which the distribution of forest types does not differ from the present one, and the unlimited dispersal scenario, which allows forest types to expand their range to fully occupy their climatic niche. Our results show that future warming will create climate conditions that are more prone to fire occurrence. However, unlimited dispersal of southern restricted deciduous species could reduce the impact of climate change on future fire occurrence. Hence, the use of deciduous species could be a good option for an efficient strategic fire mitigation strategy aimed at reducing fire propagation in coniferous landscapes and increasing public safety in remote populated areas of eastern boreal Canada under climate change.
by Claire Morin | 30 January 2019
Published in Agricultural and Forest Meteorology 162-163: 108-114
It is important to develop a better understanding of the climatic factors controlling the growth of boreal forests. Dendrometer measurements were used to characterize inter-annual variation in seasonal patterns of stem diameter increment of balsam fir trees (n = 3) over seven growing seasons (2004–2010) in a boreal forest of Québec, Canada. For the period studied, cumulative seasonal growth ranged from 1.1 mm to 2.9 mm. Cumulative seasonal growth was a function of the timing of tree growth initiation and cessation along with the maximum growth rate observed throughout the growing season. The start and finish of diameter growth showed variations of 21 and 53 days, respectively, and duration of the growing season ranged from 38 to 107 days while maximum growth rates observed throughout a season ranged from 36.0 μm d−1 to 57.6 μm d−1. Interestingly, similar cumulative annual growth can be achieved through very different seasonal growth patterns, depending on the inter-annual variation of the three factors mentioned above. Air temperature and photosynthetically active radiation appeared to regulate the initiation of tree growth with high photosynthetically active radiation and cool spring conditions delaying the start of growth. The maximum growth rate within a given season was related to snowpack depth and the timing of snowmelt. Deeper snowpack that melt earlier in spring was associated to higher maximum growth rates during the following season. Apart from exceptional climatic conditions that led to a very early growth cessation in 2006, the timing of growth cessation cannot be explained by climatic variables, suggesting that this phenological event is internally controlled by a physiological mechanism. Overall, the results indicate that the onset of growth as well as the maximum growth rate were regulated by climatic triggers. Consequently, changes in climate seasonality may have considerable effects on both seasonal pattern of growth and tree growth itself.
by Claire Morin | 30 January 2019
Published in Tree Genetics & Genomes 9(1): 129-143 https://doi.org/10.1007/s11295-012-0540-5
Estimations of genetic parameters of wood traits based on reduced sample populations are widely reported in the literature, but few investigations have considered the consequences of these small populations on the precision of parameter estimates. The purpose of this study was to determine an optimal strategy for sampling subgroups, by varying either the number of families or the number of individuals (trees) per family, and by verifying the accuracy of certain genetic parameters (across-trials analysis). To achieve this, simulations were conducted using random resampling without replacement (k= 1,000/pair of varying factors) on datasets containing 10-year total height of two coniferous species (Larix laricina and Picea mariana), as well as pilodyn measurements of wood density evaluated on a 26-year-old population of P. mariana. SASÒ 9.2 Macro Language and Procedures were used to estimate confidence intervals of several genetic parameters with different reduced samplings. Simulation results show that reducing the number of trees per family per site had more impact on the magnitude and precision of genetic parameter estimates than reducing the number of families, especially for half-sib heritability and type B genetic correlations for height and wood density. A priori determination of an optimal subsampling stragety to evaluate the accuracy of genetic parameters should become common practice before assessing wood traits, in tree breeding strudies or when planning juvenile retrospective progeny trials for forest tree species.
by Claire Morin | 30 January 2019
Published in Remote Sensing of Environment 113: 880-888 https://doi.org/10.1016/j.rse.2009.01.002
We used daily MODerate resolution Imaging Spectroradiometer (MODIS) imagery obtained over a five-year period to analyze the seasonal and inter-annual variability of the fraction of absorbed photosynthetically active radiation (FAPAR) and photosynthetic light use efficiency (LUE) for the Southern Old Aspen (SOA) flux tower site located near the soutern limit of the boreal forest in Saskatchewan, Canada. To obtain the spectral characteristics of a standardized land area to compare with tower measurements, we scaled up the nominal 500 m MODIS products to a 2.5 km x 2.5 km area (5 x 5 MODIS 500 m grid cells). We then used the scaled-up MODIS products in a coupled canopy-leaf radiative transfer model, PROSAIL-2, to estimate the fraction of absorbed photosynthetically active radiation (APAR) by the part of the canopy dominated by chlorophyll (FAPARchl) versus that by the whole canopy (FAPARcanopy). Using the additional information provided by flux tower-based measurements of gross ecosystem production (GEP) and incident PAR, we determined 90-minute averages for APAR and LUE (slope of GEP:APAR) for both the physiologically active foliage (APARchl, LUEchl) and for the entire canopy (APARcanopy, LUEcanopy).
The flux tower measurements of GEP were strongly related to the MODIS-derived estimates of APARchl (r2=0.78) but only weakly related to APARcanopy (r2=0.33). Gross LUE between 2001 and 2005 for LUEchl was 0.0241 mmol C mmol-1 PPFD whereas LUEcanopy was 36% lower. Time series of the 5-year normalized difference vegetation index (NDVI) were used to estimate the average length of the core growing season as days of year 152-259. Inter-annual variability in the core growing season LUEchl (mmol C mmol-1 PPFD) ranged from 0.0225 in 2003 to 0.0310 in 2004. The five-year time series of LUEchl corresponded well with both the seasonal phase and amplitude of LUE from the tower measurements but this was not the case for LUEcanopy. We conclude that LUEchl derived from MODIS observations could provide a more physiologically realistic parameter than the more commonly used LUEcanopy as an input to9 large-scale photosynthesis models.
