It is a given. You will have a few trees die on your watch. Nobody wants it, but it happens and it is part of the life-death cycle that underpins all living things. We do not kill our trees deliberately and it therefore is important to take note of the things that do increase the chances of this unfortunate event.
This relates to keeping the tree healthy through everyday practices, inclusive of hygiene, watering, light and nutrition requirements.
In my experience watering requirements is the most important of these aspects. Get that wrong and your tree will deteriorate and then just die. This relates to both overwatering as well as too dry conditions. Every tree will have different requirements based on species, soil medium, size and exposure to sun. Study this for each specie, make notes, learn and apply. Overwatering is the dangerous one as it generally leads to root rot and by the time you become aware of this it is too late. Be aware of irrigation systems. A power outage, a flat battery or a broken pipe, all lead to disaster. The best way to water is by hand and by studying each tree and adjust the amount of water to the daily requirements. It is labour intensive, but a safe way to keep your trees alive.
When repotting, pay special attention to the roots. Some species do not handle complete removal of soil well. Check for tangled roots, check for bugs, caterpillars and ants nests in the soil at this time.
Seasonal and Climate requirements
At the time of writing this, it is winter in New Zealand and after quite a dry period, it has now been raining for three days solidly and the temperature has dropped into single digits in places. Irrigation systems are now turned off and the focus now shifts from preventing trees from drying out to ensure they do not drown. Half of my trees now have pots at an angle to allow water to run off, rather than sit in the pot. Back to basic horticulture as your soil medium and its drainage ability now plays a big role in keeping trees healthy and alive.
I have mentioned temperature and the accompanying conditions now come into play as well. Snow can actually insulate trees, but frost is just nasty. Your local climate and specific species will dictate if a tree needs to be under cover or not. Frost has killed many a tree. Very harsh high temperatures, accompanied with little to now sun protection and not enough water will kill trees. Take note of your pot and soil temperatures. The pot construction, size and colour can have an effect on the temperature inside the pot. Two pots next to each other can have very different temperatures.
Be careful when bringing plants inside, especially non-dormant trees. I have seen and heard of many trees that just did not like artificial heating systems, wood fire burners and even being cooked on a window sill, resulting in the tree ending up on the compost heap.
Light requirement of a tree is a big one. Suitable species for sunny areas are Celtis, Junipers, Chinese Elms, Cotoneaster, most Pine species, Holly, pyracantha and others. For a more shady aspect look at Azalea, Maples, Beech, Zelkova and others. Acclimatisation plays a big role here. You can gently and over time get some species to tolerate a wider range of light and temperatures if your care is spot on. Generally, if you get this wrong, a tree will deteriorate over time and if not corrected will say goodbye.
This is not just a beginners issue. Many a tree has succumbed to being overworked. The safe mantra here is do little bits of work more often, rather than massive root work at the same time as huge pruning and styling jobs. This goes with seasonal changes as well. Certain things, like defoliation, should take place at a specific time of year. Same for root work and repotting. It could be different for different species. Study, learn and apply.
Diseases and Pests
That is self-explanatory. The best is to use preventative methods to stop disease and pests from getting to your trees. This could include preventative sprays, either organic or not. That depends on your philosophy, but either way, keep an eye out for tell tale signs. Things like ants can point to aphids, sooty mould and other issues. Yellowing and spots on leaves, droppings of bugs and caterpillars, chew marks on leaves, sudden leave drop, are all signs of things going wrong. Keep an eye out and this can easily be combined with your daily watering routine. A big part of prevention is basic hygiene around your trees. Get rid of fallen leaves, clean underneath your benches and check the undersides of pots. Many an issue can be prevented by just cleaning often.
Seal wounds, tie trees down in pots when repotting, check old wounds, look for new holes in the trunk, marks like ring barking and especially underneath the leaves.
This is just a snapshot of things that can go wrong and is not intended to scare or put Bonsai enthusiasts off, just a reminder that a Bonsai tree is like having a pet. The tree has basic requirements and if these are not taken care of, it will deteriorate and can then die as a result of neglect or just not being bale to pick up on adverse things early enough. The solution is to arm yourself with knowledge, practice the skills involved in keeping a tree alive and constantly take great care of your trees.