The Entangled Task of Studying Ancient Forests

Tropical Rainforest (TR) is, by far, the richest place on Earth with a vast diversity of birds, insects, reptiles and more than 90,000 species of plants. Flowering plants (FP), are the dominant plant type within those tangled forests where they  are represented by >95%. They are also the main character in many colombian festivities reminding us of their influence in several aspects of humankind.

Despite its environmental importance, especially now that we’re facing several effects concerning climate change, how and when TR orginated remain as mysteries. From low extinction rates to rapid speciation during Quaternary times, many have been the ambitious hypotheses trying to solve this foggy puzzle about richness and diversity, certainly without any explanation concurred amongst the scientific community yet (some examples here, here and here).

The origin of new species and changes in diversity patterns occur in geologic time scales; this is why, to understand how terrestrial communities behave DNA and fossil-related studies are needed. Carlos Jaramillo, Palaeobotanist at the Smithsonian Tropical Research Institute, settled in Panama, have been studying pollen grains to unveil the history behind Tropical Rainforests over the past decades and -specially- one of his research papers, published in 2012, give us valuable information to grasp the entire picture.

Start at the beggining

What exactly is a Tropical Rainforest? According to Jaramillo there have been loads of definitions for TR but…He follows Tropical Rainforest as a multiestratified forest within lowlands, with an average precipitation of 1800 mm/yr, an average annual temperature of >18°C and absolutely dominated by big-leaf angiosperms (here mostly flowering plants).

History of neotropical terrestrial communities suffered a split up because of that famous asteroid hitting our planet 66 Million years ago. That event marked the decease of the then-dominant vertebrates Dinosauria and the end of the period where flowering plants came into being: the imposing Cretaceous. However, the day after this dramatic episode it turned out that FP  and all sort of small furry creatures took advantage of that new climatic conditions to lauch their career into stardom.

The oldest record of flowering plants comes from Clavatipollenites and Walkeripollis, two genuses of pollen grains recovered from rocks Barremian in age (about 130 Million years ago), but macrofossils, like leaves or fruits, have been  found since Aptian times (122Ma), leaving a 10 million-year gap. Those kind of things add up to the lack of data analysis in tropical areas, leave us with an uncertainty about diversity and abundance patterns of flowering plants up to the end of the Cretaceous.

But that’s not all: there’s also a mismatch between DNA, which shows birth and development of FP even since Jurasic, and palaentological evidence, which doesn’t. DNA  shows forests within tropical palaeolatitudes having a multiestratified structure dominated by flowering plants just as modern Tropical Rainforests since Cenomanian (let’s say 100Ma), however this idea is poorly supported when it comes to study fossil evidence like fossil wood, leaves and seeds.

Lower Cretaceous record (from Hauterivian to Albian) found in Colombian rocks steadily shows forests dominanted by other plants different to angiosperms, sometimes even with the latter not exceeding 10 percent. Cenomanian microfossils also make evident pollen grains with features absent until that moment as a result of a greater ecological variability within flowering plants, without this meaning their dominance (contrary what DNA says).

During Upper Cretaceous things didn’t change a lot. Despite evidence showing co-dominance between angiosperms, ferns and gimnosperms during Maastrichtian (Just remember it as the time when a big rock killed Dinos), most of flowering plants were small-sized plants still far from having the modern importance.

Boom!

Paleogene was, however, a time for gaining ground. After that collosal impact that ended with Dinos, ammonites (probably), and about 75% of plants in the Tropics, the forest’s structure and diversity  reached levels very similar to now. What  Jaramillo and colleagues found peering into Paleocene fossil material found in Cerrejón, the world’s biggest open pit coal mine, is an incredibly richer, more humid and warmer scenario full of giants reptiles and dominated by angiosperms (80%), but still less diverse and slightly hotter than the ones we have today (+2°C).

As well as this, plants  were able to survive in much higher temperatures and CO2 levels within neotropical areas throughout the Cretaceous and the most part of Cenozoic than their modern counterparts. Interestingly, the fossil record shows us how  PETM (Paleocene Eocene Termal Maximum) and other temperature-rise events like the one that took place during the most part of Eocene coincide with a diversity augment in Neotropical Rainforests, even exceeding current levels.

By the Neogene (say  about 9Ma) savannas became an important player into the tropical ecosystems match. But what changed for this to happen? Well, in the Tropics, there is a determinant  factor choosing vegetation types: rain. Being the central belt of the planet where winds really meet at huge scales doesn’t make things easier for weather forecasters. Winds coming from north and south create incredibly powerful rains during several months a year, and everything seems to say that it has been in that way for several Million years now. The raising of Andean Mountains affected this wind circulation for sure, raising seasonality and, probably, promoting savannas’ expantion.

The lifting of that mountain range created new spaces for plants to colonise, resulting in a diverse but complex group of ecosystems. Another great change in the Neotropics history came into scene after the stablishment of the Panamanian land bridge. That event, now knew as GABI (Great American Biotic Interchange), lead plants and mammals from South America to meet with the ones settled in the northern part of the continent. Because of its importance, GABI has drawn attention from scientist all around the world; many books and research papers have been written over the past decades and several are the positions.On the one hand, the latest evidence seems to show that  it happened at least 10 Ma before we believed. On the other hand,  a lot of science men and women still say that the Panama isthmus closure happened later (an example here). So, what idea should you follow?  My recommendation is to read a lot and to stay open to the posibilities and facts. You can always study another possibilities and propose new ones :D.  [we will be discussing GABI evidence later, by the way].

Neotropical vegetation is the result of an innumerable quantity of geological transformations and I can assure you that  they won’t stop in the near future. There is an incredibly amount of evidence to be studied; besides that the rain forests hide an amazing history, the only  truth is that the debate is still alive and it’s how it should remain.

Reference

Jaramillo C.: Historia geológica del bosque húmedo neotropical. Rev. Acad. Colomb. Cienc.
36 (138): 57-77, 2012. ISSN 0370-3908.
Feature image: Tropical Rainforest, Costa Rica. Credit:WikimediaCommons

I’d like to thank my friend and Colleague Cristian Benavides for his comments.

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