Turkey Travelogue 3d - Why are there trees on ancient cultic sites?

If you haven't noticed already, we were intrigued by trees in Pergamum--and not simply because we hail from southern Saskatchewan. I've already commented here and here on the trees now growing on the Great Altar to Zeus :
And the tree in the precinct of Athena (top left of next picture):
There is also a tree growing at the site of the open-air altar in the Asclepion:

To be sure, trees grow in Turkey. Give them a hundred years and decent soil, and they will no doubt flourish in excavated archaeological sites. Nevertheless, the presence of trees at three cultic sites in Pergamum struck me as curious. I probably would not have considered the matter further if it were not for the strips of plastic we noticed tied to the tree in the precinct of Athena. Unfortunately, I have no close-up shots of the tree in question:
If you zoom in to the above photograph, the plastic is barely visible:
Was this someone's creative response to tourist litter, or does it suggest that veneration of ancient cultic sites continues on in modern Turkey? We didn't notice trees connected to altars, let alone plastic tied to trees, anywhere else on our trip, but then we only got to a fraction of Turkey's archeaological sites.

A related, much more common, and much less puzzling phenomenon is the transformation of ancient temples into churches after Constantine, and the the transformation of Christian churches into mosques after the Ottoman conquest. There are many examples of the latter in Istanbul--a practice that mercifully preserved beautiful churches from destruction. An example of the former is the Kizil Avlu in modern-day Bergama. Originally built in the first half of the 2nd century A.D., it was apparently first a temple to the Egyptian god Serapis, and later reused as a church dedicated to the apostle John:

Update (09/07/07): Is the medieval Egyptian practice of burning milk-thistle in front of the Sphinx analogous? Click here for details (and an intriguing suggestion about the loss of the Sphinx's nose).
Update (10/08/12): In a book published in 1897, W.H.D. Rouse mentioned rags on trees as one example of the persistence of ancient cultic practices in 19th century Greece:

"Nor have the dryads gone, nor the nymphs of the streams; witness this chapel over some sacred well, or that tree with its tribute of rags and onions. All over the land, I might say in nearly every field, often far from any now inhabited spot, are ruined shrines or simple enclosures, each with its patron saint; recalling the corners set apart for Pan and the nymphs in a Greek farm of old." (HT: Michael Gilleland)