Rocks and vegetation can help hold a slab in place, we call these anchors. In order to be effective anchors they need to be well spaced, if anchors are widely spaced they can actually work as dot to dots. Moreover it is important these anchors penetrate deep into the snowpack, most importantly through the weak layer. With regards to vegetation as anchors, it is most effective if they have branches which are low, as they can freeze into the snowpack, both Spruce and Fir have low branches. Finally it is also important to note that anchors work most effectively with hard slabs than softer slabs.
In Depth Hoar snowpacks, such as a Continental snowpack, rocks are common trigger points for avalanches, ie. they are not to be thought of as anchors. This is because rocks conduct the ground heat, and thus the snow around them is thin and often faceted.
-Faceted Snow, angular and large grained snow, a result of large temperature gradient in the snowpack, it typically has poor bonding qualities.
-Depth Hoar, large grained and faceted, it forms often near to the ground and is the result of large temperature gradients in the snowpack. This is why depth hoar commonly forms in the early season where the temerature between the ground and this snow is at it's greatest.