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Why plant non-native trees? They can have a positive impact on degraded land and companion crops.

One of the most common questions we are asked is why we plant the Empress Splendor tree (of the genus Paulownia) in regions where it is non-native. While it is true that the Empress tree is non-native to our planting regions (being endemic to Eastern China and Taiwan), it is important to distinguish between invasive and non-native, otherwise known as exotic, species.

Is it invasive or is it just exotic?

A toucan rests on a World Tree farm in Costa Rica.

In forestry practice, and sometimes in theory, the word “invasive” is often interchanged with the words “exotic” or “non-native.” This confusion of terms might have led to the generalized perception that all exotic species are undesirable and problematic in reforestation and restoration projects. It is true that aggressive, invasive exotics can damage and disrupt ecosystems. However, not all exotic species are invasive and there are many circumstances in which using non-native species is practical and safe. For instance, non-native trees are commonly used in agroforestry. 

The Empress Splendor trees used by World Tree (P. elongata and P. fortunei) are noninvasive. Our findings from eight years of research across 300 farm locations agree with those published in 2019: Empress trees have meager survival rates in natural ecosystems and are often overtaken by native plants if left unmanaged.

Non-native trees can "nurse" crops and degraded land

Many exotic trees can be ideal for agroforestry as they can often act as “nurse trees” by restoring degraded sites where it may be difficult to grow native species. Farming non-native trees on degraded land can improve soil fertility, providing the conditions for companion crops to flourish and opportunities for later planting native species to possibly arise.

Empress Splendor trees have a history of being used as nurse trees for other crops, particularly in intercropping systems. Intercropping with Empress trees provides various benefits, such as an improved microclimate. For example, they have been shown to contribute towards a reduced wind speed and an increase in relative humidity in summer, both of which can positively impact the growth of intercropped plants.

The ideal nurse tree

What exactly makes the Empress tree an ideal nurse tree? There is range of attributes that a nurse tree should have, but they include the following: 

  • Fast Growth: A rapid growth rate allows the nurse tree to provide quick canopy cover and shade, aiding weed suppression and reducing competition for light.  
  • Nitrogen Fixation: Nitrogen-fixing capabilities are highly desirable as they improve soil fertility by converting atmospheric nitrogen into a usable form that benefits companion plants.  
  • Biomass Production: Biomass can be utilized for mulch, compost, or as a source of organic matter to enhance soil structure and nutrient cycling, making nurse trees that produce substantial biomass advantageous. 
  • Non-Invasiveness: Nurse trees should not threaten the local ecosystem by outcompeting native species or causing ecological disruptions.  
  • Drought Tolerance: In regions with limited water availability, selecting a species that can tolerate drought conditions reduces water stress and maintains the overall health of the agroforestry system.  
  • Pest and Disease Resistance: Resistance to common pests and diseases is desirable as it minimizes the need for intensive management and reduces the risk of spreading pests or diseases to other plants.  
  • Ecological Functionality: Nurse trees should provide ecosystem services such as carbon sequestration, habitat creation, and support for beneficial organisms like pollinators and wildlife. 
  • Compatibility with Companion Plants: Compatibility with desired companion plants in the agroforestry system ensures optimal growth and productivity for all species involved.

To plant or not—our choice is a simple one

Despite being a non-native tree, our choice to plant the Empress Splendor tree was fairly simple. We asked can the tree enhance the overall connectivity of the landscape? Can the tree be used within an agroforestry system to catalyze the regeneration of degraded soil? Can it complement the natural ecosystem and help facilitate the establishment of some native species within the agroforestry system?

The answer to these questions is a resounding yes.