Forests respond to climate change

This is a story that passes for good news at a time of global warming. A new study based on data from 21 broadleaf forests in northern latitudes over a 20 year period reports that forests in some places seem to be capable of achieving normal rates of growth while using less water. For the moment, the assumption is that increased levels of carbon dioxide are essentially acting as a fertilizer, promoting growth with less water. This suggests that at least in some locations, it might be possible for forests to survive through the droughts caused by climate change.

Broadleaf forest.  Blue Ridge Parkway
Broadleaf forest. Blue Ridge Parkway

Like most changes in the environment, there are pros and cons to forests using less water because forests recycle the water into the atmosphere where it becomes rain clouds. If the forests take up less water, they will probably supply less moisture to agricultural areas downwind of the forests.

Where forests exist on the perimeters of their climatic ranges, they are not faring as well. In the American West, for example, there are massive tree die-offs caused by less rainfall and snow as well as beetle infestations where temperatures are no longer cold enough to kill them in the winter.

Scientists had predicted some growth benefit from higher levels of carbon dioxide, but this study found the benefit to be far greater than previously predicted. Higher growth rates also predict that forests will be capable of absorbing more carbon dioxide because carbon storage is mainly proportionate to biomass.

Trees absorb carbon dioxide through the pores in their leaves, called stomata. Scientists hypothesize that trees don’t need to open their pores as wide when carbon dioxide levels are higher. Since moisture is lost when the pores open, less moisture is lost if the pores don’t open as wide. That’s the working theory of this new research.

Harvard Arboretum
Harvard Arboretum

The forest at the Harvard arboretum was one of the forests included in this study. It has the longest continuous record of forest growth in the world.

Many questions remain. Which species are becoming more efficient in their water use? Are there intervening factors that are reducing water use? Will this trade-off between water use and carbon dioxide levels have an upper limit?

“Some Trees Use Less Water Amid Rising Carbon Dioxide, Paper Says,” New York Times, 7/11/13

“Trees Use Water More Efficiently as Atmospheric Carbon Dioxide Rises,” Science Digest, 7/10/13

4 thoughts on “Forests respond to climate change”

  1. Stomata size and density is used as a proxy for historic CO2 levels.
    Incidentally, Eucalyptus Globulus does not slow it’s transpiration during drought.

    Most eucalypts grow in localities where there is marked water shortage for substantial parts of the year. Therefore, they are adapted to seasonal drought stress associated with dry summers. Eucalypts develop an abundance of hard tissue called sclerenchyma which gives them the ability to endure severe wilting without lasting damage (Pryor 1976). They do not economize in the use of water but have wide-ranging root systems and an ability to extract water from the soil at even higher soil moisture tensions than most mesophytic plants. Transpiration rates remain high even when water supply from the soil is dwindling. It is only when severe permanent wilting occurs that there is stomatal closure which inhibits water loss (and, of course, also prevents gas exchange and photosynthesis) and enables the plant to survive a critical water balance situation for some time (Pryor 1976).

    1. We did not post Mr. Strayer’s comment earlier because previous attempts to respond to his comments have quickly degenerated into insults and threats. However, Mr. Strayer has now turned his comments into attacks in other venues. Therefore, we post this belated response in the hope that he will stop his attacks..

      Mr. Strayer’s comment is a non-sequitur. The article on which he is commenting is about the response of trees to increased levels of carbon dioxide. The study on which the post is based has found that some species of trees are capable of meeting their moisture needs without opening the pores in their leaves as wide as needed when levels of carbon dioxide are lower. Therefore, these trees are using less water than they have in the past. The study said nothing about eucalyptus and the article on which Mr. Strayer is commenting says nothing about eucalyptus.

      The article Mr. Strayer cites was written in 1976 at a time when elevated levels of carbon dioxide were not considered a crisis and the article is silent on the reaction of eucalyptus to higher levels of carbon dioxide. The fact that eucalypts “extract water from the soil at even higher soil moisture tensions” is not relevant to the study because the statement does not answer the analytical question: Does eucalyptus use less water when carbon dioxide levels are higher? We don’t know the answer to that question and neither does Mr. Strayer.

  2. I asked a naturalist at Mt. Diablo last night if it really was true that the beautiful endemic Pinus Sabiniana (Grey Pine, Ghost Pine, Foothill Pine, etc.) which grows inland also at Sunol, Pinnacles, etc. and is extremely drought tolerant seemed to be dying. I was told that yes, they were really suffering from the drought. Someone else said that the wineries are draining ground water that would have helped trees.

    So, if even these drought-tolerant trees are dying, even more reason to NEVER KILL ANY TREE, and especially not those who are doing the best with increasing drought and heat. We are lucky to have any trees who are surviving.

  3. I’d like to add a further response to Bob Strayer’s non-sequitur, since I responded to his complaints in another forum about your (wise) choice to not publish his comments when he made them.

    Mr. Strayer went further in his comments on that other forum as well as on his personal blog and made the inane claim that eucalypti “exacerbate drought conditions, making the local forests drier and more fire prone.” He referred to the same source as he did in his comment here as proof for his claim.

    When I read the source, it was clear that Mr. Strayer didn’t understand what he was reading and I pointed out that the source in no way stated that eucalypti make droughts worse or forests drier and more fire prone, and, in fact, the source said the opposite-that eucs are drought resistant.

    Mr. Strayer then lost his composure (to the extent it existed) and began attacking me, while insisting that his source said that which it didn’t.

    He went on his personal blog to try to contort the source into supporting his claims but he just continued to show how he doesn’t understand the basic notions of cause and effect and controlled experiments. The most amazing thing was that he confused a scientific study that said that eucalyptus MAY suppress herbaceous plant growth underneath its canopy by reading “herbaceous plant growth” to mean trees. He clearly didn’t understand that the trees in a forest are not “herbaceous plants”.

    So where a study stated that a euc MAY suppress the growth of herbaceous plants, and it’s known that herbaceous plants die each year and thus can create fuel loads for wildfire he somehow used that as support for his claims that eucs exacerbate droughts and make forests more prone to fires.

    I’m not a scientist so I don’t know the answer to the question of whether eucalypti exacerbate drought conditions. I am, however, able to read and comprehend scientific studies and the one presented by Mr. Strayer as support for his desired conclusion clearly contradict his assertions.

    It looks like Mr. Strayer is in the business of cherry picking words from scientific reports that he doesn’t comprehend and using those words in an attempt to scare people into clearcutting euc and pine forests.

    Webmaster: Yes, Mr. Belvedere, you have comprehended the difficulty of debating with Mr. Strayer. I’m pleased that you discovered our reply to Mr. Stayer which was posted for the benefit of people who read for information, rather than for rhetorical fuel.

    Unfortunately, amongst nativists, such lack of understanding–or perhaps unwillingness to understand–is common. They have gone unchallenged for too long. Their destructive projects have finally reached such huge proportions that people like you are stepping forward to challenge the unfounded assumptions on which their demands are based.

    Thank you for your visit and your insightful comment.

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