How big is a leaf? It’s an important question as leaf size affects many properties, such as its ability to capture light in shade or susceptibility to herbivores. It might seem a simple task to find out, but leaf size data has been catalogued for only around 8% of species. Schrader and colleague note that for many other species, there are taxonomic descriptions. They have developed a new technique for estimating leaf area based on length, width and, because leaves are not rectangular, a description of the shape.
Understanding leaf size is vital because larger leaves can have very different traits to smaller leaves. Schrader and colleagues point out that leaf size will also correlate with larger flowers, larger internodes and thicker twigs. They add that other factors like leaf temperature can vary, affecting photosynthesis, transpiration and respiration.
Before image recognition software, working out leaf area was a labour-intensive task. This demand means there is a large gap in knowledge in described species. Even now, it’s not always practical to use computerised techniques in situ.
The current practice is to multiply length and width by ⅔ to get leaf area, after Cain & Castro. Schrader and colleagues comment that even for Brazilian trees, where the approximation was developed, this would lead to over- or under-estimation of size in most cases. For this reason, the authors tried to quantify the effect of leaf shape to improve the accuracy of estimates and compared their results against image recognition to test the estimation method’s validity.
“Leaf shape-specific CF [correction factor] performed better in both accuracy and precision than the CF of 2/3 to nearly all leaves regardless of their shape, which was, especially for lobed leaves, highly biased,” write the authors. “We see great potential in applying our approach to data obtained from species descriptions or large databases. In addition, this method can be used for fieldwork or on sensitive herbarium vouchers, especially when non-destructive in-situ measurements are needed. When only taxonomic information is available, family-specific CF can be used as an alternative to leaf shape. As such, leaf size estimation based on the product of leaf length, width and a leaf shape or taxonomic-specific CF can fill gaps in leaf sizes for many plant species worldwide with confidence.”
Other factors like the smoothness of the edges of the leaves can also have an effect. Serrations on the edges will also reduce the leaf area, but the team found this had a much smaller effect.
“Theoretically, CF could be provided for each combination of leaf shape, level of incision and other leaf morphologies. This may enhance precision of leaf size estimation, but would also require stricter categorisation of leaf shapes and adoption of standard morphological terminology globally. Given that current categorisations offer different advantages across the breadth of leaf enthusiasts we refrain from suggesting such an approach. An intention with our approach was to provide an easily applicable and reliable solution to estimate leaf size across different data resources. Very fine categorisation of leaf morphologies would offset this making easy and fast leaf size estimation cumbersome,” write Schrader and colleagues.
As well as speeding up work on modern habitats, the team also see applications elsewhere. For example, the formulae can be applied where no current live leaves exist – like finding leaf areas for plants preserved only as fossils. In this way, the team believe their work can support more research. They conclude, “We hope that understanding scaling functions of plant dimensions could help to fill major gaps in knowledge bringing us closer to a complete understanding of morphological variation in in the world’s plants.”