Bark

Concept Explanation

Bark is a tree's armor—the protective outer layer that shields living tissues from the outside world. It's not just dead material; it includes both living transport tissue (inner bark) and protective dead layers (outer bark).

Structure

Inner Bark (Phloem):

• Living tissue
• Transports sugars from leaves to roots
• Active growth layer
• Usually thin

Outer Bark:

• Dead protective tissue
• Insulation and protection
• Constantly renewed from inside
• Variable thickness (mm to 30+ cm)

Cambium (beneath bark):

• Growth layer
• Produces new bark outward
• Produces new wood inward

Bark Types and Patterns

Smooth:

• Thin, tight bark
• Common on young trees
• Example: Guayabo, young Ceiba
• May have lenticels (breathing pores)

Furrowed:

• Deep grooves and ridges
• Thick, corky bark
• Fire-resistant
• Example: Oaks, old conifers

Scaly/Flaky:

• Peels in small scales or flakes
• Constantly renewing
• Example: Eucalyptus, Sycamore

Plated:

• Large, flat plates
• Example: Pines

Papery:

• Thin, peeling in sheets
• Example: Birch, Gumbo Limbo

Spiny:

• Thorns or spines on bark
• Defense mechanism
• Example: Pochote, Javillo

Smooth Green:

• Photosynthetic bark
• Young branches
• Example: Palo Verde

Functions of Bark

Protection:

• Physical armor against injury
• Insulation from temperature extremes
• Fire resistance (thick bark)
• Waterproofing
• Defense against insects/disease

Transport:

• Phloem moves sugars/nutrients
• Essential for tree survival
• "Girdling" (removing bark ring) kills tree

Storage:

• Some bark stores nutrients
• Cork tissue in outer layers
• Reserve materials

Gas Exchange:

• Lenticels allow oxygen in
• Necessary for living tissues
• Visible as small pores or bumps

Bark Identification

Bark is a KEY identification feature:

Young vs. Old:

• Bark changes dramatically with age
• Young: Often smooth
• Old: Develops characteristic pattern
• Check bark on trunk, not twigs

Color Variations:

• White: Birch
• Red/Cinnamon: Gumbo Limbo, some Cedars
• Green: Palo Verde (photosynthetic)
• Gray/Brown: Most temperate trees
• Black: Some Oaks

Texture:

• Smooth vs. rough
• Furrowed depth
• Plate size
• Peeling pattern

Costa Rican Bark Examples

Distinctive Barks:

• Pochote (Pachira quinata): Green, spiny when young
• Guayabo (Psidium guajava): Smooth, peeling copper-brown
• Guarumo (Cecropia): Smooth, pale gray with rings
• Gumbo Limbo (Bursera simaruba): Red, peeling "tourist tree"
• Ceiba (Ceiba pentandra): Spiny when young, smooth gray when mature

Bark Uses

Commercial:

• Cork: Cork oak bark
• Cinnamon: Inner bark of Cinnamomum
• Quinine: Bark of Cinchona
• Tannins: Oak bark for leather tanning
• Dyes: Various colors from different barks
• Medicine: Willow bark (aspirin source)

Traditional:

• Medicine (antipyretics, astringents)
• Fiber for rope, cloth
• Tannins for tanning hides
• Dyes and pigments

Ecological:

• Wildlife habitat (insects, lichens, mosses)
• Food for some animals
• Nesting material

Bark Damage

Natural:

• Lightning strikes
• Wind/ice damage
• Animal feeding (deer, porcupines)
• Woodpecker excavation
• Elephant rubbing (Africa)

Human-caused:

• Carving/vandalism
• Construction damage
• Lawnmower/trimmer wounds
• Improper pruning

Consequences:

• Entry point for disease
• Insect infestation
• Structural weakness
• Death if girdled completely

Bark and Fire Adaptation

Thick Bark = Fire Survival:

• Longleaf pine: 5 cm+ bark
• Giant Sequoia: up to 60 cm!
• Cork oak: thick, insulating
• Allows tree to survive ground fires

Costa Rica Context:

• Dry forest trees may have thicker bark
• Fire is less common in rainforest
• Some species sprout from roots after fire

Seasonal Changes

In Temperate Climates:

• Bark expands/contracts with temperature
• Can split in winter (frost cracks)
• Some trees shed bark in summer

In Tropical Climates:

• Less seasonal variation
• Growth flushes create bark patterns
• Wet/dry seasons affect appearance

Conservation Note

Bark Harvesting:

• Sustainable: Cork (regrows)
• Destructive: Cinnamon (kills tree if ring-barked)
• Overharvested: Some medicinal barks
• Need careful management

Fun Facts

• Cork oak bark can be harvested ~9 times in tree's life
• Redwood bark is fireproof and can be 30 cm thick
• Some bark contains latex (rubber tree)
• Aspirin was first derived from willow bark
• Girdling a tree kills it by blocking phloem