Can I plant a orange tree next to a lemon tree since they are both citrus fruits? If not, how far apart must the two trees be? (I live in Israel if that makes a deference in halacha)

Shalom, Thank you for your question. How wonderful it is for you to be involved in the mitzvah of planting trees in Israel! It is permitted to plant trees together, and there is no prohibition of keyalim (mixing species). The prohibition of keyalim in trees refers to grafting trees together – or planting things near grape vines. Apart from these prohibitions, it is permitted to plant all trees even right next to each other. (Vegetables and grains are where the laws of keyalim are found in the field). Of course you will recall the prohibitions of Orlah (the fruit being forbidden for three years); the special laws of fourth year fruit; the laws of Shmitah (the seventh year); and the laws of trumah and ma'aser. May you merit to be blessed to continue to make the land of Israel sprout with every holiness!