Let's begin by saying that, you're right, the connection of the past to the present is VERY important in Pedro Páramo. Keep in mind also that this is one of the top modern Mexican novels written in the late 1900s. As such, the style reflects a fragmentation of past and present that is often only connected by nature.
We also need to take a brief moment to look at the style of Pedro Paramo in order to figure out why the nature to past/present connection is so important. The story begins simply. We find Juan Preciado has promised his mother, who is dying, that he will return to Comala in order to meet the father that he has never known. We see the town in ruins and the characters hiding in shadows. Near the middle of the story (especially when Juan Preciado dies), the author switches (wisely) to the third person omniscient point of view. Depending on the past or present viewpoint, the narrator (as first or third person) continues to change, disrupt time and space (and confuse the reader!). This is why nature is so important here.
In short, there is no "present" because we discover soon that all of the characters in this novel are already dead and this story, in essence, is a ghost story.
First, let's look at nature in regard to the past and present of Comala. Take the description of the past first.
There you'll find the place I love most in the world. The place where I grew thin from dreaming. My village, rising from the plain. Shaded with trees and leaves like a piggy bank filled with memories. You'll see why a person would want to live there forever. Dawn, morning, mid-day, night: all the same, except for the changes in the air. The air changes the color of things there. And life whirs by as quiet as a murmur...the pure murmuring of life.
Ah, Comala in the past! Look at how nature is described here. The whole place is "rising from the plain." It is "shaded with trees" and acts as an oasis. It is so beautiful that all parts of the day are just as gorgeous to experience. Even the colors at different times of day are beautiful. It is LIFE. In contrast, let's look at the description of the present.
There was no air; only the dead, still night fired by the dog days of August. Not a breath. I had to suck in the same air I exhaled, cupping it in my hands before it escaped. I felt it, in and out, less each time…until it was so thin it slipped through my fingers forever. I mean, forever.
Note how nature changes. The past air that ushered in beautiful colors is now non-existent. The narrator had to "suck in the same air I exhaled," stale and dead. It is DEATH. (And, sooner or later, the author describes it as "hell.")
However, probably the best concrete example of nature ushering the reader from present to past is the example of the stallion amid the adventures of the character of Miguel Páramo. Miguel Páramo is quite wild and, as such, is the only child even acknowledged by his father, Pedro Páramo. Miguel, at best, is a philanderer who takes the virginity of all the young maidens of the Mexican town. At night he wanders on his stallion that is a beautiful brown chestnut color (and becomes known as the "chestnut stallion"). Miguel Páramo and his chestnut stallion are soon associated with one another, in that they are always riding together at night. The story changes when the chestnut stallion returns one night without a rider! This switches the reader from past to present. For Miguel Páramo is now dead and found on the side of the road while his chestnut stallion roams on. It seems that the reason for Miguel Páramo's death is a fall from the beloved chestnut stallion.
No one knows better than I do how far heaven is, but I also know all the shortcuts. The secret is to die, when you want to, and not when He proposes. Or else to force Him to take you before your time.
Miguel Páramo, the reader learns then, is a being of the past. The present holds only his dead body.
Here we find that the voices of nature in Pedro Páramo (and especially in the case of Miguel Páramo and his chestnut stallion) surpass people, circumstance, passion, and even morality.