Plants/organisms that live together share different types of relationships that either benefit both or benefit at least one partner or one where the host is harmed.
Symbiotic relationships wherein one partner is benefited, but the other is harmed are the Parasitic relationships. There are many type of fungi that act as parasites on plants. A common example of a plant considered to be parasitic is Mistletoe. It shares a parasitic relationship with its host. It doesn't kill the host, but it is certainly is harmful to the host while it is hitching a ride for its growth.
'Galls' seen on some plants are a good visual that a parasite has been at work at the host tree, eventually harming it.
Then there are symbiotic relationships that can be mutually beneficial to both, for example, Lichens. The algae live inside the filaments of fungi and form a composite organism called Lichens. Leafy Lichens are in fact a good sign that the air around that organism is clean and healthy. But "Lichens are often blamed for killing a tree or shrub but this is not true. They do, however, grow on slow growing and sometimes declining trees and shrubs" ( Source: https://extension.umd.edu/hgic/topics/lichens)
The relationship that doesn't get much attention is 'commensalism,' basically seen as one plant 'hitching a ride' on another, where one is benefited, but the other isn't harmed. The plants that receives benefits is kinder to the host, at least for most of its life cycle. Most vines are a good example, where the climbing vine needs the support of trees to climb, but the vine isn't harming the host or deriving nutrients from the host and killing it slowly internally. Ivy growing on vines can be eventually detrimental to the host, but initially it is just hitching a ride. Other examples of commensalism include Spanish Moss on trees and Orchids and many other epiphytes growing on host tree
One commensalism relationship I noticed on my walk in the gardens recently was the relationship between creeping hydrangea and Persimmon trees in the gardens. See the picture where the two trunks of the creeping hydrangea vine (Hydrangea petiolaris) seem to be choking the host tree, but they aren't really harming the tree. They are simply hugging the tree, hitching a ride for their upright growth in order to have a fuller life cycle. Based on the size of the trunks you can tell they have shared a commensalism relationship for many decades. In fact, the Persimmon tree has aged and is on a decline where it could drop any minute, but I wonder if the roles have reversed and the hitchhiker is giving the host its much needed support.
On a different note, a simple example of commensalism between animals and plants is frogs using Lily pads to sit on waiting for a prey, wherein it isn't harming the Lily pad but using its support. I don't have a picture of that cute frog, but I am sharing today a picture of the ornamental gardens with the south river as a back drop to the Lilies. Summer is gorgeous! Visit London Town's deep shade woodland gardens to beat the heat this weekend.