Final answer:
The increase in the height of young aspen around the year 2000 can be partially attributed to the higher levels of carbon dioxide in the atmosphere acting as a fertilizer, beneficial fire ecology practices that reduce competition from conifers and reduced air pollution leading to healthier growth conditions.
Step-by-step explanation:
The increase in height of young aspen trees around the year 2000 can be attributed to multiple environmental factors. One key factor is the additional carbon dioxide in the atmosphere, which has been linked to a 'greener' earth with increased leaf cover. this rise in atmospheric CO2 over the last 200 years likely acted as a natural 'fertilizer' helping plants like the aspen to grow taller and thrive even in conditions of low water availability.
Furthermore, fire ecology plays a significant role in the aspen's growth. Fires can eliminate competing conifers that are slower in regeneration, thus allowing aspens that sprout from underground stems to quickly re-colonize an area. Finally, efforts to reduce air pollution, like those in late 1990s England that led to cleaner air, can contribute to healthier growth conditions for trees such as the aspen.