What makes the leaves fall off trees?
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Factors Influencing Leaf Fall in Trees
Leaf fall, or abscission, is a complex process influenced by various environmental, physiological, and biological factors. Understanding these factors can help explain why and how leaves fall off trees.
Environmental Factors: Temperature and Photoperiod
Temperature and Frost Impact on Leaf Fall
Temperature plays a significant role in leaf fall. Early frost can cause leaves to remain on trees longer, particularly in exposed areas where only the lower leaves are affected by frost, while higher leaves fall normally. This suggests that temperature variations within a tree can lead to differential leaf fall.
Photoperiod and Light Exposure
The duration of daily illumination, or photoperiod, also affects leaf fall. Trees exposed to prolonged illumination, such as those near street lights, tend to retain their leaves longer than those experiencing natural light cycles. This phenomenon has been observed in various species, including sumac and tulip trees, where extended light periods delay leaf abscission.
Physiological Processes: Water Status and Nutrient Retranslocation
Water Availability and Leaf Retention
Water status is crucial for leaf retention. In agroforestry systems, trees like Cordia oncocalyx maintain better water status due to less competition for water, leading to delayed leaf fall compared to trees in secondary forests. This is because these trees can sustain photosynthesis longer under severe climatic conditions, thus retaining their leaves for an extended period.
Nutrient Retranslocation and Chlorophyll Degradation
During autumn, deciduous trees retranslocate nutrients from leaves back to the tree, impacting the timing of leaf fall. The degradation of chlorophyll and the formation of the abscission layer are key processes in this phase. Different species exhibit varying rates of chlorophyll degradation and abscission layer formation, which influences the timing of leaf fall. For instance, maple species start abscission earlier and degrade less chlorophyll compared to oak and beech species .
Biological Factors: Herbivory and Leaf Mining
Herbivory Reduction through Leaf Fall
Deciduous leaf fall can reduce herbivory. For example, oak saplings that shed their leaves in winter experience reduced herbivore attacks in the following spring. This suggests that leaf fall can be an adaptive strategy to minimize damage from herbivores like cynipid gall makers.
Impact of Leaf Mining Insects
Leaf mining insects, such as Phytomyza ilicis, can cause premature leaf fall in holly trees. This premature shedding helps the tree manage the frequency of mined leaves, potentially spreading out nutrient release throughout the year. However, the impact of leaf miners varies among tree species and individual trees .
Conclusion
Leaf fall in trees is influenced by a combination of environmental conditions, physiological processes, and biological interactions. Temperature, photoperiod, water availability, nutrient retranslocation, and herbivory all play critical roles in determining when and how leaves are shed. Understanding these factors provides insight into the adaptive strategies trees use to survive and thrive in their environments.
Sources and full results
Most relevant research papers on this topic
The effect of a consumer, Phytomyza ilicis, on seasonal leaf-fall in the holly, Ilex aquifolium
Trees Lose Their Leaves Later in Agroforestry Systems
Urban Ecosystems: Urban ecosystems and the science of ecology
Synchrony in fall leaf drop: chlorophyll degradation, color change, and abscission layer formation in three temperate deciduous tree species.
Deciduous leaf drop reduces insect herbivory
Retention of Leaves by Deciduous Trees
Degradation of chlorophyll and synthesis of flavonols during autumn senescence—the story told by individual leaves
Leaf mines: their effect on leaf longevity
THE EFFECT OF STREET LIGHTS IN DELAYING LEAF‐FALL IN CERTAIN TREES
Litter fall and decomposition in primary, secondary and plantation forests in the Brazilian Amazon.
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