Douglas fir Physical Structure in (WHZ) Old Growth Forest

Douglas fir Physical Structure in (WHZ) Old Growth Forest


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Douglas fir Physical Structure in (WHZ) Old Growth Forest

Ecological succession in a forest ecosystem is defined at the end by an apex system. However, the climax system is shaped by many environmental and geographical variables that ensure they retain certain structures and compositions. As such, different apex forest systems occupy different climatic and altitudinal zones and are composed of differing tree species adapted to those climatic conditions. The Western Hemlock Zone [WHZ] is characterized by different subzones among which is the Douglas fir -Western hemlock -Salal Zone. Because the Douglas fir old-growth forest occupies the uppermost stratum in the WHZ, their physical structure is not only important for the sustenance of the apex forest ecosystem but also that of other animals and plants and as a commercial resource. Thus, the classification of being old growth carries with it certain characteristics that define the structure and composition of the old growth Douglas fir forests that not only set them apart due to their ecological importance but also because of other intrinsic values. Nonetheless, understanding the physical structure of this old growth forest is integral to unlocking its ecological dynamic secrets.

Old Growth Douglas fir Forests

Old growth forests as the name suggests are old forest stands that occupy an ecological stratum and are characterized by various structural components. Some of the more known old growth forests are located at the WHZ and are predominantly dominated by the Douglas fir. Although the Douglas fir old growth forests historically occupied millions of aces, subsequent exploitation by humans for logs and timber has reduced the area they occupy downwards (Binkley & Fisher, 2013). Thus most of the new forest cannot be classified as old growth forest.  However, the Douglas fir old growth forests are characterized by trees ranging from 200 to over 1000 years thus providing a wide range in the age of the trees (Smith et al., 2002). In addition, size in girth and height of the trees varies with some being very tall and wide while others are small in size. Further, the forests exhibit dense canopies caused by the large trees. Additionally, the forest is characterized by rich dead wood consisting of standing and fallen trees. Besides, the forest is characterized by several canopy layers and different kinds of understory components (Poage & Tappeiner, 2005). Consequently, the characterization of a Douglas fir forest as old growth not only encompasses individual characteristics of the trees found therein but also the collective character of the entire system.

Multiple Canopy Layers

Old growth Douglas fir forests are characterized by multiple canopy layers. These canopies are as a result of the varying ages of the Douglas firs and other plant species that occupy the same niche. Appearing as a result of different causes such as wildfire, volcanic eruptions, deaths of single large trees, gaps in old growth forests vary in size which also influences the ecological response to the gap (Spies & Franklin, 1989). However, gaps in the canopy form an important feature of the old growth Douglas fir forests and lead to accelerated growth of the understory occasioned by the emergence of a micro climate (Odion & Sarr, 2007).

The gaps open the understory to increased exposure to climatic elements leading to the emergence shrubs, herbs, and other plant materials that are characteristic of such habitats and at the same time attract corresponding vertebrates and invertebrates (Poage & Tappeiner, 2005). As a result, gaps in old-growth Douglas fir forests ensure the continued evolution in the physical characteristics of the forest.

Due to the long time, it takes for the natural succession to occur in an old growth Douglas fir forest gaps appearing in it last for long periods of time before they are filled with the new growth. However, gaps in these mature old-growth Douglas fir forests show a patchier distribution compared to young stands of the same type of forests (Sprugel et al., 2009). During this extended period and depending on the size of the gap, the diversity of both flora and fauna between the gap area and the old growth forest varies. Following a succession paradigm, the diversity is higher in the gap area compared to the closed canopy area of the old growth (Franklin & Spies, 1991). As such, gaps in old-growth harbor diverse plants and animals that help to maintain the physical characteristic of the old growth Douglas fir forests.

Apart from enabling growth in the understory and being a micro habitat within the forest for diverse animals and plants, gaps in old-growth Douglas fir forests have an impact on the soil characteristics in the gap. For instance, in their study on soil properties in old-growth Douglas fir forests in Western Cascade Mountains of Oregon Griffiths, Gray & Spies (2010) found that Soil moisture was elevated in gaps of 50 meters. These elevations in moisture in the soil in gap areas were attributed to decreased demand due to reduced evapotranspiration caused by loss of fine root mass were also documented in European beach forest gaps (Galhidy et. al., 2006) and a tropical wet forest (Denslaw et. al., 1998). Consequently, the loss of canopy cover increases the amount of moisture available to the emerging understory.

The loss of canopy cover in an old growth forest exposes the created gap to increased light and solar intensity. As such, temperatures in the gap usually rise and in the process affect the microclimate in the gap compared to a forest area that does not have a gap. In addition, the denitrification potential in the gap area is also elevated when compared to forest area with no gap (Gray & Spies, 1997). A direct consequence of the exposure of the gap to changing biotic factors is the determination of the succession process. Nonetheless, gaps in old-growth Douglas fir forests are not only important for the growth of the understory but also help to shape soil structure for the long-term survival of the entire system.

The exposure of the previously covered area to radiation also has an effect on the diversity of the plant communities in the old growth forest. Plant seeds that have been lying dormant due to a lack of solar radiation for germination are exposed to it and germinate. This leads to patch areas of old growth forests having a higher diversity of plants compared to the closed canopy area and the beginning of understory reinitiation (Spies & Franklin, 1989). Nonetheless, the conditions created in the gap areas influence the rate of succession in these patches of old growth. In addition, new physical and chemicals activities are initiated on the newly exposed forest floor.

Large Snags and Logs

Although gaps are an important feature in an old growth Douglas fir forest that is necessary for regeneration and succession, the factors leading the creation of the gaps to vary. While most gaps are as a result of forest fires that consume these coniferous forests now and then, other gaps are as a result of the death of the canopy trees and understory (Sprugel et al., 2009). As a result of the deaths, large snags and logs become an essential aspect of old growth Douglas fir forests.

Although large live trees in old-growth Douglas fir forests play a unique role in the forest ecosystem, their death is a natural process that is also the beginning of life for other trees, the release of nutrients back into the soil, and creation of new habitats for various animals and plant. For instance, the death of a large canopy tree creates a gap in the forest for other plants in the understory to grow and succeed the dead one (Sprugel et al., 2009). However, apart from just making space for new growth, the dead trees are a source of support for climbing plants which use them for their upward mobility.

The death and fall of large Douglas fir in an old growth forest is usually the beginning of the many processes that impact the structure of the forest. For instance, the process of decay that is essential for the release of nutrients back into the soil in the long term begins after the death of tree (Sprugel et al., 2009). Through both chemical and physical breakdown, nutrients and humus are released back into the soil and in the process not only influences its fertility but also improves its structural characteristics (Gray & Spies, 2010). Consequently, the death of old growth trees is not entirely bad as long as the process thereafter follows a natural pattern of decomposition.

In addition, dead logs are also responsible for reducing soil erosion by offering cover to slow down the action of runoff water. As such, the water is given enough time to permeate the soil and recharge the underground water sources. Further, as stores of large organic matter, fallen logs enable the soil to absorb more water and in the process encourage diverse plant species to start colonizing the decomposing log. As such, apart from improving the nutritive content in the soil, decaying logs and snags improve the water retention capability of the surrounding soil as their woody substrates breaks down.

However, the most common attribute of the death of a large fir tree in an old growth forest is the creation of a habitat that attracts different animals to utilize the snag as food or habitation (Spies & Duncan, 2012). Primary borers like wood peckers will create nests in the tree by making holes. Further, boring insects will also make holes and lay eggs in the tree and help with its degeneration. In addition, snags in old-growth Douglas fir forests provide an ideal habitat for bats to roost in. For instance, Arnett & Hayes (2009) in their study observed that three species of bats that they studied regularly used Douglas fir snags as roosting sites. However, among the three species of bats, one species showed a preference for larger sized and older snags. As such, snags in old-growth Douglas fir forests are not only important for soil structure but also as a source of home and food for other animals.

As part of the structural composition of an old growth Douglas fir forest, dead logs and snags play a critical role in providing the necessary habitats for diverse vertebrates and invertebrates (Frasnklin & Spies, 1991). For instance, the presence of roosting sites for bats in old snags leads to the deposit of guano that becomes available and integrated into the soil as the snag breaks down with time. Furthermore, fallen logs provide a habit for many vertebrates and invertebrates such as insects and wood rats. The presence of these organisms also attracts their natural predators such as the Northern Spotted Owl which is dependent on old growth forests for its survival (Zabel, McKelvei & Ward, 1992). As a result, the fallen logs and snags as a structural component of old growth Douglas fir forests play a key role in ensuring ecological integrity for the survival of other species.

General Public Piece

The Douglas fir forests are an integral component of the forest ecosystem in the Western Hemisphere. However, their spatial distribution and age character have been severely compromised due to human actions. The old-growth Douglas fir forests have been harvested for wood and timber leaving small patches of old growth that pale in comparison with former range. However, scientists have tried to understand the structure of these forests to give guidance on the best management techniques to preserve and also allow for regeneration. In addition, understanding what an old growth forest is relative to age has enabled scientists to establish why a natural succession process is better than commercial plantations and in the process helped preserve biodiversity.



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