Re: tree roots: how do they harness the energy to break a sidewalk?

Plants parts, such as roots, enlarge by absorbing water. Living roots absorb 
water because they contain dissolved substances that attract water. The 
pressures roots can generate can amount to several atmospheres which is enough 
to lift or crack a sidewalk. One atmosphere is about 15 pounds per square foot. 
A textbook on plant physiology discusses water potential, which is how 
botanists quantify forces involved in water movement in plants and soils.

An analogy would be a large, empty airbag placed under an object such as a car 
or sidewalk. When the airbag was inflated, it could lift the car or sidewalk.  
Instead of airbags, plants have cells surrounded by strong cell walls, and use 
water, rather than air, to enlarge their cells. 

Arborists have many ways to prevent trees from cracking sidewalks including 
tree placement, root barriers to direct roots deeper into the soil, use of deep 
rooting tree species, use of metal grates around bases of trees, etc.

Dry plant materials such as seeds and wood, can create great pressures when 
they are confined and wetted. Swelling seeds have been experimentally 
determined to be capable of generating pressures of over 900 atmospheres.  It 
is possible to split blocks of stone by driving dry wood wedges tightly in 
cracks or holes drilled in the stone and then wetting the wedges causing them 
to swell.

References


Water Potential

Salisbury, F.B. and Ross, C.W. 1985. Plant Physiology. Belmont, CA: Wadsworth.

Meyer, B.S. et al. 1973. Introduction to Plant Physiology. New York: Van 
Nostrand. (imbibition info)