One of the most significant signs of spring is the budding of leaves. It is a beautiful sight to see the leaves burst, curing seasonal depression one leaf at a time! Leaf development is a natural process triggered by rising temperatures and increasing daylight. However, the process is cyclical, meaning that each stage of development is highly dependent on the previous stages. Before we get into the stages of leaf development, we need to understand why leaves are so important.
Leaf importance
Get ready for a science lesson! Leaves are responsible for photosynthesis, which is the process that converts sunlight, water and carbon dioxide to oxygen and glucose (fig. 1). Special cells in the leaves called chlorophyll, absorb light energy. CO2 and water vapor pass through openings in the leaves called stomata. Once inside the leaves’ epidermis (skin), the chlorophyll cells combine the 3 ingredients to produce glucose, which is either used directly by the plant for food, or stored in the roots as starch. Oxygen is a by-product of the process, being released back into the atmosphere through the stomata. This process is critical to tree health as glucose is used as food and energy. But photosynthesis is also critical to us humans since we need the oxygen to breathe. Without proper foliage and leaf development, a tree would be unable to photosynthesize. This would leave the tree malnourished and highly susceptible to disease and death.
Leaf development stages (fig. 2.)
During the summer months, when leaves are at their prime, conditions are optimal for photosynthesis due to the long days and warm temperatures. This period is called the paradormancy stage of leaf development (fig. 2a). High amounts of energy are produced for overall healthy growth rather than used directly for bud/leaf growth [1].
Eventually, summer has to end. Temperatures drop and the days get shorter, causing all growth to stop. This growth cessation is called the dormancy induction or “chilling” period (fig. 2b). The shorter days reduce the amount of sunlight available for photosynthesis, slowing down the production of energy. As a result, the leaves fall in order to conserve energy for survival during the winter. The reduced temperatures trigger what is called bud setting [2]. You might think that buds start to grow in the spring time when we first see them emerge. You would be wrong!! New buds start their development in the late summer. Then during the “chilling” period of autumn, a waxy scale is formed on the buds for protection [1]. This chilling period is critical to leaf development. Without sufficiently cold temperatures to initiate the bud setting, buds are left unprotected and susceptible to the winter elements, reducing the chances of growth come spring.
By the time winter arrives, the trees have entered endodormancy (fig. 2c). This is the fully dormant phase of the growth cycle (aka no growth occurs). The cold temperatures limit cell communication, essentially shutting off the tree [2].
Endodormancy release is triggered by rising temperatures, which restarts cell communication. Increased hormone and sugar levels in the tree have been associated with endodormancy release, suggesting that the increase in temperature is directly related to the end of dormancy in perennial trees [3].
After the endodormancy release, ecodormancy occurs in late winter (fig. 2d). During this phase, sap begins to flow from the root storage to the rest of the tree to resume normal metabolism and growth functions.
Once the weather finally starts to feel like spring and the days start to get longer, the bud burst occurs (fig. 2e). This is the official unfolding of the leaf from its bud. The awakening! The timing of budburst varies depending on the year, temperature, and species [1].
Impact of climate change on leaf development
You might have picked up on the fact that this process is HIGHLY dependent on weather. Living in Canada, we know that weather can change at the drop of a hat. Over the past few years, we have experienced much milder winters than we are used to, thanks to climate change and global warming. This might be nice if you despise cold weather and snow, but for the trees, climate change is extra confusing. For example, everyone loves a beautiful warm day in February where the temperatures get well above freezing. It is great to be reminded that sunshine and blue skies do exist! However, we know these unseasonably warm days do not last, and a re-freeze is inevitable. These early thaws have been detrimental to leaf development [4]. If the temperatures rise too quickly during endo- or eco- dormancy, bud burst will occur early. Subsequent frost can cause serious damage to newly bursting leaves, causing them to be underdeveloped or simply die off. As a result, the tree is shocked and left with less healthy foliage for summer. The lack of foliage will inevitably lead to tree death since the tree is unable to photosynthesize properly. Similarly, if there is a weak chilling period (late freezing) during autumn, unprotected buds will die off during winter, stopping leaf development all together, causing the tree to die come spring/summer.
The trees are here to tell you that no matter what you think about climate change, things are changing faster than they can keep up (fig. 3). Trees naturally adapt to changing conditions over time in order to maintain a balance with the environment in which they live. It has been proven that leaf development and bud burst is directly related to a healthy ecosystem. “Synchrony” is essentially the functionality of an ecosystem based on all developmental cycles [5]. For example, leaf development and flower development are often synchronized due to the need for cross pollination by bees. If the leaf development is disrupted by changing temperatures, flower development is also delayed, leading to reduced pollination. Reduced pollination limits food sources for both animals and humans. Many observations of this loss of synchrony have been made around the world over recent years. Genetic studies have also shown that interrupted synchrony and improper leaf development has caused offspring of mature trees to be genetically weaker [1,5]. Adaptations to environmental changes do occur, but the process of acclimation is much too slow to keep up with the warming temperatures of earth. In 2018, a study proved that bud burst or “leaf-out” was reduced by 55% due to climate change alone [5]. That’s inevitable death to over half of the trees just because of bad leaf development.
So, this spring, when you see the leaves bursting from their buds, make sure you cheer them on! Leaves are not only very important to the trees, but also important to you and the rest of the environment. Leaf development is a complex natural cycle that is highly sensitive to temperatures and sunlight. With the rapidly changing climates, the trees need all the help and encouragement they can get!!
Resources
Pletsers, A. et al. (2015). Chilling temperatures and photoperiod influence the timing of bud burst in juvenile Betula pubescens Ehrh. and Populus tremula L. trees. Annals of Forest Science 2015(72); 941–953. https://doi.org/10.1007/s13595-015-0491-8
Fadon, E. et al. (2020) A conceptual framework for winter dormancy in deciduous trees. Agronomy 10(2):241. DOI:10.3390/agronomy10020241
Maurya, J.P. and Bhalerao, R.P. (2017). Photoperiod- and temperature- mediated control of growth cessation and dormancy in trees: a molecular perspective. Annals of Botany, 120(3); 351–360, https://doi.org/10.1093/aob/mcx061
Charrier, G. et al. (2021). Interactions of drought and frost in tree ecophysiology: rethinking the timing of risks. Annals of Forest Science 78(2) DOI:10.1007/s13595-021-01052-5
Zohner, C.M. et al. (2018) Global warming reduces leaf-out and flowering synchrony among individuals. eLife, 2018(7):e40214. doi: 10.7554/eLife.40214.
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