ABSTRACT
Many plants require a certain amount of ‘warmth’ during the year. The total ‘warmth’ depends on the growing season length and the growing season temperature. These two factors are combined as ‘daydegree totals’ (Woodward 1992). Day-degree totals are the product of the growing season length (the number of days for which the mean temperature is above standard temperature, such as freezing point or 5ºC), and the mean temperature for that period. The Iceland purslane (Koenigia islandica), a tundra annual, needs only 700 day-degrees to develop from a germinating seed to a mature plant producing seeds of its own. The small-leaved lime (Tilia cordata), a deciduous tree, needs 2,000 day-degrees to complete its reproductive development (Pigott 1981). Trees in tropical forests may need up to 10,000 day-degrees to complete their reproductive development.
