I might be able to help a bit:

For the first question: abiotic factors that make up a climate zone would be temperature, elevation (affecting atmospheric oxygen levels), humidity, pH, etc.

Plants in a desert are generally few and far between because there is a limiting resource of moisture in the desert. Therefore, any area already occupied by a plant is most likely ‘gutted’ for h2o resources by that plant, disabling any other seed from taking in the area.

hope this helps