A non-native tree species regains prominence after extreme weather

The long-term impacts on forests from more extreme climate events, as well as other drivers of forest change, are highly uncertain. A new study of rainforests across Puerto Rico and the US Virgin Islands (USVI), spanning 19 years, found that after Hurricane Maria in 2017, the total biomass of a fast-growing non-native species, the African tulip tree (Spathodea campanulata), may again exceed that of the more common group of native tree species, although, at least for young and small trees, non-natives die at twice the rate of natives. The paper is published in the journal PLOS ONE.

Extreme weather events are becoming more common across much of the planet. Rainfall records have increased worldwide in recent decades. Hurricane Maria was the most intense rainfall event for Puerto Rico since 1956 and has been linked to climate change. The most severe drought in the Caribbean since 1950 also occurred recently, from 2013-2016.

In addition to climate change affecting forests, extensive historical habitat loss on the Caribbean islands, combined with many species endemic to only one or a few islands, have led scientists to rank the Caribbean islands among those areas where the conservation of biodiversity is more urgent, as human population density and associated pressures on forests remain relatively high.

In Puerto Rico, deforestation peaked in the early 1900s. This happened around the late 1800s in the USVI. Since the 1950s, forest cover has increased on many Caribbean islands. Meanwhile, many fast-growing, non-native tree species have been introduced to the Caribbean islands. They are now common throughout Puerto Rico and the USVI, as many can quickly colonize deforested and sometimes degraded lands that were formerly cultivated or grazed. Non-native tree species originate from Eurasia or Africa, although some originate from South or Central America or larger Caribbean islands.

A team of foresters, ecologists, statisticians and geographers from the USDA Forest Service compared mortality rates of native and non-native small tree species in Puerto Rico and the USVI over 19 years of Forest Service Census and Analysis data there.

They focused on small trees to explain the influence of neighbors with large trees and because many small tree species become canopy trees when larger trees die, influencing which species dominate future forests. Overall, non-native small tree species died at twice the rate of natives, and hurricanes increased mortality rates for all small trees.

They then combined the forest census data with data on other factors that affect tree survival, asking which factors best predict the survival of small trees. They used a computer learning algorithm that takes into account many combinations of potentially important factors, such as individual tree dimensions, other tree species traits, neighboring tree factors, and factors from satellite imagery and maps of climate, topography, geology, terrain, and land use. Land use maps date back to 1950.

They found that since 2001, even after taking other factors into account, non-natives, including the African tulip tree, were more likely to die. With Hurricane Maria, small trees of species with less dense wood survived less, including non-natives.

Finally, they estimated trends in the total wood biomass of small and large trees of different groups of tree species before and after Hurricane Maria, assigning the African tulip tree to its own group due to its extremely fast growth rates and high seed dispersal. Estimates suggest that the total biomass of the African tulip tree was declining before the typhoon, to the point where it was comparable to the biomass of the more common group of native species, but increased afterwards. Hurricane damage leaves light in the forests, likely benefiting this fast-growing species.

Before this research, most knowledge of tropical tree mortality in the region came from a handful of intensively studied research plots in forest reserves, where fast-growing species also increase after hurricanes but are mostly native trees. This is the first study to examine the wide range of conditions outside forest reserves.

The study concluded that how climate extremes will affect future forests will depend on their frequency, severity and type. More frequent hurricanes could perpetuate the community of fast-growing non-native tree species such as the African tulip tree, reversing the recovery of native tree species from previous deforestation. However, native trees (and possibly some non-natives) in the harsher, slower-growing environments survive hurricanes more easily and may withstand drought better.