Adam, as the beginning of all humanity, represents all men. (The beginning of something embodies within it the totality of that thing, a principle taught a number of times in the Maharal. Therefore, if we want to understand the essence of a concept, we look for the first time that concept is mentioned in the Torah. The Maharal is teaching that Adam, as the first man, embodies within him the totality of humanity, and his actions impact all of mankind.)
The “decree” referred to in “land which Adam decreed…” means the places in the world which were connected to Adam’s essence. Adam, as the root of all humanity, can be compared to a tree whose root is connected to the land, and it is from that place that all development flows. The places in which man could take root to produce his “fruit” were the places that Adam “decreed” to be settled.
The appropriate place of something or someone is a function of the characteristics of that thing or person. Since not every place on earth embodies within it elements that are fundamental to man’s existence, there were places that were not decreed by Adam for man to inhabit. It wasn’t an arbitrary choice by Adam to decree some places and ignore others. (The Maharal understands that this decree wasn’t a proclamation of intent, but a consequence which reflected reality.) Just as there are organs located in one part of the body, and not somewhere else, all dictated by G-d’s infinite wisdom, there are places on earth in which man can be located and others that are not appropriate for him.
(Places are viewed as having specific characteristics, and there is a match between the needs of the object and the characteristics of its “place.” The classic example of this is the Land of Israel as the “place” of the Jewish people. In Netzach Yisrael, Ch. 24, as well as in Ch. 1, the Maharal has a lengthy discussion on this subject, and teaches that the Jewish people living outside of the Land of Israel is an unnatural state, just as any organism which is removed from its natural habitat. The outcome of an organism being removed from its natural habitat is either a) extinction, b) adaptation, which means it really becomes a different organism than it originally was, or c) an instinctive return to its original habitat. Since the Jewish people can’t become extinct, and “adaptation” means they cease to be the authentic Jewish people, which is equivalent to extinction, there exists an instinctive drive to return to the Land of Israel throughout exile. G-d needed to impose upon us special oaths to prevent us this instinct from forcing a return to the Land of Israel before the redemption would begin. And He had to activate special protection to enable us to exist in exile without becoming extinct. (Seee Ketuboth 111a.) But that the Jewish people WOULD be returning to the Land of Israel was self-evident to the Maharal, by nature of every organism having its own unique place.)
The concept of specific places for man on earth is alluded to in the Gemara Sanhedrin (38a-b). Rav Oshia said in the name of Rav: Adam’s torso [was created from the dust of] Babylonia. His head [was created from the dust of] the Land of Israel. His limbs [were created from the dust of] the rest of the lands…
You see clearly that the land relates to the essence of the person. Man’s head relates to the Land of Israel, his torso to Babylonia, and his limbs to other lands. (The Maharsha on this section of the Gemara, and the Maharal in Netiviot Olam, Netiv Gemilut Chasadim, the beginning of Ch. 5, elaborate. Since the Land of Israel is the most important of all the lands, as well as the most spiritual, it serves as the source for man’s head, his most important part, and the part which is most connected to Divine Wisdom.) And there are other lands to which man has no connection, leading them to be desolate from civilization.