>> Background information

Development

The length of growing season varies a great deal in different years but on average it has lengthened in Finland during 1960–2010. In Southern and Central Finland (Helsinki and Jyväskylä) it has increased more than ten days, in Northern Finland (Sodankylä) about five days.

Heat summation is a sum of temperatures that exceed +5 centigrades. Unit of heat summation is a degree day. In 2011 heat summation was in Helsinki 1829, in Jyväskylä 1455 and in Sodankylä 1057 degree days. The values were clearly higher than the average in 1981–2010.

On average the heat summation remained approximately on the same level in 1960–1990, but has started to rise since. After 1990 the moving average of heat summation has risen over a hundred degree days on each three measurement locations.

Impact on biodiversity

The lengthening growing season and increasing heat summation has a broad effect on biodiversity. Especially in Northern Finland temperature is the main limiting resource for flora. The vegetation has recently started to expand in the northern high latitudes according to remote sensing research.

The lengthening growing season and increasing heat summation improve fitness of many species. The vegetation zones are moving northward. Even though species richness grows on many areas this might be harmful for some specialized species. For example alpine species cannot move northwards when climate changes.

Lengthening growing season and increasing heat summation have an effect also on insects. Multivoltinism in moths has increased since 1970s. The change has accelerated 1990s onward. Moth multivoltinism has been clearer in Central and Southern Finland. In Northern Finland multivoltinism has occurred only in some species. Multivoltinism correlates with high mean summer temperatures and increasing heat summation.

It is possible that multivoltinism occurs also in other insect species. For example multivoltinism in pest insects might cause growing populations and larger environmental detriment.

The lengthening growing season and increasing heat summation influence the whole food chain. The impact might be positive or negative, depending on the species. By and large the general species benefit from changes, but species that require very specific conditions suffer.