In snowy surroundings, Elderwood-trees might have Snowy Elderwood Leaves that will turn into ordinary Elderwood Leaves when you harvest them. Placing common Elderwood Leaves into cold environments will not change them into snowy ones.
Blocks of leaves can be harvested/pulled with your arctek-gauntlet without needing any Power Cells. If you use Stone Mining Cells or better to pull leaves, these stronger Power Cells will not even lose durability.
In Creativerse leaves will not decay / vanish by themselves when you "cut" down the stem of the tree nor if you take away all the wood-blocks that the tree is made of.
Instead the blocks of leaves (as well as all Flower-blocks, Beeswax, Vines, etc.) will prevail and float in the air until you take them down manually or use Excavators on them to collect at least half of them.
You can of course also throw Fire Bombs at leaves to burn them, up to 7x7x7 blocks with one bomb (usually a bit less though). As leaves are flammable, burning them might work overly well in warm environments and can lead to forest fires in hot biomes.
Be careful when setting fire to Elderwood leaves in hot biomes like Savannahs where giant Elderwood trees grow. The fire can spread and set all flammable blocks on fire (including most crafted wooden blocks) that touch each other, and the flames can even leap over gaps of 1-2 blocks.
To stop forest fires, either create gaps large enough so that the fire cannot leap over, or claim the area and disable the option for fire spreading. Fire spread can be disabled for a gameworld by their owners only if they have bought the "Pro" upgrade.
On the other hand burning attempts might stay below expectations in cold environments. Simple torches cannot even set fresh leaves aflame in snowy biomes (as of R25 in November 2015) that are typical for Elderwood trees.
One block of Elderwood Leaves can be made into 4 blocks of Shredded Leaves in the Processor. Shredded Leaves, that are useful to craft Beds, Carpets, Thatched Walls and Thatched Roofs burn even easierly than fresh leaves when being placed.
Elderwood Leaves can be placed as solid blocks for decoration and can also be used for building; however they can make creatures spawn, like Leafies during the day, and Night Leafies in darkness, sometimes also Chizzards.
Elderwood Leaves can be used as Fuel for the Forge, however 4 blocks of leaves are needed and used up for each melting-/hardening-process. Also each melting-/hardening process in the forge will take quite long when you use leaves as fuel.
Elderwood Leaves can be corrupted into Corrupted Elderwood Leaves, for example by using Corrupt Bombs on them. By that they will turn into better fuel, but they have to be picked up using Lumite Mining Cells, which will reduce the durability of the Mining Cells quite a bit (just like pulling all other kinds of corrupted blocks).
You can lay out Elderwood Leaves to make Blue Flowers grow on them, but that will take quite some time (a few RL-days for each new block of blue flower).