After hiking through some ancient, old growth neotropical rainforests on my recent trip to Costa Rica, I have gained a new appreciation for forest age. Upon my homecoming, I have been also preparing for my new teaching position at the University at Buffalo, where I will be teaching my students about old growth forest indicators and characteristics in the first week. As I began to prepare for the upcoming lectures, I stumbled upon a podcast from my close friend Matt and his channel “In Defense of Plants,” where he interviewed dendrochronologist Dr. Valerie Trouet. My travels, preparation for the semester, and curiosity about tree rings all coincided this week, which inspired me to put these underlying themes on paper. The goal of this article is to compare old growth forests in temperate vs. neotropical ecosystems and to better understand the interactions of tree growth rings and habitat.
Hiking through some sections of these neotropical ecosystems, even before my research, I was able to identify forest sections that are considered old growth. So first off, what is an old growth forest? Depending on the ecosystem your looking at, that definition changes. For example, on California’s coast, redwoods live long lives, with mature individuals living over 1,000 years. These redwood forests need around 500 years of little to no human interactions to be considered an old growth forest. On the contrary, in the boreal ecosystems of northern regions of Alaska, trees have a lifespan of 100-200 years so undisturbed 100 year old forests are considered old growth systems. Here in the temperate deciduous Northeast, our forests are considered old growth after 150 years of unperturbed growth.
Cross section of a coastal redwood in the Muir Woods.
Dendrochronology can help elucidate the categorization of old growth forests. By looking at tree growth rings, one can count the rings from a tree core, and date that specific specimen. These data are concrete evidence when it comes to dating tree individuals within a forest. In latitudes away from the equator, there are seasons the plants favor for growth, and seasons in which plants grow very little. In my neck of the woods, low temperatures limit plant growth in the winter, so tree cores from my area will have two distinct regions of the single ring; a thick portion of growth during the spring and summer, and a thin region showing limited growth in the fall and winter. Dr. Valerie Trouet in the podcast explains that in Tucson Arizona near where she lives, seasonal tree growth is limited by precipitation during the dry summer months instead of temperature.
Me with the giants.
Now this field may seem rather simple to the outsider, but it isn’t as easy as taking tree cores and counting rings. Some species are extremely difficult to date in this manner. Some environments provide conditions and resources year-round, so there isn’t a seasonal growth pattern at all inside the wood. And here is where we shift to the neotropics; the cloud forests I just visited in Costa Rica. Hiking in the Monteverde Cloud Forest Biological Reserve I stumbled upon some clear indication of old growth forests. Growth here is exceedingly fast given the optimal temperatures and moister year-round. One indication of an old growth forest is the presence of several large trees that don’t have low lying branches.
In urban environments, in most planted roadside trees, you can reach your hand up and touch the first limb. This reveals that there is ample available light, so the individual sends limbs all over to capture this incoming radiation. In an old growth forest, trees must grow up tall to access light. There’s no point in branching five feet from the forest floor if there’s not enough light, so individuals in old growth forests allocate more resources to grow tall as opposed to growing out. While exploring this cloud forest, there were sections of clear old growth as I found shade-tolerant species and large trees with their first limbs nearly 60 feet up.
My favorite indicator I found was this completely massive strangler fig. These parasitic plants actually germinate from the treetops of their host plant as birds and monkeys disperse fig seed containing excrement. These strangler fig vines grow downward, and once they finally reach the forest floor, they really start to grow fast. Eventually, as their common name suggest, these strangler figs widen in girth and choke their host tree. With their host unable to send water up to the canopy, they end up starving to death.
Getting a closer look at another Ficus tree.
Similar to plant growth in general within these areas, decomposition rates are exceedingly fast! Like crazy fast. Not only because there is ample water and optimal year-round temperatures, but because the soils here are extraordinarily nutrient-poor. For this reason, the nutrients locked up inside any dead organism are rapidly mobilized and absorbed by the living organisms nearby. For these reasons, in just a few decades after the strangler fig kills its host, there are no solid remains of the deceased plant.
I have found my fair share of strangler figs throughout my three excursions to Costa Rica, but this one takes the cake. Many times, you can sneak inside where the host plant used to be and look up through the living Ficus column. This individual could easily fit five adults on the inside of the tree, showing how large the host tree actually was. With these characteristics, I knew I was exploring an old growth section. This specimen has been estimated to be over 500 years old, and who knows how old its giant host tree was. I don’t know how dendrochronologists would study a core of a strangler fig, but by looking at the sheer size of its host plant, the size of the living Ficus, the lowest branches being 70-80 feet up, and the dense mats of epiphytic growth, this individual, along with other large well established Ficus and Ceiba trees, there was no question I was looking at a forest largely untouched by humans for hundreds of years.
Check out this awesome podcast [Link 1]