When your study organism fights back...

I explained back in this previous post about how important decomposition is for an ecosystem. Without the recycling of dead plants through decomposition, new plants wouldn't have enough nutrients to grow! It's such an important process that scientists have been studying it for decades. We want to understand what determines the speed of decomposition, and the way that the nutrients are recycled.

Of course, decomposition in the desert can be very different from other ecosystems, because it is very dry and hot, so biological processes (like decomposition) can be much slower. Another reason that desert decomposition can be different is that we have a lot of unique plants that become that "plant litter" that decomposes. Our plants are well-defended against herbivores, which means they are also (often accidentally) well-defended against decomposers! Decomposition studies don't often look at those unique plants. For example, there are only very few studies that have measured cactus decomposition. Even though cacti are an incredibly abundant plant in the Sonoran Desert, we don't know much about their life-after-death role in the ecosystem!

Students in my lab got curious about cactus decomposition, so we set up an experiment. 

Miranda and Ephraim building cactus decomposition cages.

The normal way to study decomposition is using litterbags, like I showed you here. But that's hard to do with a whole piece of cactus, because of the spines that stick out! It's hard to bag a plant that fights back! Ouch! So we built "cages" on the ground made out of the same material that is used for litterbags. These cages trap the cactus in place, so that we can refind each piece to measure its decomposition.

We put two different species of cactus into the cages. Both species are common in the Sonoran Desert. Prickly pear cacti have big, flat segments (called "clades") with large spines but are very juicy inside. Cholla clades are hard and cylindrical, and while they still have soft insides, they have a thicker skin on the outside that might make it hard for decomposers to eat through. So we hypothesized that these two species would decompose at different rates, and release different amounts of nutrients.

The big, flag cactus in the cages on the left are prickly pear, and the skinny, round cactus in the right-side cages are cholla.

Once our fresh cacti were in their cages, we let them decompose for a year. They went from looking like this at the beginning of the experiment...

The holes in these prickly pear clades are from cores that we used to measure their chemical properties at the start of the experiment.

...to this one year later!

Over the course of the year, we collected some of the cacti every few months so that we could track how much mass and nutrients had been released during decomposition. So, every few months, we brought the cacti back into the lab to measure their weight and chemistry:

Guillermo, Coby, and Chase breaking apart a decomposing cholla clade to measure its chemistry.

What did we learn about cactus decomposition? We learned that decomposing cacti recycle nutrients just as well as leaves from deciduous shrubs and trees. We also noticed that cacti recycle a LOT of calcium, way more than other types of plants (almost 10 times as much as other leafy plants!). That's because cacti have a lot of compounds called "calcium oxalates", which means there's a lot of calcium stored in their clades to be recycled when they die. 

We also learned that, despite the differences between prickly pear and cholla, their decomposition is pretty much the same... at least over the first year that we investigated in this study. There are a few differences between the species, though. Prickly pear released more water than cholla, and it also released potassium (an important nutrient for new plants!) faster.

There is still a LOT left to learn about cactus decomposition. We only looked at two species in one desert site, which is just a "drop in the bucket" for understanding how important cacti are in desert ecosystems. But what we do know is that cacti have an important role in nutrient recycling, so it is worth learning more about it!

The results of this study are published in: Bilderback, A.H., A.J. Torres, M. Vega, B. Ball. 2021. The structural and nutrient chemistry during early-stage decomposition and desiccation of in the Sonoran Desert. Journal of Arid Environments. 195: 104636. DOI:10.1016/j.jaridenv.2021.104636

Sonoran Desert BioArt

Last year I told you about our course in BioArt. Students work in teams to conduct research in the Sonoran Desert, and they communicate that research through a creative work of art. In the Spring semester of 2021, we had another great group of students work on projects in this course, creating wonderful works of Sonoran Desert art!

Anastasia and Rachael studied the role of desert vertebrates in the spread of cholla, a common cactus species in the Sonoran Desert. Cholla spreads to new habitats by dropping segments of the plant (called "tubercles") that can take root in the soil where they land. These tubercles are covered in hooked spines that work like velcro: they catch on the fur of coyotes, rabbits, and other mammals to be carried around and dropped somewhere new, away from the parent plant. So these animals are important to help spread this cactus, but humans are changing the abundance of these animals! How will urbanization impact the spread of this desert plant species? Anastasia and Rachael used camera traps and other survey techniques to learn which animals were associated with cholla plants. They learned that coyotes, deer, rabbits, and packrats were active in the area around chollas in the Sonoran Desert. Learn more about it in their research poster.

To demonstrate the action of a tubercle being removed from the cactus and spread to new habitats, Anastasia and Rachael created a soundscape video and an interactive cactus model. You can play the video to hear the sound of a tubercle being ripped from the cactus as it would be by a passing desert mammal. Tubercles stick to mammalian pelts because of tiny barbs on the cactus arms, ripping the tubercle off the main body like Velcro is ripped apart. This is the reasoning behind creating the interactive cactus model – arms are attached to the cactus body via Velcro, so viewers can experience the sensation of ripping a cactus arm off the main body. 

Joe and Mikayla explored whether the number of arms on saguaro cacti are related to water availability. Scientists don't actually know what signals saguaros to grow their iconic arms. We know it doesn't happen until they are older, and some saguaros don't grow arms at all! One hypothesis is that the arms are for extra water storage, in which case the number of arms would relate to water availability. Mikayla and Joe measured soil moisture along gradients from sources of water, and counted the number of arms on the saguaros at those locations. They did not find a relationship between soil water (or distance from the source of water) on the number of arms on the saguaros growing there. So... the reason for saguaros to grow arms remains a mystery! Learn more about it in their research poster.

To communicate their research, they created a glass tile mosaic that highlights the impact water availability has on Saguaro cactus branching. The recycled glasses contain blue marbles, representing the availability of water to each mosaic Saguaro it sits below. Both glasses contain the same amount of marbles, due to the research determining no significant difference between the water available to a Saguaro and its number of branches. This beautiful mosaic also sheds light on the complexity of Saguaro growth and the need for more research in this area. 

Jared and Irvin designed a rain garden that can maximize plant productivity with efficient water use in the Sonoran Desert. Rain gardens are a common form of "urban agriculture" that rely on rainwater to survive, but is that actually feasible in a desert city that doesn't receive a lot of rain? They used an infrared gas analyzer to measure the water use efficiency of native desert plants, and used their results to select the best plants to include in their design for a Sonoran Desert rain garden. Learn more about it in their research poster.

Irvin and Jared then designed the landscape for the garden using these efficient native plants, focusing on those that have tangible benefits to the community (like being a source of food or fiber for people). Of course one semester is not enough time to actually construct an entire garden, but in their blueprint, they designed it to contain artistic elements that would make it beautiful for visitors. The garden consists of three terraces: the ground level, level two at 2’ down, and level three at 3’ down. The blueprint features natural colors from the biotic elements with highlights of the pink milkweed flower and purple fruit from prickly pear. The abiotic elements are made to match the Sonoran Desert aesthetic.