Describe the relationship between bird species richness and tree species richness

Species richness is related to the number of species living in a particular community. More the number of species present in a community more will be its species richness.  

Trees provide the natural habitat for most of the birds living in this world so a higher tree species richness can provide a wide range of natural habitat for bird species.

Different species of birds rely on different source of food to reduce competition so a high species richness of trees can provide a different source of food which can support high species richness of birds.

Therefore there is a positive relationship between bird species richness and tree species richness.  

for the points

1: most common garden plants prefer loam — soils with a balance of different-sized mineral particles (approximately 40% sand, 40% silt, and 20% clay) with ample organic matter and pore space. however, some plants grow better in sandy soils, while others are well-adapted to clay soils.

2: available water capacity is the maximum amount of plant available water a soil can provide. it is an indicator of a soil's ability to retain water and make it sufficiently available for plant use. available water capacity is the water held in soil between its field capacity and permanent wilting point.

3: it is the same for all living things (plants and animals). it also includes very tiny organisms that our eyes cannot see. they are known as microorganisms, and include viruses, bacteria and fungi.