The Influence of Recent Climate Change on Tree Height Growth Differs with Species and Spatial Environment

Tree growth has been reported to increase in response to recent global climate change in controlled and semi-controlled experiments, but few studies have reported response of tree growth to increased temperature and atmospheric carbon dioxide (CO2) concentration in natural environments. This study addresses how recent global climate change has affected height growth of trembling aspen (Populus tremuloides Michx) and black spruce (Picea mariana Mill B.S.) in their natural environments. We sampled 145 stands dominated by aspen and 82 dominated by spruce over the entire range of their distributions in British Columbia, Canada. These stands were established naturally after fire between the 19th and 20th centuries. Height growth was quantified as total heights of sampled dominant and co-dominant trees at breast-height age of 50 years. We assessed the relationships between 50-year height growth and environmental factors at both spatial and temporal scales. We also tested whether the tree growth associated with global climate change differed with spatial environment (latitude, longitude and elevation). As expected, height growth of both species was positively related to temperature variables at the regional scale and with soil moisture and nutrient availability at the local scale. While height growth of trembling aspen was not significantly related to any of the temporal variables we examined, that of black spruce increased significantly with stand establishment date, the anomaly of the average maximum summer temperature between May-August, and atmospheric CO2 concentration, but not with the Palmer Drought Severity Index. Furthermore, the increase of spruce height growth associated with recent climate change was higher in the western than in eastern part of British Columbia. This study demonstrates that the response of height growth to recent climate change, i.e., increasing temperature and atmospheric CO2 concentration, did not only differ with tree species, but also their growing spatial environment

Overstory influences on light attenuation patterns and understory plant community diversity and composition in southern boreal forests of Quebec

We have characterized overstory light transmission, understory light levels, and plant communities in mixedwood boreal forests of northwestern Quebec with the objective of understanding how overstory light transmission interacts with composition and time since disturbance to influence the diversity and composition of understory vegetation, and, in turn, the further attenuation of light to the forest floor by the understory. Overstory light transmission differed among three forest types (aspen, mixed deciduous-conifer, and old cedar-dominated), with old forests having higher proportions of high light levels than aspen and mixed forests, which were characterized by intermediate light levels. The composition of the understory plant communities in old forests showed the weakest correlation to overstory light transmission, although those forests had the largest range of light transmission. The strongest correlation between characteristics of overstory light transmission and understory communities was found in aspen forests. Species diversity indices were consistently higher in aspen forests but showed weak relationships with overstory light transmission. Light attenuation by the understory vegetation and total height of the understory vegetation were strongly and positively related to overstory light transmission but not forest type. Therefore, light transmission through the overstory influenced the structure and function of understory plants more than their diversity and composition. This is likely due to the strong effect of the upper understory layers, which tend to homogenize light levels at the forest floor regardless of forest type. The understory plant community acts as a filter, thereby reducing light levels at the forest floor to uniformly low levels

Variation of Maximum Tree Height and Annual Shoot Growth of Smith Fir at Various Elevations in the Sygera Mountains, Southeastern Tibetan Plateau

Little is known about tree height and height growth (as annual shoot elongation of the apical part of vertical stems) of coniferous trees growing at various altitudes on the Tibetan Plateau, which provides a high-elevation natural platform for assessing tree growth performance in relation to future climate change. We here investigated the variation of maximum tree height and annual height increment of Smith fir (Abies georgei var. smithii) in seven forest plots (30 m×40 m) along two altitudinal transects between 3,800 m and 4,200/4,390 m above sea level (a.s.l.) in the Sygera Mountains, southeastern Tibetan Plateau. Four plots were located on north-facing slopes and three plots on southeast-facing slopes. At each site, annual shoot growth was obtained by measuring the distance between successive terminal bud scars along the main stem of 25 trees that were between 2 and 4 m high. Maximum/mean tree height and mean annual height increment of Smith fir decreased with increasing altitude up to the tree line, indicative of a stress gradient (the dominant temperature gradient) along the altitudinal transect. Above-average mean minimum summer (particularly July) temperatures affected height increment positively, whereas precipitation had no significant effect on shoot growth. The time series of annual height increments of Smith fir can be used for the reconstruction of past climate on the southeastern Tibetan Plateau. In addition, it can be expected that the rising summer temperatures observed in the recent past and anticipated for the future will enhance Smith fir's growth throughout its altitudinal distribution range

Spatial variability of humus forms in some coastal forest ecosystems of British Columbia

The spatial variability of 5 humus form properties (thickness, acidity, total C, total N and mineralizable-N) was examined in 3 coastal forest sites of different tree species composition (western hemlock, Douglas-fir and western redcedar), humus forms, and ecological site quality using variogram and kriging. Humus form properties were found spatially dependent and the kriging interpolation between sample locations unbiased for all 5 properties and in all 3 sites. The overall range of spatial dependence ranged from 46 to 1 251 cm, but varied with property and site. The average range for the humus form properties increased from 109 cm (total N) to 704 cm (mineralizable-N), and that for the sites increased from 275 cm (western hemlock) to 581 cm (Douglas-fir). It appears that humus forms in each site occur in polygons with the lateral dimension ranging from 100 to 700 cm. The spatial pattern of each property in each site was portrayed in contour maps.Variabilité spatiale des types d'humus dans quelques écosystèmes forestiers côtiers de Colombie britannique. La variabilité spatiale de 5 caractéristiques de l'humus (épaisseur, acidité, carbone total, azote total et minéralisable) a été étudiée dans 3 sites forestiers côtiers, différant par l'espèce dominante (pruche de l'Ouest, douglas et thuya géant), le type d'humus et le type de station. Elle est analysée par variogramme et krigeage. Ces propriétés des types d'humus sont dépendantes spatialement, et l'interpolation par krigeage entre les points d'échantillonnage est non biaisée pour les 5 propriétés et les 3 sites. La portée globale de dépendance spatiale varie de 46 à 1 251 cm, mais dépend de la propriété considérée et du site. La portée moyenne pour les propriétés de l'humus varie entre 109 cm (pour l'azote total) à 704 cm (pour l'azote minéralisable), et cella des sites varie entre 275 cm (sous pruche de l'Ouest) à 581 cm (sous douglas). Il apparaît que les types d'humus dans chaque site sont groupés en polygones dont la dimension varie entre 100 et 700 cm. La variabilité spatiale de chaque propriété dans chaque site est illustrée par des cartes obtenues par krigeage

Site-specific height curves for white spruce (Picea glauca [Moench] Voss) stands based on stem analysis and site classification

Polymorphic height curves have been widely used to predict dominant stand height from site index or any known pair of height and age. To provide an alternative to this conventional approach, height modelling was linked to site classification using stem analysis and site data obtained from 102 naturally established white spruce (Picea glauca [Moench] Voss) stands in the Sub-Boreal Spruce zone of British Columbia. The study stands were stratified according to their soil moisture, aeration and nutrient regimes, and a site-specific height curve was developed for each of the 7 delineated groups without using site index as a predictor. Although less precise, the curves developed were comparable to the conventional height curves that use site index as a predictor. Testing against independent data indicated that the site-specific height curves were reliable and applicable over a large area of the sub-boreal forest for predicting dominant heights of white spruce stands.Courbe de croissance en hauteur de l'épinette blanche (Picea glauca [Moench] Voss) par l'utilisation de données d'analyse de tige et de typologie des stations. L'utilisation de courbes polymorphes de croissance en hauteur est très courante pour prédire la hauteur dominante d'un peuplement connaissant un indice de fertilité ou un couple hauteur-âge. Nous proposons une alternative à cette méthode en reliant directement un modèle de croissance en hauteur aux conditions de station, par l'utilisation de données d'analyse de tige et de typologie des stations dans 102 placettes de peuplements naturels d'épinette blanche (Picea glauca (Moench] Voss) en région sub-boréale de Colombie britannique. Les peuplements choisis ont été stratifiés selon le régime hydrique du sol, la compacité, la qualité nutritive, et des courbes de croissance spécifiques ont été construites pour chacun des 7 groupes sans utiliser l'indice de fertilité comme paramètre. Bien que moins précises, les courbes obtenues sont comparables aux courbes plus conventionnelles qui utilisent l'indice de fertilité comme paramètre. La liaison entre les types de station et les courbes est significative, comme le montre un essai où cette hypothèse a été testée comme l'indépendance entre les courbes et les types de station. Ce modèle est applicable dans une grande partie de la forêt sub-boréale pour prédire la hauteur dominante des peuplements d'épinette blanche

Trembling aspen site index in relation to environmental measures of site quality at two spatial scales

To evaluate the variation in trembling aspen (Populus tremuloides Michx.) productivity at a large geographic scale, we examined the relationships between site index and environmental factors from 142 even-aged, fully stocked stands located on a variety of sites across interior British Columbia. Site index was derived from stem analysis and the environmental measures included climate surrogates (latitude, longitude, and elevation), biogeoclimatic zone, slope– aspect, actual soil moisture regime (SMR), and soil nutrient regime (SNR). The spatial gradients (latitude, longitude, and elevation), slope–aspect, SMR, and SNR affected aspen site index, but their relationships greatly varied with biogeoclimatic zone. At the provincial scale, these relationships were weaker than on the zonal scale. Among the mod- els developed for predicting aspen site index, we recommend the zone-specific all-factor model for application, which explained 82% of the variation of site index and provided unbiased and precise predictions