Soumya Mitra wrote:
It is accepted that an artist is free to design his work as he desires and that is not my point .
Obviously, we are on the same page on this one. As Will, Howard and Soumya all stated, an artist is free to use any species to express any design idea. But as Soumya said it, that's not the point.
The question still remains, can we find in nature tropicals that may grow into the shape of a full cascade? This is an important question for me as well, because I personally would design a tree into a certain shape only if it is credible to occur with such a shape in nature. I don't intend to copy nature to the letter, but it is much more attractive to me to respect the growth patterns that a certain species displays when grown in the wild. This of course is is just my personal taste, and has nothing to do with the artistic side of bonsai.
Back to the original question, at first I thought that the answer is NO, a tropical species would not grow as a cascade in nature...unless it is a vine of some sort, or a ground-covering shrub, that can hang in a draping manner. But that's not the real cascade shape we are talking about.
After giving much more thought, my conclusion is that theoretically there may be circumstances under which a tropical can grow as a full cascade.
Here is my reasoning.
A cascade form can occur when a species grows at the upper limits of its natural range. Beyond a certain altitude the conditions become increasingly hostile. The trees become stunted. There comes a point where the tree can oly survive if it hugs the ground and grows within a foot above it. I experienced that myself during my hiking trips: at high altitudes when you lay down on the ground, it is much warmer and less windy. So, on steep slopes, hugging the ground may lead to the cascade shape. And snow is not always a factor: some of these trees have very sparse foliage, with little surface to hold much snow, which will be blown off by the winds anyway. Therefore, snow in most cases will not be sufficiently heavy and long-lasting enough to bend a branch downward. It's rather the tree's ability to seek out the optimal growing conditions that guides it to grow downhill.
In the tropics, we can often find tall mountains where, as we move uphill, the tree population gradually changes from a certain mix to another, finally reaching the tree line where the trees stop growing. Every tropical species has an upper zone, where it becomes too cold and this will inevitably stunt the tree. At the very limit of its 'survival zone", this tropical species may become just a shrub, looking for the warmer crevices on the surface of the ground. When growing on a steep rocky terrain in these conditions, a cascade shape may well be an option for a certain individual to survive the cold. It doesn't have to be very cold in absolute terms, but just too cold for that particular species. And it doesn't have to be at the tree-line, in the sub-alpine and alpine zones, but can be at much lower elevations, where it is not very high in absolute terms but just too high for a certain tropical species.
This is just pure theoretical speculation. The only real answer has to come from an individual who actually travelled in those tropical mountains and saw it for himself. The literature available for us rarely deals with unusual growth forms of a certain species, so it is not likely that we will have an answer from books and articles.