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Rapid environmental change at high latitudes is predicted to greatly alter the diversity, structure, and function of plant communities, resulting in changes in the pools and fluxes of nutrients. In Arctic tundra, increased nitrogen (N) and phosphorus (P) availability accompanying warming is known to impact plant diversity and ecosystem function; however, to date, most studies examining Arctic nutrient enrichment focus on the impact of relatively large (>25x estimated naturally occurring N enrichment) doses of nutrients on plant community composition and net primary productivity. To understand the impacts of Arctic nutrient enrichment, we examined plant community composition and the capacity for ecosystem function (net ecosystem exchange, ecosystem respiration, and gross primary production) across a gradient of experimental N and P addition expected to more closely approximate warming-induced fertilization. In addition, we compared our measured ecosystem CO 2 flux data to a widely used Arctic ecosystem exchange model to investigate the ability to predict the capacity for CO 2 exchange with nutrient addition. We observed declines in abundance-weighted plant diversity at low levels of nutrient enrichment, but species richness and the capacity for ecosystem carbon uptake did not change until the highest level of fertilization. When we compared our measured data to the model, we found that the model explained roughly 30%-50% of the variance in the observed data, depending on the flux variable, and the relationship weakened at high levels of enrichment. Our results suggest that while a relatively small amount of nutrient enrichment impacts plant diversity, only relatively large levels of fertilization-over an order of magnitude or more than warming-induced rates-significantly alter the capacity for tundra CO 2 exchange. Overall, our findings highlight the value of measuring and modeling the impacts of a nutrient enrichment gradient, as warming-related nutrient availability may impact ecosystems differently than single-level fertilization experiments.

Leaf dark respiratory CO2-release (RD) is, according to some literature, dependent on the rate of leaf transpiration. If this is true, then at a given vapor pressure deficit, the leaf stomatal conductance (gs) will be expected to be a controlling factor of measured RD at any given time. We artificially lowered leaf gs by applying abscisic acid (ABA). Although leaf RD generally covaried temporally with gs, artificially lowering gs by applying ABA does not affect the measured leaf RD. These results indicate that observed diel fluctuations in gs are not directly influencing the measured leaf RD, thereby simplifying both future studies and the interpretation of past studies of the underlying environmental- and physiological drivers of temporal variation in leaf RD. [ABSTRACT FROM AUTHOR]

This article examines the changes in leaf respiration during the night, specifically focusing on the decrease in leaf respiratory O2-uptake and CO2-efflux. The authors analyze data from various studies and find that these decreases indicate a decrease in leaf mitochondrial electron transport and ATP demand. The article emphasizes the need for further research to understand the underlying mechanisms and species-specific differences in nocturnal changes in leaf respiration. The authors also discuss the implications of changes in the respiratory quotient (RQ), suggesting that the relative composition of respiratory substrates may shift during the early hours of the dark period. They highlight the importance of studying factors that affect RQ, such as substrate composition and demand for respiratory intermediates. Overall, this information is valuable for understanding the relationship between environmental and metabolic regulation of respiration and its impact on carbon uptake. [Extracted from the article]

In a range of plant species from three Californian vegetation types, we examined the widely used "normalized difference vegetation index" (NDVI) and "simple ratio" (SR) as indicators of canopy structure, light absorption, and photosynthetic activity. These indices, which are derived from canopy reflectance in the red and near-infrared wavebands, highlighted phenological differences between evergreen and deciduous canopies. They were poor indicators of total canopy biomass due to the varying abundance of non-green standing biomass in these vegetation types. However, in sparse canopies (leaf area index [LAI]°0-2), NDVI was a sensitive indicator of canopy structure and chemical content (green biomass, green leaf area index, chlorophyll content, and foliar nitrogen content). At higher canopy green LAI values (>2; typical of dense shrubs and trees), NDVI was relatively insensitive to changes in canopy structure. Compared to SR, NDVI was better correlated with indicators of canopy structure and chemical content, but was equivalent to the logarithm of SR. In agreement with theoretical expectations, both NDVI and SR exhibited near-linear correlations with fractional PAR intercepted by green leaves over a wide range of canopy densities. Maximum daily photosynthetic rates were positively correlated with NDVI and SR in annual grassland and semideciduous shrubs where canopy development and photosynthetic activity were in synchrony. The indices were also correlated with peak springtime canopy photosynthetic rates in evergreens. However, over most of the year, these indices were poor predictors of photosynthetic performance in evergreen species due to seasonal reductions in photosynthetic radiation-use efficiency that occurred without substantial declines in canopy greenness. Our results support the use of these vegetation indices as remote indicators of PAR absorption, and thus potential photosynthetic activity, even in heterogeneous landscapes. To provide accurate estimates of vegetation-atmosphere gas fluxes, remote NDVI and SR measurements need to be coupled with careful estimates of canopy photosynthetic radiation-use efficiency.

As global climatic changes increase plant susceptibility to large-scale disturbances such as drought and pathogens, understory responses to these disturbances will become increasingly important to long-term forest dynamics. To better understand understory responses to canopy disturbance, we measured changes in the growth and physiology of the dominant understory shrub, American witch-hazel (Hamamelis virginiana L.), in response to girdling of canopy oaks in a temperate hardwood forest of the northeastern United States. Changes in the growth and physiology of H. virginiana may be important to the regeneration of northeastern temperate forests, as this common shrub largely shapes the microenvironment for seedlings on the forest floor where it occurs. Canopy disturbance by girdling resulted in significant increases in light and soil nitrogen availability. In response to these environmental changes, basal-area growth of H. virginiana increased by an average 334%. This growth increase corresponded to significant increases in foliar nitrogen, respiration, and leaf chlorophyll and carotenoid concentrations. These findings indicate improved environmental conditions and increased growth for this understory shrub following the loss of dominant canopy trees. This study suggests that following large-scale canopy disturbance, H. virginiana and shrubs like it may play an important role in competing for soil N and shading seedlings of regenerating canopy species. Key words: Black Rock Forest, canopy disturbance, foliar chemistry, photosynthesis, respiration. A mesure que les changements climatiques a l'echelle planetaire augmentent la sensibilite des plantes aux perturbations a grande echelle, telles que la secheresse et les agents pathogenes, les reactions du sous-bois face a ces perturbations vont devenir de plus en plus importantes pour la dynamique forestiere a long terme. Dans le but de mieux comprendre les reactions du sous-bois aux perturbations du couvert forestier, nous avons mesure les changements dans la croissance et la physiologie de l'arbuste de sous-bois dominant, l'hamamelis de Virginie (Hamamelis virginiana L.), en reaction a l'annelation des chenes de l'etage dominant dans une foret feuillue temperee du nord-est des Etats-Unis. Les changements dans la croissance et la physiologie de l'hamamelis pourraient avoir des repercussions importantes sur la regeneration des forets temperees du nord-est etant donne l'influence determinante de cet arbuste commun sur le microenvironnement des semis au sol ou il est present. La perturbation du couvert forestier par l'annelation a entraine une augmentation significative de la disponibilite de la lumiere et de l'azote dans le sol. En reaction a ces changements environnementaux, la croissance en surface terriere de l'hamamelis a augmente en moyenne de 334 %. Cette augmentation de croissance correspond a des augmentations significatives de l'azote foliaire, de la respiration et des concentrations de chlorophylle et de carotenoide dans les feuilles. Ces resultats sont un indice de meilleures conditions environnementales et d'une croissance accrue pour cet arbuste de sous-bois a la suite de la perte des arbres de l'etage dominant. Cette etude indique qu'a la suite d'une perturbation a grande echelle du couvert forestier, l'hamamelis et les arbustes semblables pourraient jouer un role important en competitionnant pour l'azote dans le sol et en ombrageant les semis des especes de l'etage dominant qui se regenerent. [Traduit par la Redaction] Mots-cles: foret de Black Rock, perturbation du couvert forestier, chimie foliaire, photosynthese, respiration., Introduction Temperate hardwood forests play an important role in regulating water and nutrients, sequestering carbon (C), and providing habitat for biodiversity. In the United States (US) and southern Canada, many [...]