The influence of leaf size and shape on leaf thermal dynamics: does theory hold up under natural conditions?

• Published in: Plant, cell and environment Vol. 40; no. 2; pp. 237 - 248
• Main Authors: Leigh, A., Sevanto, S., Close, J.D., Nicotra, A.B.
• Format: Journal Article
• Language: English
• Published: United States Wiley Subscription Services, Inc 01.02.2017
• Subjects: Air; boundary layer; cooling time constant; effective leaf width; Image Processing, Computer-Assisted; infrared imagery; leaf dissection; leaf shape; leaf size; leaf temperature; Models, Biological; Plant Leaves - anatomy & histology; Plant Leaves - physiology; Proteaceae; Proteaceae - anatomy & histology; Proteaceae - physiology; Temperature; thermal dynamics; Time Factors; cooling time constant; leaf dissection; boundary layer; leaf shape; infrared imagery; leaf size; leaf temperature; effective leaf width; thermal dynamics
• ISSN: 0140-7791; 1365-3040
• DOI: 10.1111/pce.12857

Laboratory studies on artificial leaves suggest that leaf thermal dynamics are strongly influenced by the two‐dimensional size and shape of leaves and associated boundary layer thickness. Hot environments are therefore said to favour selection for small, narrow or dissected leaves. Empirical evidence from real leaves under field conditions is scant and traditionally based on point measurements that do not capture spatial variation in heat load. We used thermal imagery under field conditions to measure the leaf thermal time constant (τ) in summer and the leaf‐to‐air temperature difference (∆T) and temperature range across laminae (Trange) during winter, autumn and summer for 68 Proteaceae species. We investigated the influence of leaf area and margin complexity relative to effective leaf width (we), the latter being a more direct indicator of boundary layer thickness. Normalized difference of margin complexity had no or weak effects on thermal dynamics, but we strongly predicted τ and ∆T, whereas leaf area influenced Trange. Unlike artificial leaves, however, spatial temperature distribution in large leaves appeared to be governed largely by structural variation. Therefore, we agree that small size, specifically we, has adaptive value in hot environments but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection. Summary Leaf thermal dynamics are strongly influenced by the two‐dimensional size and shape of leaves through boundary layer effects, so hot environments are expected to favour selection for small, narrow or dissected leaves. Using thermal imagery of leaves under field conditions, we found that leaf dissection had no or weak effects on leaf thermal dynamics, but effective leaf width strongly predicted both the cooling time constant and leaf‐to‐air temperature difference. Leaf area influenced the temperature range across the laminae, apparently governed largely by structural variation within leaves. Therefore, we agree that small size has adaptive value in hot environments but not with the idea that thermal regulation is the primary evolutionary driver of leaf dissection.