Bidets save trees primarily because they reduce the amount of toilet paper needed for personal hygiene after using the bathroom. Here's how:

1. Reduced Toilet Paper Usage: Bidets use water to cleanse after using the toilet, significantly reducing or even eliminating the need for toilet paper altogether. This reduction in toilet paper usage directly translates to less demand for paper products made from trees.
2. Less Paper Waste: By using a bidet, individuals produce less paper waste from toilet paper, which means fewer trees are cut down to produce the paper products and less energy is consumed in the manufacturing process.
3. Environmental Impact: The production of toilet paper involves the harvesting of trees, the use of water, energy, and chemicals in the manufacturing process, and transportation. By using bidets and reducing toilet paper usage, the environmental impact associated with these activities is significantly reduced.
4. Sustainable Practice: Bidets promote a more sustainable bathroom hygiene practice by reducing reliance on single-use paper products. This aligns with efforts to conserve natural resources and mitigate environmental damage.

Overall, while bidets themselves require water for operation, the reduction in toilet paper usage associated with their use leads to significant environmental benefits, including saving trees.