The situation in autumn is much more complicated. There is no lack of general
hypotheses about which environmental factors might trigger the autumnal
phenophases, such as leaf colouring and leaf fall (Estrella & Menzel, 2006). Nevertheless,
no physiologically based model is yet available that may simulate observations
in this season. In addition, the attribution of observed changes to potential
factors by statistical methods is more complex. There are some indications thatwarm
weather in May and June will advance leaf colouring, whereas a warm late summer
and early autumn in August and September delays leaf colouring (Menzel, 2003a).
Other possible triggering factors, such as drought, soil moisture, atmospheric pollutants,
constant growing season and critical day length, resist detection by statistical
analysis.