Think of the energy stored in the vascular system as concentrated sap, or sap high in solids with very little water as the vehicle. In late summer and autumn, sap surplus to the needs of the plant to grow buds for next year stays in the roots and is used to grow roots. As less and less transpiration occurs, the sap in the upper vascular system "parks" where it is and becomes more concentrated as modest transpiration through the bark reduces the % of water in the sap. The drier it becomes, the less damage-prone the upper plant is from freezing air. The dry parts of trees regularly take whatever low temperatures come with the zones they are native to while the root zones acclimated to the same zones are not subjected to lows anywhere near what the air temperature are. The more water normally stored in the root zones, the less able to withstand low temperatures, especially freezing. Many trees that can stand 20 or even 10 or 0°F will be damaged or die from root zones at 20°F. Air temperatures change quickly, but the ground is a poor conductor and temperatures change slowly, especially dry soil. A dry autumn is gardener-friendly. Snow cover is also a poor conductor and insulates land from deep cold, if and only if, the ground has not frozen deeply before the snow covers it. A dry autumn and early and continuous snow cover are gardener-friendly. A wet autumn followed by poor snow cover and extra-low air temperatures will freeze the soil deeper and would be bad for trees like Japanese Maples and others with fleshy, shallow roots. SPF (Spruce-Pine-Fir) have fibrous roots that are not fleshy and will stand much lower temperatures. SPF do not have to re-foliate completely every spring and the needle's waxy coating reduces water loss, so along with the thick sap uses and stores less water. You can tell when you're getting close to the treeline anywhere, -all the trees are needle trees.
So, the energy for plants is stored in sap, some watery in warm climates, some thicker in temperate zones. Concentrated at the ends of growing seasons, in spring, just add water.