Inter-annual variability and controls of plant phenology and productivity at Zackenberg

Results from monitoring and experimental studies representing various elements of climatic change are presented to evaluate the concurrent dynamics of vegetation types as well as species-specific responses to climatic variations. The studies were carried out in the high-arctic valley of Zackenbergdalen in Northeast Greenland. Vegetation type dynamics and land surface phenology were studied through the use of hand-held sensors of reflection of vegetation, from which the far red normalised difference vegetation index (NDVI-FR) can be inferred. Furthermore, species-specific dynamics were studied through measures of timing and magnitude of flowering through a period of 10 years. Time of snowmelt and temperature were the major controlling factors for the timing of the phenology in the six vegetation types: fell-field, Dryas heath, Cassiope heath, Salix heath, grassland, and fen. Snowmelt had a linear positive effect on the timing of the maximum of the growing season, with late snowmelt, causing a later occurrence of the maximum. While higher summed temperatures during the green-up period (time from snowmelt to maximum) also was shown to be positively related to the timing of the maximum, enhanced flower production seemed to cause lower vegetative biomass production and hence a later maximum. The seasonal vegetative production, expressed as the seasonal integrated NDVI-FR (SINDVI), had a linear negative relation with the temperatures during the previous summer. This was probably due to higher temperatures causing more flowers the following year, leading to a lower NDVI-FR. A strong negative trend in maximum NDVI-FR is documented in all six vegetation types during the years from 1999 to 2006 with a decrease of 0.01 NDVI-FR per year. The main reason could be drying of the upper soil layers due to earlier snowmelt and higher evapotranspiration during recent years. Some general trends in phenological characteristics were recognised, although responses varied among species. The time of snow disappearance was the main determinant for onset of flowering. Shrubs seemed to be superior in taking advantage of an early snowmelt with respect to initiation of flowering. In addition, most species developed flowers and seed capsules faster when temperatures increased. More flowers increase the chance of cross-pollination and hence, offer an increased possibility for sexual reproduction and genetic exchange. Most species increased the number of flowers in years following a warm growing season in the previous year, although some species also depended on climatic conditions in the current year prior to onset of flowering. Experimental manipulations of growing season length, temperature and incoming radiation in plots with the dominant dwarf shrub, white arctic bell-heather Cassiope tetragona, confirmed that this species significantly reduced flowering when subjected to shading or reduced growing season length. Based on current predictions for climatic changes in the Zackenberg area (Stendel et al., 2008, this volume), it is expected that the plant species currently present will be able to put more effort in sexual reproduction, thus ensuring increased species adaptedness to changing climate through enhanced genetic variance. However, this is provided that seeds are able to germinate, that species are not out-competed by faster growing and canopy-forming invading species, and that species are able to continue flowering even under reduced light levels caused by an increased cloud-cover.