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pattern tends to maximize the availability of light during the entire growing season. It is interesting <br />that the trees where there is a wide variation in tree condition are in a local population that is <br />unusually crowded for this stand of cottonwoods. Cover in these local populations approaches 100%. <br />But where the trees are not so crowded and cover is about 30% to 60% the trees are much healthier. <br />High cover is not necessarily good if the local environment cannot provide the necessary growth <br />resources to maintain that density. This is probably one of the reasons why very mature cottonwood <br />forests on ephemeral drainages like Coal Creek tend to be composed of a few very large individuals <br />rather than lots of medium to large individuals as found along perennial streams and rivers. <br />Resources along ephemeral streams are simply less reliable and more limited. <br />Change in Height: The same pattern is seen here as was seen in Exclosures 3 and 4. The <br />gains between 2006 and 2007 were much greater than the gains between 2007 and 2008. This would <br />seem to indicate some uniform factor is acting on all the groves. It could be the dryness of 2008, or it <br />could be declining space and increasing competition, or it could be a return to a normal growth <br />pattern after an initial surge in growth as a result of the removal of browsing impact. <br />Adjacent Growths: Just to the west of the exclosure there is a very extensive growth of <br />willow and cottonwood in the stream channel. This growth may be providing some clues as to the <br />pattern of growth in Exclosures 3 through 5. <br />This area was very heavily browsed by cattle, so much so cottonwood almost disappeared and <br />the willows were in very poor condition. Now, however, this growth is so dense it is difficult to even <br />walk through it and both willow and cottonwood are growing strongly. <br />This stand is in the stream channel and has a very reliable supply of water under the sands <br />that form the surface layer of the channel. It appears that when browsing pressure is removed there is <br />an immediate response in increased growth at a very rapid rate. But as the total biomass reaches a <br />given level, limitations in moisture and other growth resources starts to reduce the rate of growth due <br />to competition. Willow tends to do the best in the wettest areas while cottonwood does better in <br />slightly drier locations, but as a unit all the population components are competing for the same <br />resources and this results in a reduction in the growth rate of all the individuals. <br />Thus, where the habitat is highly favorable growth is strong and dense, but as favorability <br />declines so to does the vigor and the density. Such a shifting in the utilization of the growth <br />resources results in a masking of the effects of the former limitation of browsing pressure. That is, <br />browsing pressure was limiting prior to 2006 in the exclosures and continued in unprotected areas <br />through 2006. But when browsing pressure was removed in 2007 that limiting factor declined to zero <br />resulting in an ability of the vegetation to surge upward to the next limiting factor. After a couple of <br />years of growth without browsing pressure limiting the growth a new limit is reached which is <br />basically a habitat limitation in the form of water resources and nutrients. The surge in growth after <br />2008 Annual Report Coal Creek Wetland Mitigation Permit DA 198811488 Page 18