Despite a minority of die-hard deniers and their corporate enablers in the fossil fuel industry, most scientists have quit debating that the climate has changed and will continue to change. Nor is there much doubt that the primary cause of climate change is the significant increase in the greenhouse gases that trap heat on the Earth’s surface. Scientists have now turned their attention to the huge task of understanding the consequences of a changed climate and predicting its future course. Our best hope is that such knowledge can help us to devise strategies for coping with the consequences.
In this post, we will share with our readers some of the recent research about how plants and trees are responding to climate change.
Non-native plants are more responsive than natives to higher temperatures
The State of Ohio has one of the most complete climate records in the country. They have had weather stations in stable locations throughout the state since 1895. From 1895 to 2009, these weather stations reported an average increase in temperature of 1.7 degrees Fahrenheit. All of the weather stations were outside of urban areas, so we can be confident that the data were not confused by the separate, but associated, phenomenon of the urban heat affect as population and development in urban areas increased during this period.
These data were combined with an equally rich source of information, the herbarium of the University of Ohio which contains 500,000 plant specimens. These two sources of information enabled a graduate student, Kellen Calinger, to assemble “one of the six-largest such data sets in the world tracking the history of the wildflower life cycle in response to climate change.” (1)
Ms. Calinger compared the bloom time of 141 species of plants with the temperature at the time of bloom. She reports that “…46% of the 141 species showed significant advancement in flowering in response to increased temperatures. And more of this advancement was seen in introduced species [AKA non-natives] than in native plants.”
Ms. Calinger predicts that the non-native plants that bloom before their native neighbors have a competitive advantage. Presumably, they are growing and occupying ground prior to the natives. If, indeed, climate change is giving non-native plants an advantage that would help to explain why attempts to eradicate non-native plants and replace them with native plants are often unsuccessful.
However, the report of this research then enters muddy territory. It speculates, but without offering evidence, that there may also be disadvantages to blooming earlier:
- Is the flower blooming prior to the arrival of its pollinator thereby decreasing its reproductive success?
- Will the early bloom only become the victim of a subsequent frost because the growing season is not yet stable?
- Will migrating birds pass through only to find that the nectar sources they have depended upon in the past have now completed their blooming period?
What do we know about the response of plants in urban areas?
So, how does this information apply to our urban area? In general, temperatures in urban areas are higher than in rural areas because so much of our ground is covered with buildings and hardscape that absorb and retain heat. This is called the urban heat affect. It seems logical to assume that what has been observed in the rural setting would be exaggerated in the urban setting. That is, plants in urban areas are likely blooming even earlier because the temperatures are higher, although there is probably an upper threshold, beyond which there is no growth benefit.
However, there are other factors in climate change that are more important in urban areas which are also affecting the growth of plants and trees. Greenhouse gases are greater in urban areas than in rural areas because of industrial and transportation emissions.
Carbon dioxide concentrations have increased 24% globally since 1960. We should assume that increase is greater in urban areas. Carbon dioxide is the primary fuel of photosynthesis, so we should not be surprised to learn that higher concentrations of carbon dioxide are associated with faster plant growth. (2)
Kevin Griffin (Columbia University) compared the growth of the native red oak in rural New York with their brethren in New York City over a period of 8 years. The average minimum temperature in August was 71.6 degrees at the city site and 63.5 degrees in the country. He also found elevated levels of nitrogen in the leaves of the trees in the city. Nitrogen is a plant nutrient. Griffen reported that, “The urban oaks, harvested in August 2008, weighed eight times as much as their rural cousins, mostly because of increased foliage.” (2)
Unfortunately, like most stories about climate change, this one is also a mixed blessing. While carbon dioxide and higher temperatures may benefit plants in the city, other elements in urban air do not. Higher levels of ozone can severely damage plant pores, which slows their growth and some trees are more susceptible to this damage than others. Cottonwoods are particularly susceptible to ozone damage. Ironically, ozone levels are actually higher in rural areas than in urban areas because some of the ozone is converted to oxygen in the city, while the remaining ozone “blows out to the country.” (2)
What are the implications for readers of Million Trees?
Here’s our take-away message from these research reports:
- The consequences of climate change are complex and are incompletely understood.
- Climate change and air quality conditions in the urban setting are probably giving non-native plants a competitive advantage over native plants which helps to explain the frequent failures of attempts to eradicate non-native plants.
- There are pros and cons to every change in the environment. To call change “good” or “bad” is to over-simplify the complexity of nature.
- Finally, our usual rhetorical question, “Do the managers of native plant installations understand the complexity of their undertaking?” We don’t think they do.
(1) “Non-native plants show a greater response than native wildflowers to climate change,” October 5, 2012. Available here.
(2) Guy Gugliotta, “Looking to Cities, in Search of Global Warming’s Silver Lining,” New York Times, November 26, 2012. Available here.