The difficulty of I and Thou led Buber himself to offer some clarifications in the second edition (1957). Here's what he says about the question of relation with nature. His account of knowing a tree as a "thou"was already famous (for some infamous). It's not pantheism or mysticism.
[I]f we are to suppose that the beings and things in nature that we encounter as our You also grant us some sort of reciprocity, what is the character of this reciprocity, and what gives us the right to apply to it this basic concept?
Obviously, no sweeping answer can be given to this question. ... [W]e must consider [nature's] realms separately. ... Animals are not twofold, like man: the twofoldness of the basic words I-You and I-It is alien to them although they can both turn toward another being and contemplate objects. We may say that in them twofoldness is latent. In the perspective of our You-saying to animals, we may call this sphere the threshold of mutuality.
It is altogether different with those realms of nature which lack the spontaneity that we share with animals. It is part of our concept of the plant that it cannot react to our actions upon it, that it cannot "reply." Yet this does not mean that we meet with no reciprocity at all in this sphere. We find here not the deed of posture of an individual being but a reciprocity of being itself - a reciprocity that has nothing except being. The living wholeness and unity of a tree that denies itself to the eye, no matter how keen, of anyone who merely investigates, while it is manifest to those who say You, is present when they are present: they grant the tree the opportunity to manifest it, and now the tree that has being manifests it.Our habits of thought make it difficult for us to see that in such cases something is awakened by our attitudes and flashes toward us from that which has being. What matters in this sphere is that we should do justice with an open mind to the actuality that opens up before us. This huge sphere that reaches from the stones to the stars I should like to designate as the pre-threshold, meaning the step that comes before the threshold.
Obviously, no sweeping answer can be given to this question. ... [W]e must consider [nature's] realms separately. ... Animals are not twofold, like man: the twofoldness of the basic words I-You and I-It is alien to them although they can both turn toward another being and contemplate objects. We may say that in them twofoldness is latent. In the perspective of our You-saying to animals, we may call this sphere the threshold of mutuality.
It is altogether different with those realms of nature which lack the spontaneity that we share with animals. It is part of our concept of the plant that it cannot react to our actions upon it, that it cannot "reply." Yet this does not mean that we meet with no reciprocity at all in this sphere. We find here not the deed of posture of an individual being but a reciprocity of being itself - a reciprocity that has nothing except being. The living wholeness and unity of a tree that denies itself to the eye, no matter how keen, of anyone who merely investigates, while it is manifest to those who say You, is present when they are present: they grant the tree the opportunity to manifest it, and now the tree that has being manifests it.Our habits of thought make it difficult for us to see that in such cases something is awakened by our attitudes and flashes toward us from that which has being. What matters in this sphere is that we should do justice with an open mind to the actuality that opens up before us. This huge sphere that reaches from the stones to the stars I should like to designate as the pre-threshold, meaning the step that comes before the threshold.
(trans. Walter Kaufman [NY: Scribners: 1970], 172-73)
Does this help? It helps him move on to the equally difficult relation with the divine, which he defines as an "over-threshold." I may be misreading it but I think part of Buber's gift to us is an understanding of relation which is neither humanist nor antihumanist. The human way of relating - to other humans but also to the nonhuman - is part of a broader spectrum of relations, in each part of which we also participate. This doesn't relativize the human, making it of value only under particular circumstances or only to us, but places it in a context which shows that being human is the human way of participating in the greater-than-human...
Earlier in the course we considered that words are not the only form of interpersonal dialogue or even necessary to it, though there is a sense in which words make explicit what dialogue is about even where it is wordless. With Buber we consider that the non- and superhuman may be part of the story too, even in human words.