The Gardening StackExchange question Renovating mature pear trees asks about the techniques for keeping existing old (75-100 years) trees alive. How does the choice between renovating old trees vs removing them and buying new trees affect "a lifestyle that can be maintained indefinitely without depleting available resources"? What are the issues?
Trees have a finite life span. Commercially apples and pears are usually replaced after about 40 years. A private tree isn't under the same pressure to produce, so it can be used for a longer period.
Removing the tree is difficult, especially the stump. Sustainably you could cut it flush, let it rot, use the top for firewood. Move over a few feet to plant the new tree.
Rejuvenating the tree may or may not be possible depending on the condition of the root system and lower trunk. If it is just the top that is in trouble, you may be able to cut it down to 3-4 feet, and tongue graft several scions to the trunk. My uncle recently converted a 40-acre orchard from, I think, winesap to fuji apples this way. He was producing apples in 2 years.
The other option at this point is to use scions from multiple varieties of pear, and grow a chimera tree.
Rejuvenating this way is quite cheap. Scions are typically $2 per section. The grafting would take 10-15 minutes from someone experienced. Ask around in your gardening circles for a fruit nut.
Replacing the tree depends on your access. I sell 3 year old pear trees, about 7 feet high, in a #10 pot for $75. Add that to your costs to come pick it up. 1 year old grafts are typically about 4 feet high and run about $40 retail. Wholesale, bare root, are $3-10 each in lots of 100-500.
Generally, to compare two choices from a sustainability viewpoint you want to identify how much non-renewable resources are being used by either option, or how much pollution is created. Since you often don't know this kind of information from products you buy, you either need to do research and/or do some guesswork.
In this answer I'll make various assumptions and purely look at the impact of both actions, so I won't be discussing the end result (if you will get more peaches again).
Option 1: renovating
- Assumption 1: Trimming the tree is enough to renovate it
- Assumption 2: The energy used to trim the tree is renewable (so no petrol-powered chainsaw will be used for example)
- Assumption 3: The cut-off branches will be left to rot and not burned (burning releases approximately the same amount of CO2 as rotting, but burning also releases nitrogen dioxide, a strong greenhouse gas, as well as particulates)
Based on these assumptions this option has little to no impact. There will be CO2 emissions caused by the decay of the cut-off branches, but this will always happen eventually when trees die.
Option2: removing and buying new
- Assumption 1: The new trees are grown organically and no synthetic fertilizers were used (otherwise there may be a negative impact due to the use of pesticides or use of non-renewable resources)
- Assumption 2: You will be replacing the removed trees with the same amount of new trees (otherwise the net effect is more CO2 emissions)
- Assumption 3: The trees are transported by a petrol-powered vehicle.
- Assumption 4: The energy used to remove the tree is renewable.
- Assumption 5: The removed trees will be left to rot and not burned.
Based on these assumptions there is a small impact for transport, so this option is slightly worse from a sustainability viewpoint compared to option 1.