Nothing articulates autumn like comforting , warm , orange and red tones : sweet potatoes , carved pumpkins , fallen leaves , a night by the fireplace . But what is the chemistry behind autumn ’ s colours ? Why do some trees lose their leaves ? In contrast to deciduous plants , some trees retain their leaves all year long . The Latin names for these trees usually consist of the word ‘ sempervivens ’ which means ‘ always alive ’. Most perennial trees grew in tropical climates , where cold seasons are shorter , or even non-existent . While perennial trees do drop their leaves , they at all times replace them instantly . Most trees in cold climates are deciduous because the energy cost of possessing leaves alive is far too high . Also , large leaves behave like sails on those wintery breezy days , putting the tree in risk of being blown over . Even so , some evergreens like conifers can last the cold , but they are an exception rather than the norm . How do trees ‘ decide ’ to ditch their leaves ? The response to this question is hormones . When you hear the word hormones , you might think of sullen teenagers , the menopause or sexual attraction . But hormones are simply molecules that transport chemical messages in both animals and plants . When autumn arrives , chlorophyll levels decrease , a signal that activates the release of a series of hormones . For years , scientists thought that the main molecule that led to trees