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Now, Corinne, I'll answer your questions. Thanks for the great questions. If you have any other questions about ecology or related topics, I'd be happy to answer.
In terms of questions that I have, how have humans affected the progression of droughts, and how is it affecting urban plant life.
Scientists expect that climate change will make everything hotter, and will change rainfall patterns. Some areas that are wet may become wetter, while areas that are dry may become drier. For one of my projects, I looked at how urban plants respond to drought in the greater Los Angeles region. As part of this, I analyzed more than 100-years of rainfall data. Since about the mid-1980s the L.A. region has experienced a statistically significant drying trend, where wet years are progressively less wet, and dry years are significantly drier.
However, not all places will get drier. In fact, with warmer temperatures, the air can hold more moisture, so we expect an overall increase in rainfall globally with climate change. However, the timing of the rainfall is likely to change. If the rains come a few months too late they might miss the growing season, which would have very negative impacts (such as for agriculture). Also, with warmer temperatures, more precipitation falls as rain instead of snow. This has a big impact for places like California, which heavily rely on the winter snowpack for its water needs.
Asking how drought affects urban plant life is great. We have a pretty good understanding of how plants in non-urban ecosystems (like a forest) respond to changing rainfall: generally, ecosystems globally positively respond to increasing rainfall up to a certain point (which is about 2500 mm of rain per year). Above this point, water is no longer the limiting factor in the ecosystem- increasing rain above this level will, by itself, have no effect.
Cities are unique because of all the people- we term cities "socio-ecological systems." To fully understand an urban ecosystem you have to look at the social part and the natural part together. People make choices about what to plant, how to care for plants, how often to water, and on and on. About 20 years ago ecologists discovered something called the luxury effect, which is a fundamental concept in urban ecology. Basically, wealthier regions of a city will have greater species diversity and higher biomass than poor regions of a city. Although the luxury effect was originally shown for plants, we now know that higher income leads to higher insect diversity, bird diversity, lizard diversity, and pollinator abundance, to name a few. This is really interesting- in cities, a social variable like income has a more powerful role in shaping these biological communities than traditional ecological variables like temperature.
Cities are also unique environments for a plant to live in. Compared to a rural landscape, cities are hotter, windier, have higher CO2 concentrations, have compacted and disturbed soil, and have salts and heavy metals, as just a few examples. So, we should expect that urban plants would respond differently to drought when compared to non-urban plants.
Nobody, however, has looked at how urban plants respond to drought- that's where I come in. The importance of this work comes down to something called ecosystem services. An ecosystem service is a free benefit that we receive from nature from just letting nature do its thing. These include air and water filtration, pollination, and carbon sequestration (removal of carbon dioxide from the atmosphere). Plants in cities also provide benefits- its been estimated that all urban plants globally provide about $33 billion in ecosystem services annually. This means that if we wanted to replicate everything these plants are already doing, like cleaning the air, then it would cost us $33 billion every year. However, plants that are stressed will provide fewer services than plants that aren't stressed. Therefore, its important to understand how urban plants respond to stress.
Additionally, does air pollution and/or climate change play a role in the lives of these plants, if you have data pertaining to that?
Yes- air pollution is not good for plants. Still, the plants that we have in cities are "hardy" in the sense that they can handle the unique urban environment. Not all plant species would be happy living in a city. Plants remove air pollution in two ways- first, through what is termed "dry deposition", when the pollutant lands on the leaf. Also, a plant can clean the air by sucking in the pollutant through its stomata (this is accidental and happens when the plant takes in air for the carbon dioxide). Ozone is particularly bad for plants.
Climate change is also bad for urban plants. Due to climate change species all over the world are migrating- either up in elevation or closer to the north or south pole, in order to follow its ideal climate. Plants "migrate" by extending their range in the north, and contracting their range in the south. Even species in the ocean are migrating, which in this case usually means diving deeper to access cooler water. The problem that climate change poses is that the climate is changing faster than species are able to move to track their ideal climate. If a species can't migrate to a new, suitable location, then it will likely go extinct. (The topic of "assisted migration", where we deliberately move species to a new area we know will be suitable for it, is controversial). Evolution could help species adapt to a new environment, but, again, climate change is occurring too rapidly for evolution to help.
Urban plants are also affected- we're already seeing instances where a plant's ideal climatic conditions are no longer present in the city its planted in. Urban trees, however, can live for decades. An important area of research is in trying to determine what to plant now so that 30 years from now, when the climate is very different from what it is today, the plant is still happy. In non-urban ecosystems we expect to see many new biological interactions, as species from all over the world are migrating, but at different rates. Climate change is basically taking a blender to the world's ecosystems. Similarly, due to climate change, the plants you'll find in a city in 30 years will be different from what you'll find there today.
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