Hopefully spring is here to stay! Spring is my favorite season for many reasons, but I suspect the primary reason is the promise that comes with warming temperatures and increased day length.
As discussed in the previous article, spring in Wisconsin is when trees break dormancy and begin growing again. From a biological perspective, the process of renewing growth can be thought of in terms of metabolic activity and the energy necessary to support it. There are several things going on inside the tree before the first leaves start to appear. From a management perspective, the biggest and most critical is the mobilization of stored energy reserves and essential mineral elements to support the first flush of growth. The principle source of the energy used to fuel renewed growth is stored carbohydrates. In deciduous trees, excess sugar produced during photosynthesis last year was converted to a starch and stored in various locations such as sapwood in the trunk and roots. Conifers tend to store excess sugar in a form of oil. In the spring, stored starches/oils are converted back to sugars, the breakdown of which supplies energy to support increased activity within living cells and cell division in the apical meristems (the growing points on the ends of twigs). As mentioned, all of this occurs before the leaves have formed. As a result, the period of time just prior to bud break represents a low point in the tree's energy cycle. For this reason, we do not prune living branches while the trees are breaking bud. The process of removing live branches creates a wound that the tree must respond to. Similar to growth, the wound response requires energy and the energy used in wound response is diverted away from growth. As you probably suspect, the removal of dead branches during this period of time is still acceptable.
Similar to starch and oil, the tree relies on stored essential mineral elements, most notably nitrogen, to support early season growth. To understand this process we must go back to last fall. Prior to leaf drop, approximately 50% of the nitrogen within the leaves is stored for future use. The reason for the removal is because nitrogen is an energy-expensive element to acquire and synthesize. Nitrogen is used to make amino acids. Amino acids are used to make proteins. Proteins keep the tree alive. The most abundant proteins in a tree are associated with photosynthesis, namely chlorophyll and the protein that captures carbon dioxide from the atmosphere. As a result, there is a strong positive relationship between the nitrogen content in leaves and rates of photosynthesis, and tree growth is one of underlying reasons we fertilize trees. With that being said, proper tree fertilization requires a fair amount of evaluation. The arborists at First Choice Tree Care evaluate the site's ability to supply critical resources against the tree's demand for resources and then make decisions regarding when the fertilizer should be applied. My general rule of thumb: fertilizers should available when the tree has a high demand for external resources.
The next article will explore tree fertilization in more detail.
Dr. Les Werner, University of Wisconsin - Stevens Point