Christina Sheldon

Photosynthesis Rates are Falling

For our experiment, we tested the question ‘Do deciduous trees’ rates of photosynthesis decrease in fall as compared to coniferous trees?’ Our hypothesis was that the change in CO₂ in deciduous trees declines in fall because photosynthesis rates slow. Since deciduous trees have to go almost dormant in the winter, we predicted that the rate of photosynthesis would slow down in fall and would be lower than the photosynthesis rate in a tree that stays green and active all year long.

To test our hypothesis, we measured the photosynthesis rate of juniper, our coniferous tree, in a gas chamber with juniper needles sealed by a CO₂ meter. We used the average from the entire class’ data (-16.76 ppm/min/g), which was obtained through five test groups which measured juniper needles CO₂ levels in a light-dark-light trial; the CO₂ rate from the dark period (respiration) was subtracted from the average of the light trials to isolate the rate of photosynthesis. We then measured the CO₂ levels of two trials of hawthorn leaves (our deciduous tree) using the same light-dark-light process. Our average for these trials was -0.57 ppm/min/g.

We ran a t-test on our data and determined that the photosynthesis rate of the juniper needles was significantly greater than that of the hawthorn leaves (p-value: 6.3e-5). This result validated our hypothesis, but only on a very small scale. The main problems with our experiment were the small sample range and the time of year. We only used one kind of each coniferous and deciduous tree for our experiment, which limits the applicability of our results. While the results did correlate with our hypothesis, to truly test whether deciduous trees’ photosynthesis rates decline in fall, we would need to take many measurements of different kinds of trees throughout the year to determine the ‘normal’ summer levels and how they compare to those in the fall. Because our samples were so limited, we cannot say that our hypothesis is completely valid.

Our results were consistent with those achieved by Holly Schor and Stanley Wun (Rates of Photosynthesis of Broad Leaves and Needle Leaves), in that there is a significant difference in the rate of photosynthesis in coniferous and deciduous trees. Their experiment was performed in the same way as ours, and they achieved very similar results. Because our results were similar, it is more likely that our hypothesis could be valid, but much more extensive experimentation is required before a conclusion can be made. If the hypothesis is valid, then it could be argued that coniferous trees are more beneficial to the atmosphere than deciduous trees, because they photosynthesize year-round, instead of only during warmer weather.

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