Since the early 1990s, non-native pine trees have been used to reforest areas in Trinidad and Tobago that have been affected by forest fires.
Initially, these pines were used in reforestation efforts because of their fast-growing nature and drought resistant characteristics, which meant they were ideal to withstand forest fires and reduce the need to replenish native species that perished easier during fires.
However, research in recent decades has found that these non-native pine species may be doing more harm than good as undergrowth of different plant species is limited due to the trees' effect on the soil pH.
At The UWI St Augustine Geography Department, Senior Lecturer Dr Kegan Farrick and graduate student Jesse Francis, both hydrologists, sought to further understand the effects of pine trees on the ecosystems they are planted in by researching their impact on hydrology.
The study was Francis’s main MPhil research with Farrick serving as his supervisor.
Farrick explained that he was always curious about the ecological impacts the pine trees would have as they were not native.
He added, “From just my experiences going to Mount Saint Benedict, for instance, you would see tons of pine needles on the forest floor with very little vegetation, or anything else growing.
“I started reading more, and while we had an idea for the ecological impacts, we had very little idea about the hydrology and how inevitably that's going to impact things like water resources.”
Farrick began searching for funding to carry out the research.
During that time, he was also in discussions with Francis, who wanted to pursue his MPhil at the department after successfully completing his undergraduate studies there one year earlier.
With Farrick eventually securing funding from the Campus Research and Publication Fund for the study, and Francis interested, the two started the study in 2017.
Using pine forests in St Ann’s as the sample population, Francis and Farrick turned their attention to data collection.
In the field, Francis measured the rate of rainfall interception, which can be defined as water captured by the canopy of the pines that would not reach the ground. He also measured the inception rate of the undergrowth below the pines, given that their thin needle covered branches allowed more sunlight in than native forests.
More hardy brushes and shrubs in the undergrowth of pine forests would also mean more water being intercepted before it reaches the ground.
But to compare how the inception rate of pine forests differed from other forests, Francis also measured the inception rate of native forests with native trees in the same area.
Describing the results, Francis said, “When we look at [the data collected] quantitatively, and given the amount of rainfall, there was a tendency for there to be more interception in the pines.
“The literature showed that pine needles and the bark of the pine trees have a greater capacity to hold more water and prevent water from getting into the soil.”
The “rugged” barks of pine trees also meant a greater surface area for water to be intercepted while the “smoother” barks of trees intercept less.
However, it should be noted that the inception rate is still dependent on the intensity of rainfall or the duration of rainfall.
Francis said estimates from the study show 4.3 millimeters of rainfall was needed in pine forests before any rainfall came through the canopy to the ground while only 2.6 millimeters were needed in the native forest.
“So less rainfall was needed before we would have rainfall penetrating down to the forest floor,” he said. “Looking at the leaf litter—which would be all the branches, leaves, twigs on the forest floor that would also intercept—this was thicker for most of the year in the pine forests and therefore had a greater interception rate because native forests had a greater rate of decomposition from our measurements.”
He added, “Overall, the pine forests intercept 30 percent more rainfall than the native forest.”
The study also looked at what happens when water reaches the pine forest floor and penetrates into the soil using water sensors at various depths in the soil.
Looking at the soil water repellency (the tendency for water droplets to remain on the soil surface before being absorbed by the soil), it was less likely for water to percolate through the pine forest floor, meaning it would remain on the soil for a longer period of time.
Because pine needles have a waxy chemical compound, it was theorised that when they break down and enter the soil, it helps to encourage soil water repellency to occur, especially in dry conditions and thick leaf litter.
On the real-world effects of the results gathered, Farrick explained, “You can have impacts on things like streams. Because the pine forests reduce the water reaching the forest floor, you're going to have less water making its way into streams and rivers and recharging groundwater. During drier periods with limited rainfall, this can also affect living organisms in these waterways.”
While the research may have ended in 2023, conversations are ongoing about the future of pine forests in the country.
Both Farrick and Francis agree the pines are not best suited for these ecosystems. However, they admit there is uncertainty about how to replace them and what to replace them with.
Francis explained, “Alternatives can be planted but then there's a lot of care that will be needed for those trees to get a certain level of maturity, and I don't know if the investment is there for that.”
“In these areas [where the pines are planted], there are consistent forest fires, so even if you replant the native species and you have a fire, there's a big likelihood that those native species aren't going to survive.
“If you don't have any trees, then you're going to have just bare soil which is worse than pine trees.”
