An investigation into photosynthesis in deciduous and coniferous trees

Michal Ford, Amy Turner


Coniferous trees and shrubs retain their leaves throughout the year, thus they continue photosynthetic processes. However, as deciduous trees shed their leaves in the fall, they enter a state of dormancy. The goal of this study is to determine whether these trees need a higher rate of photosynthesis because of the reduced time frame of growth compared to coniferous trees. Based on previous research, we hypothesize that deciduous trees have an increased rate of photosynthesis in contrast to coniferous trees to compensate for a shorter growth period.

Three different samples each of deciduous and coniferous leaves were collected. Each paired leaf sample was obtained under similar environmental conditions. This ensured reliability by collecting samples from trees with comparable soil, sunlight strength, and water sources. Each sample was prepared, weighed, and placed in a beaker with a CO2 sensor. The samples were tested in both dark and light conditions, each with a temperature buffer between the light source and the beaker, for 10 minute intervals. The lighted trial measured the rate of cellular respiration and the rate of photosynthesis while the dark trial measured only the rate of cellular respiration. Measurements were recorded throughout the trials, indicating the rising or falling amounts of CO2 within the beaker. If the rates of photosynthesis were significantly different, the samples should show a markedly different CO2 percentage remaining within the beaker.

The rate of photosynthesis was significantly lower for the coniferous samples, averaging at -3.38 g CO2 ppm/min, compared to the deciduous sample mean of -13.36 g CO2 ppm/min (P=0.047). These results are consistent with the reasoning that deciduous foliage could accelerate the photosynthetic process to compensate for the reduced growth time period. The p-value shows the significant differences in the rates of photosynthesis. The validity of our experiment was shown by a consistent effective measurement of the rate of photosynthesis. We conducted three separate trials on each type of tree, each trial produced similar outcomes. This evidence supports our hypothesis; therefore, we failed to reject our hypothesis.

A source of possible error could be the anatomical differences of the leaves. The different surface areas of the leaves and needles may affect the rate of photosynthesis. In addition, the light and dark conditions applied were artificial, perhaps causing the samples to absorb CO2 differently than under natural conditions. In the future, we would like to explore whether the structure of the leaves or needles affect the rate of photosynthesis. Also, do different species of leaves conduct photosynthesis at different rates? The structures of the leaves and needles may differ in order to utilize different photosynthetic mechanisms in different seasons. Although the more compact needles of the coniferous trees have a slower absorption of CO2 and sunlight, perhaps the needles are able to retain more warmth, which prevents a significant loss of heat during the winter while the deciduous trees are unable to do this. All these possibilities still tend to support our conclusion that deciduous and coniferous trees have clearly different rates of photosynthesis.