What makes a resource renewable?
A renewable resource is one that nature can make again in a human lifetime. Sunshine, wind and water are renewable. Coal and oil take millions of years, so they are not.
Wood is renewable only when we are careful. A tree takes 10 to 100 years to grow. If we cut a whole forest and never plant it again, the wood is gone. So the rule is simple: do not cut faster than the forest regrows. This is called sustainable use.
Properties of wood
Wood has many useful properties.
- Light for its strength: most dry wood has a density of 0.4 to 0.8 g/cm³, less than water (1 g/cm³), so it floats.
- Strong along the grain: a pole carries heavy loads, but it splits easily along the grain.
- Poor conductor of heat and electricity: this is why pan handles and house roofs use wood.
- Easy to work: we can saw, drill, nail, glue and carve it.
- Absorbs water: wood swells when wet and shrinks when dry, so it must be dried and sealed.
- Burns: it catches fire, so it needs care.
Softwood (pine, deodar) comes from conifers and grows fast. Hardwood (teak, sal, oak) comes from broadleaf trees, grows slowly and is usually denser.
Uses of wood
Wood is used in four big ways:
- Building: beams, doors, floors, roofs, bridges.
- Furniture and tools: chairs, tables, handles, musical instruments, toys.
- Paper and boards: pulp for paper, plywood and particle board.
- Fuel: firewood and charcoal. Around the world, about half of all wood cut is burned as fuel, mostly for cooking and heating.
Carbon storage, recycled resources and the environment
While growing, a tree takes carbon dioxide from the air and builds it into wood. About half of the dry weight of wood is carbon. A wooden roof or table keeps this carbon out of the air for as long as it lasts. That helps slow climate change.
Use wood in this order, known as the wood cascade:
- Reuse the product as it is (old doors, crates).
- Recycle it into new material (chips become particle board or paper).
- Burn it for energy at the very end. Burning gives the carbon back to the air, so do it last.
Using recycled wood means fewer trees need to be cut. Replanting and protecting forests also protects soil, water and wildlife.
Try it: count the rings
Find a cut log, a stump or a picture of one. Count the rings from the centre. Each ring is one year. Wide rings mean a good year with plenty of rain. In the 3D, move the slider and write down: how many trees can you cut every year without the forest shrinking? (Answer: the same number that grows back.)
Key formulas and definitions
- Renewable use: trees cut per year ≤ trees grown per year
- Density = mass ÷ volume (wood: about 0.4 to 0.8 g/cm³; water: 1 g/cm³)
- Carbon in dry wood ≈ 50% of its dry mass
- CO₂ stored ≈ carbon mass × 44 ÷ 12 (about 3.7 × carbon mass)
- Wood cascade: reuse → recycle → burn for energy
Worked examples
1. A log shows 42 rings. About how old was the tree?
One ring forms each year, so the tree was about 42 years old.
2. A wooden block measures 10 cm × 10 cm × 5 cm and has a mass of 250 g. Find its density. Will it float in water?
Volume = 10 × 10 × 5 = 500 cm³. Density = 250 ÷ 500 = 0.5 g/cm³. This is less than 1 g/cm³ of water, so it floats.
3. A forest grows 300 trees a year. A mill cuts 240 trees a year. Is the use sustainable? What if it cuts 400?
At 240 cut and 300 grown, the forest gains 60 trees a year: sustainable. At 400 cut and 300 grown, it loses 100 trees a year: not sustainable.
4. A table has 20 kg of dry wood. How much carbon is locked in it? How much CO₂ did the tree take from the air for this?
Carbon ≈ 50% of 20 kg = 10 kg. CO₂ = 10 × 44 ÷ 12 ≈ 36.7 kg of carbon dioxide.
5. Put these in the best order of use for an old wooden door: burn it, make it into chips for board, hang it in a new house.
Reuse first (hang it in a new house), then recycle (chips for board), and burn only at the end.
Common mistakes
- Thinking wood is always renewable. It is renewable only if forests are allowed to regrow.
- Believing all wood floats. A very dense wood such as ebony can sink; most common wood floats.
- Forgetting that burning wood gives carbon back to the air. Burn it last, not first.
- Mixing up softwood and hardwood. Softwood is from conifers, not simply "soft to touch".