Sandeep,
Good question. Eucalyptus has a special quality, it injects a growth inhibitor into the land where it grows. This chemical stops other competing plants (native species) from growing as long as the Eucalyptus tree is alive. Once the tree dies, there is no long term impact.
There are other myths propagated about this species saying they dry up the land by absorbing all ground water and so on. This is not entirely true. In plantations, since these trees are cut down every 3-6 years, their roots do not go beyond 5-6 feet depth. Eucalyptus and Acacia are fast growth species, so they obviously soak up nutrients at a faster rate compared to slow growth species but they do not cause a lot of damage to land.
It’s an entirely different issue that the leaves and fruits of these two varieties are not consumed by birds and animals. So cutting down natural forests to plan these trees is environmentally damaging as it disturbs the ecosystem. There is no harm in growing these trees in private farms and lands which would be barren otherwise.
Planting trees (any trees) helps enrich ground water and keep the surrounding areas cool.
Hope this helps!
Cheers,
AK