Warming Arctic forests diverge over nutrients

Humans continue their unique uncontrolled experiment to see how increased atmospheric carbon dioxide and the resulting warmer temperatures influence biomes worldwide.  One expected outcome of this global experiment is that trees in the extreme north will show improved growth resulting from a more benign physical environment.  As it turns out, some regions of the north do show this trend, while others don’t – this lack of consistent response is known as divergence.  For her graduate work, Sarah Ellison, working with Patrick Sullivan, Sean Cahoon and Rebecca Hewitt, wanted to document divergence in northern Alaska, and to figure out what might be causing it.

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The Wind River, near Arctic Village in the Arctic National Wildlife Refuge, is the easternmost site in the study. Credit: Patrick Sullivan.

The researchers established four study sites in four watersheds across the Brooks Range in northern Alaska.  They knew from previous studies that white spruce (Picea glauca) in the western Brooks Range have shown increased growth in response to climate warming, whereas those in the central and eastern Brooks Range have not responded. Some researchers hypothesized  that warmer temperatures caused moisture limitation in the eastern Brooks Range, but previous plant physiological studies done by this research team show no evidence of water stress, even in the extreme eastern portion of the range.

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The four study sites within the Brooks Range from west to east: Agashashok, Kugururok, Dietrich and Wind River.

So what’s causing divergence?

At each study site, the researchers set up climate stations which collected continuous data on air and soil temperature, wind speed and direction, solar radiation, snow depth and precipitation. (Check out the following (you may need to copy and paste into your browser) for an entertaining look at the challenges of doing this research: https://youtu.be/ty6vwio9LvU).

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A weather station at the Wind River sight. Credit: Sarah Ellison.

They discovered that soil temperatures were consistently warmer in the western part of the range over the course of the season.

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Temp.

(deg. C)

 

 

Colder soils are often associated with low levels of available nutrients, because bacteria are less active at colder temperatures, and thus less capable of breaking down nutrients into a form that can be taken up by roots.  In 2014, Ellison and her colleagues measured soil nutrient levels at each site and found generally lower levels in the central (Dietrich) and eastern (Wind River) sites.

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From top: ammonium, nitrate, phosphate and total free primary amine (TFPA – a proxy for amino acids) availability in the soils at the four sites during the 2014 growing season. Error bars are +/- 1 SE.

There were several other important physiological pieces to this puzzle.  Plants in the west grew more quickly than did plants in the east. Rapid growth was associated with greater photosynthetic rates in the western watersheds. The researchers measured needle nutrient concentration and found that it decreased from west to east. Each year, there was a strong correlation between needle nutrient concentration and branch extension (the measure of tree growth), but the correlation with phosphorus was generally stronger than the correlation with nitrogen.

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Branch extension in relation to needle phosphorus concentration (left graph) and needle nitrogen concentration (right graph) for three years of the study.

Armed with these findings, Ellison and her colleagues decided to experimentally test whether nutrient availability was limiting growth, particularly in the eastern regions of the Brooks Range.  If so, this would support the hypothesis that cold temperatures and the resulting decrease in nutrient availability were primary factors causing divergence across this vast ecosystem. In June, 2015, the researchers fertilized five trees at each site with a mixture of nitrogen, phosphorus, and potassium fertilizer, and left five similar nearby trees as untreated controls. After one year, branch extension was greatly enhanced at the most eastern site, and only slightly (insignificantly) enhanced at the most western site.

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Mean annual growth (branch extension) before and after fertilization experiment for fertilized trees (gray circles) and control trees (black circles). Bars are +/- 1 SE.  Fertilization occurred in 2015 (indicated by vertical dotted lines).

For many years, forest ecologists have believed that forests in young glacial soils are nitrogen limited.  This study, and a few other recent studies, thrust phosphorus into prominence as a factor that can limit forest productivity.  Over time, as the climate continues to warm, soils in the eastern Brooks Range will enjoy increased microbial activity, and may no longer suffer as much from nutrient limitation.

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The Agashashok mesic treeline sits on a gentle slope above the Agashashok river. Credit Sarah Ellison.

One surprising finding was that mycorrhizal growth on fine roots was more extensive in central and eastern watersheds.  Abundant mycorrhizae were associated with reduced branch extension, suggesting that these mycorrhizae may be parasitic, rather than mutualistic. The researchers are in the process of expanding their study to an even greater spatial extent of 20 sites distributed across the Brooks Range, which will allow them to further explore how general their findings are across this vast biome.

note: the paper that describes this research is from the journal Ecology. The reference is Ellison, S. B. Z.,  Sullivan, P. F.,  Cahoon, S. M. P., and  Hewitt, R. E..  2019.  Poor nutrition as a potential cause of divergent tree growth near the Arctic treeline in northern Alaska. Ecology100( 12):e02878. 10.1002/ecy.2878. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Drought differentially diminishes ecosystem production

Sometimes, even the most carefully conceived experiment is thrown for a loop by Mother Nature.  Good scientists must embrace the unexpected.  Ellen Esch, David Lipson and Elsa Cleland set out to explore how plant communities responded to high, normal and low rainfall conditions.  The researchers set up rainfall manipulation plots that were covered with a clear plastic roof that would allow most light to pass through, but intercept all of the water.  They then reapplied the intercepted water, with each plot receiving either 50%, 100% or 150% of the fallen rain.  The plan was to simulate drought, normal and wet conditions. The natural world had other plans, however, as 2013-2016 were unusually dry years. Fortunately the researchers adjusted, by refocusing their question on how plant communities respond to severe drought  (50% of intercepted rainfall), moderate drought (100%) and normal rainfall (150%).

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Herbaceous plant community being irrigated (notice the rainbow). Credit: Ellen Esch.

Esch and her colleagues set up their experiment at the San Diego State University Santa Margarita Ecological Reserve, which has a Mediterranean-type climate with mild, somewhat moist winters and hot dry summers.

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Exotic grasses (here showing recently senesced Bromus madritensis) dominated the herbaceous sites. Credit: Ellen Esch

They wanted to know how climatic variability brought about by climate change would influence plant phenology (the timing of periodic ecological events), specifically green-up date (when plants begin turning green) and senescence date (when they turn brown and curtail photosynthesis). They expected that the native species, primarily sage-type shrubs, would be more drought-resistant than the exotic herbaceous vegetation, which was dominated by brome grass.  Climate change is predicted to increase climatic variability, which should increase the frequency and intensity of severe droughts (and also of unusually wet years).

An important measure of ecosystem functioning is its productivity – the amount of carbon taken up by an ecosystem, usually by photosynthesis.  More productive ecosystems have more energy available to feed consumers and decomposers.  More productive ecosystems also take up and store more carbon dioxide from the atmosphere, which can help reduce climate change. The researchers used a reflectance radiometer to calculate the Normalized Difference Vegetation Index (NDVI), which essentially calculates how green an area is, and is a good measure of productivity.  Esch and her colleagues hypothesized that drought would reduce overall ecosystem NDVI, but that native vegetation would be more buffered against the negative effects of drought than would the invasive exotic vegetation.

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A student from a plant physiology class at San Diego State University measures NDVI. Credit: David Lipson

Each year from 2013 – 2016, the researchers set up 30 3X3 meter plots; 15 plots were dominated by exotic herbaceous species such as brome, and 15 plots had mostly native shrub species such as sage. Plots were treated the same, except for receiving either 50%, 100% or 150% of the fallen rain, which corresponded to severe drought, moderate drought and normal rainfall, respectively. Periodically, the researchers used a radiometer to measure NDVI for each plot.  They discovered that, as expected, drought reduced NDVI much more in the plots dominated by exotic herbaceous species (top graph below) than in the plots dominated by native shrubs (bottom graph).

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NDVI on each measurement date for plots dominated by (top graph) exotic herbaceous species and (bottom graph) native shrub species. Red square = severe drought treatment, green circle = moderate drought, blue triangle = normal precipitation. Error bars = +/- 1 standard error.

What caused this difference in response to drought between exotic plant-dominated and native plant-dominated communities?  Mechanistically, the native shrubs have deeper roots than the exotic grasses, which may allow them to take up more water.  But how does this translate to differences in green-up date and senescence date?

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A student measures stem elongation on a senescent native shrub, the black sage Salvia mellifera, near the very end of the growing season. Credit: Ellen Esch.

The researchers used two different NDVI measures to help answer this question.  Maximum NDVI is the greatest daily NDVI measure over the course of the growing season.  It is correlated with the maximum productivity of the plant community (at its greenest!).  In contrast seasonally integrated NDVI is a measure of productivity summed over the entire growing season.  Keeping those distinctions in mind, under extreme drought maximum NDVI was much lower in the exotic plots than the native plots.  But exotic plot performance increased with rainfall, so that under the wettest conditions (normal rainfall), exotic plot maximum NDVI was similar to native plot maximum NDVI (graph a below). However, when considered over the entire growing season, native plots were consistently more productive than exotic plots (graph c below).

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Effect of rainfall on (a) maximum NDVI (top left), (c) seasonally integrated NDVI (top right), (b) green-up date (bottom left) and (d) senescence date (bottom right). Colors indicate dominant plot community composition (yellow = herbaceous, green = shrub) and point shape indicates growing season year (circle = 2013, square = 2014, diamond = 2015, triangle = 2016).

Phenology played an important role accounting for these differences in seasonally integrated NDVI.  At all rainfall levels, the native plant communities greened-up well before the exotic plant communities (graph b above). Exotic plants greened-up somewhat earlier as rainfall increased, while native plant green-up date was independent of rainfall. At all rainfall levels, native plots senesced about one month later than exotic plots, with increased rainfall delaying senescence in both native and exotic plant communities (graph d above).

Esch and her colleagues conclude that species composition (native shrub vs. exotic herbaceous plants) and drought both influence phenology and productivity in this important ecosystem. Climate change is predicted to increase the frequency of extreme droughts in this and other ecosystems.  Consequently, drought coupled with invasion by herbaceous species threatens to sharply reduce ecosystem productivity, which will decrease the food available for consumers and decomposers, and simultaneously reduce the amount of carbon dioxide taken up and stored by the ecosystem, thereby contributing to further climate change.

note: the paper that describes this research is from the journal Ecology. The reference is Esch, E. H.,  Lipson, D. A., and  Cleland, E. E.  2019.  Invasion and drought alter phenological sensitivity and synergistically lower ecosystem production. Ecology  100(10):e02802. 10.1002/ecy.2802. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2019 by the Ecological Society of America. All rights reserved.

Vacation’s changing tides

Cindy and I and our dog (Cheyanne) recently returned from a two+ week vacation at North Carolina’s Outer Banks.  We stayed in Avon, which is about eight miles north of the iconic Cape Hatteras lighthouse in a large house with a great ocean view.  We got a large house, because we thought our kids might join us, but it turns out that one disadvantage of kids getting older is that their lives become more complex.  Anyhow, several friends stayed with us for a few days, and a grand time was had by all.

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Cheyanne and I ponder the ocean’s vastness. Credit: Cindy Miller

But the point of this post is the trip home.  On Friday, we packed everything into our car, including Cheyanne, and began the eight-hour drive back to our home in Radford, VA.  At Rodanthe (about 15 miles north), traffic just stopped.  We sat in our car for a few minutes, disembarked, and spoke with many people walking by, who told us that the road (NC12) was flooded and covered with sand.  We had heard rumors of flooding, but since the sun was out and the wind relatively calm, we assumed that was all in the past.  Apparently the flooding was so bad that a motor home and the boat it was towing got totally caught up in the sand and water, and was wedged so efficiently that they could not even be towed out until serious excavation happened. That was not going to happen until Saturday.

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Moving the dunes off of the road. Credit: Cindy Miller.

Saturday at 6 PM we got the call that the road was open and we could head home.  We repacked the car, re-experienced Cheyanne’s baleful look, and set out, with an ETA of 3 AM at the earliest.  Alas the high tide came in, water breached the dunes, and a very kind police officer knocked on our window, imploring us to return to Avon and wait for a better day.  Cheyanne gave him a baleful look, but we obeyed.

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Reconstituted dunes.  Notice the tire tracks left by earth-moving machines. Credit: Cindy Miller.

The next morning we set out again; by now we could pack a car in just a few minutes.  Our peanut butter on toast dinner of the previous night had left us a bit peckish, so we stopped off for some pastries and cappuccinos. We headed north once again and this time we were able to pass through the Rodanthe flood, and several others along the way.  The water level was high, but our car had good ground clearance and our escape was relatively uneventful, but done at sub-breakneck speed.

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Riding away through Rodanthe’s rising tides. Credit: Cindy Miller.

 

Why was this happening?  The weather was beautiful – no rain, no wind and sunny skies.  It just doesn’t get any nicer than this at the Outer Banks.  As it turns out, there were two provocateurs.  First, there was subtropical storm Melissa several hundred miles to our east, passing harmlessly out to sea, but increasing sea levels.  Second, there was almost a full moon, which also tends to increase sea levels.  But that’s it!

That shouldn’t be enough.  In past years those two events might cause waves to crash to the dunes with increased vigor, but would not cause them to breach the dunes and spill onto the roads.  But those were past years, and now is the present, and sea levels along the North Carolina coast have risen by about one foot in the past 50 years.  Here are some data from Wilmington, NC – about 150 miles south of Avon.

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Rising sea levels measured at Wilmington, North Carolina. Credit: National Oceanic and Atmospheric Administration and SeaLevelRise.org

You should note two things.  First, there is substantial year-to-year variation in sea levels. Second, rates of sea level rise are accelerating.  Scientists at the National Oceanic and Atmospheric Administration and the US Army Corps of Engineers expect this trend to continue.  Here is the prognosticated change in sea levels between now and 2050 at Oregon Inlet (just a few miles north of Rodanthe).

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Forecast sea level change between 2016 and 2050. Credit: National Oceanic and Atmospheric Administration, U. S. Army Corps of Engineers and SeaLevelRise.org

This is very bad.  I’ve been vacationing at the Outer Banks for about 25 years; it has become a part of who I am.  I don’t want to give up on this spectacular part of the world, but we must act.  We cannot continue sticking our heads in the sand (which we can now oftentimes find on NC12), pretending that climate change is a construct of the liberal press or elite intelligentsia.

The first step in dealing with a problem is acknowledging that it exists. Climate change is here, and its impact is increasing. An estimated 50 million climate change refugees around the globe are being forced to abandon their homes. More will follow, including our neighbors from North Carolina’s Outer Banks. For their sake, and ours, let’s acknowledge the problem, and focus our resources, energies and talents to reducing the damage in the short term, and dealing with the causes of climate change over the next decades and centuries.

Decomposition: it’s who you are and where you are

“Follow the carbon” is a growing pastime of ecologists and environmental researchers worldwide. In the process of cellular respiration, organisms use carbon compounds to fuel their metabolic pathways, so having carbon around makes life possible.  Within ecosystems, following the carbon is equivalent to following how energy flows among the producers, consumers, detritivores and decomposers. In soils, decomposers play a central role in energy flow, but we might not appreciate their importance because many decomposers are tiny, and decomposition is very slow.  We are thrilled by a hawk subduing a rodent, but are less appreciative of a bacterium breaking down a lignin molecule, even though at their molecular heart, both processes are the same, in that complex carbon enters the organism and fuels cellular respiration.  However. from a global perspective, cellular respiration produces carbon dioxide as a waste product, which if allowed to escape the ecosystem, will increase the pool of atmospheric carbon dioxide thereby increasing the rate of global warming. So following the carbon is an ecological imperative.

As the world warms, trees and shrubs are colonizing regions that previously were inaccessible to them. In northern Sweden, mountain birch forests (Betula pubescens) and birch shrubs (Betula nana) are advancing into the tundra, replacing the heath that is dominated by the crowberry, Empetrum nigrum. As he began his PhD studies, Thomas Parker became interested in the general question of how decomposition changes as trees and shrubs expand further north in the Arctic. On his first trip to a field site in northern Sweden he noticed that the areas of forest and shrubs produced a lot of leaf litter in autumn yet there was no significant accumulation of this litter the following year. He wondered how the litter decomposed, and how this process might change as birch overtook the crowberry.

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One of the study sides in autumn: mountain birch forest (yellow) in the background, dwarf birch (red) on the left and crowberry on the right. Credit: Tom Parker.

Several factors can affect leaf litter decomposition in northern climes.  First, depending on what they are made of, different species of leaves will decompose at different rates.  Second, different types of microorganisms present will target different types of leaves with varying degrees of efficiency.  Lastly, the abiotic environment may play a role; for example, due to shade and creation of discrete microenvironments, forests have deeper snowpack, keeping soils warmer in winter and potentially elevating decomposer cellular respiration rates. Working with several other researchers, Parker tested the following three hypotheses: (1) litter from the more productive vegetation types will decompose more quickly, (2) all types of litter decompose more quickly in forest and shrub environments, and (3) deep winter snow (in forest and shrub environments) increase litter decomposition compared to heath environments.

To test these hypotheses, Parker and his colleagues established 12 transects that transitioned from forest to shrub to heath. Along each transect, they set up three 2 m2 plots – one each in the forest, shrub, and heath – 36 plots in all. In September of 2012, the researchers collected fresh leaf littler from mountain birch, shrub birch and crowberry, which they sorted, dried and placed into 7X7 cm. polyester mesh bags.  They placed six litter bags of each species at each of the 36 plots, and then harvested these bags periodically over the next three years. Bags were securely attached to the ground so that small decomposers could get in, but the researchers had to choose a relatively small mesh diameter to make sure they successfully enclosed the tiny crowberry leaves. This restricted access to some of the larger decomposers.

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Some litter bags attached to the soil surface at the beginning of the experiment. Credit: Tom Parker.

To test for the effect of snow depth, the researchers also set up snow fences on nearby heath sites.  These fences accumulated blowing and drifting snow, creating a snowpack comparable to that in nearby forest and shrub plots.

Parker and his colleagues found that B. pubescens leaves decomposed most rapidly and E. nigrum leases decomposed most slowly.  In addition, leaf litter decomposed fastest in the forest and most slowly in the heath.  Lastly, snow depth did not  influence decomposition rate.

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(Left graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath, shrub and forest. (Right graph) Decomposition rates of E. nigrum, B. nana and B. pubescens in heath under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars are 1 SE.

B. pubescens in forest and shrub lost the greatest amount (almost 50%) of mass over the three years of the study, while E. nigrum in heath lost the least (less than 30%).  However, B. pubescens decomposed much more rapidly in the forest than in the shrub between days 365 and 641. The bottom graphs below show that snow fences had no significant effect on decomposition.

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Percentage of litter mass remaining (a, d) E. nigrum, (b, e) B. nana, (c, f) B. pubescens in heath, shrub, or forest. Top graphs (a, b, c) are natural transects, while the bottom graphs (d, e, f) represent heath tundra under three different snow depths simulating snow accumulation at different vegetation types: Heath (control), + Snow (Shrub) and ++ Snow (Forest) . Error bars represent are 1SE. Shaded areas on the x-axis indicate the snow covered season in the first two years of the study.

Why do mountain birch leaves decompose so much more than do crowberry leaves?  The researchers chemically analyzed both species and discovered that birch leaves had 1.7 times more carbohydrate than did crowberry, while crowberry had 4.9 times more lipids than did birch. Their chemical analysis showed much of birch’s rapid early decomposition was a result of rapid carbohydrate breakdown. In contrast, crowberry’s slow decomposition resulted from its high lipid content being relatively resistant to the actions of decomposers.

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Researchers (Parker right, Subke left) harvesting soils and litter in the tundra. Credit: Jens-Arne Subke.

Parker and his colleagues did discover that decomposition was fastest in the forest independent of litter type. Forest soils are rich in brown-rot fungi, which are known to target the carbohydrates (primarily cellulose) that are so abundant in mountain birch leaves.  The researchers propose that a history of high cellulose litter content has selected for a biochemical environment that efficiently breaks down cellulose-rich leaves. Once the brown-rot fungi and their allies have done much of the initial breakdown, another class of fungi (ectomycorrhizal fungi) kicks into action and metabolizes (and decomposes) the more complex organic molecules.

The result of all this decomposition in the forest, but not the heath, is that tundra heath stores much more organic compounds than does the adjacent forest (which loses stored organic compounds to decomposers).  As forests continue their relentless march northward replacing the heath, it is very likely that they will introduce their efficient army of decomposers to the former heathlands.  These decomposers will feast on the vast supply of stored organic carbon compounds, release large quantities of carbon dioxide into the atmosphere, which will further exacerbate global warming. This is one of several positive feedbacks loops expected to destabilize global climate systems in the coming years.

note: the paper that describes this research is from the journal Ecology. The reference is Parker, T. C., Sanderman, J., Holden, R. D., Blume‐Werry, G., Sjögersten, S., Large, D., Castro‐Díaz, M., Street, L. E., Subke, J. and Wookey, P. A. (2018), Exploring drivers of litter decomposition in a greening Arctic: results from a transplant experiment across a treeline. Ecology, 99: 2284-2294. doi:10.1002/ecy.2442. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Climate changes a bird’s life in shrinking grasslands

Back in graduate school, a couple of my grad student buddies and I would get together to fish for brown trout in the Kinnickinnic River in western Wisconsin.  We were students at the University of Minnesota (Twin Cities), but the Kinni was the closest trout stream.  Tired of catching small brown trout, we consulted a trout fishing map and discovered that the headwaters of the Kinni were rich in brook trout. So early one morning, map in hand, we followed strange paths and found our sacred brook trout haven. Alas, the only thing it was rich in was corn, now about two feet high – though there was a modest depression where trout waters once had flowed. Our personal depression was perhaps more than modest – having been robbed of brook trout, and the opportunity to experience some pristine waters flowing through a beautiful grassland.

Grasslands, one of the biomes native to parts of Wisconsin and Minnesota, are globally one of the most endangered biomes, because they usually are relatively easy to convert into farmland and suburban developments. Native grasslands harbor a wide biological diversity; consequently conservation biologists are concerned about their continued loss.

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Cool-season grassland in southwest Wisconsin. Credit: John Dadisman.

Ben Zuckerberg, Christine Ribic and Lisa McCauley wanted to know how environmental factors influenced the nesting success of grassland birds, in particular, because as obligate ground nesters, they might be susceptible to changing  weather conditions that will be affecting the climate in coming decades.  A nest built on the ground is much less insulated from the environment than one built in or on a tree or even a ledge.

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Seven day old bobolink chicks in a ground nest. Credit: Carolyn Byers.

Zuckerberg and his colleagues used Google Scholar and the ISI Web of Science to comb the literature (1982-2015) for studies that explored the nest success of obligate grassland birds in the United States. They identified 12 bird species from 81 individual studies of 21,000 nests. Based on their experience and the literature, both precipitation and temperature were likely to influence nest success, which is the proportion of nests that fledge at least one young. They considered three precipitation time periods: (1) Bioyear – previous July through April of the breeding season, (2) May of the breeding season, (3) June – August of the breeding season. They considered breeding season temperatures during May, and during the period from June-August. The researchers were also interested in the size of the grassland (grassland patch size), reasoning that a larger grassland might provide more diverse microclimates, so, for example, a bird might be able to find a dry microhabitat for nesting in a large grassland, even in a wet breeding season.

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Map of the identity and location of species considered for this study.

The researchers discovered that both temperature and precipitation were important.  Nest success increased steadily with bioyear precipitation (Figure (a) below).  Presumably, more rain led to more plant growth and more insect survival, which would help feed the young.  Taller plants could also help shade or hide the nests. In contrast, nest success declined sharply with precipitation during spring and summer of the breeding season (Figure (b) and (c)). Heavy rains during the breeding season can flood nests, and also decrease the foraging efficiency of parents who might need to spend more time incubating nests during rainstorms. Lastly, extreme (low or high) May temperatures depressed nest success, which was highest at intermediate temperatures (Figure (d)). Egg viability depends on maintaining a constant temperature, and the parents may be more challenged to thermoregulate at extreme temperatures.  Temperatures later in the breeding season did not affect nest success.

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Effects of (a) bioyear precipitation (previous July – April of the breeding season), (b) May precipitation during the breeding season, (c) June – August precipitation during the breeding season, and (d) May temperature on nest success. Shaded area represents 95% confidence interval.

But all is not straightforward in the grassland nest success world. These main findings about precipitation and temperature interacted with grassland size in interesting ways.  For example high bioyear precipitation, which overall increased nest success, only did so for smaller grassland patches (dashed line in top graph below), but not for larger patches (solid line).  Extreme May temperatures had different effects on nest success in relation to grassland patch size.  Low May temperatures were associated with high nest success in small patches (dashed line in bottom graph) and with low nest success in large patches (solid line).  High May temperatures were associated with high nest success in large patches, and with low nest success in small patches.

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Predicted nest success of grassland birds in relation to bioyear precipitation (top graph) and May temperature (bottom graph) in relation to grassland patch size.  Solid lines represent large grasslands, while dashed lines represent small grasslands.  Shaded area is 95% confidence interval.

The researchers were surprised to discover that patch size affected how weather influenced grassland bird nesting success. Some of the patterns seem intuitively logical; for example, in unusually hot breeding seasons birds had higher nest success in larger grasslands than in smaller grasslands.  Presumably, birds were more likely to find a cooler microclimate for their nests in a large grassland.  However it is puzzling why in unusually cold breeding seasons birds had higher nest success in smaller grasslands. The researchers are planning a follow-up study to better document and measure the existence of microclimates in grasslands of different sizes, and explore how different microclimates influence the nesting success of vulnerable grassland birds.  Finding that warmer temperatures and drought generally reduce nest success to the greatest extent in small grassland patches is strong incentive for conservation mangers to establish large core grasslands as a tool to maintain bird populations in the wake of present and future changes to the climate.

note: the paper that describes this research is from the journal Conservation Biology. The reference is Zuckerberg, B. , Ribic, C. A. and McCauley, L. A. (2018), Effects of temperature and precipitation on grassland bird nesting success as mediated by patch size. Conservation Biology, 32: 872-882. doi:10.1111/cobi.13089. Thanks to the Society for Conservation Biology for allowing me to use figures from the paper. Copyright © 2018 by the Society for Conservation Biology. All rights reserved.

 

Carbon dioxide’s complex personality

Carbon dioxide (CO2) deservedly gets a lot of bad press because it is responsible for much of the global warming Earth is currently experiencing.  Less publicized, but perhaps equally important, CO2 is acidifying oceans, thereby threatening the continued existence of some critical biomes such as coral reefs and kelp forests (acid interferes with the ability of many marine organisms to build their shells).  But carbon dioxide also has a kinder, gentler side, as it is an essential resource for plants, and in some cases higher CO2 levels can increase a plant’s ability to carry on photosynthesis.  Sean Connell and his colleagues explored this complex personality by studying a marine ecosystem that experiences naturally varying levels of CO2. High CO2 levels and acidity exist near CO2-emitting vents at the study site – a volcanic island (Te Puia o Whakaari) off the coast of New Zealand.

White_Island_James Shook [CC BY 2.5 (https-::creativecommons.org:licenses:by:2.5)], from Wikimedia Commons

The volcanic Te Puia o Whakaari off the coast of New Zealand’s north island. Credit: James Shook [CC BY 2.5 (https-//creativecommons.org/licenses/by/2.5)], from Wikimedia Commons.

The major players in this ecosystem are the kelp, Ecklonia radiata, several species of turf-forming algae, and two grazers, the snail, Eatoniella mortoni, and the urchin, Evechinus chloroticus.  The typical vegetation in the region is a mosaic of kelp forest, some scattered small patches of algal turf, and sea urchin barrens – hard rock without significant vegetation, a result of overgrazing by sea urchins.  In contrast, extensive algal mats carpeted the rocks near these vents, and the researchers hypothesized that high CO2 levels caused this shift in dominant vegetation.

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Sean Connell collects data in a habitat dominated by algal turf (and numerous fish). Credit: anonymous backpacker.

Connell and his colleagues chose two vents and two nearby control sites at a depth of 6-8 meters. The CO2 levels and acidification near the vents were approximately equal to the amount projected for the end of the 21stcentury, but there were no differences between vents and controls in temperature, salinity or nutrient concentrations. The researchers estimated photosynthetic rates for kelp and turf algae by measuring the rate of oxygen production. They also estimated snail consumption rates by caging them for 3 days and measuring how much algal turf they removed.  They used an analogous approach to measure sea urchin consumption rates.

Conditions at vents had a major impact on both producers and consumers.  Kelp production decreased slightly, while turf production increased sharply at vents (Figures A and B below).  Urchin density declined (almost to nonexistence) while gastropod density increased markedly at vents (Figures C and D).  Lastly, consumption rates (on a per individual basis) by urchins plummeted, while consumption rates by snails increased sharply at vents (Figures E and F).

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Comparison of production and consumption at control sites vs. carbon dioxide emitting vents.

These patterns converted the normal mosaic of kelp forest, small algal turf patches and urchin barren into turf-dominated habitats.  Algal turf increased in size and frequency near the vents, while kelp forest shrank into near oblivion.

ConnellFig2

Frequency of patches of turf (light gray bars), urchin barren (medium gray) and kelp (black) in relation to patch size (diameter in meters) at control sites (top graph) and sites near vents (bottom graph).

These results can be pictured visually by the graph below.  Under conditions of present-day pH and CO2 levels, gross algal production is relatively low and urchin consumption is relatively high, which results in negligible net algal turf production (net production = gross production – urchin and gastropod consumption).  High CO2 levels sharply increase gross algal turf production while dramatically decreasing consumption by urchins.  Even though gastropod consumption increases slightly at vents, the overall effect on vents is a dramatic increase of net algal turf production. Consequently, the ecosystem experiences regime shift from kelp to algal turf domination.

ConnellFig1

Summary of effects of CO2 release by vents (bottom) vs Controls (top). Net algal production (red circle) = Gross algal production – urchin and gastropod consumption.  Net algal production in dark green zone is predicted to be turf-dominated (as is found near vents), light green is a mosaic, while white zone represents urchin barrens (low production and high consumption). Error bars are 1 standard error. 

Under current conditions, kelp is the dominant producer over turf algae in the near offshore ecosystem. High consumption by urchins keep the turf algae in check.  But near CO2 emitting vents, high levels of carbon dioxide have a dual effect on this ecosystem, disproportionately increasing turf algae production rate and decreasing urchin abundance and consumption rate.  This allows the competitively subordinate turf algae to replace the competitively dominant kelp, resulting in a dramatically changed ecosystem.  This occurs in the absence of an increase in ocean temperature.  Given that ocean temperature will increase sharply by 2100 (along with CO2 levels), many species interactions are expected to change in the next century, and ecosystem structure and functioning will be very different from what we observe today.

note: the paper that describes this research is from the journal Ecology. The reference is Connell, S. D., Doubleday, Z. A., Foster, N. R., Hamlyn, S. B., Harley, C. D., Helmuth, B. , Kelaher, B. P., Nagelkerken, I. , Rodgers, K. L., Sarà, G. and Russell, B. D. (2018), The duality of ocean acidification as a resource and a stressor. Ecology, 99: 1005-1010. doi:10.1002/ecy.2209 Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.

Too much of a good thing is killing Monarch butterflies

There was a time in the mid-Pleisticine when a photo of an ecological event was an awesome novelty, and a movie of an ecological event even more so.  Dodderers of an ecological bent (myself included), can vividly recall viewing a series of photos or a movie, either in a seminar or in an ancient ecology text, of a blue jay consuming a monarch butterfly, Danaus plexippus.  Consumption is immediately followed by explosive vomiting, as the cardenolides within the monarch butterfly claim another victim.  The monarch sequesters these cardenolide toxins from its larval food (milkweed), and incorporates them into its tissues as a means of protecting itself from predators – presumably blue jays learn from this very aversive experience.  I should point out that the individual sacrificial butterfly enjoys no fitness from this learning event – which raises some evolutionary questions we will not explore at the present.

Karen Oberhauser

Five instars (stages of development) of monarch caterpillars on a milkweed leaf. Credit: Karen Oberhauser

Rather we turn our attention to the relationship between milkweed, monarchs, and climate change. In several places in this blog we’ve talked about how climate change has influenced the behavior or physiology of a single species. For example, my first blog (Jan 31, 2017) discusses how increasing temperatures create more females in a loggerhead turtle population. But there are fewer studies that explore how climate change influences the ecological landscape, ultimately affecting interactions between species.  Along these lines, Matt Faldyn wondered if increased air temperature would change the chemical constitution of milkweed in a way that might influence monarch populations.  As he describes, “With milkweed toxicity, there is a ‘goldilocks’ zone where monarchs prefer to feed on milkweed that produce enough toxins in order to sequester these (cardenolide) chemicals as an antipredator/antiparasite defense, while also avoiding reaching a tipping point of toxicity where feeding on very toxic milkweeds negatively impacts monarch fitness.” He expected that at higher temperatures, milkweed would become stressed, and be physiologically unable to sustain normal levels of cardenolide production.

Faldynnative

Monarch butterfly feeds on a native milkweed, Asclepias incarnata. Credit: Teune at the English Language Wikipedia.

For their research, Faldyn and his colleagues worked with two milkweed species.  Asclepias incarnata is a common, native milkweed found throughout the monarch butterfly’s range in the eastern and southeastern United States.  Asclepias curassavica is an exotic species that has become established in the southern United States.  In contrast to A. incarnata, A. curassavica does not die back over the winter months; consequently some monarch populations are no longer migratory, relying on A. curassavicato provide them with a year round food supply.

Faldynexotic

The exotic milkweed, Asclepias curassavica. Credit: 2016 Jee & Rani Nature Photography (License: CC BY-SA 4.0)

To protect against herbivory, milkweeds have two primary chemical deterrants: (1) the already-mentioned cardenolides, which are toxic steroids that disrupt cell membrane function, and (2) release of sticky latex, which can gum up caterpillar mouthparts and actually trap young caterpillars.

field_noborderii.jpgThe researchers wanted to simulate climate change under field conditions, so they created open-top chambers with plexiglass plates that functioned much like mini-greenhouses, into which they placed one milkweed plant that was covered with butterfly netting.  This setup raised ambient temperatures by about 3°C during the day and 0.2°C at nighttime.  Control plots were single milkweed plants with butterfly netting. Half of the plants were native milkweed, and the other half were the exotic species.

For their experiments, Faldyn and his colleagues introduced 80 monarch caterpillars (one per plant) and allowed them to feed normally until they pupated.  Pupae were brought into the lab and allowed to metamorphose into adults.

MattGood

Matt Faldyn holds two monarch butterflies in the laboratory. Credit Matt Faldyn.

At normal (ambient) temperatures, monarchs survived somewhat better on exotic milkweed.  But at warmer temperatures, there is a strikingly different picture. Monarch survival is unaffected by warmer temperatures on native milkweed, but is sharply reduced by warmer temperatures on exotic milkweed (top graph below). The few that managed to survive warm temperatures on exotic milkweed grew much smaller, based on their body mass and forewing length (middle and bottom graph below)

FaldynFig1

Survival (top), adult mass (middle) and forewing length (bottom) of monarch butterflies raised under normal (ambient) and warmed temperatures.  Error bars are 95% confidence intervals.

Both milkweed species increased production of both types of chemicals over the course of the experiment. But by the end of the experiment, the exotic species released 3-times the quantity of latex and 13-times the quantity of cardenolides than did the native milkweed species.

FaldynFig2

Average amount of latex released at the beginning and end of the experiment.  Error bars are 95% confidence intervals.

FaldynFig2

Average cardenolide concentration at the beginning and end of the experiment.

The researchers argue that the exotic milkweed, Asclepias curassavica, may become an ecological trap for monarch butterflies, in that it attracts monarchs to feed on it, but will, under future warmer conditions, result in dramatically reduced monarch survival. Interestingly, these results are not what Faldyn originally expected; recall that he anticipated that temperature-stressed plants would reduce cardenolide production. The tremendous increase in cardenolide production in exotic milkweed at warmer temperatures may simply be too much toxin for the monarchs to process. The researchers predict that as climate warms, milkweed ranges will expand further north into Canada, and lead to northward shifts of monarch populations as well.  They urge nurseries to emphasize the distribution of native rather than exotic milkweed, so that monarchs will be less likely to become victims of this ecological trap.

note: the paper that describes this research is from the journal Ecology. The reference is Faldyn, M. J., Hunter, M. D. and Elderd, B. D. (2018), Climate change and an invasive, tropical milkweed: an ecological trap for monarch butterflies. Ecology. doi:10.1002/ecy.2198. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2018 by the Ecological Society of America. All rights reserved.