Light Pollution and Plants

I love walking around my town at night. Things really seem to slow down when the sun sets. Growing up in the country, my evening walks were lit only by the moon. Now that I live in civilization, however, street lights punctuate the darkness on every block. Walking around I can't help but wonder what all of this artificial light is doing to our photosynthetic neighbors. 

The vast majority of plants need light to make food. It doesn't matter if this light comes from the sun or a high powered electric light, as long as it is strong enough for photosynthesis. Even weaker wavelengths of light serve a purpose for our botanical friends. Plants can sense the relative length of uninterrupted darkness in their environment and they use that information for myriad internal processes. Its this dependence on light that makes many plant species vulnerable to our addiction to artificial lighting.

Just because a light isn't strong enough for photosynthesis doesn't mean it isn't affecting nearby plants. This is especially true for plants that use day length for timing events like bud break, flowering, and dormancy. The type of lighting favored by most municipalities emit wavelengths that peak especially high in the red to far-red ratio of the electromagnetic spectrum, which makes them particularly adept at disrupting plant photoperiods.

One of the most obvious effects of artificial lighting on plants can readily be seen in street trees growing in temperate regions. Though light sensitivity varies from species to species, trees growing near street lights tend to hold onto their leaves much longer in the fall than trees farther away. Because artificial lighting is enough to trick the red to far-red receptors in plants, it can "convince" trees that the days are longer than they actually are. Additional photosynthesis may not seem that bad but holding onto leaves longer makes trees more susceptible to ice damage. 

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The effects of artificial lighting continues into spring as well. Trees growing near lights tend to break buds and flower earlier in the spring. This too makes them susceptible to frost damage. Early flowering plants run the risk of losing their entire reproductive effort by blooming before the threat of frost is gone. This can really mess up their relationship with pollinators. 

The effects of artificial lighting can even influence the way in which plants grow. Research has found that plants growing near street lights had larger leaves with more stomatal pores and these pores remained open for considerably longer than plants growing under unlit night conditions. This made them more susceptible to pollution and drought, two stressors that are all too common in urban environments. This issues is made much worse if the artificial lighting never turns off throughout the night. 

Artificial lighting affects more than just plant physiology too. Scaling up, the effects of night lights can have whole ecosystem consequences. For instance, researchers found that artificial lighting was enough to change the entire composition of grassland communities. Some plants responded well to artificial lights, producing more biomass and vegetative offshoots to the point that they pushed out other species. This was compounded by the change in reproductive output, with certain species showing higher seed production than others.

Photo by Lamiot licensed under CC BY-SA 4.0

Changes in plant physiology, phenology, and composition also affect myriad other organisms in the environment. Changes in the timing of flowering or bud break can disrupt things like insects and birds that rely on these events for food and shelter. Research even suggests that forest regeneration is being altered by artificial lighting. Seed dispersers such as bats often will not fly into well-lit areas at night, therefore reducing the amount of seeds falling in those areas. Such research is still in its infancy meaning we have a lot more to learn about how artificial lighting is disrupting natural events.

Light pollution is so much more than an aesthetic issue. Artificial lighting is clearly having pronounced effects on plant life. Disrupt plants and you disrupt life as we know it. Certainly more work is needed to tease out all the ways in which lights influence plants, however, it is clear that we must work hard on reducing light pollution around the globe.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

On Soil and Speciation

Lord Howe Island. Photo by John Game licensed under CC BY 2.0

Many of you will undoubtedly be familiar with some variation of this evolutionary story: A population of one species becomes geographically isolated from another population of the same species. Over time, these two separate populations gradually evolve in response to environmental pressures in their respective habitats. After enough time has elapsed, gradual genetic changes result in reproductive isolation and eventually the formation of two new species. This is called allopatric speciation and countless examples of this exist in the real world.

At the opposite end of this speciation spectrum is sympatric speciation. Under this scenario, physical isolation does not occur. Instead, through some other form of isolation, perhaps reproductive or phenological, a species gives rise to two new species despite still having contact. Examples of this in nature are far less common but various investigations have shown it is indeed possible. Despite its rarity, examples of sympatric speciation have nonetheless been found and one incredible example has occurred on a small oceanic island off the coast of Australia called Lord Howe Island.

Howea belmoreana and Howea forsteriana [SOURCE]

Lord Howe Island is relatively small, volcanic island that formed approximately 6.4–6.9 million years ago. It is home to four distinct species of palm trees from three different genera, all of which are endemic. Of these four different palms, two species, Howea belmoreana and Howea forsteriana, are quite common. Interestingly enough, H. forsteriana, commonly known as the kentia palm, is one of the most commonly grown houseplants in the entire world. However, their horticultural value is not the most interesting thing about these palms. What is most remarkable is how these two species arose. 

Multiple genetic analyses have reveled that both species originated on Lord Howe Island. This is kind of odd considering how small the island actually is. Both palms can regularly be found growing in the vicinity of one another so the big question here is what exactly drove the evolution of their common ancestor? How does a single species growing on a small, isolated island become two? The answer is quite surprising.

Howea belmoreana Photo by John Game licensed under CC BY 2.0

When researchers took a closer look at the natural histories of these two species, they found that they were in a sense isolated from one another. The isolation is due to major phenological or timing differences in their reproductive efforts. H. forsteriana flowers roughly six weeks before H. belmoreana. Flowering time is certainly enough to drive a wedge between populations but the question that still needed answering was how do such phenological asynchronies occur, especially on an island with a land area less than 12 square kilometers? 

As it turns out, the answer all comes down to soil. Individuals of H. belmoreana are restricted to growing in neutral to acidic soils whereas H. forsteriana seems to prefer to grow in soils rich in calcarenite. These soils have a more basic pH and dominate the low lying areas of the island. Growing in calcarenite soils is stressful as they are poor in nutrients. This physiological stress has caused a shift in the way in which the flowers of H. forsteriana mature. When found growing on richer volcanic soils, the researchers noted that the flowers mature in a way that is more synchronous, not unlike the flowers of H. belmoreana.

Thanks to their attention to detailed life history events and conditions, researchers were able to show that soil preferences caused a phenological shift in the flowering of these two related species. Because they flower at completely different times when growing on their respective soil types, enough reproductive isolation was introduced to disrupt the random mating process of these wind pollinated palms. As soon as such reproductive biases are introduced, speciation can and will occur.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Spring Has Sprung Earlier

Phenology is defined as "the study of cyclic and seasonal natural phenomenon, especially in relation to climate, plant, and animal life." Whether its deciding when to plant certain crops or when to start taking your allergy medication, our lives are intricately tied to such cycles. The study of phenology has other applications as well. By and large, it is one of the best methods we have in understanding the effects of climate change on ecosystems around the globe. 

For plants, phenology can be applied to a variety of things. We use it every time we take note of the first signs of leaf out, the first flowers to open, or the emergence of insect herbivores.  In the temperate zones of the world, phenology plays a considerable role in helping us track the emergence of spring and the onset of fall. As we collect more and more data on how global climates are changing, phenology is confirming what many climate change models have predicted - spring is starting earlier and fall is lasting longer.

Researchers at the USA National Phenology Network have created a series of maps that illustrate the early onset of spring by using decades worth of data on leaf out. Leaf out is controlled by a variety of factors such as the length of chilling temperatures in winter, the rate of heat accumulation in the spring, and photoperiod. Still, for woody species, the timing of leaf out is strongly tied to changes in local climate. And, although it varies from year to year and from species to species, the overall trend has been one in which plants are emerging much earlier than they have in the past.

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https://www.usanpn.org/data/spring

For the southern United States, the difference is quite startling. Spring leaf out is happening as much as 20 days earlier than it has in past decades. Stark differences between current and past leaf out dates are called "anomalies" and the 2017 anomaly in the southern United States is one of the most extreme on record.

How this is going to alter ecosystems is hard to predict. The extended growing seasons are likely to increase productivity for many plant species, however, this will also change competitive interactions among species in the long term. Early leaf out also comes with increased risk of frost damage. Cold snaps are still quite possible, especially in February and March, and these can cause serious damage to leaves and branches. Such damage can result in a reduction of productivity for these species.

Changes in leaf out dates are not only going to affect individual species or even just the plants themselves. Changes in natural cycles such as leaf out and flowering can have ramifications across entire landscapes. Mismatches in leaf emergence and insect herbivores, or flowers and pollinators have the potential to alter entire food webs. It is hard to make predictions on exactly how ecosystems are going to respond but what we can say is that things are already changing and they are doing so more rapidly than they have in a very long time. 

For these reasons and so many more, the study of phenology in natural systems is crucial for understanding how the natural world is changing. Although we have impressive amounts of data to draw from, we still have a lot to learn. The great news is that anyone can partake in phenological data collection. Phenology offers many great citizen science opportunities. Anyone and everyone can get involved. You can join the National Phenology Network in their effort to track phenological changes in your neighborhood. Check out this link to learn more: USA National Phenology Network

Further Reading: [1] [2]