Tree-Species Density across Environmental Gradients Zones - Research Paper Example

Tree-Species Density across Environmental Gradients Zones Abstract A selection of five common tree varieties were tested to determine environmental preferences. Among three regions of terrain, Valley, Mid-slope, and Ridgetop, counts were conducted of Oak, Maple, Beech, Pine, and Tulip Poplars . The tests determined whether these species prefer any one of the three zones over another. After quantification, differences were found to reject the Null hypothesis, indicating environmental factors that create differential survival outcomes for the five species based on which of the three regions the tree grows in. Introduction The purpose of this paper is to test and document the performance of five tree species and elucidate the environmental factors that define plant communities. Vegetative populations proliferate in a given section of land when conditions of moisture, temperature, and soil composition favor certain species at the expense of others. Experiments of this sort are crucial to enhancing our understanding of species diversity. Similar ecological conditions can exist between both mountainous regions and ravine systems. Either of these landscape systems can yield effects on communities of vegetation, providing niches where they may not otherwise exist. Landscape effects can be hydrological, thermal, and these locations are known to produce microclimates with influences on soil moisture and nutrient availability. The hydrological cycle in the region can experience variations in water infiltration within the soil, proclivities towards erosion/soil retention, movements of air fronts and dry adiabatic lapse rates; all relating to the slope of the terrain, or lack thereof. Terrain slopes affect soil types, which impact the growth of a variety of plants, which itself influences sunlight availability, which influences the success of subsequent vegetation. These combinations cause vegetation to arrange in communities favoring undergrowth/ground cover, shrubs, and canopy trees. Trees are of specific importance for this study, which will document tendencies of five species to colonize either of three environmental gradient-levels upon sloping terrain. These gradient-levels form zones that will naturally distribute similar plants along a horizontal axis to take advantage of the slope conditions most favorable to them, while the most variation will occur along a vertical axis, as soil and air conditions vary with elevation. The importance of all forms of biodiversity, including a range of plant species provides a variety of benefits to an ecosystem beyond simply targets for scientific study. Evidence exists that a more and diverse ecosystem is also more resilient to stress-factors. As species richness increases, stability of plant communities increases regardless of location. As a plant-diverse region is also more productive biologically, it is in the interest of investigators to understand plant community structure and diversity throughout a variety of environments. (Tilman et al. 2006) Five species across three zones will be quantified to determine whether any preference exists for one tree species over any others between the valley, mid-slope, and ridgetop regions. It is predicted that differential survival and growth outcomes will demonstrate that each of the three zones will favor some species over others. METHOD Five tree-canopy species were measured in three distinct growth zones across several categories to determine their relative ecological success. Oak, Maple, Beech, Pine, and Tulip Poplars were identified throughout the valley-bottom, mid-slope, and ridgetop areas for growth patterns. A sixth category was also documented for other trees species not included in the five main varieties. Plots of land will be divided into subplots, 50 meter tape will be used to determine a line perpendicular to the slope of the hill. A 10 meter area will be defined on either side of the slope. Flags will mark four corners of the plot. Each species found within the plot will be documented. For each species, Density and Dominance were measured in both absolute and relative terms, calculated in form of percentages. Absolute Density is the whole integer representing the number of individuals of each species. Relative Density is measured as the number of a particular species per square plot of land, divided by the total number of all other species on a plot of land, expressed as a percentage. Dominance for trees is measured by comparisons of trunk size and diameter, the cumulative basal area of the trunk. This provides an objective quantification of the competitive success of the five target species. Calculations were performed based on the total numbers found for each species, and compared with anticipated values to determine Chi squared values, in order to test the hypothesis. RESULTS Occurrence of each of the five species was documented, and sums were calculated for each of the three zones. Expected values were included to contrast with the actual trees reported. Chi squared values were computed based on differences between actual and expected incidences. Based on the total Chi squared values, a sufficient variance was detected to reject the Null hypotheses; being that no preference would be shown by any species for any zone. Different species do prefer certain environmental types. Oak: Oak trees grew best at the Mid-slope level with 19% density. Their percentages where 7% at the valley bottom, and just 1% density at the ridgetop. Maple: Maple was strongest at the Mid-slope level with 10% density. Their growth was 8% at the Ridgetop, and 3% density at the valley bottom. Beech: Beech was the most dominant overall, with 54% density at the Valley bottom. 42% density at Mid-slope, and 34% at Ridgetop. Pine: Pine exhibited no detectable density at any levels below the Ridgetop, where it constituted 29%. Tulip Poplar: Tulip Poplar had the greatest density at the Valley Bottom, at 23%. It exhibited 20% at the Ridgetop and 10% density at the Mid-slope. Others: Other varieties of trees beyond the five varieties under investigation where found most often at Mid-slope level, at 19% density. They occurred at 13% on the Valley bottom, and 8% density on the Ridgetop. DISCUSSION Pines in particular, had the most stringent requirements, being present only on the Ridgetop zones, this occurrence in and of itself, would prove sufficient to falsify the Null hypothesis, which states that none of these tree varieties prefer one elevation over the other. While beyond the immediate scope of this study, a graphic comparison opens the possibility that at the highest elevation, pine may be competing with Beech. While Beech grows at all zones, it is sharply reduced at Ridgetop level. Other species may also be in competition with pine to some extent, but Tulip Poplars are stronger at Ridgetop than Mid-level, while Maple suffers only a 2% decline at the higher elevation. There is the possibility that Pines may deprive resources from Oak at Ridgetop level, as the Oak population drops to 1%, but it is apparent that Oak’s most favored zone is Mid-slope. Greater total species diversity is often found in lower, valley regions, but for hardwood canopy trees, the results demonstrate that greater tree diversity is found at Mid-slope. This is evidenced by the dumping ground category of non-affiliated ‘other’ trees besides the five principle species. It may be theorized that the Mid-slope is the most tree-populous region due to diminished competition from grasses as soil conditions change. This allows for a gradient zone with useable nutrients, but with a more hospitable temperature and humidity for deciduous varieties. At the higher elevations closer to mountain peaks, the percentages of fertile topsoil diminishes as temperatures plummet; creating another specialized niche in which the Pine flourishes, at the expense of other varieties. In addition, other studies indicate that conifers may prefer higher elevations for the simple reason of seed survival. Pinecones and other conifer seeds are slower growing, and are outpaced by angiosperm seeds from deciduous trees. When temperature and soil conditions no longer favor angiosperm seeds, coniferous seeds are allowed to take root. (Coomes et al. 2005) While an adult tree is typically able to whether all but the most extreme environmental shifts; seed survival can never be excluded from studies of the establishment of plant communities. Moisture across environmental gradients of course, is crucial to seed germination and later survival. Other factors that can affect the growth of young trees, shrub plants, which in turn affect trees in turn also include litter, or leaf cover on the forest floor. Litter is mainly a factor in tree survival in space/time conditions where moisture is a limiting resource. Substantial ground coverage has the potential to enhance tree survival with low, or transient moisture. This is presumably due to the leaves trapping water, increasing its availability at the most critical time in a tree’s lifecycle. (Albrecht, et al. 2009) But litter on the forest floor is also influenced by decomposition rates, which can in turn influence tree growth, as described above. Bryophytes and Lichens have great variability in decomposition, with Bryophytes on average exhibiting a lower rate than either lichens or most vascular plants. All these factors influence the available soil, which in undeniably critical in the growth of survival of the target tree species across the three principal environmental gradients studied herein. (Lang et al. 2009) Other studies indicate that Oak and Beech exert an important influence the in Mid-slope zones for vascular plant ground cover. Beech trees are beneficial for understory plants benefiting from higher nitrate levels, while Oak may exhibit greater sunlight interception, influences grasses and shrub plants, which in turn affect the nutrient content available for other trees. (Bengtson et al. 2006) Plants of all species, especially those in or near areas of differing elevations take advantage of – and are in turn influenced by varying conditions. The environmental shifts in temperature, moisture, and soil composition depending on elevation form gradients, or niche-locations that delineate non-generalist plant communities. Rue Anemone and Skunk Cabbage, for instance – require precise conditions that limit them to a single gradient zone within a ravine or mountainous slope. (Fleischmann et al. 2006) The species diversity upon which an ecosystem’s resiliency depends requires this mix of ecosystem-specialist and generalist. (Levine, et al. 2009) In Himalayan and Trans-Himalayan mountainous ecosystems, for instance, types of vegetation can be delineated into 11 broad functional groups including 414 species of vascular plants. (56 families and 202 genera) The majority of course, are limited to the Valley bottoms, which nurture a wider range of vegetation by comparison with extreme elevations the world over. (Joshi et al. 2006) Even forest fires can lay bare necessary mineral-layers in forest soil and permit greater sun exposure for pine cones that can prove essential to additional, specialized niches in Ridgetop Pine communities. It is hardly an overstatement to say that some form of vegetation benefits from virtually any natural, environmental vagary. (Waldrop et al. 2000) Plant communities take on other discrete forms, in a stable vegetative eco-system. Three essential layers are readily identifiable in which known species can be grouped; based not only upon their appearance, but functional niche as well. A definitive identifier for whether a plant belongs to a particular layer would be its physiognomy; immediate physical traits and pattern of growth. The groundcover, including but not limited to grasses, is often taken for granted, but contains a rich mix of interdependent plant, animal, and microbial species. In most woodlands, the groundcover is likely to have the highest number of distinct species. Up to 80% of plant biodiversity may be in this layer alone. Shrub layers are typically beneath the trees, often with a maximum height of five meters, but most are far smaller. The density of the shrub layer affects the habitats it is able to support. Localized differences in the ecosystem become apparent depending on whether the growth is thick or patchy. The third layer of course, would constitute the forest canopy of hardwood trees such as those five included in this investigation. The rejection of the Null Hypothesis and subsequent data underscores the multi-various complexities of topographical environmental gradients as they pertain to forestation. Ideas for additional studies might include moisture measurements of the soil, as correlated with density for the principle tree species. These potential studies could verify far-reaching hypothesis concerning climactic variances along the slopes of mountainous regions and adjacent foothills, in addition to similar conditions found in ravines. REFERENCES Albrecht, M. A. and McCarthy, B. C. (2009), Seedling establishment shapes the distribution of shade-adapted forest herbs across a topographical moisture gradient. Journal of Ecology, 97: 1037–1049. doi: 10.1111/j.1365-2745.2009.01527.x BENGTSON, Per. FALKENGREN-GRERUP,Ursula. BENGTSSON, GÖRAN 2006. Spatial distributions of plants and gross N transformation rates in a forest soil. 24 MAY 2006. Journal of Ecology Volume 94, Issue 4, pages 754-764 DOI: 10.1111/j.1365-2745.2006.01143.x COOMES, David A. ALLEN, Robert B. BENTLEY, Warren A. BURROWS, 2005. The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering. Journal of Ecology Volume 93, Issue 5, pages 918–935, October 2005 Fleischmann, Eileen. and Keys, Robert. 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Nature 461, 254-257 (10 September 2009) | doi:10.1038/nature08251; Received 22 May 2009; Accepted 29 June 2009; Published online 12 August 2009 Tilman, David. Caldwell, Martyn.Reich, Peter. Knops, Johannes. National Science Foundation (2006, June 3). Ecosystems With Many Plant Species Produce More And Survive Threats Better. The National Science Foundation, Press Release 06-092 Waldrop, Thomas A. Welch, Nicole W. Brose, Patrick H. Elliott, Katherine J. Mohr, Helen H. Gray, Ellen A. Tainter, Frank H. and Ellis, Lisa E. (2000) Current Research on Restoring Ridgetop Pine Communities With Stand Replacement Fire. Proceedings: Workshop on Fire, People, and the Central Hardwoods Landscape GTR-NE-274 Read More