Trees have different adaptations to cope with winter. Bud scales or cataphylls (outermost leaf forms of a bud) have been said to provide a physical barrier that protects the new leaves from the harsh environment but over 40 flowering plant families do not have them. This raises the question why some woody plants have bud scales and others do not.
Kristel Schoonderwoerd and William Friedman at Harvard University found that bud scales are not necessarily an adaptation to winter environments of temperate trees such as walnuts and hickories. The researchers delicately followed the formation of over 2,000 leaves, used micro-CT scanning and reconstructed an evolutionary tree of terminal bud-types in Juglandaceae. The team proposes that bud scales did not evolve to protect buds from winter conditions primarily but rather, to protect the buds from environmental conditions in the growing season.
The experiments were set up at Arnold Arboretum of Harvard University in 2019 and consisted of six tree species: Bitternut hickory (Carya cordiformis), Shagbark hickory (Carya ovata), Black walnut (Juglans nigra), Japanese wingnut (Pterocarya rhoifolia), Chinese wingnut (Pterocarya stenoptera) and Platycarya strobilacea (commonly called Platycarya).
Between April and October, the researchers non-destructively measured every leaf and resting bud every month. The measurements of individual leaves along every axis (petiole) allowed the scientists to track if all leaves grow at the same rate, to the same size or whether perhaps in the middle, the leaves are wider and longer than the ones at the base.
The team also collected terminal buds in winter, spring and summer months for micro-CT scanning. Based on previous research, a molecular evolutionary tree was reconstructed of 48 species in Juglandaceae.
Tracking bud formation can reveal how trees adapt to different conditions. For example, previous research has shown that plants are flowering earlier than in the past due to climate change, and phenological studies can help farmers chose later or earlier flowering varieties of crops in order to reduce frost damage.
Schoonderwoerd and Friedman found that bud and leaf growth varied over multiple seasons on the six tree species. The starting bud shape did not determine how the rest of the leaves would grow but overall, the overwintering leaves were typically smaller and had shorter lives. The relative flexibility in growing patterns shows different adaptations and strategies throughout a tree’s life.
The leaves grew differently on the naked bud-type tree compared to the bud scale-bearing trees. The leaves of the naked bud-type species (Pt. stenoptera) had the same proportions along the petiole but the proportions varied on the three bud scale-bearing species (Pt. rhoifolia, Pl. strobilacea and C. ovata). The bud scale formation leads to transitional leaf forms in the growing season that could actively respond to environmental cues, followed by the production of foliage leaves. The trade-off between delaying the foliage leaves and having bud scales seems to be in the growing season, rather than the dormant stage (winter). So, maybe bud scales did not evolve to protect buds from winter conditions primarily but rather, to protect the buds from environmental conditions in the growing season.
“With this work, we aim to show that the evolution of resting bud structures in some temperate trees has not been shaped by selective pressures of the winter environment, such as cold and freezing temperatures, alone”, Schoonderwoerd and Friedman write.
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Schoonderwoerd, K.M. and Friedman, W.E. (2022) “Interspecific morphological variation in Juglandoideae resting bud organization: a winter’s tale?,” Annals of Botany, 129(6), https://doi.org/10.1093/aob/mcac050