Pics of the trees.

I do not know what Changi Airport does but most commercial establishments (malls, airport, offices, etc) contract out to professional nurseries. They swap out plants on a regular basis and send them back to greenhouse conditions to recover for a year or two before rotating them back for display. Changi is supposed to be spectacular. In a tropical climate it is lot easier too.

One of our neighbors uses the same service from a friend who runs such a business. They have spectacular potted tropicals in huge pots (eg 20-40 gallon pots). Every fall they come to pick up the plants and truck it 700 miles to a Vermont greenhouse and in early summer they bring them back in excellent condition all refreshed, with slow release fertilizer ready to put on a show. All that our neighbor does all summer is water them.

without digging into the planter ... there is no way to know whether the sand/pebbles are anything more than just a decorative top dressing ...

and if the theory of cycling them is correct... i wouldnt be surprised if they arent in pots ... so they dont have to move the larger cement? planter ... though i suppose the rock.. would be heavy enough to hold the pots inside secure .... wouldnt want them tipping over ...

all speculation ....

thx for the pix... ken

They're plastic.

LOL

Often they are in individual pots with provision for water to drain out the bottom into the outer planter. So the outer planter is just for looks. I have not seen the planter having a drain hole. When they change they bring appropriate equipment to lift out the inner pot(s). They are pretty quick and efficient. The outer planter stays in place - they just clean it out and fill out the gaps with whatever decorative material they use.

I will ask some ground staff about it next time I go through the airport. I think there must be a pot inside and all that sand and pebbles are just decorations.

Edit: Found some info

https://fivedotzerohorticulture.wordpress.com/2017/09/28/changi-airports-nursery/

The pebbles are leca beads.

short for Light Expanded Clay Aggregate beads. These beads are little spheres with the diameter of about 1-2 centimetres, made from heating clay. Water can easily seep through the gaps between these beads, and its compactness prevents it from making a mess if these beads spill out of their pots.

https://qz.com/410561/the-worlds-best-airport-has-a-horticulture-team-a-butterfly-garden-and-500000-plants/

http://www.changiairport.com/corporate/media-centre/resources/publication/issue-22/ever-wondered-how-changis-gardens-look-so-lush.html

As explained above, they almost certainly swap the plants out regularly.

I vaguely remembered that Singapore has a whole in-house team to maintain their plants. Supposed to be a wonderful experience.

Leca beads are generally available in hydroponics stores and I believe pet-supply stores too.

Also rainforest soils are often highly degraded and mineralized so that if plants from that environment get other factors they need the soil may not need to be very good. Otherwise look at all the foliage houseplants that are able to linger on for decades in temperate climate interiors where they don't even get much light or humidity.