Pawpaw trees form dense patches that outcompete other plants

Pawpaws, the largest native fruit in North America, have become a favorite among foragers and food enthusiasts.

Found in their characteristic dense patches, they are known for their creamy, custard-like texture and sweet flavor – described as a mix of mango and banana – and offer a unique culinary experience.

However, the ecological impact of pawpaws extends far beyond their popularity as a delicious treat.

Pawpaw patches take over the forest

Pawpaws, the state fruit tree of Missouri, naturally grow in groups called patches. These patches form because pawpaw trees can reproduce clonally, meaning they spread through their roots.

New stems grow from these roots, close to the original tree, in a way similar to how aspen trees create groves. This process allows a single pawpaw tree to establish a larger group of genetically identical trees, which is why these patches are often celebrated in American folk songs.

However, these patches don’t just grow – they take over. Pawpaw trees spread quickly, creating dense clusters that dominate the forest understory.

Their thick growth and clonal expansion leave little space or resources for other plants to thrive, effectively reducing plant diversity and altering the local ecosystem. While these patches are great for pawpaws, they can significantly limit the growth of other species around them.

Pawpaws’ effect on plant diversity

New research from Washington University in St. Louis highlights how pawpaw patches influence biodiversity.

“Pawpaws create a habitat where the rules of competition are more random than when they’re absent,” noted Anna Wassel, a graduate student and lead author of the study.

The researchers found that pawpaw patches have a significant impact on plant diversity within their boundaries. Specifically, the variety of herbaceous plants, which include smaller, non-woody species like grasses and wildflowers, was 49% lower inside pawpaw patches compared to areas outside.

This indicates that nearly half of the usual types of these plants struggle to survive in the environment created by pawpaw trees. The dense shade and competitive conditions within the patches make it difficult for many plant species to thrive.

Pawpaw patches decrease plant density

In addition to reduced diversity, the total density of plants – combining both woody species like shrubs and herbaceous species – was found to be 29% lower inside pawpaw patches.

This means not only are there fewer types of plants, but also fewer individual plants overall. The dominance of pawpaw trees and their resource consumption leave less room and fewer resources for other plants to grow.

The study also revealed that pawpaw patches increase the randomness of which plants survive, particularly at larger scales.

In typical forest ecosystems, plants compete for sunlight, water, and nutrients in predictable ways, with certain species better adapted to the conditions prevailing. However, inside pawpaw patches, these usual rules of competition break down.

The survival of plants becomes more random, disrupting the natural balance of the ecosystem. This unpredictability makes it harder for a stable and diverse plant community to form within pawpaw-dominated areas.

Why pawpaw patches ruin other plants

Pawpaw trees have a strong ability to outcompete other plants, thanks to several factors.

Their large, tropical-looking leaves create dense shade, blocking sunlight that many smaller plants need to grow. Without enough light, these plants struggle to survive and thrive under the pawpaw canopy.

Additionally, pawpaw trees spread through underground roots and produce multiple stems in one area. This high stem density increases competition for soil nutrients and water, leaving fewer resources for other plants in the vicinity.

Another factor is that deer tend to avoid eating pawpaw trees. Instead, they graze on other plants nearby, further reducing the variety of plant species in and around pawpaw patches.

This combination of shading, resource competition, and selective deer browsing makes pawpaw patches a tough environment for other plants to grow – significantly lowering local biodiversity.

Implications for forest management

The study’s findings offer important lessons for managing forest ecosystems.

“If you are monitoring a plot of land with the aim to encourage the growth of understory species, unfortunately, pawpaws are not your friends,” Wassel noted.

The dominance and unpredictability of pawpaw patches make them a challenge for land managers focused on promoting plant diversity.

However, for those interested in harvesting pawpaw fruit, the approach is different. To ensure cross-pollination and healthy fruit production, managers should encourage as many genetically distinct patches as possible.

“The presence of pawpaws is never inherently good or bad, it’s just natural. Nature’s goals are never as linear as ours,” noted Wassel.

The study is published in the journal Ecosphere.

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