A confounding aspect of bonsai is the variability in care from species to species. Each tree species has their own preferences and quirks. To help break down this confusion we categorize trees into four different types: conifers, deciduous, broadleaf evergreens, and tropical trees. These categories group trees by visual and horticultural attributes. 

Conifers are the most common category used in bonsai cultivation. Some common species are Junipers, Japanese black pine, and White pine. These are generally hardy and robust species with immense resistance to harsh growing conditions. They can withstand cold temperatures and have tremendous longevity. Additionally, they never drop their foliage and maintain a nice green canopy year-round. Below is a picture of a Japanese black pine. 

Deciduous trees are radically different from conifers with their seasonal nature. They will lose their leaves during winter dormancy and then leaf out in the spring. This is a protective adaptation against frost damage. Sugars move from the leaves into trunk tissues which creates a higher concentration of solutes in the plant. More solutes in the trunk tissues reduces the temperature at which the tree will freeze. This does not mean all deciduous are invulnerable to frost damage, so a greenhouse is almost always used to protect these trees in the winter. Additionally, deciduous trees often display showy flowers and fruits which make them attractive and refreshing to observe. Below is a photo of a dormant Japanese Beauty Berry bonsai displaying diminutive purple fruits.

Broadleaf evergreens are trees that have leaves year-round. Some popular species include silverberries, coast live oaks, olives, and firethorn. These species have variability in tolerance to freezing conditions, but generally they need greenhouse protection. Below is an olive bonsai tree in training.  

Tropical trees are fast growing species that have leaves year-round and thrive in intense tropical climates. These species can tolerate incredible heat and humidity. They are also some of the few species that can be maintained indoors for long periods of time. Tropical trees have no tolerance for freezing temperatures and must be kept in greenhouses during the winter. Some popular varieties are ficus, hibiscus, buttonwood, pemphis and Brazilian rain trees. Below is a photograph of a defoliated Ficus salicaria.

How to Improve horticultural Bonsai care

To become truly proficient at bonsai care it will take doing some personal research and hands on experience to understand each species. I put an emphasis on personal experience because reading books alone does not account for the limitless variables you will encounter. Instead, first identify the species you wish to care for and then buy several inexpensive trees of that type. This will give you room to experiment and master that species without financial pressure. Once you feel comfortable with general care, pruning and repotting then you can move onto better trees.  Some people get excited and want to buy the best tree possible right away, but without proper experience the tree will often become weak and die.