by Audrey Verreault | 30 January 2019
Published in Ecology Letters 9: 399-409
The genus Populus comprises some of the most commercially exploited, pioneer forest trees distributed throughout the northern-hemisphere. The high level of morphological diversity, extensive inter-species hybridization, and low level of DNA sequence variation among species in this group have impeded the progress of taxonomic and phylogenetic studies. We used nuclear genomic data based upon inter simple sequence repeat (ISSR) variability, a highly variable class of molecular markers to determine the genetic relationships among species of the genus Populus. Species of the section Populus (Leuce) clustered together suggesting monophyly of the section Populus. The Eurasian members of section Populus (P. alba, P. davidiana, and P. tremula) showed closer genetic relationships to each other than to two North American aspens (P. tremuloides and P. grandidentata) of the same section. In contrast to previous phylogenetic studies, P. nigra showed a close genetic relationship to species of the section Tacamahaca. This relationship is in agreement with various phenotypic traits, interfertility and chemistry of bud exudates and serves as evidence for introgression between P. nigra and species of the section Taccamahaca. Overall, the genetic relatedness estimates based on nuclear ISSR data were congruent with phylogenetic trees based on other molecular (RFLP and DNA sequence) and morphological data, but provided better resolution in assessing the genetic relatedness among closely related taxa, and provided genetic evidence for previously suspected introgressions.
by Audrey Verreault | 30 January 2019
Published in Forest Ecology and Management 242: 553-557
We evaluated the effects of a major ice storm on understory light conditions (%PPFD, photosynthetic photon flux density) in an old-growth Acer–Fagus forest in Quebec, based on pre- and post-disturbance light measurements taken until the seventh growing season after the event (which occurred in January 1998). Before the ice storm, most microsites received between 2 and 4%PPFD. Following the ice storm, the stand-level mean %PPFD increased four- to five-fold, ranging from13.8 to 20.5%PPFD, from 0.3 to 4 m aboveground. Despite its magnitude, the post-ice storm increase in light-transmission was short-lived. By 1999 (2-year+), the mean light levels had decreased by half, and recovery to pre-storm conditions occurred within 3–7 years, depending on height. The decrease in light transmission during the post-disturbance years followed an inverse J-shape trend, indicating more dynamic changes early after disturbance. By 2004 (7-year+), light levels at <2 m had become slightly but significantly lower than before the ice storm, with most microsites receiving <2%PPFD. The ice storm led to a synchronized increase of the light levels at almost all understory locations, which might allow a high proportion of the advanced regeneration to experience a release. However, due to the rapid recovery of the light conditions to levels similar or lower than before the ice storm, this disturbance should be more advantageous to shade-tolerant species.
by Audrey Verreault | 30 January 2019
Published in Forest Ecology and Management 262: 863-872. doi : 10.1016/j.foreco.2011.05.027
In the present context of global climate changes and the continuous development of forest management strategies based on the concept of sustainable use, it is important to develop a better understanding of the climatic factors controlling the growth of boreal forests. In this study, we report the results of a five-year field research within which day-to-day balsam fir (Abies balsamea (L.) Mill.) stem diameter variation was measured with dendrometers and examined in relation with various daily climatic variables. A model built with data from three growing seasons that included solar radiation, relative humidity, temperature and precipitation explained 84% of the variance in day-to-day stem diameter variation from June to September. The model has approximately the same predictive capability when validated with independent daily data from two other growing seasons. The model captured both the cumulative increment associated with the irreversible growth and the high frequency variation of day-to-day fluctuations associated to changes in the stem water content. In general, rainy days during which relative humidity was high and solar radiation was low favored stem diameter expansion (growth and swelling) while stem diameter decreased during periods of low relative humidity and high solar radiation. Similar models were obtained when the June–September period was divided into two parts (June–July and August–September) to better represent the period during which most of the cumulative annual stem increment is observed (June–July). Inter-annual variation in stem growth computed from the modeled day-to-day variation in stem diameter was significantly correlated to the inter-annual variation in annual growth determined from tree core measurements over a 10 year period (p = 0.023). The model was notably able to capture a particularly poor growing year (2006) presumably due to a short-term heat stress period. Results suggest that the inclusion of daily data in growth–climate models may contribute to improve predictions of the potential tree growth response to climate by identifying particular climatic events that may escape to a classical dendroclimatic approach.
by Marie-Claude Boileau | 30 January 2019
Published in Forest Ecology and Management 259: 867-874
Ecophysiology of black walnut (Juglans nigra L.) along a vertical canopy gradient was studied in a mixed species plantation for the first (2007) and second (2008) growing seasons after thinning to understand response mechanisms to increased resource availability. Parameters were affected by the thinning treatment in 2008 only. Thinned trees showed increased light-saturated maximum photosynthesis (Amax) from 2007 to 2008 compared to non-thinned trees. This response was likely due to increased light levels near thinned trees (vs. changes in water or nutrient status), because thinning did not affect midday leaf water potential (Ymd), average daily soil water content (SWC), or leaf nitrogen content per unit area (Na). Plantation thinning did not increase relative diameter growth during the experimental period. This may be due to low thinning intensity and mortality prior to thinning that reduced competition from firsttier neighbors. Certain leaf traits such as leaf mass per unit area (LMA) and Na increased from the bottom to the upper canopy position, but did not influence thinning responses. Distribution patterns of photosynthetic parameters through the vertical canopy gradient were less defined than leaf structural traits such as LMA and Na. Findings reflect black walnut’s large variability in response to thinning.
by Marie-Claude Boileau | 30 January 2019
Published in Canadian Journal of Forest Research 40: 928-942
The knowledge of natural disturbance dynamics and preindustrial landscapes is essential to implement sustainable forest management. Recent findings identify the lack of a forest dynamics model, different from the standard cyclic model of Baskerville (1975. For. Chron. 51: 138-140), for balsam fir (Abies balsamea (L.) Mill.) ecosystems of maritime eastern Canada. With the use of historical forest maps and dendrochronology, we reconstructed the range of variability of the preindustrial landscape (6798 km2) and inferred on the natural disturbance dynamics of the balsam fir forest of Anticosti Island. The preindustrial landscape was characterized by a forest matrix of overmature softwood stands with inclusions of younger softwood stands ranging from 0.1 to 7837 ha in size. Widespread stand-initiating events were apparently rare in the preindustrial landscape over the last 160 years. Since our results were not well represented by the cyclic model, which predicts the occurrence of a mosaic of stands in different age classes, we proposed an alternative forest dynamics model for eastern balsam fir ecosystems near the Gulf of St. Lawrence. Forest management inspired by this alternative model may be more appropriate to maintain or restore ecological characteristics of balsam fir forests of this region within their range of natural variability.
