"After the treaty had been made at Beersheba, Abimelek and Phicol the commander of his forces returned to the land of the Philistines." NIV translation
The king had given Abraham Beersheba as a part of the treaty and so it was considered to be a separate land and no longer a part of his kingdom.
"Abraham planted a tamarisk tree in Beersheba, and there he called on the name of the Lord, the Eternal God." NIV translation
This small verse may seem somewhat insignificant but it contains a big lesson. The tamarisk tree grows in desert areas and uses a lot of water so it has very deep roots. When insects feed off of the leaves and branches, they give off a white residue that covers the ground. This is a somewhat salty residue that prevents other plants from growing and competing for the scarce water resources. Mesopotamian tradition held that the tamarisk tree served as a boundary between the unclean and the holy. Throughout the Bible, water is a symbol (picture) of the Holy Spirit. That Spirit is a boundary which designates us as God's children by its presence in us. Abraham would have been surrounded by Philistine territory but Beersheba would have been sort of an island in the midst of their idolatry.
As Christians, we have a tamarisk tree of faith that separates us from the world around us and is fed by the Holy Spirit. Jesus told us (in John 13:35) that we would be known to be His disciples (Christians) by our love for others. This is a sacrificial love (like His) that puts the needs of the less fortunate above our own.
"And Abraham stayed in the land of the Philistines for a long time." NIV translation
Finally, we notice that Abraham stayed in the midst of the Philistines. Many Christians want to isolate themselves from the evil world around them but that is not what Jesus told us to do. Every day Abraham's neighbors would have seen the tamarisk tree and perhaps they asked about it and the God that Abraham worshiped there. The same thing will happen when people see our love for others. When they ask us why we are doing loving things for the less fortunate, we can explain that the love is from God through the power of the Holy Spirit.