by Claire Morin | 30 January 2019
Published in Remote Sensing of Environment 98: 212-224
The relationships between ecosystem-level light use efficiency (LUE) obtained from an eddy covariance flux tower and MODIS-derived values of a scaled Photochemical Reflectance Index (sPRI) were investigated for a boreal aspen stand (Populus tremuloides Michx.) in Saskatchewan, Canada. Using MODIS ocean band 11 at 531 nm, we tried to detect variations in canopy reflectance related to the xanthophyll cycle. We tested several other MODIS bands as the reference band because the 570 nm reference band that had been determined to be optimum for calculating PRI in earlier studies is not available on MODIS.
While LUE varied greatly within the 2001, 2002, and 2003 growing seasons, the Normalized Difference Vegetation Index (NDVI) calculated from tower sensors remained stable. LUE had a negative exponential relationship with vapor pressure deficit and air temperature, and was at a maximum when absorbed PAR was <200 µmol m-2 s-1. The range and magnitude of tower-based LUE values were smaller on clear days, when MODIS acquisitions were possible, than they were overall. Furthermore, the orbital parameters of the Terra and Aqua satellites restricted MODIS acquisitions to a 2.5-h period in early afternoon at our study site when LUE values were typically lower than they were earlier in the day.
Strong correlations between MODIS-sPRI and LUE were found only for backscatter reflectance scenes when band 13 (667 nm) was used as the reference band. The correlations were higher for sPRI calculated from top of atmosphere reflectances than for surface reflectances (r2=0.76 and 0.53, respectively). The absolute backscatter reflectance of bands 11, 12 (551 nm), and 13 all decreased with increasing LUE. The decrease in band 13 suggests that the correlation between sPRI and LUE that we observed was caused by reductions in canopy chlorophyll content from 2001 to 2003 and/or by increased visibility of brighter non-photosynthetic material. Regional values of sPRI from 260 deciduous forest pixels in the 10,000 km2 vicinity of the tower for two contrasting days, one in 2001 and one in 2003, were consistent with that observed for the flux tower footprint.
by Marie-Claude Boileau | 30 January 2019
Published in The Sports Turf Manager Spring: 18-21.
by Claire Morin | 30 January 2019
Published in Annals of Forest Science 68(8): 1303-1313. https://doi.org/10.1007/s13595-011-0138-3
Context Root growth is a characteristic to which nursery personnel is particularly attentive. The increase in root growth of white spruce seedlings in the autumn relies on the current season’s photosynthates. Needle hardening or a decrease in the mass of photosynthetically active foliage as a result of early frost may negatively affect the seedling’s photosynthetic capacity and its ability to fuel root growth.
Aims This study evaluated the relationship between cold hardiness, root growth potential, and photosynthesis of 2-year-old white spruce seedlings.
Methods At the end of their second growing season under standard cultural practices in a forest nursery, seedlings were hardened under natural conditions of photoperiod and temperature. After being subjected to artificial freezing tests on four sampling dates during the fall, the seedlings were compared for cold tolerance and recovery of gas exchange.
Results The effect of artificial freezing treatments on seedling photosynthesis varied with frost intensity and degree of needle hardening. The mass of new roots formed over a 21-day cultivation test period following the freeze tests was positively related to the photosynthetic capacity of the seedlings and negatively related to the proportion of damaged needles.
Conclusion These results imply that autumnal cold damage to needles directly affects seedling root growth and emphasize the importance of maintaining seedlings in an environment favorable for photosynthesis during the fall to promote root growth.
by Claire Morin | 30 January 2019
Published in Agroforestry Systems. https://doi.org/10.1007/s10457-012-9487-7
The implementation of a management-protection program, using suitable cultivation practices, contributes to rehabilitating marginal areas prone to prolonged waterlogging and preserving biodiversity. Three hybrid poplar clones (‘I-488’, ‘Rimini’ and ‘D-64’) were tested to assess their degree of tolerance to waterlogging by imposing four water regimes over a 1 year period: control (C), non-preconditioned (NPr), and two levels of flood preconditioning (Pr10 and Pr20). During the 2nd year, waterlogging (1.9 ± 0.7 mg/lO2) was imposed on plants from NPr, Pr10 and Pr20 treatments for 60 days followed by drainage. Survival rate, dry mass production, net photosynthesis and relative damage to membrane structural integrity were evaluated. The treatment response varied among clones and levels of preconditioning. Less pronounced damage to ‘I-488’ clones in comparison to those of ‘Rimini’ and ‘D-64’ indicates its superior tolerance to waterlogging. Preconditioning treatments (Pr20) and, to a lesser extent (Pr10), significantly improved the ability of the clones to tolerate flooding, suggesting that the application of a Pr20 preconditioning treatment in the nursery before outplanting would give hybrid poplar clones the morpho-physiological advantages necessary to counteract the arduous conditions on inundated sites.
by Audrey Verreault | 30 January 2019
Published in Oikos: 1-11. doi : 10.1111/j.1600-0706.2011.19777.x
The two major disturbance types of boreal black spruce forest in north-central Québec, Canada – natural disturbance by wildfire and anthropogenic disturbance by harvest – may affect processes of recovery differently and leave distinct post-disturbance soil and vegetation spatial patterns. We tested whether 1) spatial patterns of physico-chemical soil organic layer properties, black spruce diameter and density, and understory ericaceous shrub cover, differ between these two principal disturbance types; 2) operations associated with forest harvest result in distinct, regular spatial patterns of these same variables related to presence of machine trails; and 3) ericaceous shrub presence is a potential factor contributing to the legacy of spatial patterns after harvest. We explored these patterns on black spruce-feathermoss forest stands, including fire-origin stands (18 and 98 years) and stands originating from harvest (16 and 62 years) in central Québec, Canada. We used two spatial analysis methods, spectral analysis and principal component analysis in the frequency domain, to characterize and relate spatial patterns of these soil and vegetation variables, measured along 50-m transects on each site. Spatial patterns of distribution of soil and vegetation variables were different on the burned and the harvested forest sites. Wildfire gave rise to spatial patterns in soil and vegetation variables at multiple scales, reflecting the complexity generated by variable burn intensity. Patterns following forest harvest were mainly related to the regular structure defined by trails created by logging operations. In contrast to burned sites, ericaceous shrub patterns on harvested sites were strongly associated with spatial arrangements of spruce diameter and density, promoting absence of canopy closure and persistence of trails. Moreover, different spatial signatures did not converge in the long-term (62-98 years) between the two disturbance types. The divergence in spatial structure between natural and anthropogenic disturbances has implications for ecosystem structure and function in the longer term.
