Sapriver is another poem in my sugar maple cycle; when I first began working on this poetry series and thinking about trees more deeply, I came to the conclusion that trees wouldn’t obey our seasons. So I created what I thought were important ‘seasons’ for trees: Sunleaves, Deepnight, Sapriver, Budbreak, and Windborne. Sapriver occurs as the ground begins to warm and winter (Deepnight) starts to fade into spring.
In fall, trees store sugars in their roots before losing their leaves and laying dormant through the short, cold days of winter. ABA, abscisic acid, helps the tree acclimate to the cold winter temperatures and be ‘frost ready‘ (entering a period of dormancy with fallen leaves, closed stomates, and other cellular changes).
During those winter days, sunlight can warm the cells just under the bark, causing them to expand. When night comes, the bark cools and contracts faster than the cells underneath, causing a vertical seam to split the bark open as it tightens over an expanded layer of cells. This heat stress can cause significant cell death and cracks in the barks of trees, sometimes called ‘frost cracks’ or ‘radial shakes’ (though there are also other causes of these wounds). Smaller trees can even die from these wounds, as they have fewer cell layers overall. This is just one challenge faced by trees due to weather conditions.
Longer days cause snow to melt, saturating the soil, and also raises ground temperature. The daytime heating of the ground causes sap stored in the roots (created through photosynthesis by leaves the tree lost in the fall) to also heat up; the sap expands due to the heating, creating pressure inside the finite space of the roots, causing some of the sap to flow up the trunk of the tree through the xylem. At night, when everything cools, there is now negative pressure in the roots – causing water to be pulled into the roots from the environment to equalize the pressure again. This sap and water is used by the tree to begin creating buds that will eventually become flowers and leaves, a process helped along by ‘gibberelins’ – hormones that stimulate stem elongation, breaking and budding, and seed germination after periods of dormancy (in response to cold).