Rock  |  Water  |  Wood
The forests of Yosemite range from foothill-woodland to alpine, the result of elevations in the Park that extend from less than 2,000 feet to the highest point, Mount Lyell, at 13,111 feet.  Different zones have different mixes of trees and vegetation.  In some areas the forest provides fairly dense cover and welcome shade on hot days, while in other places trees are dotted about amidst the granite.  In alpine areas of the Yosemite, there are no trees at all.  Among the most dramatic sights in the Park are several giant sequoia groves; the Mariposa Grove includes such trees as the 210 foot tall Grizzly Giant.  (I did not manage to get to the sequoia groves on this visit).  Factors big and small, from lightening, wind and other weather to insects and fungal infections, fell individual trees.  Additionally, each season trail maintenance crews cut smaller trees to keep the paths clear, and the Park Service has engaged in efforts over many decades to prevent conifers from encroaching on meadows.  On a larger scale, rockfalls can flatten large swaths of trees, fire can blacken many acres, and insects can lay bare thousands of trees.
A view from above Olmstead Point includes a Western juniper in the right foreground.  These juniper trees are probably my personal favorite among Yosemite's trees.  The older trees are like works of art scattered about on a vast granite canvas.  They have broad trunks and light reddish bark; often there are exposed sections of wood.  In some places the wood is texturized gray and black, elsewhere it may be eaten away by dry rot, and in other places looks as if it had been carved by a chisel from a fresh piece of timber.  At the top are small craggy branches and tufts of greenery.  Oftentimes there will be lichens as well.  The trees have been growing for hundreds of years out there on the granite, surviving through hot summer days and cold winter snowstorms.
The trunk of a Western Juniper in the Olmstead Point area.
The trunk of a Western Juniper frames a scene in the Olmstead Point area.
The trunk of this Western Juniper in the Olmstead Point area looks as if it has been carved with a chisel.

Below: Details of fallen trees (top two from the Lembert Dome trail; lower two from the Dewey Point trail).

Bits of wood from a fallen tree along the Dog Lake trail.

According to the National Park Service, Yosemite has 37 species of native trees. 
One common tree is the Sierra lodgepole pine (tamarack pine), photo taken in the Olmstead Point area.
Not all of Yosemite's trees are conifers; there are also aspens, photo taken on the trail from Dewey Point to the Glacier Point Road, as well as oaks and others.
Smoke rises from small fire, seen from near the Sentinel Dome trailhead off Glacier Point Road.

Notes:
Famous trees of the past include the Wawona Tunnel Tree, a giant sequoia which fell in 1969, and a twisted Jeffrey Pine that grew atop Sentinel Dome and was the subject of an Ansel Adams photograph.

Links:
Plants - Yosemite National Park