What is Senescence in Trees and Why Is It Important?

• Senescence occurs at the cell, tissue, organs, and individual levels.
• Organ senescence is consequential in trees as it helps “recycle materials” within an individual tree to maintain function and productivity.
• Organ and whole tree senescence are also crucial for nutrient cycling, wildlife diversity, and forest productivity.

Senescence is an integral part of all living organisms, including trees. Plants and trees show various types of senescence patterns that are crucial for plant fitness and ecosystem functions. Learn more about this topic and how it is essential for natural processes and productivity.

What is Senescence?

Senescence leads to the death or end of the functional life of the entire individual or a part of the plant through a process called apoptosis or programmed cell death.

Senescence is the final stage of a plant or its organs’ development, allowing the reclamation of valuable cellular building blocks used during growth. During senescence, the increase in cellular breakdown reduces metabolic activities and functioning. Unlike animals, plants lose older organs through regulated senescence and replace them with new ones. So, resources reclaimed by senescence are used again. Therefore, the process is critical for ensuring plant fitness and survival of the individual and future generations.

Plant development integrates senescence at various levels and could affect only cell-organelles like chloroplasts, cells like xylem, organs like leaves and flowers, or the entire plant.

Senescence is not synonymous with aging, which is the process of growing old and involves structural and chemical changes. Aging is one of the factors that causes senescence. The other causes of senescence are internal factors like reproductive development and ethylene and abscisic acid phytohormone production. External factors that trigger senescence are environmental signals like light, temperature, season, photoperiodism, shading, stresses like wounding, drought, waterlogging, and nutrient deficiency.

The process is complex, regulated by gene expression, involves signaling pathways, and is integral to plant development.

Figure 1: Types of senescence, Joshi et al. 2020.

Types and Functions of Senescence

The four patterns of senescence that determine its role and benefits to plants are discussed below; also see Figure 1.

1. Overall /whole plant senescence: The entire plant dies in this case and can be seen in all plants and trees. In monocarpic senescence, the end occurs after a single cycle of vegetative growth, reproduction, and seed ripening in annuals (rice, mustard) and biennials (cabbage). Polycarpic senescence, seen in trees, occurs after many years of growth and reproduction.
2. Top /shoot senescence: The aboveground shoots are produced yearly and die after flowering and fruiting. This senescence pattern occurs in plants where the top dies, but the stem and roots are underground and perennial, for example, in bananas, ginger, etc.
3. Organ senescence: The entire plant is perennial, as in trees that live for many years. Only specific organs like leaves, flowers, and fruits show senescence. The separation of leaves from trees occurs through leaf abscission. Two types of organ senescence patterns exist:

• Sequential /progressive senescence: Old leaves, twigs, and branches periodically undergo senescence since leaves have a limited lifespan. The organs’ senescence happens progressively, for example, starting with older and lower leaves in a tree, which is common in many tropical species.
• Simultaneous /synchronous senescence: The organs undergo senescence simultaneously, as in deciduous trees during a particular season in temperate regions, for example, in maple, beech, etc.

Organs like leaves, flowers, and fruits are separated from trees through abscission. Abscission starts at the leaf petiole base and proceeds transversally across the base. Abscission results in a hardened deposition at the petiole base, allowing the trees to seal their vascular system to prevent water and nutrient loss, and entry of pathogens.

Figure .2: All cellular building blocks, like the nucleus and cytoplasm, are degraded and removed from a cell during apoptosis, Joshi et al. 2020.

Importance of Senescence in Trees

The various patterns of senescence are integral to the growth and development of trees, starting with shaping the lifecycle and size.

Tree Lifecycle

Plant habit and lifecycle are strongly connected to timings and rates of senescence and organ formation. In biennials and annuals, tips with meristematic tissue that allow indeterminate growth stops the production of new structures due to apical tissue senescence or change into determinate floral tips. Seed formation is followed by whole plant senescence.

Evergreen perennials reproduce several times before death. Their apical tissue becomes temporarily determinate and dormant, as in deciduous trees. Trees retain meristematic tissue that can produce new buds and leaves, allowing indeterminate growth and achieving large sizes that are habitat-specific.

Senescence Benefits to Trees

In trees, organ senescence patterns are most important since entire tree death occurs after several years. Organ senescence is crucial for trees for the following reasons:

1. Reuse of materials: Senescence breaks down the cellular organelles and contents to release the constituent nutrients they are made of, such as simple sugars, amino acids, proteins, amides, nucleotides, and minerals, see Figure 2. These nutrients are removed from the dead cells and organs, transported into the main trunk, and used later to produce new organs like leaves, flowers, etc. Over 80% of phosphorus and nitrogen are reclaimed. Recycling and reusing materials within an individual tree helps it maintain and grow its biomass and productivity.
2. Maintains tree functions: Removal of old inefficient leaves due to age or environmental factors, through senescence and replacement by new organs through recycled materials, maintains plant functions. As trees grow taller, lower and older leaves shaded by upper foliage and have less photosynthesis experience senescence through environmentally controlled signals (shade).
3. Perennation: Organ senescence, which causes leaf fall, allows trees to perennate (survive from one growing season to the next) during unfavorable seasons (autumn and winter) or periods of stress.
4. Reduces transpiration: Simultaneous leaf fall in autumn before winter reduces transpiration in temperate regions when water absorption from frozen ground is difficult for trees.

Lower-level cellular senescence is also required during reproduction for gamete formation.

Ecosystem Implications of Organ and Whole Senescence

Organ senescence is vital also at larger levels for ecosystems.

1. Nutrient cycling: Organ senescence of leaves and twigs produces litter, an essential source of organic matter and minerals for cycling all the minerals in the soil and back into the ecosystem to benefit the source tree and other vegetation.
2. Niches for wildlife: When the trees age, the whole tree also undergoes senescence, going through several stages from live mature trees to standing dead stumps, as shown in Figure 3. It can start as a hollowing of trees. These hollow but standing trees provide shelter and breeding for numerous wildlife, such as birds, mammals, amphibians, and reptiles. The availability of senescent trees can determine animal diversity and abundance in a forest.
3. Tree productivity: Both organ and tree senescence can affect tree productivity. The length of the growing season in spring and summer influences productivity in temperate forests, so the time of leaf senescence is critical. Climate change-related temperature and light changes are expected to advance it by 3 to 6 days, reducing growing time and carbon uptake by trees. Ultimately, forest productivity will be affected.

As a tree ages, fecundity or fruiting ability falls around five years before tree senescence. The phenomenon has implications for orchards that age and forests, where rare large trees contribute significantly to forest productivity.

Since senescence is valuable to forests and trees, it is measured using the changes that occur during the process.

Figure 3: “Senescence-class evaluation for all sampled trees, modified from stylized trees found in wet sclerophyll forests in Smith and Lindenmayer (1988). Class 1: mature, living trees; Class 2: mature, living trees with some dead branches, particularly at the top of the crown; Class 3: mature trees with most branches intact but only minimal foliage remaining; Class 4: dead trees with most branches intact; Class 5: dead trees with none or few branches intact,” Owens et al. 2014. (Image credits: http://dx.doi.org/10.1071/WR14168)

Changes During Senescence

Physiological and structural changes during organ senescence occur in a highly controlled and regulated program, and some essential steps are as follows.

1. Protein biosynthesis is reduced, and the protein and lipid breakdown increases to release nitrogen and other minerals.
2. The degradation of chloroplasts, which have high protein and lipid content, reduces chlorophyll content in leaves, causing yellowing.
3. The amino acids and other minerals from protein degradation are withdrawn from leaves and transported to the phloem.
4. Photosynthesis rate falls, and the starch content in cells is reduced.
5. Synthesis and accumulation of anthocyanin pigments lead to leaf color changes to red, yellow, or brown.
6. Breakdown of RNA and DNA releases phosphorus, which is also transported away from the dying cell. DNA remains intact until the last stages of cell death.
7. Loss of genes changes cell property.
8. The membranes around organelles are disrupted.
9. Cell size reduces.

Senescence does not proceed evenly in leaves. It starts at the edges and tips and spreads inwards. The tissue closest to veins and the base of leaves is the last to die to allow for the mobilization of materials away from the dying leaf (see Figure 4).

Figure 4: “Leaf senescence showing the differential progression of senescence in the leaf. Areas close to the veins senesce last.” Adapted from Buchanan-Wollaston 2007. (Image credits: https://www.esalq.usp.br/lepse/imgs/conteudo_thumb/mini/Senescence-in-plants.pdf)

Measuring Tree Senescence

Both organ and whole senescence are measured for trees. Two standard methods are discussed below.

Spectroscopy

The color change in leaves during organ senescence can be measured in the field through hand-held spectrometers such as CI-710s SpectraVue Leaf Spectrometer or remote imagery to study leaf loss due to seasons or stress.

Pigments interact with light in the range of visual and near-infrared wavelengths. The portable CI-710s uses leaf spectroscopy to measure and analyze light absorption, reflection, and transmission to give a color analysis of leaves.

Spectroscopy is also used to measure the stage of whole tree senescence using aerial multispectral imagery and vegetative indices into five senescence classes for trees (See Figure 3). These surveys can identify individual senescent trees, which is useful in biodiversity monitoring programs and commercial forest inventories.

Canopy Analysis

Canopy loss due to simultaneous and progressive organ senescence during autumn and stress can be measured by canopy analysis. For example, CID Bio-Science’s CI-110 Plant Canopy Imager uses hemispherical canopy photography analyzed by the software and light measurements to give canopy parameters and Leaf Area Index (LAI).

Gradual canopy loss due to crown form changes in whole tree senescence can also be recorded through this method.

Both these methods give non-destructive and quantitative results in real-time to advise forest management and forestry operations.

Sources

Buchanan-Wollaston, V. (2007). Senescence in Plants. Encyclopedia of life sciences. John Wiley & Sons, Ltd.

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Joshi, U., Rana, D. K., & Tanuja, K. B. (2020, October). Senescence in Plants, its Patterns, Types, and Events Associated with it. Retrieved from https://www.researchgate.net/profile/Udit-Joshi-3/publication/344448140_Senescence_in_Plants_its_Patterns_Types_and_Events_Associated_with_it/links/5f76a90292851c14bca7a8ca/Senescence-in-Plants-its-Patterns-Types-and-Events-Associated-with-it.pdf

 

Jajic, I., Sarna, T., & Strzalka, K. (2015). Senescence, Stress, and Reactive Oxygen Species. Plants, 4, 393-411. https://doi.org/10.3390/plants4030393

 

Owers, C. J., Kavanagh, R. P., Bruce, E. (2015). Remote sensing can locate and assess the changing abundance of hollow-bearing trees for wildlife in Australian native forests. Wildlife Research 41, 703-716.

 

Qiu, T., Aravena, M. C., Andrus, R., Ascoli, D., Bergeron, Y., Berretti, R., … & Clark, J. S. (2021). Is there tree senescence? The fecundity evidence. Proceedings of the National Academy of Sciences, 118(34), e2106130118.

 

Thomas, H. (2013). Senescence, ageing and death of the whole plant. New Phytologist, 197 (3), 696-711. https://doi.org/10.1111/nph.12047.

 

Woo, H. R., Masclaux-Daubresse, C., & Lim, P. O. (2018). Plant senescence: how plants know when and how to die. Journal of experimental botany, 69(4), 715–718. https://doi.org/10.1093/jxb/ery011

 

Zani, D., Crowther, T. W., Mo, L., Renner, S. S., & Zohner, C. M. (2020). Increased growing-season productivity drives earlier autumn leaf senescence in temperate trees. Science, 370(6520), 1066-1071.