The responses of radial and height growth, plant architecture, and the probability of mortality of saplings to varying light levels were quantified for six tree species in temperate conifer forests of the Bhutan Himalayas. Increases in growth with increasing light were comparable with those of high latitude tree species but lower than those of tropical tree species and temperate species in North America. The shade-tolerant species Tsuga dumosa (D. Don.) Eichler showed the strongest increase in radial growth at low light and reached asymptotic growth early. It had the deepest crowns in low light and a low decrease of leader growth with decreasing light. It represents a continuous growth type, which invests in height rather than lateral growth under low light conditions. Betula utilis D. Don. showed greater increases in radial growth and a higher mortality at low light than the more shade-tolerant Abies densa Griff., in keeping with the trade-off between survivorship and growth at low light. Picea spinulosa Griff, Larix griffithiana Carriére, and Pinus wallichiana A.B. Jackson showed small increases in growth at low light levels. The latter two species showed no capacity to adapt their morphology in response to changing light levels, which resulted in higher probabilities of mortality at lower light levels. Differences in the probability of mortality at different light levels were more pronounced than differences in the light-growth response, underlining the importance of survivorship at low light for successional dynamics.Résumé : Les effets de différents niveaux de lumière sur la croissance radiale et la croissance en hauteur, sur l'architecture des arbres et sur les chances de survie des jeunes tiges ont été quantifiés chez six espèces d'arbres dans les forêts tempérées de conifères de la zone himalayenne du Bhoutan. Les augmentations de croissance dues à un niveau de lumière plus élevé étaient comparables à celles qu'on observe chez les espèces d'arbres des latitudes élevées mais infé-rieures à celles des espèces tropicales ou tempérées d'Amérique du Nord. L'espèce tolérante Tsuga dumosa (D. Don.) Eichler a connu la plus forte augmentation de croissance radiale sous une faible intensité lumineuse et a rapidement atteint une croissance asymptotique. Cette espèce avait la cime la plus profonde sous faible intensité lumineuse et subissait une légère diminution de croissance apicale avec la réduction du niveau de lumière. Cette essence est typique du type à croissance continue qui investit dans la croissance en hauteur plutôt que latérale sous faible intensité lumineuse. Betula utilis D. Don. a connu une plus forte augmentation de croissance radiale et de mortalité sous faible intensité lumineuse qu'Abies densa Griff., une espèce plus tolérante, conformément au compromis entre survie et croissance sous faible intensité lumineuse. Picea spinulosa Griff., Larix griffithiana Carriére et Pinus wallichiana A.B. Jackson ont connu de faibles augmentations de croissance sous faible intensité lumineuse. Les deux dernières e...

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Forests in mountain ecosystems provide a diversity of services and goods in mountain landscapes, and the immediate utility of the forest to upstream residents must be balanced with the insurance forests provide for downstream residents in the form of, especially, the services of water regulation and soil stabilization. Little empirical data exist for Bhutan that confirm a causal link between upstream forest use and downstream security. To help to fill this gap, we summarize the results of two literature reviews related to forest ecosystem services and human well-being, using the framework of Bhutan’s Gross National Happiness (GNH). We report preliminary findings of three field-based assessments of forest ecosystem services on hillsides. We conducted an assessment of the perceptions of local residents about the forest ecosystem services in three distinct forest areas of Bhutan. The studies were carried out in high-altitude oak forests, forest management units (FMUs) in government reserve forests, and community plantations. We engaged villagers in focus group discussions and conducted key informant interviews. The strongest evidence in the literature for linkages between forests and GNH was found for the role forests play in socio-economic development and good governance, particularly through the community forestry program. Regulating services of forests were cited as crucial to several aspects of human well-being, but little empirical evidence was provided. To local residents in the three study locations, the provisioning services of forests were highly ranked, as were water regulating services. At the plantation sites, residents felt that the new forests did improve soil stability and water provisioning services. Cultural services were identified but not highly prioritized. Awareness of forest ecosystem services was high among participants in the study, but understanding of the causal links between forest use and human well-being was mixed. Lack of direct evidence about causal pathways between upstream forest condition and downstream security leaves gaps in our knowledge and even perpetuates myths and misconceptions about the role that forests play. We encourage further research at multiple scales and using mixed methods to test hypotheses on the linkages between forests and human well-being in mountain landscapes.

We investigated whether species richness, diversity and density of understory herbaceous plants differed along logging (gap) and grazing (primarily by cattle) disturbance gradients, and sought to identify drivers of richness, diversity and density of understory vegetation of logged sites. A factorial experiment was conducted in the mixed conifer forest of Gidakom in Western Bhutan. Levels of the logging treatment included small (0.15 -0.24 ha), medium (0.25 -0.35 ha) and large (0.36 -1.31 ha) gaps. The grazing treatment included grazed (primarily by cattle) and ungrazed (where herbivores were excluded by a fence) plots nested within each gap. Data were collected from 12 gaps (4 replicates at each level of logging) using the point intercept method. Shannon Weaver Diversity and Margalef's indices were used to estimate species diversity and describe species richness, respectively. Soil samples were analyzed for pH and nutrients. The interaction effect of logging and grazing was significant (p≤0.001) only on species diversity. Relative to ungrazed areas, species diversity was significantly higher (0.01≤p≤0.05) in medium grazed gaps. Under grazed conditions, soil P was negatively correlated with gap size and species diversity. While species diversity was positively correlated (0.01≤p≤0.05) with soil N in grazed plots species richness was positively correlated (0.001≤p≤0.01) with soil N in ungrazed plots. Relative density of Yushania microphylla and Carex nubigena were higher under ungrazed conditions. Our study suggests that the combined effect of cattle grazing and logging results in higher species diversity of understory vegetation in medium and grazed gaps in mixed conifer forests of Bhutan,whereas increase or decrease in relative density of major species is determined primarily by the independent effects of grazing and logging. From management perspective, forest managers must refrain from creating large gaps to avoid loss of nutrients (mainly P and N), which may eventually affect tree regeneration. Managers intending to maintain understory vegetation diversity must consider the combined effects of grazing and logging, ensuring low to moderate grazing pressure.

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