I knew that sometimes multiple trees of the same species fuse their roots to share water and nutrients. Elm trees do this in urban settings; it's one of the reason why a diseased elm needs to be "trenched" to sever its roots from surrounding elms, to keep the disease from circulating into other trees and killing a whole yard or park full of mature trees. Roses also graft together below ground and spread nutrients, as well as diseases, through a network.
In nature, Redwood trees are known to form massive conglomerated root systems combining hundrets of individuals - and whole forests - into nutrient-sharing systems. One demonstration of how thorough this network is, is "albino" trees; Redwoods are the only tree species in which individuals that are genetically mutated so can't photosynthesize (and appear white) actually survive and grow in the nature because other trees feed them sugar through root connections.
(More info: http://en.wikipedia.org/wiki/Albino_redwood ... pretty cool huh?)
(In other circumstances, there are parisitic plants that don't photosynthesize, but they cannot parasatize thier own species because none of the others photosynthesize either. In this case, it's less like parasitism and more like welfare.)
But it goes beyond that. I JUST READ: in most forests, ALL trees connect to large underground mycelial (fungus) networks to pass minerals, water and sugars... through the networks... EVEN TO "COMPETING" TREE SPECIES!
Here's one source for more info which in turn contains references to further scientific data: http://www.mykoweb.com/articles/MycorrhizalNetworks.html
Studies showed that seedlings that would have died ended up surviving when they tapped in to the mycorrhizal network and fed off competitors. But this isn't "parasitism" because when they grow they end up giving back, and it's much more than "symbiotism" because the whole forest is plugged in to the network.
Take a minute to comprehend the implications of this. It basically undermines everything we were taught about the nature of... well, nature. Since Darwin we've thought of the wild as a cruel place in which aiding your competitors would mean disadvantage and even death, where the "fittest" survive and selfish motives drive success. Humans were a rare exception because many of us choose to care for the sick, the elderly, adopt children who are not our own, choose not to reproduce genetically, make altruistic sacrifices - even sacrifice our own lives for causes - and value the common good.
As it turns out, well yes competition still exists in nature (no one would deny that), but nature also "cooperates" and forms much more than occasional relationships. Entire eco-communities of common interest develop; civilizations basically.
Both competition and cooperation can aid survival through natural processes, and if your rivals are cooperating you'd damn well better get on board if you want to make it because they have a huge advantage in doing so.
And what if your willingness to cooperate was so thorough that you were willing to help anybody and everybody who would team up with you? Would that be the ultimate trump card in survival?
I did learn about symbiotic relationships in school biology (the classic example is lichens http://en.wikipedia.org/wiki/Lichen ), but we were taught that it was an anomaly in nature. We never learned the "rare exception" would be an individual organism that is NOT in a directly-symbiotic relationship.
The movie Avatar IS TRUE. On Earth.
I have a hypothesis. I think an unexplored tendency in metabiology is that, over eons, ecosystems evolve from competitive to cooperative.
Think about how life "colonizes" a barren place like a new island or forest wiped out by fire; pioneering plants creep in and begin growing in a rough environment. They live, die and add organic material to soil, which in turn allows other plants to grow there. Slowly, ecological communities develop.
The Earth itself has followed that route as a whole: the biosphere has gradually expanded since life's beginning, due to species aiding each other. It took a long time for any life at all to exist on land, and when it started, only wet swamps were hospitable enough for plants to grow or animals to live; the fossil record shows that even when life was evolving rapidly on Earth, huge expanses of dry land were barren. Nothing we now see as plains, deserts, or subarctic regions would have any species at all.
Slowly, a few plants moved in - plants that could better regulate their water and get through the heat of the day, plants with more developed roots so they could stay green long after rain, plants that could store water for weeks, months, even years. Plants that could go dormant in rough times and spring back to life when rare rains came. Even plants that could get water from the air.
Places we now know as plains, prairie, Savannah, and even some forests are actually quite dry in the scheme of things, and would be barren if not for innovative strategies.
Those plants improved soils, provided food sources, altered weather patterns to make them rainier, and diverged into even more species that filled even more niches.
I'd suspect that in another half billion years, the concept we currently term "deserts" will no longer exist on Earth. More adaptave species of plants will colonized them and be able to grow lush without water. You might find enormous canopies of the descendants of joshua trees and saguaros that grow so thick they shade the ground. Maybe plants will be able to extract water exclusively from the air and share it with other plants, or they'll be able to purify salty water, or water from extremely deep underground reserves.
And perhaps in so doing they will cause rain to fall more often, or at least make rain more or less irrelevant to the ecosystem.
I don't think there's even anything far-fetched or speculative about what I am saying; it's a pretty well-known concept that ecosystems can gradually tame adverse settings.
So to go along with my hypothesis - ecosystems evolve to become cooperative - perhaps Homo Sapiens, with our ability to domesticate & partner with thousands of plants and animals are unwittingly part of that process. We "cooperate" with our crop plants, our pets, animals we have domesticated, ornamental plants, bacteria and fungus we use in food processing as well as creating industrial products. I would suggest that human beings are probably the most symbiotic species on Earth, forming mutual relationships with new species all the time.
Maybe human beings are a natural process, of cooperation, that will someday allow Earth's biosphere to expand into space.
Imagine it. By the same process that oceanic life came to live on land (species taking small steps and aiding each other), Earth's life could grow and thrive in greenhouses on the Moon, on Mars, on Saturn's moons, and beyond, because in this case, Humans had the survival strategy of technology, to transport, and build habitat for, other life forms.
That's sort of an "expansionist" mindset that is very American; perhaps it's more pertinent to talk about our species' ability to symbiotize as a call to nurture and protect natural spaces in land that we already live in, to improve the quality of life for everybody, and to have the wisdom to check some of our worst impulses. But it's worth noting that the most relevant criticism of American expansionism is that it wasn't cooperative, but exploitative, and that doesn't fit the arrow of nature's natural process.
In a Darwinistic sense, though we aid other species by breeding them for slaughter or growing them as crops, we aren't exactly doing very much for the organisms as individuals; it surely sucks to be a farmed chicken or pig. I'll note that's not what I'm talking about here, but there is still - interestingly - a process going on (a political one, albeit) to be more aware of these kinds of human impacts and reduce them.
...but even if we weren't around, Earth's biosphere would still be driven to expand as it becomes increasingly cooperative; to the poles, to deep crevices, to high altitudes, and so forth.
1) Nature is secretly communist
2) AVATAR IS A TRUE STORY. (Not the "Cowboy/Indian" social commentary, just the part about the trees.)
3) Humans are the ultimate symbiote.
Or ignore all that, and just know that trees feed each other, which is still pretty awesome.