Experimental evidence for correlation between leaf shape and temperature in different plant species: suggestions for inferring paleoclimate

Experimental evidence for species-dependent responses in leaf shape to temperature: Implications for paleoclimate inference

by: Melissa L. McKee, Dana L. Royer, Helen M. Poulos

Summarized by: Mckenna Dyjak

What data were used?: Four species of seeds from woody plants were used: Boxelder Maple (Acer negundo L.), Sweetbirch (Betula lenta L.), American Hornbeam (Carpinus caroliniana Walter), and Red Oak (Quercus rubra L.). Three species from transfered saplings were also used: Red Maple (Acer negundo), American Hornbeam (Carpinus caroliniana Walter), and American Hophornbeam (Ostrya virginiana  K.Ko(Mill.)ch). The types of species were chosen because they each exist naturally along the east coast of the United States and have leaf shapes that vary with climate.

Methods:The seeds and saplings were randomly divided into either warm or cold treatments. The warm treatment cabinet had a target average temperature of 25°C (77°F) and the cold treatment cabinet had a target average temperature of 17.1°C (63°F). After three months, five fully expanded leaves were harvested and photographed immediately. The images from the leaves were altered in Photoshop (Adobe Systems) to separate the teeth (zig-zag edges of leaves) from the leaf blade (broad portion of the leaf). The leaf physiognomy (leaf size and shape) was measured using a software called ImageJ. The measured variables were tooth abundance, tooth size, and degree of leaf dissection. The degree of leaf dissection or leaf dissection index (LDI) is calculated by leaf perimeter (distance around leaf) divided by the square root of the leaf area (space inside leaf). The deeper and larger the space between the teeth of the leaf, the greater the LDI.

Results:The leaf responses to the two temperature treatments are mostly consistent with what is observed globally: the leaves from the cool temperature treatment favored having more teeth, larger teeth, and a higher LDI (higher perimeter ratio). However, it was found that the relation between leaf physiognomy (leaf size and shape) and temperature was specific to the type of species. 

Fig.1 Tooth abundance in relation to temperature. The colder temperature (in blue) correlates with an increasing number of teeth in generally all of the plant species tested. The differences can also be observed in the pictures present below the graphs. More leaf teeth (serrated edges along the margins) can be seen under the cold treatment.

Why is this study important?: Paleoclimate (past climate) can be determined by using proxy data which is data that can be preserved things such as pollen, coral, ice cores, and leaves. Leaf physiognomy can be used in climate-models to reconstruct paleotemperature from fossilized leaves. This study supports the idea that leaf size changes correlate with temperature change. However, the responses varied by species and this should be taken into account for climate-models using leaf physiognomy to infer paleoclimate. 

The bigger picture: Studying paleoclimate is important to see how past plants reacted to climate change so we have an idea how plants will respond to modern human-driven climate change.

Citation: McKee ML, Royer DL, Poulos HM (2019) Experimental evidence for species-dependent responses in leaf shape to temperature: Implications for paleoclimate inference. PLoS ONE 14(6): e0218884. https://doi.org/10.1371/journal.pone.0218884

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