Prune, Prune, Prune Part I

     With Hurricane Sandy and its calamitous after effects still fresh on the mind of many there are those particularly concerned about the state and condition of their large, established trees- either trees inhabiting private property or the open-grown curbside public street trees in front of their homes. As an outcome to this storm an interesting human response has evolved with regard to the care and maintenance of many of these amenity trees. I am perplexed by the recent proposal by many community leaders, politicians and public voices who, fearful of the instability of large trees and branches through the printed media have called for increasing street trees branch pruning across the urban forest. I interpret this as their assumption that massive street trees branch pruning (also known as “de-branching”) is good for trees. By this prescription- increased de-branching activity as a solution to reduce future tree and tree branch failures from future winds and storm is clearly misguided and a poor choice of action in the aftermath of this storm. With that, I thought it timely by this mid-winter blog to place some needed light and accuracy about trees and the practice of “de-branching”.

     Tree failures across the metropolis by this storm and others are not only attributed to high wind forces and intensity, but a conglomeration of interacting tree health and environmental factors that lead to the loss of wood and soil strength. Some factors are- the presence of advanced decay with seen or unseen cavities, past root removal and loss by mechanical excavation or, poor root architecture, compacted soils, wet or poorly drained soils, excessive street trees branch pruning etc. Whereby trees and their branches fail when a wind force applied to the overall tree structure exceeds the strength of the wood and soils and the tree’s ability to structurally support itself. Trees and their branches grow and develop in response to their exposure to two principle forces acting upon them- gravity and wind. As with gravity, dynamic and static wind (also known as a load) applied to a tree through its life span induces many developmental and structural changes to branches, stems and leaves, the circumferential branch Vi frarader imidlertid a spille denne formen for roulette nar du spiller casino , da den eneste forskjellen fra Europeisk Roulette er at hjulet inneholder et ekstra felt med tallet 0, altsa dobbel null. distribution throughout the tree canopy, the affects upon the biomechanical properties of the tree canopy, trunk and root morphology- in a highly beneficial way contributes to the 3-dimensional architecture and the balancing act of the urban tree amid those forces, especially wind turbulence.

figure1 - street trees branch pruning

Figure 1. Urban trees such as this parkland Copperleaf beech (Fagus sylvatica) exemplify a well-balanced casino online tree with a high live-crown ratio.

figure2 - street trees branch pruning

Figure 2. Tree failure due to high wind force and intensity and the contributing weak wooded condition by decay at the most vulnerable place of the tree- the base.

     Having evolved in part for the purpose of counteracting gravity and wind loads, branch development patterns and the overall live-crown are vital for tree health and stability. Branch development makes up the live-crown and tree architecture and is closely associated to the production of the overall tree. A live-crown of 66% provides the aesthetic appeal of a balanced, well-tended and well-branched tree. More importantly the live-crown ratio (a ratio of the live crown to the total live tree height) as well as tree opacity1 (among other criteria) assures ample photosynthetic surface for a potentially robust and vigorous tree, if site resources are not constrained. For a variety of tree health issues live-crown ratio has been successfully used to gauge potential tree reactions to resource changes and the probability of dealing with stress conditions2, wind load as a stress included.

     There are two branches types that have differing roles. There are sun-branches that place leaves into position to intercept solar radiation that drives photosynthesis, and there are structural- branches upon which subordinate sun-branches grow. Leaf production across the perimeter of the upper canopy as well as the interior canopy ensures optimization of photosynthesis and governs physiological growth. However the presence of interior leaves and their productivity is especially noted by their contribution to tree productivity and health. By adaptation, on extreme hot weather days combined with water stress conditions the upper canopy sun-leaves shut down (by the closing of stomata and halt vapor loss from evapotranspiration) and the mass of cooler interior shade-leaves take full charge of the photosynthetic process. But only if they are allowed to remain.