While lichens are epiphytic (non-parasitic) and do not directly consume the tree as a food source, they are highly selective about their habitat because they rely on the bark for anchorage, moisture retention, and specific chemical conditions
. The specificity of certain lichens to specific tree species is driven by environmental, physical, and chemical interactions with the bark surface.
Here is why lichens are specific to certain trees and the interaction between the two:
1. Chemical Composition of the Bark (pH)
Acidic vs. Alkaline: Bark pH varies significantly between tree species. Some lichens require high alkalinity found only on older bark, such as Ash (high pH). Other lichens, like those on Larch or Pine, prefer highly acidic bark.
Nutrient Leaching: Lichens absorb nutrients (phosphorus, potassium) that are dissolved from the bark surface by rain. The chemical composition of the bark acts as a nutrient source, making different trees attractive to different species.
2. Bark Structure and Physical Characteristics
Texture: Rough bark (e.g., Oak, Elm) provides more cracks, crevices, and stable surfaces for spores to attach and hold water compared to smooth bark (e.g., Beech, Young Birch).
Water Retention: Some lichens are drought-tolerant while others are not. A tree with bark that stays damp longer (e.g., Poplar, Willow) will attract different species than one that dries out quickly.
Age and Stability: Older trees have more stable, nutrient-rich bark, which allows for the slow growth of specialized lichen communities (often 1-2mm a year).
3. Light and Microclimate
Canopy Structure: The type of tree determines the light levels reaching the trunk. Lichens that need sunlight prefer sparse, light-loving canopy trees, whereas others prefer shaded conditions.
Position on the Tree: Some lichens are specifically found at the base of the trunk where humidity is higher, while others live on higher, drier branches.
4. Direct and Indirect Interaction (Substrate Conditioning)
Chemical Signaling: There is evidence of chemical crosstalk between lichen partners and their environment. Phenolic compounds in the bark may act as cues for attachment.
Bark Chlorophyll: Some trees, like Beech and Holly, have chlorophyll in their bark. Crustose lichens growing on these trees can create a unique microhabitat by influencing light absorption, and in turn, may be specialized to live on such surfaces.
"Dripzone" Effects: The chemistry of the water dripping from the canopy (leachates) can modify the bark chemistry lower down, influencing which lichens can survive there.
Summary Table of Specificity
Factor
Influence on Specificity
pH
Acidic vs. Alkaline loving lichens select different trees (e.g., Pine vs. Ash)
Texture
Rough bark allows faster colonization by foliose/fruticose lichens
Moisture
Water-holding capacity determines if a lichen survives dry periods
Nutrients
Chemical leachates from bark nourish the lichen
In short, while not parasitic, lichens are sensitive to the "substrate chemistry" and "microclimate" provided by different tree species, making the tree a specialized ecological home rather than just a place to sit.
Lichen Ecology
For saxicolous species the type of rock on which they grow may be important, with siliceous (acid) and basic rock types each supporting specific lichens.
Do Lichens Harm Trees?
Non-Parasitic: Lichens do not penetrate the inner bark or cambium layer, nor do they steal nutrients from the tree.
Sign of Existing Stress: While harmless, excessive lichen growth is often a symptom that a tree is already in decline, stressed, or dying. A thinning canopy allows more light and moisture to reach the branches, which encourages lichen to grow.
Minimal Impact: In very rare cases, extremely heavy growth on smaller branches might cause slight issues, but they do not cause the decline of a healthy tree.
Do Trees Take Chemical Measures?
Trees do not typically produce specific chemical "defenses" against lichens in the way they do for pathogens or herbivores. However, they manage lichen growth through physical and biological means:
Bark Shedding: Rapidly growing, healthy trees (like many pines) shed their bark regularly, preventing lichens from establishing a permanent foothold.
Canopy Density: A healthy, dense canopy blocks sunlight and prevents the high-moisture conditions that lichens prefer.
Are There Chemical Controls for Lichen?
No Natural Tree Chemicals: There are no widely recognized, active chemical defense mechanisms trees use to kill lichens.
Artificial Controls: While not a "natural" measure by the tree, gardeners sometimes use copper sulfate or potassium salts (soaps) to kill lichen on trees, though this is usually unnecessary.
Best Management: Instead of chemical treatment, the best way to reduce lichen is to improve tree health through proper watering, pruning, and fertilization to increase vigour and canopy density.
Note: Removing lichen manually can cause more damage to the tree's bark than leaving it alone.
Lungwort