In this post we’ll discuss worldbuilding subtropical climate zones, where they’re found, what they’re like, and the flora and fauna that can be found within them.

Hey everyone, my name is Matthew, at least when I’m not drowning in humidity, and this discussion is part of a series where I will be going through a science-adjacent worldbuilding process step-by-step. Last time we discussed the deserts of our fictional world of Locus, looking at how the flora and fauna there have adapted to the exceptionally dry conditions.

For today’s discussion, we’re going to be moving out of the tropics and discussing the first climate of the temperate zones, the humid subtropical climate, with hot wet conditions that allow for a diverse range of wildlife to thrive.

As Subtropical climates are the first climate of this series that are considered temperate, let’s quickly establish what temperate means in climatological terms. Temperate zones, according to the Koppen Climate Classification system, are categorised as areas where the coldest month has temperatures between 0 and 18 degrees Celsius, and at least one month where temperatures exceed 10 degrees Celsius. They are differentiated between climate zones that have a dry summer, a dry winter, or no dry season at all. Temperate zones usually have wider temperature variations throughout the year compared to tropical climates, and these temperature variations create distinct seasonal changes. Not only are temperatures influenced by latitudinal changes, but also by ocean currents, prevailing winds, how big a continent is, and altitude. This has led to the further differentiation between temperate zones that have a hot summer, a warm summer, and a cold summer.

What this means is that by combining the distinctions between dry seasons, and distinctions between summers, there are multiple type combinations that lead to distinct climate zones that we’ll need to differentiate between. However, the good news is that while there are nine unique climate zones, they fit neatly within three main climate categories; the subtropical climates, Mediterranean climates, and oceanic climates, so we’ll only need to worry about placement for these three groups.

The Subtropical climates we’re focusing on include two main climate zones: Humid Subtropical and Monsoon-influenced Humid Subtropical. Both climates have hot summers, but while Humid Subtropical regions have no dry seasons, Monsoon-influenced Humid Subtropical regions have dry winters due to monsoonal activity. On earth, many places across most continents experience some humid subtropical conditions, including but not limited to the southeast states of the United States, much of eastern China, eastern South America, and most of the east coast of Australia. These climates have long, hot, and humid summers, with temperatures reaching the highest seen in temperate zones, and rainfall often peaks in summer, where convective thunderstorms are common. A convective storm is a storm formed when surface heat causes moisture to rise into the atmosphere, forming single-celled localised storms that often generate thunder, lightning, significant winds, and sudden temperature changes. At the cold-warm air boundaries around the subtropical ridges, the strong temperature gradients combined with lots of moisture generate a lot of potential energy through convection, which is the perfect recipe for tornadoes. Add to this that the Humid Subtropical climates are right in-line for tropical cyclones that form over the ocean, and these regions can be regularly battered by extreme weather conditions.

This is the map of Locus, the fictional earth-like planet that we’re creating throughout this series. As you can see, we’ve already placed our tropical climate zones as well as our deserts on the map. Humid Subtropical climates are placed along areas with warm currents between the 25 to 45 degrees mark. If your planet has similar ocean currents to earth, like Locus does, then your Subtropical climates should form on the eastern side of your continents. The subtropical climates that are closer to the coast are more likely to be Humid Subtropical, while the subtropical climates that are further inland are more likely to be Monsoon-influenced Humid Subtropical.

For any planets warmer than earth, Humid Subtropical regions will extend further towards the equator, down to around 20 degrees north and south, eating into the savanna regions. Subtropical regions will also extend further inland, expanding the reach of both climate types. For a colder planet like Locus, Humid Subtropical regions don’t extend as far poleward, giving way to other climates at around 35 degrees north and south, looking something like… this. Notably, there will also be a significant reduction in the monsoon-influenced Humid Subtropical regions on colder planets, with the inland regions simply being too cold to be considered subtropical.

The hot and humid environment of the Subtropical climates provide excellent growing conditions for many plants and trees, which thrive without fear of wildfire or frost. Tropical evergreens such as ferns, pines, and palm trees successfully grow here within the subtropics, just as they do within the tropical rainforests, so any plants and trees that you’ve already designed for tropical rainforests can be placed here too. However, while tropical rainforests have similar temperatures all year-round, subtropical climates do still have seasons, which means that plants here have growing seasons, though these growing seasons can be as long as three quarters of the year, making subtropical climates more forgiving than other temperate zones in terms of when plants can grow. Generally speaking, if something can grow in the tropics, there’s a good chance it can here too.

On Locus, perhaps the most interesting organism among the trees is not a plant at all, but rather a bacteria-like organism called Fulmeneques, which is capable of a fictional process called severance, specifically electro-severance. You can find a video linked in the corner explaining how this process works. Simply though, electro-severance absorbs electrical energy from its environment, with Fulmeneques taking advantage of the frequent thunderstorms present in the subtropical regions to fuel this process. Fulmeneques is a parasite, growing on the tops of the tallest trees and acting as a lightning rod. When lightning storms occur, infected trees are more likely to be struck, and when they are, they release the Fulmeneques spores which are then dispersed and carried by the wind to other trees. Unsurprisingly, trees struck by lightning often die, though their remnant husks become new habitats for other species, and their decaying matter will feed many other organisms such as the fungi-like Pilea. Because Fulmeneques tend to infest the tallest trees, it acts as a control against trees becoming too large, preventing any trees from dominating the ecosystem and spreading unchecked.

Among subtropical regions, a common geographical feature are wetlands. Wetlands are usually low-lying areas that are waterlogged, with some being present permanently, while others occur seasonally. They provide some of the greatest biological diversity across the planet and are generally highly productive ecosystems that spur a lot of growth. Importantly for evolution, they are the most expansive transitional zones between land and sea, which provide a steppingstone for ocean dwelling organisms to make their way onto land. Wetlands are divided into three main types: marshes, swamps, and peatland. Simply put, marshes are areas that are permanently or frequently waterlogged, swamps are forested areas that are permanently flooded, and peatland refers to bogs or fens that have spongy peat deposits with acidic waters, with bogs receiving water through precipitation, while fens receive water through runoffs or streams. These wetlands are common in subtropical lowlands due to their high humidity and frequent storms that cause flooding, though wetlands can absolutely form in other climate zones if conditions are right.

Within wetlands, especially the nutrient rich swamps and marshes, a variety of plant life can thrive, though the most common are macrophytes. Macrophytes refer to plants that have adapted to growing and living in aquatic environments, and we separate macrophytes into four categories. Emergent macrophytes refer to plants that grow in water but have parts above the surface that are exposed to air, such as papyrus and wild rice. Submerged macrophytes grow completely underwater, though often still have segments above the water while their growing buds are below the surface, such as reeds and cattails. Floating-leaved macrophytes have root stems that are attached to the bottom of the body of water, with leaves that float on the surface, with the most iconic being water lilies. Finally, there are free-floating macrophytes, which float on the surface with no attachment to the ground, such as water lettuce. Successful flora within subtropical regions are likely to be macrophytes, due to the constantly humid conditions and regular flooding storms.

One such macrophyte on Locus is the Multagranum, or Multa grass, which specifically is an emergent macrophyte, with stems that emerge from the water that it grows in. Multa grass is not fussy with its growing conditions, able to grow across marshes and swamps, or just about any area where water is present, even being found along the shores of lakes and rivers. It is perennial, meaning that it lives for longer than 2 years, and provided its stalks are not consumed, can continue to provide seeds for many years consistently. These seeds are one of the most plentiful and abundant sources of replenishing food across the entire planet, and unsurprisingly, Multa Grass are keystone species within the subtropics, with many creatures being dependant on the reliable food source it provides.

Now that we’ve looked at the flora of the subtropics, let’s discuss some iconic fauna. Like tropical rainforests, the hot humid conditions here allow for a greater variety of cold-blooded animals to thrive, though warm-blooded mammals are still abundant in the subtropics. On earth, some of the most well-known subtropical creatures include crocodiles, alligators, turtles, snakes, and frogs, as well as big cats like tigers and panthers. Many land creatures are semi-aquatic, taking advantage of the wetlands to establish dominance over their own unique niches.

Dominating the subtropical wetlands are the hypercarnivorous Confractossis, which are cousins of the Respirafinalis we discussed in the deserts. Confractossis is a comparative giant however, reaching colossal sizes, though there is significant sexual dimorphism present, with males only reaching around half the size of females. Both sexes have similar colouration, helping them to camouflage in their waterlogged environments, from which they strike at their prey, grabbing it with their teeth. Once grabbed, the Confractossis lives up to its name by constricting its prey until death before consuming it whole. While males are dangerous, they are still preyed upon by other predators. Females however are far too big for other predators to safely manage, making them the apex predators of the subtropics.

Speaking of predators within the subtropics, I would be remiss if I didn’t include a large spider-like creature within the ecosystem, seeing as on earth spiders are abundant within subtropical climates. Though I have to say, as an arachnophobe, designing this creature was not easy. The Terrasicaris are actually not Planapedes like most other insect-like creatures on Locus, but rather are a type of Duruscutis, which are generally ocean-dwelling creatures that resemble crabs. The intertidal zones present within the swamps and marshes have allowed for Terrasicaris to transition from the seas to make the subtropics their new home. Their exoskeletons provide them with enough structural support to reach sizes of up to 1 meter, which is… just great. Terrasicaris are carnivorous, mostly feeding on Planapedes, though larger individuals will trap and feed on other creatures. They use their legs to dig burrows, and as amphibious creatures can dig these burrows within marshes and swamps underneath the water. They will then ambush prey that comes within range of the burrow, dragging their prey in before injecting it with a venom that is not designed to kill but instead to paralyse. Terrasicaris will then eat their prey alive over the course of many days, which I’ve decided is the most horrifying way a creature can die on Locus. And with that, let’s move on (SHIVER).

The final creature we’ll discuss here within the subtropics are the Visomnis, yet another ocean-dwelling creature that has made landfall. The Multarma generally include creatures akin to jellyfish, and Visomnis is one of the only land-dwelling members of their entire phylum. In terms of body structure, they are not dissimilar to an octopus, with suckers on their tentacles that they use for grasping and locomotion. They earn their name from their unique eye placement on the sides of their head, which grants them 360-degree vision, helping to keep them safe from predators. As soft-bodied invertebrates, they lack the skeletal support required to reach large sizes while on land, though have made landfall to take advantage of the thriving amount of food available within the subtropical wetlands. Visomnis feed mostly on the insect-like Planapedes, as well as the seeds of Multa grass. In fact, Visomnis are responsible for protecting Multa grass from larger herbivores, and larger creatures that threaten to consume the Multa grass in their habitat can be met with extreme hostility from the Visomnis, that will attach themselves using their suckers onto the larger creatures, before stabbing at them repeatedly with sharpened sticks. This is not the only instance of Visomnis using tools and weaponry, and the use of these tools as well as their understanding of which creatures are threats to their food source, make Visomnis one of the more intelligent creatures within the subtropics, so we’ll be keeping in mind in the future as we develop intelligent species.

So, to recap, subtropical climates are categorised as hot humid temperate regions where wetlands are common, and flora and fauna are abundant. Flora within the area is similar to those found in tropical rainforests and is often aquatic. Fauna is greatly diverse, and a unique opportunity is present within the subtropics for ocean dwelling creatures to make landfall more easily.

Join me next time when we’ll discuss Mediterranean climates, which are the temperate zones on the opposite sides compared to subtropical ones. You can find all the information for this video and other resources for worldbuilding in general over at worldbuildingcorner.com, and if you enjoyed this video don’t forget to like and subscribe to follow the world-building journey. And until next time… stay awesome!