A Wanderling is a tree grown from a seed (coiled with the hair of the elf) planted in the Wanderling Woods to remember an elf. Each Wanderling takes on some attributes of the person after their DNA (usually a hair from their hairbrush) has been wrapped around the seed. Their eye and hair colours, as well as body shape are translated into the tree‘ appearance. Some even smell like the elf they represent.
SPOILERS BELOW Edit
When she was kidnapped in the first book, she was presumed dead, and a planting took place. Her tree is still present because it is a living object. Her tree still remains like Dex’s even though she is still alive.
When he was kidnapped in the first book, he was presumed dead, and a planting took place. His tree is also present for the same reason as Sophie's tree.
One of the two Mr. Forkles died when Lumenaria was knocked down in book five, from a "wound that cut too deep", although his seed was not planted in the Wanderling Woods to keep from suspicion (that there are two of them).
Note: There were many other plantings that have occurred, but they are not known. There are still many less deaths than humans.