Chapter 5 164 showcase distinct lifestyles and evolutionary insight into shade responses. They are case studies with varied responses into how plants adapt and compete within diverse environmental contexts. Pioneer plant species are adapted to colonize harsh, sterile and sun-exposed environments and grow quickly to take advantage of available resources before larger competitors arrive (Miyazawa et al., 2014; Sottosanti, 2023). However, as the environment changes and the intermediate species grow taller, the shade they cast can deprive the pioneer species of adequate sunlight (Sottosanti, 2023). In response to this shade, the species composition undergoes a transition from pioneers to a mixed ecosystem with dominant plants favoring high light and understory species thriving in the shade of the forest canopy (Hagen et al., 2015; Avalos and Avalos, 2019). For these shade-tolerant species, it is not beneficial to produce shade avoidance responses in deeply shaded habitats, such as forest understory, as they would suffer reduced performance without any benefit to be gained. In contrast, shade-tolerant species have a slower growth rate and a more restricted capacity to respond to sudden environmental changes (Coverdale and Agrawal, 2021). Shade-tolerant species tend to have larger seeds, high wood density, high leaf specific mass, low photosynthetic rates, and slow growth (Taiz et al., 2010; Coverdale and Agrawal, 2021). Shade tolerance appears to involve efficient light harvesting at constant low irradiance and efficient harvesting of sunflecks (Henry and Aarssen, 1997). Shade avoidance responses have possibly evolved with shade, as early as the Devonian period, and are possibly more ancient than shade tolerance in plants (Casal, 2012). In contrast, the evolution of shade tolerance is associated with attenuation of shade avoidance and reduced phenotypic plasticity in certain plant species such as milkweeds and Geranium robertianum (Gommers et al., 2017; Coverdale and Agrawal, 2021). Therefore, is evidence that in these lineages, shade avoidance responses predate the evolution of shade tolerance in plants. 5.1.2 Stem evolution and diversity The evolution of terrestrial life ushered in a period of intense competition for sunlight among plants, where the ability to grow taller and reach the canopy ahead of competitors conferred a distinct advantage. However, as plants increased in height and expanded along their primary axis, they encountered new challenges. These challenges included the need for innovative structural support for aerial organs and an efficient mechanism to transport water and nutrients to greater heights, beyond the limitations of capillary forces (Hetherington et al., 1998; Parra et al., 2010).
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