3. Tree crowns. Crown Change

Transcription

1 3. Tree crowns A often under emphasized part of forestry education is the role of tree crowns in defining the size and shape of the tree. This is not a static relationship but a dynamic one that has tremendous implications for the trees in a stand. Tree crowns are the productive part of a tree converting sunlight and CO 2 and H 2 O into the material to produce all parts of a tree. Any study of tree change must take into consideration the change in this productive tissue. The size and shape of the tree s crown is one of the most easily influenced components of a tree. This is because of the various ways a tree produces and transports photosynthates, influence the size and shape of all other parts of the tree. To not study these relationships is to miss a major part of forest stand dynamics. In this chapter we will examine a number of principles for stand dynamics that effect the nature and consequences of crown change. As foresters, we know that species have differences in growth pattern and dynamics. There are, however, principles that can be applied to all tree species that help us understand the possibilities that exist. Crown Change Tree crowns can only change in a few specific ways. These ways include change through height growth, branch growth, and crown recession. These ways of change can all function in consort or independently with quite different results(figure 3.1). Largest changes in crown size are a result of height growth. If height growth is accompanied by little or no change in crown base large increases in crown size are possible. This means that the period of tree s life-cycle that has the largest height growth rates is the period where the largest crown changes are possible. Conversely, as height growth slows down at older tree ages it becomes harder for tree crowns to expand as crown expansion is depended on primary growth (growth in length dimensions, height, branches, roots) of the tree. Alternatively, large changes in crown recession can often overtake the growth and reduce the overall size of the tree s crown. Crown recession occurs when the conditions at the base of the tree crown no long support the productivity of a 15

2 Height growth Branch growth Crown recession Figure 3.1.: Diagram of the crown change tree s branch at that location. When this happens, the branch dies, moving the tree crown upward. In normal tree crowns they do not move productively downward. Epicormic branches do occur below the current tree crown when condition become favorable for their formation. They, however, seldom provide significant photosynthetic resource that that compete with an intact normal tree crown. The third type of crown expansion is branch growth. Branch growth in magnitude is general assumed to be proportional to height growth and usually less than height growth. This is one area where species difference in terms of epinastic control become important to the relative importance of branch growth. When we thin a forest, we are attempting to increase tree crowns by increasing or maintaining height growth, increasing branch growth and reducing crown recession. This combination increases crown mass so that in turn increases the photosynthetic potential of the tree. Next we will spend some time exploring dimension of crown change in detail. Height growth Height growth is the main source of crown change. The effect of height growth is to move the top of the tree crown up. Height growth is the largest single positive rate of change in any dimension of a crown. This has the interesting consequence that crowns are more dynamic during the period of active height growth, usually in the first 25 years of the tree s life. This means that during this timer period 16

3 trees can dramatically change crown size taking advantage of available space. This relationship also means that as the rate of height growth slows as trees get older the tree s ability to change and adapt is reduced proportional the it s rate od height growth. Trees that have reached the asymptote have a small capacity for the crown to respond to new space available. This is a real consideration when making decision to thin older stands. In general, larger healthy trees in a stand out compete smaller trees in all cases. Trees put a higher priority on height growth, as crowns can be rebuild in marginal trees of equal height given new space but trees seldom catch-up if they fall behind in height growth. Since trees are processors of sunlight height growth moves the tree toward that sunlight and potential competitive advantage. The net effect of this change depends on how the other two ways of crown change (branch growth and crown recession) behave. If a tree is in the fast part of the height growth curve, and growing space is available the crown will increase upward and outward. Branch growth Branch growth is a secondary source of positive crown change. In trees with active height growth and strong epinastic control branch growth is usually less than the leader growth. Crown radial growth will usually be less in length relative to height growth even is very open spacing conditions. It can be severely limited in response to limit growing space because of stand density. Branch length are limited in several ways, limited sunlight, physical abrasion, and wind whipping. Only when the height growth is relatively slow can branch growth out pace the height growth usually in flat-topped understory trees. Branch growth is limited in extent in very low density conditions by mutual crowding. I more open conditions branch length is described by maximum crown width or radius equation. These differences create crown asymmetries in radius and crown length dimensions. Just as in height growth, is an accumulation process the future size of a branch is dependent on the past size and the increment added by the current growth. In general, current growth in uncrowded conditions can not completely replace the lack of growth during previous crowded conditions. 17

4 Crown recession Crown recession refers to the upward change in crown base. In general, a crown base does not move down producing new crown with the ability to create new effective foliage, however, crown bases can remain at a constant height or move upward through the death of branches at the base of the crown. The death of branches at the base of the crown is very dependent on the densityof the neighbor trees. Higher density stands yield smaller growing space per tree promoting faster crown recession. One of the main objectives of thinning is the reduce crown recession, allowing the height growth and branch growth to expand the crown. If crown recession out paces height growth the tree s crown will become smaller. The discussion of the branch autonomy theory help to explain the process that happens to these branches at the base of the crown. Crown Asymmetries Branch growth and crown recession can vary around the stem because of differences in available growing space. These differences create asymmetries in the crown shape. Many of the asymmetries are create by competition with neighbor trees. Other asymmetries are created by breakage within the tree crown. Crown asymmetries seem to have minimal effect on growth rates, but they do create asymmetries in branch lengths and potential other parts of the a tree. The next section will explore these difference in more detail. Crown influence on tree dimensions Because tree crown are the productive part of a tree the size and shape of the tree crown can influence the size, shape and abundance of other parts of the tree such as stems, and roots. The following sections will address ways in which the crown size and shape influences. the size and shape of other tree parts. Stem increment based on crown For example, tree stem shape is know to be influenced by tree crowns. Stem increment is know to be the greatest at the crown base and increment reduces as a function of distance below the crown base Assmann (1970), Jensen (1976). This relationship has a number of very interesting consequences. If tree crown are large it encourages greater taper in the stem but also larger diameter growth 18

5 rings. This is because the with plenty of photosynthate, a tree will increase at all diameters in relation to the amount of leaf area above the diameter point. If crown size is small, however, the tree will have the largest increment at the crown base and the increment will decline with distance below the crown base. Thistendstoreducethetaperintreestemsbelowacrownbaseonsmallercrowned trees. This approach will produce slow diameter growth, tight diameter growth rings and less stem taper. These differences can be used to produce tree of very different size and shape base of how the crowns are allowed to grow or not grow. Knot distributions Tree crowns are made up of branches of various lengths and diameters and branch angles. Each branch forms a knot that extends back the stem center. These knots vary in diameter over time. This variation is recored in the stem of the tree as record of past branch size. The knot diameter near the center of the stem reflect the size of the branch diameter when the stem was smaller. This means that the history of crown dynamics is to a extent stored in the knots (Cite Maguire, Kershaw). The size and distribution of knots can be effected by past stand treatments. Kershaw 1991 found unfertilized stand that did not differ in breast height diameter fertilized stand had greater numbers of branches and greater leaf area than the unfertilized stands. the net effect of the fertilization was null in terms of breast height stem diameter. But the distribution of photosynthate with int he stems were different. Branch structures We all know from basic tree morphology that trees have different forms with decurrent (a distributed of highly branching structure) and excurrent (a single stem leading to the top of the tree) forms being the most common. Each of the structure have definite patterns that control the relative possible branch size and branch angles. Sapwood-Heartwood relationships In many species it is important to determine the amount of heartwood or sapwood for a variety of reasons. For example in many species sapwood is much less rot 19

6 resistant and therefore less desirable. In some species the color of the heartwood provides the value for the species such as walnut or gmelina, and in other species it is the characteristic of the heartwood that make the wood usable for specialty wood products. A example of this is white oak which is used for wooden barrel cooperage. In this product, sapwood has not developed the tiloces that seal the wood vessel. In white oak cooperage sapwood is removed because it leaks. Also in non-commodity outcome sapwood is less resistant to insects so is less desirable for long-term snags. All these reason would lead one to an interest in how to change the sapwood-heartwood ratio. If we assume the pipe-model theory (Valentine (1988)), we believe that a specific amount of foliage requires a specific amount of sapwood area to connect it to the root system. This model has been successfully used to define the sapwood to foliage ratio for a specific species. Branch Autonomy Theory Branch autonomy theory (Sprugel et al. (1991) ) that tree general invest in branches and then expect a branch to support itself and potential return fixed carbon to the rest of the tree. With this idea branches would consume the carbon it produces to cover maintenance respiration, reproduction and growth. excess is exported to the main stem. In this approachcarbon is used locallyfirst then excess is exported but the cells along the transport path use the carbon to cover their needs before exporting the excess further. We know that there are exceptions to this process but in many carbon labeling studies these rules seem to apply during the photosynthetic active periods of tree growth. 20