ABSTRACT
Arctic-alpine ecosystems are considered hot-spots of environmental change, with rapidly warming conditions causing massive alterations in vegetational structure. These changes and their environmental controls are highly complex and variable across spatial and temporal scales. Yet, despite their numerous implications for the global climate system, the underlying physiological processes and mechanisms at the individual plant scale are still little explored. Using hourly recordings of shrub stem diameter change provided by dendrometers, paired with on-site environmental conditions, enabled us to shed light on these processes. In this way, growth patterns in three widely distributed shrub species were assessed and linked to thermal and hygric conditions. We started our analysis with a close examination of one evergreen species under extreme environmental conditions, followed by a comparison of evergreen and deciduous species, and, finally, a comparative look at growth patterns across local micro-habitats. The results revealed distinct growth strategies, closely linked to species-specific water-use dynamics and cambial rhythms. Within the heterogenous alpine landscape these conditions were mainly attributed to the variation in local micro-habitats, defined by fine-scale topography and consequent variation in snow conditions and exposure. Thus, the overall growth success was mainly controlled by complex seasonal dynamics of soil moisture availability, snow conditions, and associated freeze-thaw cycles. It was therefore in many cases decoupled from governing regional climate signals. At the same time, exceedingly high summer temperatures were limiting shrub growth during the main growing season, resulting in more or less pronounced bimodal growth patterns, indicating potential growth limitation with on-going summer warming. While shrubs are currently able to maximize their growth success through a high level of adaptation to local micro-site conditions, their continued growth under rapidly changing environmental conditions is uncertain. However, our results suggest a high level of heterogeneity across spatial and temporal scales. Thus, broad-scale vegetational shifts can not be explained by a singular driver or uniform response pattern. Instead, fine-scale physiological processes and on-site near-ground environmental conditions have to be incorporated into our understanding of these changes.
ABSTRACT
Effects of drought and aerosol stresses were studied in a factorial experiment based on a Randomized Complete Design with triplicates on two ornamental shrubs. Treatments consisted of four levels of water container (40%, 30%, 20%, and 10% of water volumetric content of the substrate) and, after 30 days from experiment onset, three aerosol treatments (distilled water and 50% and 100% salt sea water concentrations). The trial was contextually replicated on two species: Callistemon citrinus (Curtis) Skeels and Viburnum tinus L. ‘Lucidum’. In both species, increasing drought stress negatively affected dry biomass, leaf area, net photosynthesis, chlorophyll a fluorescence, and relative water content. The added saline aerosol stress induced a further physiological water deficit in plants of both species, with more emphasis on Callistemon. The interaction between the two stress conditions was found to be additive for almost all the physiological parameters, resulting in enhanced damage on plants under stress combination. Total biomass, for effect of combined stresses, ranged from 120.1 to 86.4 g plant−1 in Callistemon and from 122.3 to 94.6 g plant−1 in Viburnum. The net photosynthesis in Callistemon declined by the 70% after 30 days in WC 10% and by the 45% and 53% in WC 20% and WC 10% respectively after 60 days. In Viburnum plants, since the first measurement (7 days), a decrease of net photosynthesis was observed for the more stressed treatments (WC 20% and WC 10%), by 57%. The overall data suggested that Viburnum was more tolerant compared the Callistemon under the experimental conditions studied.