What if a cactus grew in a rainforest?

A cactus lives where it hardly ever rains. A rainforest is where it rains and rains.

What if a desert cactus grew in the rainforest? Would it like all that water?

Cacti and other succulents mostly grow in places with little rain. A rainforest is the opposite: tropical rainforests get about 180 to 250 centimeters of rain a year, and the air is damp most of the time.

So what would happen if you planted a desert cactus, like the giant saguaro, in a rainforest? More water sounds like a gift for a thirsty plant. In this thought experiment, you will find out why it might not be, and meet some surprising cacti that do live in rainforests.

Plants are shaped by the conditions of their homes. Desert cacti store water, limit water loss and gather brief rainfall quickly; tropical rainforests receive roughly 180–250 cm of rain a year at a humidity of about 77–88 percent (National Geographic Education).

This thought experiment transplants a desert cactus into that setting and follows the consequences: for its stem, for its roots and for its photosynthesis. It then compares it with cacti that evolved to live in rainforest canopies.

A saguaro cactus stores water in its stem. When it rains, the stem gets fatter, like an accordion opening.

Its roots spread out just under the ground to catch the rain.

The saguaro cactus of the Sonoran Desert is a water-storing champion. Its stem has folds called pleats. When heavy rain falls, its roots soak up as much water as they can, and the pleats open out like an accordion to hold it. In dry times, the cactus uses its stored water and the pleats fold back in.

A saguaro's roots spread out in all directions, just a few inches under the ground, ready to catch rain quickly. A fully grown saguaro can weigh 2 tons or more.

The National Park Service describes the saguaro's pleated stem expanding like an accordion as its radial roots, several inches below the surface, absorb water after heavy rain; in dry periods the plant draws on that store and the pleats contract. An adult saguaro may weigh 2 tons or more.

Cacti also save water in how they photosynthesize. OpenStax Biology 2e explains that cacti open their stomata at night, when cooler temperatures reduce water loss, store the carbon dioxide temporarily, and use it for photosynthesis in the day with their stomata closed.

In the rainforest, the ground stays wet all the time.

Too much water can make a cactus's roots go soft and rot.

In a rainforest, the soil would hardly ever dry out. That is a problem for a desert cactus. Plant scientists say that when soil stays waterlogged, there is less oxygen in it, and roots die. Gardeners call this root rot. Too much water is one of the most common problems plant labs find in cacti and other succulents.

The cactus's water-saving tricks would not help either. Its stem would already be full, and its shallow roots would sit in soggy ground. In the warm, wet rainforest, it might also get diseases such as soft rot, which turns plant tissue soft, watery and black.

Iowa State University Extension notes that root rot typically develops from too much water in the soil, because waterlogged soil holds less oxygen and roots die; overwatered succulents show yellowing, soft, mushy leaves and stems. The Texas Plant Disease Diagnostic Lab reports overwatering as one of the most common problems in the succulent samples it receives, along with soft rot (Erwinia carotovora) that leaves tissue soft, watery and black.

A desert cactus in constantly wet rainforest soil might therefore suffer root death and rot. Its other adaptations, a water reservoir and roots built to grab brief rains, offer no advantage where water is never scarce.

Every plant is just right for its home.

A cactus is right for the dry desert. Rainforest plants are right for the rainy forest.

A desert cactus is shaped for a place where water is rare. It stores water, has roots that grab rain fast, and opens its tiny leaf pores, called stomata, at night when the air is cooler so it loses less water.

In a rainforest, the challenges are different, and rainforest plants are shaped for those conditions instead. Move a plant to a very different home, and the features that helped it can stop helping, or even cause trouble.

Adaptations are trade-offs fitted to particular conditions. A desert cactus's water storage, shallow radial roots and night-time stomatal opening all solve the problem of scarce water. In a habitat where water is abundant, those same features offer little benefit, and sitting in waterlogged soil exposes roots to oxygen shortage and rot.

The NPS adds another limit: saguaros are sensitive to freezing, which restricts them to elevations below about 4,500 feet. Each species' range reflects the conditions it can tolerate, not only those it prefers.

Surprise! Some cacti do live in rainforests. They grow high up on tree branches.

Many have no prickly spines. Their stems are flat like leaves or thin like strings.

Here is a twist: some cacti really do live in rainforests, but not on the ground. More than 100 kinds of cacti grow high in the branches of tropical rainforest trees, where they can cope with dry spells and strong sunshine.

They look different from desert cacti. Many have lost their spines, and their green, water-storing stems are flat like leaves or hang down like thin rods. Almost all cacti come from the Americas, but one rainforest group, Rhipsalis, also grows wild in Africa, Madagascar and Sri Lanka.

The Botanical Garden Munich-Nymphenburg notes that more than 100 cactus species from several genera live in the canopy of tropical forests as epiphytes on the branches of large trees, tolerating drought and strong sun. Their water-storing stems are often flattened and leaf-like or reduced to thin, hanging rods, and most have lost their spines. Like almost all cacti they are American, except Rhipsalis, which also occurs in Africa, Madagascar and Sri Lanka.

So the answer to this lab's question is two-sided. A desert cactus moved to the rainforest floor would likely struggle; cacti adapted to the rainforest canopy thrive there. The difference lies in their adaptations.