It's all around, we live within it, it's always dying but it never dies - what is it?! The title is a giveaway of course, but these are just a few of the things we could say about nature. There are many ways to describe nature: physically, scientifically, artistically, poetically to name a few and also spiritually.
It might be helpful to have some idea of what we mean when we say nature or indeed anything is 'spiritual' though. The original Latin term spiritualis indicated that something was connected to the Divine, so a holy person or a sacred ritual could be spiritual - things that were physical could also be spiritual. This term was taken up by religion to refer to things that were somehow related to God. Today the spiritual tends to refer to the non-physical realm which also includes the non-physical part of us (our soul or spirit) and so the term is a little more abstract. So what might it mean to say that nature can be spiritual? I suggest it is both abstract and concrete, that we might have a sense of something other or transcendent, but we might also sense this a being God. This means that nature - the living, breathing, dying, moving world all around us - can somehow convey to us human a sense of the transcendent - a sense of the presence of God.
I wonder if you have found yourself in nature, at some point in your life, where you were arrested by the beauty of it, or calmed by it's stillness or invigorated by it's drama? Spiritual experience (as spirituality is about experience) can sometimes come to us unwittingly when we are in nature, watching a sunset, listening to the crashing waves, smelling roses. But it can also be something that we cultivate. What I mean by this is that we can attune ourself to the spiritual nature, of nature! We can practice spiritual awareness in nature, by mindful awareness of it and of the way that transcendent, or Divine presence might come to us through nature. This means slowing down, stopping, looking, listening, hearing, smelling even tasting nature in a mindful way so that we give space to the transcendent-God that may come through nature in that moment. This is not the same as saying that nature is God (this is pantheism).
For myself, as a contemplative, I've had some intensely rich experiences of the presence of God coming through nature. There are some wonderful writers such as Francis of Assisi or more recently Thomas Merton or Gerard Manley Hopkins who give words to these kinds of experiences. However, I believe we can all access the spiritual or God through nature, it's part of what it means to be in the Imago Dei. In fact I think those of us in the racing, technological, modern world would do well to cultivate a sense of the spiritual through nature as it is incredibly life-giving and can even lead us into a close and loving relationship with the Divine. So why not take up a mindful-spiritual practice in nature on a regular basis: take two minutes every day to simply stand outside and just be present to what is around, using your senses (bodily, mental and spiritual) to tune in to how the transcendent, or perhaps God, might be present to you in that moment.