In this particular case I assume that this is an advertisement in making us `environmentally aware` and I presumed that they would want us to save more than one type of tree. If there is a tree type(name)
mentioned you can use both plural and singular

For example:

Eucalyptus Tree

One would say `Save the Eucalyptus tree`
or `Save the Eucalyptus trees`

Dictionary references

The-

1.determiner-used to refertoone or more people or things
2.
used to refer to someone or something that is the only one of its kind
3.used to refer to something in a general rather than a specific way

ぺブルビーチのローン・サイプレス・ツリーは有名ですね。