Tree roots caused mass extinctions of the past -- like humans today?

In a press release, Filippelli explained that their analysis showed that during the Devonian Period, between around 419 million to 358 million years ago, the evolving tree roots flooded the oceans of that time with excess nutrients. This resulted in massive algae growth, with “destructive” blooms depleting the oceans of oxygen. This, in turn, brought about devastating mass extinctions. 

The authors theorize that this process, called eutrophication, is much like some modern ecological catastrophes taking place in areas such as the Great Lakes and the Gulf of Mexico, where fertilizer-produced excess nutrients join other agricultural runoff in causing huge algae blooms that consume much of the oxygen in the water.

In the past, these types of events also resulted from the abundance of tree roots, which took nutrients from the land when they grew and then deposited them in the water supply upon decay.

What the scientists found through their chemical analysis were specific wet and dry cycles in Earth’s history. They looked at signs of soil formation called “weathering”, which came about from root growth. More weathering would show wet cycles with more roots, and less weathering would mean dry cycles that had fewer roots. 

The team observed that dry cycles tended to correspond to higher levels of phosphorous. This would indicate that nutrients from dying roots were being absorbed into the water at those points in time.

In our time, trees don’t tend to cause this to happen since evolution has established systems that can balance out the effects of rotting wood, according to the researchers. There are other threats, however, with the scientists pointing out that pollution from fertilizers and manure, as well as other types of organic waste like sewage, may be causing similar oxygen deprivation in the Earth’s ocean. This should serve as a warning about the extent of human impact on the oceans.

Interesting Engineering reached out to Professor Filippelli for more details on their work. The following conversation has been lightly edited for clarity and flow.

Interesting Engineering: How did tree roots of the Devonian period end up flooding the oceans with nutrients? Did they have to be near the oceans?

Professor Filippelli: Nowadays, deep roots from bushes and trees are largely responsible for the thick soils that we see in many areas. But before that evolution, soils tended to be thin, and very weakly weathered. Once trees evolved roots, they were able to delve deeper and deeper into the rock, creating soils and at the same time releasing a flood of nutrients into streams, rivers, and eventually the ocean. They did not need to be near the oceans to cause this. 

IE: How did the tree roots evolve -- what specifically changed in them to contribute to the Devonian mass extinction?

Plants began formulating more complex organic material, partly in response to increased competition for a more limited area of land. It was basically a two-dimensional competition for sunlight playing out among very short-statured plants. The only way to outcompete in this environment was to go up. As proto-trees hit upon the production of lignin, a stiff bio-protein which allowed them to stand taller but needed to be offset by a deeper anchor that roots supply (as well as provide better access to water and nutrients in a deeper soil), they began using the third dimension, height, to capture much-needed sunlight. But this transition from thin soils to thicker soils has a side-effect, namely that the soils in transition were very inefficient at capturing and recycling vital nutrients like phosphorus. The phosphorus leaked out from these newly-rooted landscapes and flooded the seaways with nutrients, triggering massive algae blooms which choked off and largely killed off the previous marine ecosystems.

IE: Are there modern human practices or natural events that pose the same danger of mass extinction to the oceans and the life in them?

Yes, and we are seeing one play out right before our eyes with excess fertilizer application in corn and soybean fields of the Midwest. Some of this fertilizer runs off the fields during heavy rains (which are occurring more frequently now due to climate change) and ends up flowing down the Mississippi River to the Gulf of Mexico, fueling massive algae blooms there and the same pattern of ecosystem destruction that we saw during the Devonian.

Abstract:

The evolution of land plant root systems occurred stepwise throughout the Devonian, with the first evidence of complex root systems appearing in the mid-Givetian. This biological innovation provided an enhanced pathway for the transfer of terrestrial phosphorus (P) to the marine system via weathering and erosion. This enhancement is consistent with paleosol records and has led to hypotheses about the causes of marine eutrophication and mass extinctions during the Devonian. To gain insight into the transport of P between terrestrial and marine domains, we report geochemical records from a survey of Middle and Late Devonian lacustrine and near-lacustrine sequences that span some of these key marine extinction intervals. Root innovation is hypothesized to have enhanced P delivery, and results from multiple Devonian sequences from Euramerica show evidence of a net loss of P from terrestrial sources coincident with the appearance of early progymnosperms. Evidence from multiple Middle to Late Devonian sites in Greenland and northern Scotland/Orkney reveal a near-identical net loss of P. Additionally, all sites are temporally proximal to one or more Devonian extinction events, including precise correlation with the Kačák extinction event and the two pulses associated with the Frasnian/Famennian mass extinction. For all sites, weathering, climate, and redox proxy data, coupled with nutrient input variability, reveal similar geochemical responses as seen in extant lacustrine systems. Orbitally forced climatic cyclicity appears to be the catalyst for all significant terrestrial nutrient pulses, which suggests that expansion of terrestrial plants may be tied to variations in regional and global climate.