Our pagan ancestors had no temple, at least not in the form of a building. The sacred places were natural sites: a clearing in the heart of the forest, an entire wood, a particular rock, or even certain trees.
Regarding the latter, they were generally the oldest subjects, sometimes several thousand years old.
The tree is in that case often represented as a link between the underworld and the celestial world. In some cultures, each species is even associated with a deity.
"According to certain dissident members of the Druidicca, Yggdrasil (the "World Tree", meeting place between the earth and skies) would be in the heart of each and everyone of us. Otherwise how could we explain the striking resemblance between our lungs and the branches of a tree reaching for the sky?" Salliah (Ysambre - The World Tree, Mickaël Ivorra and Séverine Pineaux) (this is my own translation as I don't believe this book was ever edited in English)
The practices around these sacred trees were varied: from simple thanks and contemplation to the consultation of their wisdom as oracles, and also ceremonies to call an event of one's wishes, such as rain for example.
It was sometimes customary to hang magical objects on branches or in hollow trunks, to place candles or to tie pieces of cloth on the twigs or even animal skins, or simply to decorate them. Some made altars at their feet, poured liquids as offerings, which could also take the form of food, flowers or various objects. Some trees were even the object of pilgrimages.
If it was allowed to touch these sacred trees, some used to embrace them, to kiss them, even to take parts of these trees for rituals (generally the druids); on the contrary, some trees could not be touched under any circumstances.
A practice that today would be considered harmful for the trees was to hammer nails into them like so many aches that one wanted to get rid of.
When our pagan ancestors began to be confronted with Christianization and therefore with the clergy, the latter proceeded, for the most virulent, to large-scale felling of these sacred trees or simply to prohibitions to meditate at their feet. Others have tried to capture the veneration of the faithful by placing crucifixes or virgins in the trees in question. Masses were also practiced in front of trees belonging to pagan worship.
Some spiritualities still practice what can be called dendrolatry (the cult of trees), in particular the animists, for whom each being and even each thing is endowed with a soul, a spirit.
"When I am among you, trees in these forests,
In all that surrounds me and I also hide
In your loneliness, where I withdraw myself,
I feel someone listening to me and loves me."
Victor Hugo
In France, the lime tree of Réaumont in Isère, 600 years old, hollow as often for trees of this age, is still celebrated every year on the third weekend of October during the Tree festival. The oak of the Hindrés, 450 years old and located in the forest of Paimpont (to which the mythical forest of Brocéliande is attributed), is apparently still the object of Druidic celebrations. The association A.R.B.R.E.S. (meaning "Trees" in French and standing for "remarkable trees: assessment, research, studies and safeguarding") keeps a national inventory of remarkable trees, among which are certainly hidden specimens of cult... A similar association in the UK has existed for even longer, and there are probably equivalents in many countries.
If your practice includes a cult of trees or if this spirituality appeals to you, remember that you must avoid any injury inflicted on the trees (which may eventually lead to death by the vector of bacteria, fungi or insects) and that the the use of candles near them is not recommended (or else should be done under close supervision and with abundant water at hand).