These are the rules for choosing subdomains (emphasis mine):
Subdomains—more specific focuses for clerical worship and power that allow players greater flexibility in customizing their characters. Every domain has a number of subdomains listed indented underneath it. Each subdomain replaces a granted power and a number of spells in the domain’s granted spell list.
A cleric who chooses a subdomain must have access to both the domain and its subdomain from her deity. If a cleric selects a subdomain, she cannot select its Associated Domain as her other domain choice (in effect, the subdomain replaces its Associated Domain). Subdomains are treated as equivalent to their Associated Domain for any effect or prerequisite based on domains. If a subdomain has two (or more) Associated Domains, the cleric can only select the subdomain for one of her domains. Subdomains can be selected by druids (except the metal subdomain) and inquisitors (if their deity allows it).
If a domain or subdomain ability calls for a saving throw, the DC of the save is equal to 10 + 1/2 the character’s cleric level + her Wisdom modifier.
From this, I conclude that a druid who can take a domain (via the Natural Bond Class Feature or Archetype Feature) is also allowed to instead take one of the subdomains associated with that domain.
So, a Druid could choose either the Fire Domain, or one of the associated subdomains: Arson, Ash, or Smoke.
But what happens when a Druid gets to choose other domains than the "original" elemental domains via an archetype?
For example, the Treesinger archetype's Plant Bond feature says:
The first [form] grants the treesinger one of the following domains: Plant; Growth; Jungle, Swamp. When determining the powers and bonus spells granted by this domain, the treesinger’s effective cleric level is equal to her druid level. A treesinger who selects this option also receives additional domain spell slots, just like a cleric. She must prepare the spell from her domain in this slot, and this spell cannot be used to cast a spell spontaneously.
[...]
Plant bond replaces the druid’s nature bond ability.
The Life Channeler archetype's Plant Preservation feature says:
A life channeler must choose the Plant domain with her nature bond (she can select the Growth subdomain if she wishes). When she performs a coup de grace that kills a sentient humanoid creature (not a summoned creature), she can do so ritualistically in order to affect the area centered on the body with the enrichment version of plant growth. The sacrifice is a death effect.
This ability alters nature bond.
If a Treesinger has the Plant Domain, wouldn't she be able to choose from the associated domains Decay, Growth, Leshy, or Thorns also? But only Growth is listed.
When can a druid choose a subdomain instead of an ordinary domain, and when can't they?
