Predicting germination capacity of Pinus sylvestris and Picea abies seeds using temperature data from weather stations

In Fennoscandia, both Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst.) often fail to produce mature seed, especially in the northern parts of their range. Therefore, cone and seed crop predictions are of major strategic importance for maintaining sustainable multipurpose forestry. We present functions for predicting germination capacity of Pinus sylvestris and Picea abies seed over a wide geographic area. The functions are based on germination analyses for 1297 Pinus sylvestris and 597 Picea abies natural stands in Sweden during 1971-1994. Meteorological data from 71 weather stations were used to calculate heat sums with threshold values from 4 to 10 degrees C and two durations of growing season (ending August 31 or September 30). Logistic regression was utilised for parameter estimates. Accumulated heat sum (threshold 5 degrees C) from start of growing season until August 31 in combination with number of days from estimated time of fertilisation until approximate time for embryo growth cessation gave the best function. The function shows that Picea abies has lower temperature requirements for producing mature seed than Pinus sylvestris. A germination capacity of 95% is reached at a heat sum of 875 degree-days for Picea abies and at 975 degree-days for Pinus sylvestris.