I'm not from North America, but I think there are a few factors at play here that might answer your question.

1. There are several species of tree present in your photos. The 3rd photo shows both hardwoods (probably a maple of some description or a sycamore) and a conifer species. The middle shows almost exclusively a conifer species, which I would guess is either American Larch (Larix laricina) or a Spruce (Picea species). I can't tell what the top photo shows, but it might be Balsam Fir (Abes balsamea); it's definitely a conifer. Each of these different species have different habits of growth. Conifers tend to grow a single strong trunk with a cone-shape of relatively small limbs. Hardwoods have a different structure with a trunk that spreads and divides into several strong branches to produce a wider top or middle than bottom (arrow head shaped) or a very wide flat top. These different growth forms mean that you get wider spread of the trunks at lower altitudes, where the hardwoods exist. Conifers tend to be tougher, so often grow to higher altitudes than hardwoods. There are also species of plants that are specialist alpine growers and have forms that are naturally short or procumbent.

2. Competition. In all environments organisms, including trees compete for resources. In lowland sites, resources tend to be more abundant than they are at higher altitude. This means more free water, more nutrients etc, so plants tend to grow taller. Taller also means wider in the context of trees. In all spaces, competition for light is a big factor. If you observe carefully, saplings of hardwood species under the canopy tend to be thin and not very wide until they reach the canopy, where they spread out - this is from competition for light. At higher altitudes, the plants grow slower and as height and width are somewhat correlated in trees, they tend to be able to grow more densely.

3. The climate is also gentler at lower altitude - less very strong sunlight, higher temperatures (in general), rain rather than snow/ice, less wind. This means that plants tend to grow faster and get bigger as they have longer growing seasons and are more protected from the elements than at high altitude.

4. Wind. In exposed places, particularly near ridge tops, there is less to impede the wind, so you get stronger winds. This leads to something called krummholz or wind shaping. It results in low mat like forms of the trees and often includes "tree flagging" (see last two images on Wikipedia page for krummholz above) where you get a stem with branches pointing away from the prevailing wind direction. It is a very strong factor in plant height in exposed places, resulting in shorter stem lengths, and wider and stronger trunks. This effect is thought to be a major driver in shape and height of plants at altitude (see this article for more information).

I'm not from North America, but I think there are a few factors at play here that might answer your question.

1. There are several species of tree present in your photos. The 2000' photo shows both hardwoods identified by @Eonema (thanks for all of the ID's) as Acer rubrum (Red Maple), A. saccharum (Sugar Maple), and Picea rubens (Red Spruce). The middle (4000') shows mostly a conifer species, which is Balsam Fir (Abies balsamea) with some hardwood Sorbus americana (American Mountain Ash). The top (5000') is the conifer Picea mariana (Black Spruce). Each of these different species have different habits of growth. Conifers tend to grow a single strong trunk with a cone-shape of relatively small limbs. Hardwoods have a different structure with a trunk that spreads and divides into several strong branches to produce a wider top or middle than bottom (arrow head shaped) or a very wide flat top. These different growth forms mean that you get wider spread of the trunks at lower altitudes, where the hardwoods exist. Conifers tend to be tougher, so often grow to higher altitudes than hardwoods. There are also species of plants that are specialist alpine growers and have forms that are naturally short or procumbent.

2. Competition. In all environments organisms, including trees compete for resources. In lowland sites, resources tend to be more abundant than they are at higher altitude. This means more free water, more nutrients etc, so plants tend to grow taller. Taller also means wider in the context of trees. In all spaces, competition for light is a big factor. If you observe carefully, saplings of hardwood species under the canopy tend to be thin and not very wide until they reach the canopy, where they spread out - this is from competition for light. At higher altitudes, the plants grow slower and as height and width are somewhat correlated in trees, they tend to be able to grow more densely.

3. The climate is also gentler at lower altitude - less very strong sunlight, higher temperatures (in general), rain rather than snow/ice, less wind. This means that plants tend to grow faster and get bigger as they have longer growing seasons and are more protected from the elements than at high altitude.

4. Wind. In exposed places, particularly near ridge tops, there is less to impede the wind, so you get stronger winds. This leads to something called krummholz or wind shaping. It results in low mat like forms of the trees and often includes "tree flagging" (see last two images on Wikipedia page for krummholz above) where you get a stem with branches pointing away from the prevailing wind direction. It is a very strong factor in plant height in exposed places, resulting in shorter stem lengths, and wider and stronger trunks. This effect is thought to be a major driver in shape and height of plants at altitude (see this article for more information).