Long time series of ground-based plant phenology, as well as more than two decades of satellite-derived phenological metrics, are currently available to assess the impacts of climate variability and trends on terrestrial vegetation. Traditional plant phenology provides very accurate information on individual plant species, but with limited spatial coverage. Satellite phenology allows monitoring of terrestrial vegetation on a global scale and provides an integrative view at the landscape level. Linking the strengths of both methodologies has high potential value for climate impact studies. We compared a multispecies index from ground-observed spring phases with two types (maximum slope and threshold approach) of satellite-derived start-of-season (SOS) metrics. We focus on Switzerland from 1982 to 2001 and show that temporal and spatial variability of the multispecies index correspond well with the satellite-derived metrics. All phenological metrics correlate with temperature anomalies as expected. The slope approach proved to deviate strongly from the temporal development of the ground observations as well as from the threshold-defined SOS satellite measure. The slope spring indicator is considered to indicate a different stage in vegetation development and is therefore less suited as a SOS parameter for comparative studies in relation to ground-observed phenology. Satellite-derived metrics are, however, very susceptible to snow cover, and it is suggested that this snow cover should be better accounted for by the use of newer satellite sensors.

Plant phenological observations are of increasing value as indicators of climate change and variability. We developed a robust multispecies estimate for Swiss Alpine spring phenology for the period 1965-2002 by applying empirical orthogonal function (EOF) analysis on a combination of 15 spring phases. The impact of climate parameters such as temperature and precipitation on the phenological development was investigated using a multivariate statistical model. This multispecies estimate proved to be a good approach to assess the pattern of spring appearance during the last 40 years. It revealed an earlier onset of spring in recent years, mainly since 1988 when a clear shift in spring appearance occurred. The mean overall trend of 1.5 days per decade was clearly driven by winter and spring temperatures whereas precipitation showed no significant influence. The dominant EOF patterns suggested a general climate forcing for the observed inter-annual variability independent of single plant phases. A more regional phenology signal was found in the second EOF mode, indicating slightly weaker phenological trends in southern Switzerland as well as in higher altitudes. Both, temperature and precipitation contributed to this pattern significantly. Analysis of single phases confirmed the pattern of the multispecies estimate. All species showed trends towards earlier appearance ranging from −1 to −2.8 days per decade and the appearance dates had a very high covariance with temperature.

The phenological development of plants provides information about the influence of weather on vegetation and may be assessed on both the individual plant level and on a global level. Since 2000, Switzerland has had a phenological monitoring network for forest trees which records the seasonality and is complementary to the ICP-Forests Assessments (International Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests). A comparison of leaf discoloration at the monitoring plot Kaiseraugst has shown that beech, oak and ash trees have all been affected differently as a result of the summer drought of 2003. In 2004,a digital picture data base was developed for the research project 'Phenophot' at the Geographical Institute Berne which allows the phenological observations to become objective and reproducible. A Phenological Growing Index (PGI) is being derived from the red-green-blue channel data of the digital sensor, which complements the information of satellite based vegetation indices. These indices include the Normalized Differenced Vegetation Index (NDVI) on a sub-pixel level which provides improved accuracy for the information on the character and beginning and ending of the vegetation period. The first comparison of the phenological spring index from the satellite based NDVI revealed that the annual start of spring is reproduced most accurately by determining a threshold of the NDVI.

Spring season (March–May) temperature trends show an increase in most regions of the world. In Europe, the spring temperatures important to plant physiology were the warmest during the decade 1997–2006 with respect to the past 500 years. A temperature increase of 1 °C would lead to an earlier onset of spring phenological phases of 2 to 10 days depending on the species and the region. A phenological multi-species index for Switzerland which includes several plant species is shown to be a suitable tool to describe Swiss spring variability over the past four decades. The index shows a mean trend of 1.5 days/decade from 1965–2002. The 280-year long series of the flowering of the cherry tree represents the index-variability of the past 40 years and gives the unique possibility to assess climate impact on spring phenological variability during past centuries.

No abstract