Improvement of water use efficiency is a key objective to improve the sustainability of cultivated plants, especially fast growing species with high water consumption like poplar. It is well known that water use efficiency (WUE) varies considerably among poplar genotypes, and it was recently suggested that the use of the mesophyll-to-stomatal conductance ratio (g_m/g_s) would be an appropriate trait to improve WUE. The responses of 7-week-old cuttings of four hybrid poplar clones and one native Balsam poplar (Populus balsamifera L.) to a water stress–recovery cycle were examined to evaluate the relation between the g_m/g_s ratio and transpiration efficiency (TE), a leaf-level component of WUE. A contrasting g_s response to water stress was observed among the five clones, from stomatal closure early on during soil drying up to limited closure in Balsam poplar. However in the hybrids, the decline in g_m was consistently delayed by a few days compared with g_s. Moreover, in the most water use-efficient hybrids, the recovery following rehydration occurred faster for g_m than for g_s. Thus, the delay in the response of g_m to drought and its faster recovery upon rewatering increased the g_m/g_s of the hybrids and this ratio scaled positively with TE. Our results support the use of the g_m/g_s ratio to select genotypes with improved WUE, and the notion that breeding strategies focusing mainly on stomatal responses to soil drying should also look for a strong curvilinearity between net carbon assimilation rate and g_s, the indication of a significant increase in g_m/g_s in the earlier stages of stomatal closure.

Mesophyll conductance (g_m) has been shown to impose significant limitations to net CO₂ assimilation (A) in various species during water stress. Net CO₂ assimilation is also limited by stomatal conductance to water (g_sw), both having been shown to co-vary with leaf hydraulic conductance (K_leaf). Lately, several studies have suggested a close functional link between K_leaf, g_sw, and g_m. However, such relationships could only be circumstantial since a recent study has shown that the response of g_m to drought could merely be an artefactual consequence of a reduced intercellular CO₂ mole fraction (C_i). Experiments were conducted on 8-week-old hybrid poplar cuttings to determine the relationship between K_leaf, g_sw, and g_m in clones of contrasting drought tolerance. It was hypothesized that changes in g_sw and K_leaf in response to drought would not impact on g_m over most of its range. The results show that K_leaf decreased in concert with g_sw as drought proceeded, whereas g_m measured at a normalized C_i remained relatively constant up to a g_sw threshold of ~0.15mol m^–2 s^–1. This delayed g_m response prevented a substantial decline in A at the early stage of the drought, thereby enhancing water use efficiency. Reducing the stomatal limitation of droughted plants by diminishing the ambient CO₂ concentration of the air did not modify g_m or K_leaf. The relationship between gas exchange and leaf hydraulics was similar in both drought-tolerant and drought-sensitive clones despite their contrasting vulnerability to stem cavitation and stomatal response to soil drying. The results support the hypothesis of a partial hydraulic isolation of the mesophyll from the main transpiration pathway.

Humid boreal forests are unique environments characterized by a cold climate, abundant precipitation, and high evapotranspiration. Transpiration ( $E_T$ ), as a component of evapotranspiration (E), behaves differently under wet and dry canopy conditions, yet very few studies have focused on the dynamics of transpiration to evapotranspiration ratio ( $E_T/E$ ) under transient canopy wetness states. This study presents field measurements of $E_T/E$ at the Montmorency Forest, Québec, Canada: a balsam fir boreal forest that receives $\sim 1600$ mm of precipitation annually (continental subarctic climate; Köppen classification subtype Dfc). Half-hourly observations of E and $E_T$ were obtained over two growing seasons using eddy-covariance and sap flow (Granier’s constant thermal dissipation) methods, respectively, under wet and dry canopy conditions. A series of calibration experiments were performed for sap flow, resulting in species-specific calibration coefficients that increased estimates of sap flux density by $34\%\pm 8\%$ , compared to Granier’s original coefficients. The uncertainties associated with the scaling of sap flow measurements to stand $E_T$ , especially circumferential and spatial variations, were also quantified. From 30 wetting–drying events recorded during the measurement period in summer 2018, variations in $E_T/E$ were analyzed under different stages of canopy wetness. A combination of low evaporative demand and the presence of water on the canopy from the rainfall led to small $E_T/E$ . During two growing seasons, the average $E_T/E$ ranged from $35\%\pm 2\%$ to $47\%\pm 3\%$ . The change in total precipitation was not the main driver of seasonal $E_T/E$ variation, therefore it is important to analyze the impact of rainfall at half-hourly intervals.

Multi-criteria decision-making (MCDM) methods provide a framework for addressing sustainable forest management challenges, especially under climate change. This study offers a systematic review of MCDM applications in forest management from January 2010 to March 2024. Descriptive statistics were employed to analyze trends in MCDM use and geographic distribution. Thematic content analysis investigated the appearance of MCDM indicators supplemented by Natural Language Processing (NLP). Factorial Correspondence Analysis (FCA) explored correlations between models and publication outlets. We systematically searched Web of Science (WoS), Scopus, Google Scholar, Semantic Scholar, CrossRef, and OpenAlex using terms such as ‘MCDM’, ‘forest management’, and ‘decision support’. We found that the Analytical Hierarchy Process (AHP) and the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) were the most commonly used methods, followed by the Preference Ranking Organization Method for Enrichment Evaluation (PROMETHEE), the Analytic Network Process (ANP), GIS, and Goal Programming (GP). Adoption varied across regions, with advanced models such as AHP and GIS less frequently used in developing countries due to technological constraints. These findings highlight emerging trends and gaps in MCDM application, particularly for argan forests, emphasizing the need for context-specific frameworks to support sustainable management in the face of climate change.