© 1996 Heron Publishing—Victoria, Canada
Improving the reliability of a combined phenological time series by analyzing observation quality
Tapio Linkosalo (1), Risto Häkkinen (1) and Pertti Hari (2)
1. The Finnish Forest Research Institute, Unioninkatu 40 A, FIN-00170 Helsinki, Finland / 2. Department of Forest Ecology, University of Helsinki, Unioninkatu 40 B, FIN-00170 Helsinki, Finland / Received May 17, 1995
Summary
Collecting phenological data is a slow process. Although such data have been collected by a number of organizations, the reliability
of these data is not known because the data-generating process cannot be repeated. No further observations to improve the
reliability can be obtained. However, the data usually consist of several overlapping observation series and this overlap
can be utilized to construct a combined phenological time series and to improve its reliability. We have developed two techniques
for selecting the most reliable observations or observation series and thereby improve the reliability of the combined time
series. Both techniques require that the method used to combine the separate phenological time series adjusts the individual
series to eliminate possible systematic differences between them. A data set of bud burst in Betula pendula Roth collected in Central Finland during 1896–1955 was adjusted and used to test both techiques. Both techniques considerably
improved the reliability of the combined time series; the mean of the confidence intervals of the annual means decreased by
12%. Despite the improvement in reliability, the resulting changes in the annual values of the combined time series were small,
the largest change being 2.5 days. Removing outliers was the most effective method of improving reliability, i.e., it resulted
in the greatest improvement with the smallest number of discarded observations.
Keywords:
Betula pendula, bud burst, combined time series, outliers.
