The Plant Press

From The Plant Press, Vol. 23, No. 3, July 2020.

Researchers split the birdcatcher trees (genus Pisonia) into three 

-Adapted from Pensoft

The birdcatcher trees – genus Pisonia – are infamous for trapping birds with their super-sticky seed pods that would frequently entangle the body of the ‘victim’. Left flightless, the poor feathered creatures eventually die either from starvation or fatigue, or predators. Similarly notorious are the birdcatcher trees for botanists, who have been baffled by their complicated classification for the last three centuries. 

Smithsonian Research Associate Marcos A. Caraballo-Ortiz and graduate student Elson Felipe Rossetto (Universidade Estadual de Londrina, Brazil) are working to untangle the classification of these trees. A recently published paper appears in the open-access scholarly journal PhytoKeys (152: 121-136; 2020).

The authors of the paper reestablished two genera: Ceodes and Rockia, where both had been previously merged under the name of Pisonia. Now, as a result, there are three distinct lineages of birdcatcher trees from the islands of the Pacific and Indian Oceans: Ceodes, Pisonia, and Rockia (Nyctaginaceae).

“Previous molecular studies on Pisonia species from around the world showed that species were clustered into three major groups, and here we assign names for each of them. With this new classification, a large number of the species known as Pisonia will be henceforth named Ceodes. This includes the Parapara (Ceodes brunoniana) and the birdlime (Ceodes umbellifera) trees, both native to many islands, including Hawaii and New Zealand. They are commonly planted in gardens for their lush and sometimes variegated foliage, as well as their fragrant white flowers. However, the cabbage tree (Pisonia grandis) will still be technically known as Pisonia,” adds the study’s lead author Felipe Rossetto.

Birdcatcher trees have generated much controversy in the popular media because of their seed pods (anthocarps) secreting a sticky substance that sticks to the feathers of seabirds or other animals for dispersal. Sometimes, though, too many seed pods can harm or kill birds, especially small ones, by weighing them down and rendering them flightless. This macabre practice has led to many controversies and local campaigns aiming to remove the trees, even illegally.

Despite their forbidding reputation, birdcatcher trees have positive effects on ecosystems and are important components of vegetation, especially for small islands. Sadly, there are many endemic and already endangered species of birdcatcher trees that only exist on a few small islands where they are effectively placed at the mercy of local people.

Many species of birdcatcher trees are large and tolerate harsh environments like seafronts and rocky cliffs, making them prime nesting spots for seabirds. Birdcatcher trees are also ecologically curious and could be regarded as keystone species on small islands because their soft branches can sustain many types of invertebrates; their flowers are an important food source for bees and ants; their dense leaf litter nourishes the soil; and their roots have intimate interactions with native underground fungi (mycorrhiza).

Although most people relate birdcatcher trees with beaches and coastal habitats, there are species that are only found in mountains or rainforests. For example, the species now allocated to the genus Rockia is endemic to the Hawaiian archipelago. These are small trees able to grow in dry to mesic mountain forests. Using the new classification, future studies can explore in detail the hidden diversity of these enigmatic plants, and find out how trees with high dispersal capabilities evolve into species endemic to small island ecosystems.