Case Study 2: Phenological Trends in the Federal State of Hesse

Abstract

In Hesse, a rise in air temperature of about 0.9 °C in average established comparing the climate reference period 1961–1990 and the period 1991–2009. This equals the trend in entire Germany, whilst in some regions even a temperature increase of up to 3 °C was measured. Until the end of the twenty-first century, air temperatures in Hesse are expected to rise between 3.2 and 3.7 °C compared to the reference period 1971–2000. This should also affect the onset and duration of phenological stages of plants implicating serious impacts for environment and economy. In order to deal with this issue, the research project “HeKlimPh” funded by the Hessian Competence Centre of Climate Change modelled spatiotemporal trends of plant phenology as being an indicator for climate change related biological effects. Accordingly, case study 2 describes the coupled analysis of meteorological air temperature data and phenological data indicating different phenological seasons. The data comprise observations on 35 plant phenological phases of wild growing plants, fruits, crops and vines collected at about 6500 observation sites in Germany (553 in Hesse) between 1961–2005. Within a GIS environment, also estimations on the future phenological development for the periods 2031–2060 and 2071–2100 were performed. The projections were based on air temperature grids derived from four regional climate models considering the IPCC emission scenario A1B.

The statistical association between air temperatures and onset of phenological phases for the past periods 1961–1990, 1971–2000, and 1991–2009 was investigated by means of correlation analysis, considering auto-correlation, and regression analysis. For plant phases showing a significant and at least medium (|r| ≥ 0.5) correlation between phase onset and air temperatures, phenological development was assessed by applying Regression Kriging. Future projections based on the regression models derived for the climate reference period 1971–2000. For each of the respective plant phases the according regression function was applied to air temperature grids depicting the estimated thermal development in the climate periods 2031–2060 and 2071–2100.

The calculations revealed that 31 out of 35 phases started earlier in the years 1991–2009 compared with 1961–1990. These shifts were more pronounced in Hesse (8 days) compared to the development in Germany (6 days). The onset of several plant phases was even more than 10 days earlier comparing the recent climate period and the prior one. Contrarily, plant phases indicating autumn and winter seasons tend to shift towards the end of the year yielding a prolongation of the vegetation period of up to 3 weeks. More than 70 % of the phases in each of the past periods were correlated with air temperatures by r ≤−0.5, more than 50 % even by r ≤ − 0.7. Projections gave reason to assume that the advances in phase onset to the beginning of the year should intensify in future: In many cases, shifts between 2071–2100 and 1961–1990 are expected to be at least twice as high as between 1991–2009 and 1961–1990.