Elephant proof trenches are not the solution. It stops elephants from entering into the fields for sometime, but not for ever. Over a period of time, then trenches get filled. These are difficult and costly to maintain. Also, it is not a solution as you are not attacking the root cause.

One needs to increase the quality of habitat which includes reduction/removal of exotic weeds, increasing/restoring plant diversity, fruit bearing trees etc. That will help in their nutrition levels. Elephants engage in crop raiding because of shortage of food and also because the cultivation of crops which the elephants find nutritious and love. A huge animal like an elephant needs lot of lot of forage. The elephant corridors are mostly blocked, so the elephants have to take a diversion and come in contact with humans.

I strongly detest the use of tree guards. If you are planting trees in the road dividers and other such places, then you are not creating forests. The tree guards are used as advertising boards. Apart from being costly, from a donor perspective I can say that it breeds corruption. Also, creating a tree guard out of some metal is not good for the environment.

It is better to identify degraded forest and revenue lands and plant indigenous trees. Not in rows of the same trees - as mono-culture doesn't help. Forest department often plants avenue trees like Gulmohur etc. As a kid, I remember reading in the Statesman the famous naturalist M. Krishnan lashing out against Gulmohur trees. In Oriya we call in Krushnachuda. Those have nice colourful flowers. However, it is not a native of India.

Cheers,
Sabyasachi