Several characteristics of plants have been studied to uncover why there are physical differences between and among plant species. These differences can be recorded and then applied to future research regarding how current climate change will alter the distribution and abundance of Earth’s natural resources. For example, stomata measurements can reveal the geographic distributions of plants because they are the primary means of gas exchange in vascular plants. Stomata are found on the underside of the leaf and control gas exchange by varying their densities and lengths to uptake maximum rates of CO2 (Pearson 2012). When the stomata are open, essential resources can be absorbed by the leaves of the plants, particularly in optimal environmental conditions i.e., areas with large abundances of CO2, lower atmospheric temperatures and prolonged periods of sunlight (Lawson et al. 2014).
Another commonly studied aspect is the needles of conifer trees and how their differing sizes and thickness reflect local conditions, with thinner needles signifying that the local area may lack certain resources necessary for optimal growth within that environment. One species chosen for this experiment was the Douglas-fir because they are capable of growing hundreds of feet tall and the needles of one tree can be compared to reveal how different temperatures, sunlight, and other environmental variants alter tree growth.
The aim of this study is to incorporate these two aspects by comparing conifer stomata and needle characteristics, focusing on the Pacific Yew and both the upper and lower canopy of Douglas-fir. Another important note regarding these two species is the different environments they survive best in, with Pacific Yew exhibiting similar rates of CO2 uptake in sunny or shaded environments, while Douglas-fir prefer areas exposed to increased amounts of sunlight (Mitchell 1998). I hypothesize that needle thickness, stomatal density, and stomatal abundance will be greatest in the upper canopy of the Douglas-fir because there is less competition for essential resources, however, Pacific Yew will have the greatest needle area due to their shade tolerance.
Methods
We traveled to the HJ Andrews Institute, within the Willamette National Forest, and acquired needles from a Pacific Yew and both the upper and lower canopy of a Douglas-fir. Prior to arrival, ‘LabQuest 2’ sensors were used to acquire the relative humidity and temperature of each treatment group over a course of six days, and averaged. To attain needle samples from the upper canopy of a hundred-foot-tall Douglas-fir, a rope was attached to the utmost branch and then ascended to reach the targeted area. These needles were then taken back to the University of Oregon lab, measured using calipers, and finally, the stomatal impressions were analyzed under a microscope magnification of 400x. These stomatal impressions were acquired by painting clear nail polish over the stomata, placing tape on the stomata to attain their imprint, and then the tape was transferred to a microscope slide so that individual stomata data could be recorded. This process was conducted twice for each tree species to prevent bias results and to see whether patterns emerged which connected stomatal and needle changes to their correlating tree species and location on the tree.
Results
Needle Characteristics
Needle area was significantly larger in the Pacific Yew (46.43 mm2) than in the lower canopy (30.97 mm2) and upper canopy (22.15 mm2) of the Douglas-fir. However, needle thickness was greatest in the upper canopy of the Douglas-fir (0.455mm) and proceeded to get thinner from the Pacific Yew (0.427mm) to the lower canopy of the Douglas-fir (0.39 mm). Analyzation of the mean needle area and needle thickness within the three treatment groups uncovered p-values less than 0.05, thus the null hypothesis is rejected. Results revealed that needle area was greatest in the Pacific Yew, while the upper canopy of the Douglas-fir held the thickest needles.
Stomatal Characteristics
The stomatal density data revealed that there were significantly more stomata (1.391 stomata/mm2) of larger densities (Figure 1) on the upper canopy needles of the Douglas-fir than on the needles in the lower canopy and the Pacific Yew. We found no significant correlation between stomata length and tree type or height (p ≥ 0.05).
Environmental Characteristics
Relative humidity of the environment of the Douglas-fir upper canopy (78.33%), Douglas-fir lower canopy (88.89%) and Pacific Yew (95.87%) increased, respectively. The temperatures of these treatment groups, however, resulted in contradictory findings with the upper canopy of the Douglas-fir having the highest temperature (14.63ºC) and the Pacific Yew having the lowest (4.59ºC).
Discussion
The data from this experiment supported my hypothesis that tree height and access to sunlight impact the abundance and relative size of the stomatal structures and needles. The results of this experiment revealed that the tallest treatment group, the upper canopy of the Douglas-fir, contained attributes that enabled the species to uptake as much CO2 as possible and thus undergo photosynthesis for longer periods of time. These findings are logical because there is better water use efficiency in the upper canopy of Douglas-fir, which is crucial because water flows out of leaves at faster rates than CO2 flows in. This direction of flow is due to the fact that water flows along energy gradients, from high to low energy, and thus needs to be utilized as efficiently as possible (Woodruff et al. 2009). These results also reveal that the Douglas-fir contains greater photosynthetic rates than the Pacific Yew. This is because greater stomatal densities are positively correlated with greater photosynthetic rates due to their increased storage capacity for CO2.
The one optimal trait that the upper canopy of the Douglas-fir did not contain was needle size. In turn, the data revealed that the Pacific Yew withheld the largest needles of the three treatment groups. This is explained by analyzing the texture of the Pacific Yew needles and noting the thick, waxy (cuticle) on their leaves, a trait evolved to prevent excessive water loss (Pearson 2012). This coating is imperative for the Pacific Yew because of their wide range of habitats and moisture stress can result in the closure of stomata, thus preventing further growth of the plant due to a reduced amount of CO2 (Restaino et al. 2016). This moisture stress is just one explanation as to why the Pacific Yews’ physical characteristics are significantly smaller than those of the Douglas-fir. Other factors can contribute to decreases in stomatal and needle size, but a major determinant in this study was the temperature of the tested area. The temperature of the Pacific Yew habitat was nearly 10ºC cooler than the upper canopy of the Douglas-fir which is due to the shaded location of the Pacific Yew in a closed canopy forest. This location limited the amount of sunlight attained by the Pacific Yew, and the colder temperatures reduced the rates at which the tree species could photosynthesize.
These results reveal that the Douglas-fir are better adapt to current climate conditions and are sequestering and releasing CO2 at steady rates. It is imperative that studies resembling this one be conducted annually because the environment is not static, and data of the same species can differ over time and between localities. Moreover, the findings of this experiment can be used to research how increasing global temperatures affect species, and whether these impacts could later result in reductions in tree size and photosynthesis rates. This is crucial to uncover because as the temperature continues to increase, deficit-related stress among plants will become more prevalent and tree size will begin to decline as a result. This decline in size will have harmful implications on the human population because not only will the amount of CO2 that trees remove from the atmosphere be greatly reduced, but so will the amount of oxygen that they produce. This decline in oxygen levels could cause severe impacts on ecosystems and reduce the biodiversity of species if they cannot adapt and adjust to changing environmental conditions.