What Can Trees Tell Us About Climate Change?

We hear a lot about climate change and its future effects on our ecosystems, but climate change can be hard to sense as a person.  Maybe you noticed that Pennsylvania native trees seemed to turn their color a bit late this past fall season. Observing climate change indicators is a difficult task. Better technology facilitates researchers in making accurate observations about our climate. Using tree leaves as an indicator, with historical context, we can better understand how climate change has already affected our landscapes.  How exactly do leaves tell us about our current climate?

The shape, size, and form (morphology) of a leaf can give us clues about water availability and moisture of a region. While the morphology of a single tree’s leaves can vary greatly, statistical inference can give us a broad picture of the local climate.  Average leaf sizes of an area typically scale with water availability. That is, the wetter the climate, the more likely we are to see species that support leaves with a large surface area.

The leaf edge, or margin, gives us hints about the temperature range of the region. Leaf margin analysis is not very new. Modern interpretations began early in the 1900s with published observations by Irving W. Bradley and Edmund W. Scott. With few exceptions, the researchers showed an association between colder climates and the presence of serrate or toothed-margined leaves on woody, flowering plants. The same for smooth or entire-margined leaves in warmer climates. It is not yet understood completely why this association takes place, but that hasn't stopped researchers from postulating their theories.

Phenology, the study of biology in relation to climate, can also be used to help us monitor climate change. Robust satellite networks and remote sensing technologies enable researchers to create and store massive amounts of data. The United States Geological Survey routinely collects and publishes satellite data that captures the beginning of each spring growing season. We don't have to look back too far in time to notice changes in Pennsylvania.

Lengthened local growing seasons may be the most noticeable climate change indicator and yet, without remote sensing technologies, the gradual change can be too subtle for all to notice. All except the most natural and attentive persons. This is why phenological record-keeping is important. It is thought by some that the Chinese kept phenological records of cherry tree blossoms as far back as the eighth century.

Despite our remote sensing technologies, we still cannot compete with what nature has already created. Some of the most sophisticated sensing mechanisms on Mother Earth take place within the photosynthetic tissue of plants. Leaves!

Researchers from UC Berkeley are looking under the hood of these mechanisms to get a better understanding of paleoclimates. Using 180-million-year-old fossils of conifer needles, researchers have counted the number of stomata within a square inch of leaf surface.

Stomata are a cellular part of the leaf that regulates gaseous exchange within the leaf and the atmosphere. When the stoma opens and takes in carbon dioxide it also loses water through transpiration in the process. With light, plants need both carbon dioxide and water for photosynthesis. So, if the atmosphere is already rich in carbon dioxide, plants may gain an advantage by having fewer stomata. Less stomata means a lower potential of losing that precious water. Using this logic, researchers are using stomata as an indicator of carbon dioxide levels in the atmosphere at different times in Earth’s history.

Leaf fossils can be of further use simply by relating the fossil to its nearest living relatives. Leaf fossils in the past have been found in areas where its species no longer range. In such a circumstance, impressions of a specimens past range can be compared to its modern, natural ranges. Information like this can help researchers make better judgements on paleoclimate trends.

So, what do tree leaves tell us about climate change?

• Fossil leaves of species in an area can indicate where modern, related trees might have resided in ancient times and demonstrate geographic migration due to climate change
• The larger the leaves, the wetter the climate
• The smaller the leaves, the drier the climate
• The smoother the edge of the leaves, and the more species in a region with smoother leaf edges, the hotter the climate
• The more teeth in the edge of the leaves, and the more species in a region with toothy leaf edges, the cooler the climate
• The larger and fewer the stomata of a tree within a single species, the higher the CO₂ levels were at the time of that leaf growth
• The small and denser the stomata of a tree within a single species, the lower the CO₂ levels were at the time of that leaf growth

If you have any questions or are interested in collaborating with FOCCE, please reach out to Melissa Kreye at mxk1244@psu.edu

References

Leaf Margin Analysis. (n.d.). Digital Atlas of Ancient Life. Retrieved January 31, 2023.

Peppe, D. J., Royer, D. L., Cariglino, B., Oliver, S. Y., Newman, S., Leight, E., Enikolopov, G., Fernandez-Burgos, M., Herrera, F., Adams, J. M., Correa, E., Currano, E. D., Erickson, J. M., Hinojosa, L. F., Hoganson, J. W., Iglesias, A., Jaramillo, C. A., Johnson, K. R., Jordan, G. J., & Kraft, N. J. B. (2011). Sensitivity of leaf size and shape to climate: global patterns and paleoclimatic applications. New Phytologist, 190(3), 724–739. DOI.org/10.1111/j.1469-8137.2010.03615.x

The story in the stomata - Understanding Evolution. (2021, June 5).

Will changes in phenology track climate change? A study of growth initiation timing in coast Douglas-fir | U.S. Geological Survey. (n.d.). www.usgs.gov. Retrieved January 31, 2023.