Why mosses can grow in the desert, and why their future is uncertain

Readers of this blog won't be so surprised, but most people are unaware that mosses grow in deserts and semiarid zones. The reason they can do so is that desert mosses are dessication tolerators, meaning they are capable of drying without dying. While dry, they are in a state of suspended animation, simply waiting for the next hydration period so that biological activity - and hopefully - net photosynthesis can occur. They rehydrate literally in seconds, and are immediately active. You could measure their respiration right away, for example. I always tell people that this is somewhat like spilling a glass of water on Tutankhamun, resulting in him coming back to life. A pretty cool party trick in my opinion. Biocrust organisms in general do this, but it is perhaps most dramatic in the mosses. Here's a video posted by Casey Allen of a simulated rain event.

And another from a class led by Larry Macafee at Badlands National Park. Focus is a little fuzzy, but you can see individual plants hydrating.

Neat trick for sure, but it does come with costs. Biocrust organisms do not regulate water loss with stomates (pores which a plant can open and close to regulate the rate of carbon coming in and water going out), so they lose that water they gained passively due to simply evaporation. They hang onto water very similarly to a piece of a paper towel - soaking it up via contact and losing it to evaporation. Every dry down event is thought to damage membrane integrity a little bit, so that the first order of business when rehydrating is maintenance. Many researchers have shown that upon rehydration there is a period of net respiration, and that after the photosynthetic rate may exceed the respiration rate, resulting in positive carbon uptake. If that happens growth is possible. But if that net photosynthetic threshold is not reached, the hydration event has resulted in carbon loss, and therefore dry mass loss. In other words the mosses would be shrinking a tiny bit rather than growing....bad news. So you can imagine that short hydration events due to small rain events or just high temperature driving fast evaporation, could damage mosses. Further, many such events could even kill them. So nature's tough guys have an Achilles heel, and seem to straddle a knife edge of survival. The climate is changing: what if increased temperatures or altered frequency or magnitude of rain events decrease hydration times? These mosses seem vulnerable to catastrophe.

Recent papers, including an awesome one by Reed et al. in Nature Climate Change, have induced rapid changes in biocrust community composition as a function of experimental climate change manipulations in a 5 year experiment. They induced a 2 degree C warming using infrared lamps in the field in Utah. This is a modest warming effect compared to multi-GCM projections for the area. They crossed this warming effect with a watering treatment which doubled the frequency of rain events but only slightly increased the total amount of rainfall. It's not surprising to me that this had an effect....but what is surprising is that the authors induced a 90% moss mortality in only 1 year!.

Moss dieback in response to increased frequency of small summer rainfall events (Zelikova et al. 2012)

Mosses fail to attain net photosynthesis when experiencing 1.25 mm rain events, contrasting with a 5 mm rain event (Reed et al. 2012).

This effect was essentially exclusively driven by the watering frequency treatment. The group also tracked effects rippling through the entire community. Under the high frequency summer rain treatment, cyanobacterial cover apparently expanded filling the gap left by the mosses, but pigment concentrations suggest that biomass was declining (Zelikova et al. 2012). Meanwhile bacteria and fungi were in decline (Zelikova et al. 2012), and enzyme signatures suggested that decomposition rates were faster under the frequent watering regime. In addition, changes were induced to nitrogen cycling, including increased nitrfication and a shift from an ammonium to a possibly leakier nitrate dominated regime.

Unlike temperature projections, precipitation projections from climate models are notoriously variable. Therefore, we don't know if climate changes will induce this utter tanking of biocrusts and their function. It is a plausible scenario, however,  that the summer monsoon in the Colorado Plateau region would bring a higher frequency of storms. This is exactly what we don't want. Not only would we lose soil fertility, but biocrusts would be less able to aggregate soils and prevent dust emissions which could go on to affect Western US water supplies (see previous post).

I should point out that this simulated climate change scenario is not necessarily the most plausible in all drylands, and the apparent indifference of biocrusts to warming may also not be universal. These studies from Utah, contrast nicely with another study which shows a clear negative effect of 2 - 4 degree C warming on lichen-dominated biocrusts in Spain (Escolar et al. in press).

Escolar C, Maestre FT, Martínez, I, Bowke, MA 2012. Warming reduces the growth and diversity of lichen dominated biological soil crusts in a semi-arid environment: implications for ecosystem structure and function. Proceedings of the Royal Society B: in press.

Zelikova TJ, Houseman DC, Grote EE, Neher DA, Belnap J. 2012. Warming and increased precipitation frequency on the Colorado Plateau: implications for biological soil crusts and soil processes. Plant & Soil DOI 10.1007/s11104-011-1097-z

Reed SC, Coe KK, Sparks JP, Houseman DC, Zelikova, TJ, Belnap J (2012). Changes to dryland rainfall result in rapid moss mortality and altered soil fertility. Nature Climate Change DOI: 10.1038/nclimate1596

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