DISCUSSION: Trees have a complicated two-way relationship with the environment which depends on several factors, including type of tree and location. In general, the environment influences trees in straightforward ways. For example, drought conditions (when water levels are significantly below normal) can obviously, dry a tree out, making it easier to burn and potentially making disease or insect infestation more likely. Any condition that reduces sunlight (e.g., volcanic eruption) would slow photosynthesis and tree growth.
Not only does the environment influence vegetation, but trees also impact their environment. Trees and other vegetation absorb carbon dioxide. On a global, aggregate scale, this absorption can help partially mitigate the warming caused by the increase of that gas and others like it in our atmosphere. Unfortunately, humans are releasing carbon dioxide into the atmosphere far faster than trees (or the oceans) can absorb it. But, Earth’s global mean temperature would be rising faster without vegetation and the oceans.
Trees can also influence environmental conditions on a more local scale. Abundant rain falls over tropical rainforests because the tropics are mostly covered by water, there is abundant energy to evaporate the water, and intense solar heating of the land surface generates rising motion there. Trees in the tropics readily pull moisture from the soil and transpire (evaporation from vegetation) it into the atmosphere. This enhances the local water vapor content of the atmosphere and further enhances rainfall.
A recent study suggests that certain trees could have the opposite effect and actually enhance drought conditions. When drought conditions exist and rains finally come, some trees (e.g., oak trees in California) rapidly absorb and transpire the added water from the soil, quickly drying the soil again and perpetuating drought conditions. In contrast, other trees usually found in cooler climates (e.g., cedars, pines) slowly absorb water from the soil as it becomes available, keeping soil water levels higher for longer. One of the main takeaways from the new study is that some trees may be less appropriate in drought-prone regions, or such trees should be planted with a variety of other trees that are more drought-resistant.
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© 2019 Meteorologist Dr. Kenneth Leppert II