Claude Lavoie

Invasive aquatic plants are associated with substantial economic costs. In North America, a large number of lakes have been invaded by Eurasian watermilfoil (EWM; Myriophyllum spicatum), a species widely reported to reduce lakefront property values by 1–40%. We reassessed the impact of EWM on property values using a causal identification strategy based on an eventstudy framework. This reassessment relied on more than 11,000 lakefront real estate transactions conducted in Québec (Canada) between 2000 and 2023. We tested the hypothesis that EWM exerted a significant negative effect on property values and that this effect was further amplified following media coverage of the invasion. Contrary to findings from studies conducted in the United States, we did not detect a robust or statistically significant effect of EWM on lakefront house prices. Likewise, we did not find a significant price response associated with either the initial detection of EWM or its first mention in a local newspaper. Differences may be explained by limitations in previous studies that relied on hedonic price models without an explicit causal identification strategy. In addition, the potential effect of EWM on prices may be attenuated by strong demand for lakefront properties combined with highly constrained housing supply. Overall, our findings suggest that the effect of invasive aquatic plants on property values reflects a complex buyer–seller decision-making process, particularly in settings where other amenities remain highly attractive to prospective buyers.

Une population de cypripède royal (Cypripedium reginae ; Orchidaceae), occupant une tourbière de la région du Bas-Saint-Laurent au Québec (Canada), a été suivie annuellement afin de vérifier si elle pouvait être en déclin, notamment à cause d’un boisement consécutif à un drainage et au broutement par le cerf de Virginie (Odocoileus virginianus). Chaque tige de cypripède aperçue de 2009 à 2023 a été marquée, puis ses feuilles et ses fleurs dénombrées. La présence de signes de broutement a aussi été notée. Le nombre de tiges marquées a augmenté de 62 % de 2009 à 2023 alors que le nombre de tiges observées annuellement n’a augmenté que de 10 %. En 2023, 32 % des tiges marquées lors des années précédentes étaient en dormance, ce qui explique la différence. Pour 89 % des tiges, la dormance a duré de 1 à 3 ans. La proportion de tiges avec au moins une fleur est pour sa part demeurée stable de 2009 à 2014 (25-27 %), puis a augmenté en 2015 pour se maintenir entre 34 et 39 % jusqu’en 2023. Il ne semble donc pas y avoir chez cette population d’alternance entre une année avec forte floraison et une année avec faible floraison. Aucun indice ne permet de croire que la population est en déclin. Elle semble bien tolérer l’ombrage et est, jusqu’à ce jour, très peu affectée par le broutement.

Eurasian watermilfoil (Myriophyllum spicatum L.) is an aquatic vascular plant that forms extensive dense beds in lakes. This invader competes with native plants, interferes with aquatic activities, and decreases riparian property values. In Canada, the use of aquatic herbicides is highly restricted. Environmental managers must therefore rely on physical methods such as hand pulling or benthic matting for control. Although these methods are not new, there has been little scientific investigation regarding their effectiveness and cost over multiple years. Benthic matting and hand pulling were used in Lac des Abénaquis (area: 1.2 km²) to control 3.6 ha of M. spicatum beds. Initiated by citizens in 2016, control procedures were scientifically studied in 2020 and 2021. Benthic fiberglass mats were deployed on dense M. spicatum patches for 10 wk. Isolated plants and patches 2 were hand pulled by divers, and the harvested material surfaced via a suction hose or in hand-filled bags. By August 2021, all the M. spicatum patches had been eliminated, and only 560 widely scattered plants remained. Over the last 2 yr of control, hand pulling required 243 person-hours and removed 2,245 kg of biomass. The biomass brought to the surface was 2.4 times higher per person-hour with the suction system than with bags. The use of 1,000 m² of benthic mats required 47 to 51 person-hours per summer season, including installation, removal, and maintenance. Intensive management (years 1 to 5) using benthic mats and hand pulling cost an estimated Can$185,000 (US$140,000) ha⁻¹ of M. spicatum bed. Hand pulling of scattered individuals (years 6+), estimated at Can$20,000 (US$15,000) per summer, is essential to avoid reinfestation. An invasion of M. spicatum can successfully be managed in small lakes without herbicides, but control remains a costly and long-term endeavor.

Japanese knotweed (Reynoutria japonica Houtt.) is an invasive Asian plant abundant along rivers in its introduced range. In riparian areas, floods and ice flows uproot the rhizomes, facilitating their dissemination downstream. Control of large, well-established R. japonica clones in riparian areas is difficult if the use of herbicides is prohibited. An alternative to controlling entrenched clones is the rapid detection and manual unearthing of rhizome fragments that have recently rooted after being deposited by floodwaters. We applied this strategy along a Canadian river where spring floods with abundant ice are recurrent. Two river stretches, with approximately 10 km of shoreline each, were selected for the fragment removal campaign. One of the stretches was heavily invaded by R. japonica, while the other was only sparsely invaded. In the heavily invaded stretch, 1,550 and 737 R. japonica rhizome fragments were unearthed in 2019 and 2020, respectively. Unearthed fragments had an average length of 27–32 cm. Only 21 fragments were found in the sparsely invaded stretch in 2020. Despite similar distances being surveyed, the detection and unearthing took 62% less time (overall) in the sparsely invaded than in the heavily invaded stretch. Along sparsely invaded riverbanks, a rapid response removal campaign for R. japonica cost, including transportation and labor, an estimated Can105) per aborted clone (i.e., fragment removed). A rapid response removal campaign is economically advantageous compared to the hypothetical eradication of large, well-established clones, but to be cost-effective, the time spent locating rhizome fragments must exceed the time spent unearthing them. The question is not whether rapid response unearthing is economically feasible –it is– but rather determining from which invasion level the intervention is practicable. In highly invaded river stretches generating thousands of fragments annually, finding and removing these fragments year after year would require a massive, unsustainable effort.

We present the longest tree-ring chronology to date in northeastern North America (2233 years; 227 BCE – 2005 CE), resulting from several research projects conducted at the subarctic treeline in northern Quebec. This raw chronology of tree-ring width includes 464 black spruce (Picea mariana (Mill.) B.S.P.) shrubs (krummholz) growing in wetlands and preserved within peatlands. An indexed series of 152 erect black spruce trees that have lived in wetlands is also presented, covering the period 216 BCE to 1619 CE. We compared these chronologies to a tree-ring series including 116 black spruce trees and krummholz having grown on well-drained lichen woodlands over the period 1304–2000 CE. These chronologies highlight the major climatic periods of the last two millennia. Floating chronologies dating from 2500 to 3500 years ago were also developed from trees preserved in frozen peat. Growth rings from this period are much wider than those of the last 2233 years, suggesting warm climatic conditions and permafrost-free peatlands during the transition from mid- to late Holocene. The three subarctic chronologies presented here underscore the relevance and usefulness of tree growth rings and growth forms as ecological tools to assess the influence of climate on subarctic ecosystems.

L’ objectif de cette étude était de caractériser les communautés forestières du parc national du Mont-Saint-Bruno. Plus particulièrement, nous voulions trouver les sites présentant un caractère unique quant à leur composition floristique et déterminer comment cette unicité était associée à diverses variables environnementales. Nous avons aussi évalué si certaines espèces étaient indicatrices de la présence de bordure. Pour ce faire, nous avons utilisé les données d’un inventaire floristique fait en 2013 comprenant 133 placettes d’ échantillonnage. Au total, 173 taxons ont été identifiés, y compris 21 d’origine exotique et 11 indigènes à statut précaire. Les espèces exotiques étaient en général peu abondantes et peu fréquentes. La richesse et la diversité floristiques étaient plus élevées dans les placettes situées dans les dépressions ou en bas de pente. Les communautés présentes dans 8 placettes se distinguaient significativement des autres par la présence d’espèces de milieux humides. Les espèces indicatrices des bordures étaient soit intolérantes à l’ombre, tolérantes au stress hydrique, ou rudérales. Cette étude montre que les communautés floristiques forestières du parc sont pauvres en espèces exotiques, que les sites plus humides se distinguent de la flore relativement homogène qui caractérise le parc et que les bordures ont un effet sur la composition des communautés végétales.

Aims: We investigated whether biotic homogenization of plant communities occurs over three decades in a small urban protected forest, and what the possible drivers are underlying the observed changes.Location: A 96 ha temperate forest, Montréal, Canada.Methods: We surveyed vegetation and explanatory variables (ecological conditions, disturbances, spatial structure) in 62 plant communities in 1980 and 2011. We collected plant attributes to identify functional groups (FGs). We evaluated changes in beta diversity using tests for homogeneity of multivariate dispersions. We used space–time interaction models to assess changes in FGs spatial structures. We used multivariate analyses to identify relationship between spatial patterns and explanatory variables, and variation partitioning analyses to identify the drivers involved in beta diversity changes.Results: Beta diversity declined only in the herb–shrub communities. About 35% of their FGs had significantly changed their spatial distribution over time. Those that contracted their distribution were mainly composed of plants with wind‐dispersed seeds or with low to intermediate vegetative propagation capabilities, while those that expanded were plants with fleshy fruits or extensive vegetative propagation capabilities. In 1980, communities were structured into small isolated clumps, while in 2011, clumps expanded and coalesced near disturbed areas. In 1980, the spatial assembly processes relied essentially on endogenous factors while in 2011 exogenous factors, such as disturbances, became more influential. Multivariate analyses suggested that edge effect, past agricultural disturbances, and beaver activities facilitated the decrease of beta diversity (homogenization process).Conclusions: The homogenization of herb–shrub communities likely resulted from changes in their clumped spatial structure. FGs with the most adapted attributes to disperse under urban forest conditions were those that expanded spatially. Structural changes seem to have begun with an unpredictable stage governed by dispersal limitation processes, followed by a predictable stage mostly driven by land‐use legacy and disturbances.