I’m going to try and explain how trees break their winter dormancy.
Plants exhibit various adaptations for dealing with tough weather conditions. Some of the more famous plants dealing with adverse conditions are cacti. Native to the North American southwest and Central America the Cactus Family is a famously tough group of plants that flourish in dry, hot environments. The “stem” of a typical cactus is thick and fleshy, storing a large amount of water in these tissues. Leaves, which are the primary conductors of water in plants (water being drawn from the roots, through the stems, and evaporating out of leaf pores = transpiration), have been repurposed in the cactus into hard, sclerified spines which do not conduct water. Yes, cactus spines are modified leaves.
Cacti still do conduct water through their bodies but they do differently than most other plants. Most plants conduct water during the day, opening up their leaf pores (stomates) and allowing water vapor to evaporate. This causes a suction effect that draws water from the soil into the roots and up through the plant body. At the same time air is allowed to cycle in and out of the leaf. Carbon dioxide enters and is used for photosynthesis (remember photosynthesis requires sunlight, water, and carbon dioxide to work). So when the leaf pores are open and the sun is shining photosynthesis can occur.
Because the environment where cacti grow is so hot and so dry it is dangerous for transpiration to occur during the daytime (they would dry out too quickly). Instead the cacti shut their pours during the day and open them at night (the opposite of most plants). When they open up their pores at night carbon dioxide comes in and the cactus will store the carbon in it’s cells as malic acid (or malate). At the same time the plant can take up the water it needs from the soil.
When the sun comes out again the cactus will shut it’s pours, stymy the flow of water, and then release the carbon it was storing up all night so it can be used for photosynthesis. Because cacti are so efficient at storing water in their juicy flesh (think aloe, which is not a cactus) they have plenty on hand to use for photosynthesis even though they are not actively taking up water during the day. This is an example of how plants adapt to a dry, hot environment.
In our part of the country in southern New England we have plants that deal with the opposite kind of adverse conditions: cold, wet, and dark weather. During the winter the ground freezes making it impossible for plants such as trees and shrubs to take up water from the soil. Without free water leaves cannot stay turgid and will wilt. Perennial plants around the world are adapted to these conditions by going into a type of dormancy where leaves senesce (“die back”) and abscise (“fall off”). By shedding it’s leaves the tree does not need to try and maintain transpiration and instead cannibalizes the nutrients in it’s leaves, taking them into the body of the trunk, and storing them for the spring. The dead leaves are shed, no longer of any use.
Over the course of the winter perennial plants maintain the following years growth in their buds. Fall and winter buds are the product of the growing season and represent next years twigs, leaves, and often flowers, which all unfurl the following spring. Hard scales cover the buds tightly and frost-resistant chemicals concentrate in the buds to ensure their safe passage through the dark, cold months. Take a look at the tops of oak trees in the winter and see the massive, stout buds all swaying at the summits of our mightiest trees. They are completely exposed to the wind and cold weather yet they manage to pull through.
The water table becomes very high during the spring as the ground thaws but the trees and bushes are all still dormant, without leaves, and unable to take up water in any extensive amount. Vernal pools and muddy ground is the result of the water table being so high. Once plants break dormancy and “leaf-out” transpiration begins again and the water table drops, allowing the ground to dry out. Available water (i.e not frozen) is key for plants breaking dormancy.
Plant reactions to cold weather in the fall and warm weather in the spring is mediated by a balancing act of several plant hormones. Ethylene, abscisic acid and cytokinin interact in determining when leaves begin to fade and drop, and when spring buds begin to open again. The delicate balance of this is such that the plant will protect it’s incipient new growth until it has received plenty of information confirming that it is, indeed, nice outside and will stay nice outside. Warm snaps in the winter do not immediately cause plants to break dormancy although extended periods of warmth will cause some species to flower and leaf out too early. An example of this is the extremely warm weather we had in 2012, hitting several seventy and eighty degree days in March. This extended warm spells caused the fruit trees in Rhode Island to bloom far too early. When cold weather returned in April many of these tender blossoms were destroyed by frost, greatly impacting the peach and apple crop that year.