Answer:
In general plants are adapted to overcome a few different problems.
Lets take a few which are applicable for the Acacia's.
- Hard soil/soft soil
Plants living on hard (normally clay soils with a high mineral/nutrient content) soils have a root system that spreads out without penetrating far into the soil.
Plants living on soft (normally sandy soils with a low mineral/nutrient content) soils have a root system that penetrates deep into the soil.
- Water availability
Trees living on hard ground normally have little water available. Rain water does not easily penetrates the surface and then therefore the ground water volume is low.
These trees have small leaves to reduce the water loss. Some off the trees like the Knobthorn Acacia have leavelets that close up during the heat of the day, reducing the surface area with almost 50% (like the wings off a butterfly closing together).
On the other hand trees on soft soils have often much more water available. The water penetrates easily into the ground which results in a high ground water volume. The leaves are generally much larger while they do not have to spare water. The leaves help in pumping water up from the depths. By evaporations there is an 'underpressure' in the top off the tree and this helps to pump (suck) up the water.
Some trees have waxy leaves or hairy leaves all helping in reducing water loss.
- Nutritional value
Plant with small leaves normally have leaves high in nutrients (because off the soil) and are therefore sought after by browsers. These trees protect themselves from this onslaught by herbivores through different kind of adaptations.
A family trate of the Acacia's are their thorns. One way of protecting yourself as with spines. Another way is by growing tall and keeping your leaves out of reach off herbivores.
Some plants use chemical defences. Some are toxic, some poisonous.
Some acacias use tannin to protect themselves. Pushing up tannin from the roots making the leaves untasty.
Trees with large leaves normally do not have a high nutrient content due to the soil. Normally these trees have less protection (no thorns or spines).
Some trees make use of other organisms to help them...
- Symbioses
Some trees use a symbiotic relationship in which the other organism benefits from them (in the form off housing and food for instance) while they protect the tree (or rather their food source and home).
A beautifull example of this is the sybiotic relationship between the Whistling Thorn Acacia (Acacia drepenalobium) and ants.
There are more adaptations in trees in general, but this is some basic information.