The Color of Leaves

The colour and shine of leaves in Andaman’s Havelock and Neil islands astonished me. It looked abnormal, as if somebody had painted them bright and had put extra illumination to make them shine. But after a few days I realised that this was normal. The cities have trained our eyes in a such a manner that we have forgotten the true colour of leaves.

The dust, micro particles,  grime and grease on city leaves have permanently changed their colour. The leaves are helpless. They can’t shrug off the grime easily. It has to wait for the rains. But even the rains do the partial job because any type of grease and oil is sticky. So practically the leaves never fully get rid of particulate matter and carbon soot.

Only the road side trees do not bear the brunt, the tender  leaves of plants in pots  in protected balconies of flats are also not untouched.   

2. Whether all this affects the growth of trees, its health in any manner, its capacity to absorb carbon dioxide and release oxygen?

This question led to some fact finding from already carried out studies & research. The results are gloomy and pitiable for humans.

There’s substantial experimental  evidence that persistent dust, soot and other particulate deposition on leaves harms leaf function, reduces photosynthesis and growth, and therefore can lower a tree’s carbon-sequestration (and oxygen-release) performance.

Photosynthesis and pigments drop. Dust and soot physically shade the leaf surface, change leaf reflectance, and can reduce measured chlorophyll concentration — all of which lower photosynthetic rate.

Stomatal clogging and gas-exchange impairment. Fine particles and oily/sooty residues can block or partially obstruct stomata, reducing CO₂ uptake (photosynthesis) and changing transpiration. This leads to altered leaf temperature and water relations as well as less carbon fixation. 

a.      Chemical/physiological damage. Particles often carry acidic or toxic compounds (black carbon, heavy metals, organic residues) that may accelerate chlorophyll degradation, impair biochemical processes in the leaf.

b. Net growth & carbon sequestration effects. Field studies and reviews show urban trees do capture particulate matter (a beneficial ecosystem service), but heavy or chronic leaf loading correlates with reduced growth/biomass increment in some species  — implying reduced net carbon sequestration capacity

c. Rough, hairy or waxy leaves trap more particles (good for air cleaning) but can suffer more persistent loading; smooth or tall species may shed particles more easily. Seasonal patterns (more soot in winter in many cities) change impacts.

Thus it is evident that particulate loading harms leaf physiology. 

3. The degenerative cycle:  Urban trees remove PM (benefit people) but can be impaired by chronic pollution. The impaired and particulate matter laden leaves will now absorb less PM, will absorb less carbon dioxide, release less oxygen,  and thus will be less effective in reducing pollution.

4. The table below with numeric estimates from published studies showing how particulate/dust deposition on leaves affects physiological functions (photosynthesis, stomatal conductance, yield, light interception). The numbers below capture magnitude of impact measured in real or controlled experiments — they give a sense of scale for how urban dust/soot loading can impair tree and plant leaf performance:

Effect measured	Magnitude of change (approx.)	Study context / notes
Reduction in stomatal conductance	~30% ↓ compared to clean leaves	Cotton leaves with dusty surface showed ~30% lower stomatal conductance vs control.
Reduction in plant yield/biomass	~28% ↓ vs control	In dust-applied cotton, yield/biomass dropped about 28% under heavy dust deposition.
Reduction in photosynthesis (light blocking)	~20% ↓	Dust stagnation on leaf surface blocked ~60% of incoming light, leading to ~20% reduction in photosynthesis.
Increase in leaf temperature	~2–4 °C increase	Dust covering raised leaf surface temperature by a few °C due to altered energy balance.
Reduction in transpiration rate (from similar cement dust study)	~22% ↓	Cement dust on wheat leaves reduced transpiration (proxy for stomatal and gas exchange) by ~22%.

Most studies don’t measure oxygen release directly, but photosynthesis and stomatal conductance are tightly linked to oxygen evolution and carbon uptake — so reductions in these proxies reliably indicate impaired carbon fixation and gas exchange.

Studies show that effects vary by species, particle size, and environmental conditions; fine PM (PM2.5) tends to contribute more to stomatal clogging and shading than coarser particles. 

5.  Dust versus grease-bound particulates — they behave very differently, and this difference is one of the big but under-discussed challenges for urban trees.

a.  Dust is transient; oily/sooty films are persistent

• Ordinary dust (soil, road crust, construction particles) settles loosely on leaf surfaces. Rain, wind, or even leaf flutter can remove a large portion of it.

• Grease, unburnt hydrocarbons, diesel soot, tyre wear oils, and lubricating‐oil vapour form a sticky film on the leaf cuticle.

• This sticky layer functions like adhesive, binding dust, carbon soot, and metals permanently to the leaf, turning the surface into a dark, rough, matte coating.

b.  Evidence shows oily soot is extremely resistant to wash-off

Research on black carbon, diesel particulate matter, and organic aerosol films shows:

• After rainfall, only 10–30% of soot particles are removed from leaves; the rest stays adhered due to oily organic fractions.

• Sticky hydrocarbon layers can keep particulate matter on the leaf for the entire season, effectively “locking in” the pollution.

• These films can penetrate the wax layer of the leaf and change its chemistry, making the surface permanently dull or rough.

(These findings appear repeatedly in urban tree PM-accumulation studies.)

c. Why persistent grease matters for the tree

When the coating doesn't wash off, trees get chronic instead of temporary stress:

(i) Permanent stomatal obstruction

Grease + PM creates a semi-sealed layer that can:

• block stomata for weeks or months

• reduce CO₂ entry and transpiration

• elevate leaf temperature and water stress

(ii) Long-term reduction in photosynthesis

A darker, sticky layer:

• reduces light reflectance and penetration

• leads to chronic ~15–30% lower photosynthetic activity (based on studies from heavy-traffic corridors)

(iii) Changes to leaf cuticle

The oily layer can:

• dissolve or alter the natural wax layer

• cause cracks or thickening

• reduce the leaf’s ability to self-clean in the future

  This is why trees near highways or industrial zones often look permanently greyish-black, even after monsoons.

(iv) Reduced particulate absorption & lower air-cleaning efficiency

Ironically, if the leaf is already coated in oily soot, it loses its natural ability to capture new particles (because the cuticle is damaged or saturated).This reduces the tree’s pollution-mitigation function over time.

(v). Species with waxy, hairy, or rough leaves suffer more

These leaves trap PM better (which is good for air quality), but:

• the same surfaces also bind oily particles more strongly

• grease penetrates or adheres deeply

• these species experience more long-term physiological stress

7. In Indian cities -

Traffic corridors produce a mix of:

• diesel soot

• unburnt lubricating oil aerosols

• tyre and brake wear particles

• volatile organic compounds

  
  

  Rainfall cannot fully remove these.

This is why tree leaves along major corridors look blackened even after heavy monsoon spells.

Conclusion: The Color of Leaves is not a simple matter of color.

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