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Since the mid-1990s, several empirical unsaturated shear strength equations have been proposed for use in geotechnical engineering practice. Most of the equations have been based on information associated with the measured drying soil- water characteristic curve (SWCC) in combination with the effective shear strength parameters of the saturated soil. There have been few studies, however, undertaken on verifying and comparing the suitability of the proposed unsaturated shear strength equations. This study involves consideration of a wide range of possible drying SWCCs plotted as a relationship between the degree of saturation and suction (saturation soil-water characteristic curve (S-SWCC)). The study forms the basis for comparing five methodologies that have been proposed for the estimation of the shear strength of unsaturated soils. The SWCCs are defined by the fitting parameters for the Fredlund and Xing (1994) sigmoidal equation. This study supports the use of unsaturated soil shear strength empirical estimation methods using the S-SWCC anchor points of air-entry value and residual suction of the soil. Key words: unsaturated shear strength, unsaturated soil, soil-water characteristic curve, shear strength function, anchor conditions, air-entry value, residual suction, Introduction Soils founded above the water table have increased shear strength due to negative pore-water pressures or matric suctions. The soil experiences an increase in shear strength upon drying (provided [...]

Soil water saturation affects soil behavior when subjected to external loads and thus buried pipe deformation. To investigate this effect, full-scale tests were conducted on a buried steel pipe with a diameter of 600 mm. The deformed pipe ring transforms from a 'heart' shape into an inclined 'heart' shape due to the load redistribution within the soil after increasing soil water saturation. Then, a three-dimensional finite element model was created and validated based on the experimental data. A parametric study was thereafter performed to investigate the impact of factors including soil water saturation, cover depth, and pipe wall-thickness on buried pipe responses when subjected to external loads. Soil might be simply modeled as a linear elastic material at a large cover depth (>0.6 m) or a water saturation < 80%. Soil plasticity should be considered at a relatively low cover depth ([less than or equal to] 0.6 m) and a high water saturation ([greater than or equal to] 80%). The pipe circumferential stress is inversely proportional to the square of pipe wall-thickness when the pipe-soil stiffness ratio is equal to or greater than 0.13. Key words: X52 steel, static load, saturation, cover depth, wall-thickness, 1. Introduction Steel pipe has been widely used in oil and gas industry because of its outstanding properties of high yield and ultimate strengths, ability to take a load while [...]

The Eucalyptus genus is extensively planted in Brazil for industrial use, and with the expansion of forest frontiers and climate changes, studies are needed on how these changes affect wood density (WD). The aim of this study was to evaluate the effect of some climate variables on WD of an Eucalyptus urophylla clone for a 4-5 year period. WD from trees collected in 12 sites were evaluated. Five growth rings were identified using a magnifying glass, X-ray densitometry, and forest inventory. WD was correlated with temperature (T), precipitation (P), soil water deficit, ring width, and current annual increment. There was no variation in WD in the first 20 months among sites. From that age on, WD was mainly correlated with T (r > 0.6). The 1 [degrees]C increase in T resulted in a WD reduction of 0.014 g [cm.sup.-1], and increases of 10 [m.sup.-3] [ha.sup.-1] in the final volume were related to an increase of 0.004 g [cm.sup.-1] in WD. Researchers and managers should continue to put efforts into broad experimental networks to assess the effects of climate change on the adaptation and wood quality of Eucalyptus clones. Highlights The influence of climate on E. urophylla wood density was greater after the third year of growth. The mean air temperature was the best correlated climatic variable with E. urophylla wood density. The cross-site climatic gradient was more important in wood density variation of E. urophylla than the variability among the years. Key words: wood quality, climate conditions, annual growth, densitometry, forest expansion, 1. Introduction Plant secondary (radial) growth is a dynamic process and related to external factors such as climate and soil (Pillai et al. 2013; Zhang et al. 2014; Mendivelso et [...]

Drought is a costly natural disaster characterized by water shortages that impact water availability, agriculture, ecosystems, and the economy. The driving mechanisms of drought operate on a wide range of spatial scales, from the movement of soil water on a hillslope to global atmospheric circulation. Additionally, drought impacts vary across spatial scales, from drought induced crop stress on a specific agricultural field to widespread continental water shortages. As a result, multiscalar drought monitoring and early warning systems are needed to utilize observational data sets obtained at different spatial scales and to communicate drought impacts to various levels of decision-makers in government and industry. However, scaling must be employed to translate information across scales, either to fix incongruencies in the spatial scale of input data sets or to modify the model output scale. These scaling techniques have several challenges and limitations that hinder drought accuracy and interpretability, such as the Modifiable Areal Unit Problem (MAUP) and increased model uncertainty. This paper reviews the role of spatial scale in drought monitoring and early warning systems, the associated challenges, and techniques to minimize their impact. Finally, this review identifies several knowledge gaps and future directions. Key words: drought, spatial scale, scaling, MAUP. La sécheresse est une catastrophe naturelle coûteuse caractérisée par des pénuries d'eau qui ont un impact sur la disponibilité de l'eau, l'agriculture, les écosystèmes et l'économie. Les mécanismes moteurs de la sécheresse opèrent sur un large spectre d'échelles spatiales, allant du mouvement de l'eau du sol sur une pente de colline àla circulation atmosphérique globale. De plus, les impacts de la sécheresse varient selon les échelles spatiales, allant du stress des cultures induit par la sécheresse dans un champ agricole spécifique aux pénuries d'eau généralisées àl'échelle du continent. Par conséquent, des systèmes de surveillance de la sécheresse et d'alerte précoce multi-scalaires sont nécessaires pour utiliser les ensembles de données d'observation obtenus àdifférentes échelles spatiales et pour communiquer les impacts de la sécheresse àdifférents niveaux de décideurs au gouvernement et dans l'industrie. Cependant, la mise àl'échelle doit être utilisée pour traduire l'information àtravers les échelles, soit pour corriger les incongruités dans l'échelle spatiale des ensembles de données d'entrée, soit pour modifier l'échelle de sortie du modèle. Ces techniques de mise àl'échelle présentent plusieurs défis et limites qui nuisent àla précision et au potentiel d'interprétation de la sécheresse, comme le problème d'agrégation spatiale (MAUP ; Modifiable Areal Unit Problem) et l'incertitude accrue du modèle. Cet article examine le rôle de l'échelle spatiale dans la surveillance de la sécheresse et les systèmes d'alerte précoce, les défis associés, et les techniques pour minimiser leur impact. Enfin, cette synthèse identifie plusieurs lacunes dans les connaissances et les orientations futures. [Traduit par la Rédaction] Mots-clés : sécheresse, échelle spatiale, mise àl'échelle, MAUP., 1. Introduction Drought is a pervasive and insidious natural disaster marked by a prolonged lack of precipitation relative to normal conditions, resulting in complex and costly impacts that permeate a [...]

In situ wetting-drying cycles significantly and repeatedly influence the soil water content in the active zone but not in the steady zone. To investigate effects of in situ wetting-drying cycles on the mechanical behaviour of intact loess, a series of pressure plate tests was carried out with an intact loess sampled from the active and steady zones. Results show that the specimens from the steady zone have higher yield stresses at given suctions and a larger inclination of normalized loading collapse (LC) curve than the specimens from the active zone. As evidenced by the scanning electron microscopy results, the clay particles accumulating around the contacts between silt particles are widely observed in the former specimens but not in the latter specimens. The distribution of clay particles in the former specimens results in a stabilized skeleton with mechanical properties sensitive to suction variations. On the other hand, the maximum collapse potential of the specimen from the steady zone is 37% larger than that of the specimen from the active zone, mainly because of the larger inclination of normalized LC curve owned by the former specimen. Key words: intact loess, compression, collapse, in situ wetting-drying cycles. Les cycles de mouillage-sechage in situ influencent de maniere significative et repetitive la teneur en eau du sol dans la zone active, mais pas dans la zone stable. Pour etudier les effets des cycles de mouillage-sechage in situ sur le comportement mecanique du loess intact, une serie d'essais sur plaque de pression a ete realisee avec un loess intact echantillonne dans les zones actives et stables. Les resultats montrent que les specimens de la zone stable ont des contraintes d'elasticite plus elevees a des suctions donnees et une plus grande inclinaison de la courbe d'effondrement normalisee (LC) que les specimens de la zone active. Comme le montrent les resultats de la microscopie electronique a balayage, les particules d'argile qui s'accumulent autour des contacts entre les particules de limon sont largement observees dans les premiers specimens, mais pas dans les seconds. La repartition des particules d'argile dans les anciens specimens donne lieu a un squelette stabilise dont les proprietes mecaniques sont sensibles aux variations de la succion. D'autre part, le potentiel d'effondrement maximal de l'eprouvette de la zone stable est 37 % plus eleve que celui de l'eprouvette de la zone active, principalement en raison de la plus grande inclinaison de la courbe LC normalisee appartenant a la premiere eprouvette. [Traduit par la Redaction] Mots-cles: loess intact, compression, effondrement, cycles de mouillage-sechage in situ., 1. Introduction Loess is distributed over vast arid and semiarid regions in the People's Republic of China and many other countries (Munoz-Castelblanco et al. 2012; Ng et al. 2018). Active [...]

Black spruce (Picea mariana (Mill.) BSP) is the main tree species growing within peatlands. The difficult growing conditions within peatlands are associated with low individual wood productivity, and only tree species tolerant to high ground water level can survive. We examine the effect of water-saturated soil on the growth potential of black spruce trees and specific adaptation of the root system. Experimental mesocosms were constructed with two drainage regimes (saturated and well-drained soil conditions). We measured biomass, height, and diameter at stem base of 55 black spruce saplings and noted the location of each horizontal root. Black spruce exhibited a very shallow root system located above saturated commercial peat 19 years after the experiment was initiated. Most roots were adventitious in the water- saturated mesocosms. Overall aerial and root biomass accumulation of the black spruce trees growing under saturated soil conditions was significantly lower than that of the well-drained mesocosms. Interestingly, root-shoot ratios were similar across the two drainage regimes. Soil conditions induced adaptation of the root system in black spruce trees, and physiological stress affected the entire individual with lower biomass productivity in all components (stem, branches, root system). However, biomass distribution remained similar to that of trees growing in well-drained mesocosms. Key words: peatland, black spruce, root system, adventitious root, biomass, root-shoot ratio. L'epinette noire (Picea mariana (Mill.) BSP) est la principale espece d'arbre poussant dans les tourbieres. Les conditions de croissance difficiles dans les tourbieres sont associees a une faible productivite du bois, et seules les especes d'arbres tolerantes aux niveaux eleves de la nappe phreatique peuvent survivre. Nous examinons l'effet d'un sol sature en eau sur le potentiel de croissance des epinettes noires et l'adaptation specifique du systeme racinaire. Experimental mesocosms were constructed with two drainage regimes (saturated and well-drained soil conditions). Nous avons mesure la biomasse, la hauteur et le diametre a la base du tronc de 55 gaules d'epinette noire et note l'emplacement de chaque racine horizontale. L'epinette noire presentait un systeme racinaire tres peu profond situe au-dessus de la tourbe commerciale saturee, 19 ans apres le debut de l'experience. La plupart des racines etaient adventives dans les mesocosmes satures d'eau. L'accumulation globale de biomasse aerienne et racinaire des epinettes noires poussant dans des conditions de sol sature etait significativement plus faible que celle des mesocosmes bien draines. Il est interessant de noter que les ratios racinespousses etaient similaires pour les deux regimes de drainage. Les conditions du sol ont entraine une adaptation du systeme racinaire chez l'epinette noire, et le stress physiologique a affecte l'ensemble de la plante avec une baisse de la productivite de la biomasse dans toutes les composantes (tige, branches, systeme racinaire). Cependant, la distribution de la biomasse est restee similaire a celle des arbres poussant dans des mesocosmes bien draines. [Traduit par la Redaction] Mots-cles : tourbiere, epinette noire, systeme racinaire, racine adventive, biomasse, rapport racines-pousses., Introduction Extensive peatlands occur in Canada, accounting for more than 13% of its terrestrial surface. They are mostly located (more than 85%) northward of the 51st parallel in the boreal [...]

Invasive plant species can alter litter decomposition rates through changes in litter quality, environment conditions, and decomposer organisms (microflora and soil fauna), but limited research has examined their direct impact on soil fauna. We assessed the abundance and relative contribution of soil meso- and macrofauna to litter decomposition in a forest invaded by Ligustrum lucidum W.T. Aiton and in a noninvaded forest in a subtropical mountain forest in northwestern Argentina, using litterbags (0.01, 2, and 6 mm mesh size). Additionally, we analyzed the litter quality and soil properties of both forest types. Soil fauna abundance was lower in the invaded forest than in the noninvaded forest. The contribution of soil macrofauna to litter decomposition was important in both forest types, but soil mesofauna contribution was significant only in noninvaded forest. Litter decomposition was significantly faster in the invaded forest, consistent with its higher quality litter compared with the uninvaded forest. Invaded forest had significantly lower litter accumulation, lower soil moisture, and greater soil pH than noninvaded forest. Our results showed that although soil fauna was less abundant and played a less pronounced role in litter decomposition in invaded forest, these changes did not translate into a reduced litter decomposition rate due to the higher quality of litter produced in the invaded forest. Key words: glossy privet, invasive alien species, litter decomposition, soil mesofauna and macrofauna, subtropical montane forest. Les especes vegetales envahissantes peuvent modifier le taux de decomposition de la litiere par le biais de changements dans la qualite de la litiere, les conditions du milieu et les organismes responsables (microflore et faune du sol) de sa decomposition, mais peu de travaux de recherche ont etudie leur impact direct sur la faune du sol. Nous avons evalue l'abondance et la contribution relative de la mesofaune et de la macrofaune a la decomposition de la litiere dans la foret envahie par Ligustrum lucidum W.T. Aiton et la foret non envahie dans une foret subtropicale de montagne du nord-ouest de l'Argentine a l'aide de sacs a litiere (mailles de 0,01, 2 et 6 mm). De plus nous avons analyse la qualite de la litiere et les proprietes du sol dans les deux types de foret. L'abondance de la faune du sol etait plus faible dans la foret envahie que dans la foret non envahie. La contribution de la macrofaune du sol a la decomposition de la litiere etait importante dans les deux types de foret mais la contribution de la mesofaune du sol etait significative seulement dans la foret non envahie. La decomposition de la litiere etait significativement plus rapide dans la foret envahie ou la litiere est de meilleure qualite que dans la foret non envahie. La foret envahie accumulait significativement moins de litiere, avait un sol moins humide et un pH du sol plus eleve que la foret non envahie. Nos resultats montrent que, meme si la faune du sol est moins abondante et joue un role moins important dans la decomposition de la litiere dans la foret envahie, ces changements n'ont pas entrarne de reduction du taux de decomposition etant donne la plus grande qualite de la litiere dans la foret envahie. [Traduit par la Redaction] Mots-cles : troene luisant, especes exotiques envahissantes, decomposition de la litiere, mesofaune et macrofaune du sol, foret subtropicale de montagne., Introduction One of the characteristics of the Anthropocene is that no place seems immune to biological invasions. Invasion by alien species is, in fact, considered one of the main drivers [...]

Stomatal conductance (SC) and chlorophyll concentration (CC) are sensitive to soil matric suction ([psi]). The relationship between these two plant parameters and [psi] in the root zone can help understand the water uptake efficiency and photosynthetic capacity of a plant. This paper aims to quantify the relationships between SC and CC with [psi], for Schefflera arboricola grown in compacted bare silty sand and biochar amended soil (BAS). Plant parameters (SC, CC, leaf area (LA), root length (RL)) and [psi] were regularly monitored in an atmospheric controlled room. The maximum recorded SC was greater in BAS than that in bare soil due to the higher water demand of LA. Based on measured data upon evapotranspiration, two equations are proposed to interpret the effects of [psi] on plant water uptake and photosynthesis. A fitting equation is established to relate normalized SC with [psi] to identify a threshold suction (CNSC; indicator of drought stress resistance). The magnitude of [[psi].sup.t.sub.NSC] is found to be mainly dependent on LA/RL ratio and root ends. Furthermore, a new relationship between CC and [psi] is observed and developed. A decrease in CC at higher [psi] is attributed to leaf senescence and stomatal closure, restricting plant to produce more chlorophyll. Key words: monitoring, partial saturation, suction, vegetation. La conductance stomatique (SC) et la concentration en chlorophylle (CC) sont sensibles a la succion matricielle du sol ([psi]). La relation entre ces deux parametres vegetaux et [psi] dans la zone racinaire peut aider a comprendre l'efficacite de l'absorption d'eau et la capacite photosynthetique d'une plante. Cet article vise a quantifier les relations entre SC et CC avec [psi], pour Schefflera arboricola cultive dans du sable limoneux nu compacte et dans un sol amende au biochar (BAS). Les parametres des plantes (SC, CC, surface foliaire (LA), longueur des racines (RL)) et [psi] ont ete regulierement controles dans une piece a atmosphere controlee. La SC maximale enregistree etait plus grande dans le BAS que dans le sol nu en raison de la demande en eau plus elevee de LA. Sur la base des donnees mesurees sur l'evapotranspiration, deux equations sont proposees pour interpreter les effets de [psi] sur l'absorption d'eau et la photosynthese des plantes. Une equation d'ajustement est etablie pour relier la SC normalisee a [psi] afin d'identifier un seuil de succion ([[psi].sup.t.sub.NSC]; indicateur de la resistance au stress de la secheresse). La magnitude de [[psi].sup.t.sub.NSC] depend principalement du rapport LA/RL et des extremites des racines. En outre, une nouvelle relation entre CC et [psi] est observee et developpee. Une diminution de la CC a un ft plus eleve est attribuee a la senescence des feuilles et a la fermeture des stomates, ce qui limite la production de chlorophylle par la plante. [Traduit par la Redaction] Mots-cles: surveillance, saturation partielle, aspiration, vegetation., Introduction Vegetation as a 'live protection' layer provides hydrological reinforcement for shallow slope stability in landfill covers and bio-engineered slope (Pagano et al. 2019; Liang et al. 2020). Transpiration by [...]

Given the large contribution of forests to terrestrial carbon storage, there is a need to resolve the environmental and physiological drivers of tree-level response to rising atmospheric C[O.sub.2]. This study examines how site-level soil moisture influences growth and intrinsic water-use efficiency in sugar maple (Acer saccharum Marsh.). We construct tree-ring, [[delta].sup.18]O, and [[DELTA].sup.13]C chronologies for trees across a soil moisture gradient in Ontario, Canada, and employ a structural equation modelling approach to ascertain their climatic, ontogenetic, and environmental drivers. Our results support previous evidence for the presence of strong developmental effects in tree-ring isotopic chronologies--in the range of -4.7% for [[DELTA].sup.13]C and +0.8% for [[delta].sup.18]O--across the tree life span. Additionally, we show that the physiological response of sugar maple to increasing atmospheric C[O.sub.2] depends on site-level soil moisture variability, with trees only in relatively wet plots exhibiting temporal increases in intrinsic water-use efficiency. These results suggest that trees in wet and mesic plots have experienced temporal increases in stomatal conductance and photosynthetic capacity, whereas trees in dry plots have experienced decreases in photosynthetic capacity. This study is the first to examine sugar maple physiology using a dendroisotopic approach and broadens our understanding of carbon-water interactions in temperate forests. Key words: tree ring, temperate broadleaf, dendroecology, carbon isotopes, oxygen isotopes, tree physiology, Acer saccharum Marsh. Etant donne l'importance de la contribution des forets au stockage du carbone terrestre, il y a un besoin evident d'identifier les facteurs environnementaux et physiologiques determinants de la reaction a l'augmentation du C[O.sub.2] atmospherique a l'echelle de l'arbre. Cette etude examine de quelle facon l'humidite du sol a l'echelle de la station influence la croissance et l'efficience de l'utilisation de l'eau chez l'erable a sucre (Acer saccharum Marsh.). Nous avons elabore des series dendrochronologiques pour [[delta].sup.18]O et [[DELTA].sup.13]C chez des arbres situes le long d'un gradient d'humidite du sol en Ontario, au Canada, et nous avons utilise une approche de modelisation par equation structurelle pour determiner leurs facteurs climatiques, ontogeniques et environnementaux determinants. Nos resultats supportent les indices anterieurs de la presence d'effets marques lies au developpement dans les chronologies isotopiques, dans l'ordre de -4,7% pour [[DELTA].sup.13]C et +0,8% pour [[delta].sup.18]O, pour la duree de la vie. De plus, nous montrons que la reaction physiologique de l'erable a sucre a l'augmentation du C[O.sub.2] atmospherique depend de la variation de l'humidite du sol a l'echelle de la station alors que seuls les arbres situes dans les parcelles relativement humides montrent des augmentations temporelles de l'efficience de l'utilisation de l'eau. Ces resultats indiquent que les arbres situes dans les parcelles hydriques et mesiques ont connu des augmentations temporelles de la conductance stomatique et de la capacite photosynthetique, tandis que les arbres dans les parcelles xeriques ont connu des diminutions de la capacite photosynthetique. Il s'agit de la premiere etude qui examine la physiologie de l'erable a sucre a l'aide d'une approche dendroisotopique et elle permet d'elargir notre comprehension des interactions entre le carbone et l'eau dans les forets temperees. [Traduit par la Redaction] Mots-cles : cernes annuels, feuillu tempere, dendroecologie, isotopes du carbone, isotopes de l'oxygene, physiologie de l'arbre, Acer saccharum Marsh., 1. Introduction Increasing atmospheric carbon dioxide ([C.sub.a]) is a unique component of global environmental change in that its influence on terrestrial ecosystems is ubiquitous. Since photosynthesis of C3 plants is [...]

Although it is clear that gap thinning significantly increases the soil water content (SWC) of the topsoil, less is known about whether and how this treatment affects deeper layers. From December 2008 to April 2012, we monitored the SWC at depths of 10, 20, 30, 45, 60, and 90 cm in gap creation treatments (small gap size of 30 [m.sup.2], intermediate gap size of 80 [m.sup.2], and unthinned plots) in a typical pine plantation in the eastern Tibetan Plateau. Among gap treatments, differences in SWC and its coefficient of variation (CV) at each depth and the soil water content proportion (SWCP) of the whole soil profile at specific depths were compared. Gap thinning improved SWC and decreased the CV at each depth. The SWCPs in thinned plots were lower at depths from 10 to 30 cm compared with unthinned plots but higher at depths of 45 and 60 cm. Also, in each season, the patterns were similar to the general results. In conclusion, gap thinning improves the SWC, changes the vertical soil water distribution, and decreases the SWC heterogeneity. The soil water conditions in intermediate gaps are more appropriate for local forest restoration. Key words: gap size, precipitation, season, soil layer, understory plant. Bien qu'il soit clair que l'eclaircie par trouees augmente significativement la teneur en eau du sol (TES) de la couche superficielle du sol, les effets de ce traitement sur les horizons du sol plus profonds sont moins bien connus. De decembre 2008 a avril 2012, nous avons mesure la TES a des profondeurs de 10, 20, 30, 45, 60 et 90 cm dans des traitements visant a creer des trouees (petites trouees : 30 [m.sup.2]; trouees intermediaires : 80 [m.sup.2]; et placettes non eclaircies) dans une plantation de pin representative du plateau tibetain oriental. Ces traitements ont fait l'objet d'une comparaison de la TES, du coefficient de variation de la TES (CV) a chaque profondeur, et de la proportion de la teneur en eau du sol (PTES) par rapport a celle du profil de sol complet a des profondeurs specifiques. L'eclaircie par trouees a ameliore la TES et a diminue le CV a chaque profondeur. La PTES des placettes eclaircies etait plus faible aux profondeurs de 10 a 30 cm par rapport aux placettes non eclaircies, mais plus elevee aux profondeurs de 45 et 60 cm. De plus, ce type de resultats qui est semblable aux resultats generaux a ete observe a chaque saison. En conclusion, l'eclaircie par trouees ameliore la TES, modifie la distribution verticale de l'eau du sol et diminue l'heterogeneite de la TES. Les conditions d'eau du sol dans les trouees de taille intermediaire sont plus appropriees pour la restauration des forets locales. [Traduit par la Redaction] Mots-cles : taille des trouees, precipitation, saison, couche du sol, plantes de sous-bois., Introduction Forest gap thinning is an effective protocol to improve understory environmental conditions, thereby contributing to seedling establishment and increasing species diversity (Muscolo et al. 2014). Soil water content (SWC) [...]

The genus Drosera (sundews) is represented in British Columbia (BC), Canada, by Drosera rotundifolia (L.), Drosera anglica (Huds.), and their hybrid Drosera x obovata (Mert. & W.D.J. Koch). All three can be found in Sphagnum bogs of central BC, including those within the Aleza Lake Research Forest (ALRF) located 60 km east of Prince George. Vegetation patterns in bogs are known to be correlated with light, water, and nutrient gradients, and despite information being available on the influence of light and water on Drosera occurrence, little information is known about the role of nutrients. Here, we focused on a bog containing all three Drosera species, to determine whether nutrient levels are related to the abundance of the widespread species, D. rotundifolia. Univariate regression tree analysis between soil water chemistry and D. rotundifolia numbers indicates that D. rotundifolia is a calcifuge, preferring moderately acidic soil pH (>5.5) and relatively low calcium levels ( Key words: Drosera rotundifolia, Drosera anglica, calcium, Sphagnum peatlands, carnivorous plants. Le genre Drosera (<>) est represente en Colombie-Britannique (CB), au Canada, par Drosera rotundifolia (L.), Drosera anglica (Huds.) et leur hybride Drosera x obovata (Mert. & W.D.J. Koch). Tous trois peuvent se trouver dans les tourbieres du centre de la CB, incluant les tourbieres de la foret d'enseignement et de recherche d'Aleza Lake situee a 60 km a l'est de Prince George. Les patrons de vegetation des tourbieres sont connus pour etre correles avec les gradients de lumiere, d'eau et de nutriments, et malgre le fait que l'information sur l'influence de la lumiere et de l'eau sur la presence de Drosera soit disponible, le role des nutriments est peu connu. Les auteurs se concentrent ici sur une tourbiere comportant les trois especes de Drosera afin de determiner si les niveaux de nutriments sont relies a l'abondance de l'espece repandue D. rotundifolia. L'analyse d'un arbre de regression multivariable entre la chimie de l'eau du sol et le nombre d'individus indique que D. rotundifolia est un calcifuge, preferant les sols a pH moderement acides (5,5) et a niveaux de calcium relativement faibles ( Mots-cles: Drosera rotundifolia, Drosera anglica, calcium, tourbieres, plantes carnivores., Introduction Members of the sundew genus Drosera represent a number of scientifically important species. Widely known for their carnivorous habit, sundews are key models for ecological processes such as plant [...]

Most hypotheses about controls over high-altitude forests, including treeline, the elevation for upright woody plants, or timberline, the upper elevation for aggregated forest, suggest that low temperature drives forest dynamics, either through effects on cell division and tree growth or indirectly through frost damage or nutrient availability. However, abiotic factors other than temperature, including water availability, may serve as other important controls at high elevations, particularly for seedlings. To test the hypothesis that the timing and amount of precipitation exerts a strong control over the high-elevation forest boundary on the Wasatch Plateau in central Utah, USA, we conducted a field experiment that manipulated water availability and monitored photosynthesis, growth, and survivorship in Picea engelmannii Parry ex Engelm. seedlings. Survivorship increased from the driest to the wettest conditions, whereas the timing of precipitation did not explain differences in survival. However, we found that large, infrequent rain events increased maximum photosynthetic flux density compared with small, frequent rain events. Our results highlight the potential role of growing season water availability in limiting timberline expansion below the low-temperature thermal limits of P. engelmannii. As a consequence, the infilling of trees below the treeline in this region in response to climate change is likely to be episodic and driven by multiyear periods of high water availability and frequency that overcome drought limitations. Key words: treeline, hydrology, Picea engelmannii, photosynthesis, survivorship. La plupart des hypotheses concernant les controles sur les forets de haute altitude, incluant la limite des arbres, l'altitude a laquelle on retrouve encore des plantes ligneuses dressees, ou la limite de vegetation des arbres, l'altitude superieure a laquelle il y a encore de la foret, indiquent que la basse temperature determine la dynamique forestiere, soit en ayant des effets sur la division cellulaire et la croissance des arbres, soit indirectement a cause des dommages dus au gel ou de la disponibilite des nutriments. Cependant, des facteurs abiotiques autres que la temperature, incluant la disponibilite en eau, pourraient etre d'importants facteurs alternatifs de controle a haute altitude, particulierement pour les semis. Pour tester l'hypothese que le moment et la quantite de precipitation exercent un controle significatif sur la limite de la foret a haute altitude sur le plateau des Wasatch dans le centre de l'Utah, aux Etats-Unis, nous avons realise une experience sur le terrain dans laquelle nous avons manipule la disponibilite en eau et suivi la photosynthese, la croissance et la survie des semis de l'epicea d'Engelmann (Picea engelmannii Parry ex Engelm.). La survie augmentait en passant des conditions les plus seches aux plus humides tandis que le moment des precipitations n'expliquait pas les differences dans la survie. Cependant, nous avons trouve que des episodes peu frequents de forte pluie ont augmente la densite du flux photosynthetique maximum comparativement a des episodes frequents de faible pluie. Nos resultats font ressortir le role potentiel de la disponibilite en eau durant la saison de croissance qui limiterait l'expansion de la limite de vegetation des arbres en deca des limites thermiques de temperature minimale de l'epicea d'Engelmann. En consequence, le recrutement des arbres en deca de la limite des arbres dans cette region en reponse au changement climatique sera probablement episodique et determine par des periodes de plusieurs annees de disponibilite en eau elevee et a une frequence qui surmonte les restrictions imposees par la secheresse. [Traduit par la Redaction] Mots-cles: limite des arbres, hydrologie, Picea engelmannii, photosynthese, survie., Introduction Renewed interest in upper treeline dynamics is, in part, driven by the hope that these systems will help us understand how ecosystems will respond to climate change (Harsch et [...]

Mechanical mastication is increasingly used as a fuel management treatment to reduce fire risk at the wildland-urban interface, although ignition and fire behaviour in these novel fuel beds are poorly understood. We investigated the influence of observed fuel moisture content, wind speed, and firebrand size on the probability of sustained flaming of masticated fuel beds under both laboratory and field settings. Logistic regression techniques were applied to assess the probability of sustained flaming in both datasets. Models for the field were also developed using estimated moisture from three sets of weather- based models: (i) the hourly Fine Fuel Moisture Code (FFMC) from the Canadian Forest Fire Weather Index System, (ii) the National Fire Danger Rating System (NFDRS) moisture estimates for 1 h and 10 h fuels, and (iii) a masticated surface fuel moisture model (MAST). In both laboratory and field testing, the likelihood of a successful ignition increased with decreasing moisture content and increasing wind speed; the effect of firebrand size was only apparent in laboratory testing. The FFMC, NFDRS, and MAST predictions had somewhat reduced discriminative power relative to direct moisture in predicting the probability of sustained flaming based on our field observations. Our results speak to the disparity between the fire behaviour modeling that occurs in the laboratory and the fire behavior modeling that occurs in the field, as the methodology permitted comparison of predictions from sustained flaming models that were developed for one experimental setting and applied to the other. Key words: ignition probability, fire behaviour, fuel management, moisture content, mulch. Le broyage mecanique est de plus en plus utilise comme traitement pour la gestion des combustibles dans le but de reduire les risques d'incendie en zone periurbaine bien que l'allumage et le comportement du feu dans ces nouveaux lits de combustibles soient mal compris. Nous avons examine l'influence de la teneur en humidite observee des combustibles, la vitesse du vent et la taille des tisons sur la probabilite d'une inflammation soutenue des lits de combustibles broyes tant en laboratoire que sur le terrain. Des techniques de regression logistique ont ete appliquees pour evaluer la probabilite d'une inflammation soutenue dans les deux ensembles de donnees. Des modeles de terrain ont aussi ete developpes a partir de la teneur en humidite estimee par trois series de modeles fondes sur les conditions meteorologiques : (i) l'indice horaire du combustible leger (FFMC) de la methode canadienne de l'indice foret-meteo; (ii) les estimations de la teneur en humidite du systeme national d'evaluation du danger d'incendie (NFDRS) pour les combustibles 1 et 10 h; et (iii) un modele de la teneur en humidite des combustibles de surface broyes (MAST). Tant en laboratoire que sur le terrain, la probabilite qu'un allumage reussisse augmentait avec la diminution du contenu en humidite et la l'augmentation de la vitesse du vent; l'effet de la taille des tisons etait apparent seulement dans les essais en laboratoire. Les predictions faites sur la base de la teneur en humidite estimee par les modeles FFMC, NFDRS et MAST avaient un pouvoir de discrimination quelque peu reduit comparativement a la teneur en humidite mesuree directement pour predire la probabilite d'une inflammation soutenue selon nos observations sur le terrain. Nos resultats font etat de la disparite entre la modelisation du comportement du feu en laboratoire et sur le terrain etant donne que la methodologie a permis de comparer les predictions de modeles d'inflammation soutenue qui ont ete developpes pour un contexte experimental donne et appliques a un autre. [Traduit par la Redaction] Mots-cles : probabilite d'allumage, comportement du feu, gestion du feu, teneur en humidite, paillis., Introduction The conditions under which a fuel will successfully ignite and spread are of critical importance in fire danger rating, fire behaviour modelling, and fire management operations. Models of ignition [...]

The objective of this study was to determine if the stand-level soil moisture regime had a significant effect on the reduction in black spruce (Picea mariana (Mill.) B.S.P.) radial growth during the most recent spruce budworm (Choristoneura fumiferana Clem.) outbreak in the boreal forest region of northeastern Ontario. We collected a stratified random sample of co-dominant black spruce trees from three moisture regimes and compared the reduction of radial growth during a spruce budworm outbreak between dry, moist, and wet stands. We focused on the most recent outbreak from 1975-1987, which we dated by dendrochronological analysis of black spruce increment cores from the Romeo Malette Forest near Timmins, Ontario. Samples collected from dry and moist sites showed significantly greater maximum radial growth reduction than those from wet sites. Mean growth reduction over the entire outbreak was not significantly different among moisture regimes but followed the same trend. We found no evidence of spatial autocorrelation in the growth reduction response, suggesting that the moisture effect was not confounded by location. Resume: L'objectif de cette etude consistait a determiner si le regime d'humidite du sol a l'echelle du peuplement a eu un effet significatif sur la reduction de la croissance radiale de l'epicea noir (Picea mariana (Mill.) B.S.P.) durant la plus re-cente epidemie de la tordeuse des bourgeons de l'epinette (Choristoneura fumiferana Clem.) dans la zone de foret boreale du nordest de l'Ontario. Nous avons realise un echantillonnage aleatoire stratifie des tiges codominantes d'epicea noir associees a trois regimes d'humidite du sol et nous avons compare la reduction de croissance radiale durant une epidemie de la tordeuse des bourgeons de l'epinette entre des stations seches, fraiches et mouilleuses. Nous avons mis l'accent sur l'epidemie la plus recente que nous avons datee de 1975 a 1987 a la suite de l'analyse dendrochronologique de carottes d'epicea noir provenant de la foret Romeo Malette pres de Timmins en Ontario. Les echantillons montraient une reduction de la croissance radiale maximale significativement plus grande dans les stations seches et fraiches que dans les stations mouilleuses. La reduction de la croissance moyenne pour toute la duree de l'epidemie ne differait pas significativement selon le regime d'humidite mais suivait la meme tendance. Nous n'avons trouve aucun indice d'autocorrelation spatiale dans la reponse de reduction de croissance, indiquant que l'effet de l'humidite n'etait pas masque par la localisation. [Traduit par la Redaction], Introduction Eastern spruce budworm (Choristoneura fumiferana Clem.) influences the dynamics of the eastern boreal forest, with periodic outbreaks causing large-scale damage across the landscape and sometimes compromising the supply of [...]

To better understand the availability of plant resources on the forest floor in the Amazon and also to show the effect of their heterogeneity on tree seedlings, I described the spatial and temporal variation of light and soil water along a 100 m transect in a terra firme forest for 6 months and recorded responses of three tree species planted on that transect after 1 year's growth. I found that (i) the spatial heterogeneity across the transect was greater than the temporal heterogeneity at any given microsite on the transect for both light and water and there was a positive correlation between them, (ii) Couepia obovata Ducke, the largest seeded and the only subcanopy tree, survived the best and showed both the largest relative height growth rate (RHGR) and the largest specific leaf area (SLA), while among the two early successional trees, Tapirira guianensis J.B. Aublet had the largest leaf area ratio (LAR) and the largest leaf mass ratio (LMR) and Duguethia spixiana Mart. had the largest root to shoot ratio (RTOS), (iii) for T. guianensis, SLA increased with increasing light and soil water potential predicted both increasing LMR and decreasing RTOS with increasing soil water, and (iv) soil water potential could also predict increasing LAR with increasing water for D. spixiana and, for C. obovata, soil water potential predicted more survivorship, LMR, and RHGR but less RTOS, all with increasing soil water. I conclude that some subcanopy trees may survive and grow more than open-canopy trees when presented with water stress in the forest understory and that within the ranges of light and soil water sampled here, plants responded more to spatial variation in water compared with light. Resume: Afin de mieux comprendre la disponibilite des ressources pour les plantes dans la couverture morte de la foret amazonienne et de montrer l'effet de leur heterogeneite sur les semis d'arbre, j'ai decrit la variation spatiale et temporelle de la lumiere et de l'eau du sol le long d'un transect de 100 m dans la foret de terra firme sur une periode de 6 mois. J'ai aussi note les reactions de trois especes d'arbre plantees dans ce transect apres 1 annee de croissance. J'ai trouve que (i) l'heterogeneite spatiale etait plus grande que l'heterogeneite temporelle dans n'importe quel microsite le long du transect dans le cas de la lumiere et de l'eau sol et qu'il y avait une correlation positive entre les deux, (ii) Couepia obovata Ducke, le plus gros arbre ensemence et le seul arbre de sous-bois, a le mieux survecu et avait a la fois le plus fort taux relatif de croissance en hauteur (TRCH) et la plus grande surface foliaire specifique (SFS), tandis que parmi les deux essences pionnieres, Tapirira guianensis J.B. Aublet avait les plus grands rapports de surface foliaire (RSF) et de masse foliaire (RMF) et Duguetia spixiana Mart. avait le plus grand rapport racine a tige (RRT), (iii) dans le cas du T. guianensis, la SFS augmentait avec l'augmentation de la lumiere; le potentiel hydrique du sol predisait a la fois l'augmentation du RMF et la diminution du RRT avec l'augmentation de l'eau du sol et (iv) le potentiel hydrique du sol pouvait egalement predire l'augmentation du RSF avec l'augmentation de l'eau du sol dans le cas de D. spixiana et, dans le cas de C. obovata, le potentiel hydrique du sol predisait une augmentation de la survie, du RMF et du TCRH, mais une diminution du RRT, dans tous les cas a la suite d'une augmentation de l'eau du sol. Je conclus que certains arbres de sous-bois peuvent survivre et croitre plus que les arbres dans un couvert clairseme lorsqu'ils sont soumis a un stress hydrique en sous-etage, et que dans les gammes de valeurs de disponibilite de la lumiere et de l'eau du sol echantillonnees dans le cadre de cette etude les plantes reagissent davantage a la variation spatiale de la disponibilite de l'eau qu'a celle de la lumiere, contrairement a l'opinion traditionnelle selon laquelle la lumiere est la ressource la plus limitative dans les forets tropicales. [Traduit par la Redaction], Introduction The Amazon River basin contains the world's largest tropical rainforest (Walter 1973; Myster 2009). Researchers have historically regarded this forest as a shifting mosaic (Watt 1947) composed of patches [...]

We studied the effect of water availability on basal area growth and wood properties of 11-year-old loblolly pine (Pinus taeda L.) trees from contrasting Florida (FL) (a mix of half-sib families) and South Carolina coastal plain (SC) (a single, half-sib family) genetic material. Increasing soil water availability via irrigation increased average whole-core specific gravity (SG) and latewood percentage (LW%) by 0.036 and 6.93%, respectively. Irrigation did not affect late-wood SG or wood stiffness, but irrigated FL and SC trees had more latewood due to a 29 day longer growing season. Irrigation did not affect the length of corewood production, but irrigated trees had earlier transition ages, producing outerwood ~3 years before rainfed trees. The increase in whole-core SG and LW% was moderate because irrigation promoted earlywood growth in corewood formed before canopy closure, but after year 7, rain-fed and irrigated trees had similar earlywood growth but irrigated trees had more latewood growth, increasing ring SG and LW%. The SC half-sib family had higher SG and greater LW% than trees from FL independent of irrigation due to greater yearly latewood growth. Thus, absence of soil water stress extended seasonal diameter cessation date but did not change latewood SG or wood stiffness. Resume: Nous avons etudie l'effet de la disponibilite en eau sur la croissance en surface terriere et les proprietes du bois de tiges de pin a encens (Pinus taeda L.) agees de 11 ans et dont le materiel genetique provenait soit de la Floride (FL) (un melange de descendances uniparentales), soit de la plaine cotiere de la Caroline du Sud (CS) (une seule descendance uniparentale). L'augmentation de la disponibilite en eau par l'irrigation a augmente la gravite specifique (GS) moyenne du bois de caeur et le pourcentage de bois final (BF) de respectivement 0,036 et 6,93%. L'irrigation n'a influence ni la GS du bois final ni la rigidite du bois, mais les tiges FL et CS irriguees avaient plus de bois final parce que leur saison de croissance durait 29 jours de plus. L'irrigation n'a pas influence la duree de la production de bois de caeur mais l'age de transition arrivait plus tot chez les arbres irrigues qui produisaient du bois d'aubier environ trois ans avant les arbres alimentes en eau par la pluie. L'augmentation de la GS et du pourcentage de BF du bois de caeur etait moderee parce que l'irrigation favorisait la croissance du bois initial dans le bois de caeur forme avant la fermeture du couvert. Mais apres sept ans, la croissance du bois initial etait semblable chez les arbres alimentes en eau par la pluie et les arbres irrigues alors que la croissance du bois final etait plus prononcee chez les arbres irrigues, ce qui augmentait la GS et le pourcentage de BF des cernes. La descendance uniparentale de la CS avait une GS et un pourcentage de BF plus eleves que les arbres de la FL in-dependamment de l'irrigation a cause d'une plus forte croissance annuelle du bois final. Par consequent, l'absence de stress hydrique a repousse la date de la croissance saisonniere en diametre mais n'a change ni la GS du BF, ni la rigidite du bois. [Traduit par la Redaction], Introduction The southeastern United States has more than 13 million ha of southern pine plantations (Fox et al. 2007) from which about 58% of total US timber is harvested (Jordan [...]

Early in the establishment of Pacific Northwest Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) plantations, herbaceous vegetation can decrease seedling growth through competition for soil moisture during the dry summer months. This study was designed to statistically quantify soil moisture, seedling xylem water potential ([psi]), vegetation community, and seedling growth response to six herbicide treatment regimes commonly applied over the first 2 years of establishment. When compared with the control, soil moisture and seedling kP increased in response to reductions in competitive cover, al lowing seedlings to extend productive growing time from 28 to 80 days. As a result, seedling volume growth increased from 56 [cm.sup.3] in the untreated control to greater than 250 cm 3 for the most intensive herbicide treatment regimes. Vegetation surveys revealed that treatment regimes had the potential to provide a disturbance, which could shift community composition from native to introduced species as the relationship decreased from 10:1 to 2:1. The most intense herbicide treatment regime reduced cover below 20%, retained soil moisture >30%, maintained predawn seedling kP above -1.0 MPa, and decreased height to diameter ratio below 50, increasing the likelihood of successful plantation establishment. Toot, lors de l'etablissement des plantations de douglas de Menzies (Pseudotsuga menziesii (Mirb.) Franco) dans le Pacific Northwest, la vegetation herbacee peut reduire la croissance des semis a cause de la competition pour l'humidite du sol pendant les mois secs d'ete. Cette etude a ete concue pour quantifier statistiquement l'humidite du sol, le potentiel hydrique ([psi]) du xyleme des semis, la communaute vegetale et la reaction en croissance des semis a six regimes de traitement avec des herbicides communement appliques pendant les deux premieres annees d'etablissement. Comparativement au temoin, l'humidite du sol et le potentiel hydrique des semis ont augmente en reaction a la reduction de la competition, ce qui a permis aux semis d'allonger leur periode de croissance productive de 28 a 80 jours. La croissance en volume des semis a par consequent augmente de 56 [cm.sup.3] dans le cas du temoin non traite a plus de 250 [cm.sup.3] dans le cas du regime le plus intensif de traitement avec des herbicides. Les inventaires de vegetation ont revele que les regimes de traitement avaient la possibilite de provoquer une perturbation qui pouvait modifier la composition vegetale en favorisant le remplacement d'especes indigenes par des especes introduites etant donne que le rapport entre les deux groupes d'especes est passe de 10 : 1 a 2 : 1. Le traitement le plus intensif avec des herbicides a reduit le couvert en deca de 20 %, a conserve l'humidite du sol au-dessus de 30 %, a maintenu le potentiel hydrique de base des semis au-dessus de -1,0 MPa et a diminue le rapport entre la hauteur et le diametre a moins de 50, augmentant ainsi les chances que l'etablissement de la plantation soit un succes. [Traduit par la Redaction], Introduction Harvesting merchantable trees causes disturbance to both the forest canopy and floor, thereby, changing the growing conditions as well as the structure and composition of the remaining vegetation community. [...]