What is Abscission?
Abscission is the carefully controlled process a tree uses to drop its leaves, flowers, or fruit. The tree creates a separation layer (abscission zone) at the base of the organ, which weakens until the organ falls away cleanly, leaving a sealed scar.
The Process
Step by Step
- Hormonal signal: Decrease in auxin and increase in ethylene trigger abscission.
- Zone formation: Cells at the base of the petiole differentiate into an abscission zone.
- Cell wall dissolution: Enzymes break down the cell walls in the zone.
- Separation: The organ detaches — often aided by wind, rain, or gravity.
- Scar sealing: A protective layer (wound periderm) forms over the scar.
Types of Abscission
Leaf Abscission
- Seasonal: Deciduous trees drop all leaves during the dry season (Guanacaste, Pochote).
- Continuous: Evergreen trees drop and replace leaves year-round.
- Stress-induced: Drought, disease, or herbivory can trigger premature leaf drop.
Flower Abscission
- Unfertilized flowers drop after the pollination window closes.
Fruit Abscission
- Mature fruit drops when ripe (gravity dispersal).
- Immature fruit may be abscised to redirect resources (fruit thinning).
Costa Rican Examples
Dry-Season Leaf Drop
In Guanacaste, dry-forest species like Pochote and Guanacaste undergo dramatic leaf abscission in January–March, creating a stark, bare-branched landscape.
Corteza Amarilla
Drops leaves just before flowering, allowing the spectacular yellow blooms to be fully visible on bare branches.
Why It Matters
- Water conservation: Leaf abscission reduces water loss during the dry season.
- Nutrient recovery: Before abscission, trees withdraw valuable nutrients from leaves.
- Fruit harvesting: Understanding abscission helps determine optimal harvest timing.
- Forestry: Abnormal abscission patterns can indicate tree disease or environmental stress.