The changes in plant leaf size and shape in response to the environment is a core in ecology, as it is closely related to plant carbon assimilation and growth strategies, thus influencing productivity². Though people are aware of the relationships between leaf size and temperature, and between precipitation and radiation on a global scale^3,4, little is known about the specific responses of leaf size and shape to habitat changes.

Recently, Gallaher et al.¹ found leaf shapes of these herbaceous plants are different, depending on whether they come from the forest understories, forest margins, or from open habitats¹. In general, leaves from open habitats, which are exposed to more solar radiation and more arid climates, are smaller (Fig. 1). Thus, leaf size can be used to indicate its habitat type; for example, smaller and thicker leaves can often be found in disturbed forest margins and open habitats. This information has the potential to predict the ecological consequences of rapid changes in land use⁵.

Leaf width:length ratios, leaf areas, and absolute and relative leaf widths become larger as one goes from open habitats, to forest margins, to forest understories. Thus, leaf areas of leaves from under the forest or near the forest margin are substantially larger than those from open habitats (Fig. 1). There are no differences in leaf length among the three habitat types; moreover, leaf size and shape are not significantly different among photosynthetic types. Interestingly, the relationships between leaf area, the leaf width for the highest weekly solar irradiance (the higher the value, the stronger the radiation), and the mean annual moisture index (the higher the value, the more humid the environment), are opposite. Leaf areas and widths decrease with an increase in radiation or a reduction in humidity, especially for leaves in open habitats and forest understories. These relationships, on one hand, reflect the inherent negative correlation between radiation and humidity, and suggest the important role of habitats in determining the responses of leaf size and shape to radiation, e.g., when keeping radiation constant, leaf widths are found to decrease more in forest understories than that in open habitats. The patterns showed here imply that future climate change, especially radiation changes, may exert more influence on grasses within the forests, which is crucial for understanding leaf size changes in fragmented habitats.

Fig. 1. A diagram depicting the size and shape of herbaceous plants in different habitats shape by solar radiation. From left to right are images corresponding to open habitat, forest margin and forest understory, respectively. Leaves become larger and wider, showing the effects of solar radiation on leaf photomorphogenesis. Credit: Jianguo Gao.

The effects of light on the size and traits of leaves are mainly owing to its energy and its signal attributes. Energy is a key factor in affecting leaf size. An interesting question is whether the leaves of herbaceous plants in tropical forests are smaller than those of temperate open habitats because they come from warmer environments. We think that this is probably not the case. This is because in addition to light energy, the plant’s photoperiod may also have an important influence on leaf size, which is an idea that has not been fully considered in the study of Gallaher et al¹. Therefore, future research may require further quantification of photoperiods or study the effects of daily or seasonal solar radiation on leaf morphology.

Climate change factors, such as radiation changes and precipitation, decrease the size of the leaves year after year. Recently, Stropp et al⁶ found that the leaves of six Amazonian tree species have gradually become smaller since the 1970s. Similarly, Guerin et al⁴ reported that the leaf width of Dodonaea viscosa subsp. angustissima in South Australia has decreased by 2 mm over the past 127 years. Considering the recently enhanced solar radiation⁷, and the increase in human disturbances caused by land use on a global scale⁸, we may see an increase of smaller and narrower grass leaves in the field, such as those of herbaceous bamboo species (Puelioideae and Olyreae).

References

1.        Gallaher, T. J. et al. Leaf shape and size track habitat transitions across forest–grassland boundaries in the grass family (Poaceae). Evolution 73, 927–946 (2019).

2.        Cayssials, V. & Rodríguez, C. Functional traits of grasses growing in open and shaded habitats. Evol. Ecol. 27, 393–407 (2013).

3.        Wright, I. J. et al. Global climatic drivers of leaf size. Science 357, 917–921 (2017).

4.        Guerin, G. R., Wen, H. & Lowe, A. J. Leaf morphology shift linked to climate change. Biol. Lett. 8, 882–886 (2012).

5.        Stevens, J. T., Safford, H. D., Harrison, S. & Latimer, A. M. Forest disturbance accelerates thermophilization of understory plant communities. J. Ecol. 103, 1253–1263 (2015).

6.        Stropp, J. et al. Drier climate shifts leaf morphology in Amazonian trees. Oecologia 185, 525–531 (2017).

7.        Wild, M. et al. From dimming to brightening: decadal changes in solar radiation at Earth’s surface. Science 308, 847–850 (2005).

8.        Haddad, N. M. et al. Habitat fragmentation and its lasting impact on Earth’s ecosystems. Sci. Adv. 1, e1500052 (2015).