Most of us will never see the vibrant Gomortega keule—an endangered flowering tree that produces sweet, lemon-colored fruits. The narrow slice of Chilean forest where G. keule abides is being cleared for farms and industrial pine plantations, and even if the remaining trees survive, G. keule faces many obstacles on the road to recovery. Seed germination has a low success rate, vegetative propagation is difficult, and planted trees grow slowly.
You would think that protecting native forest from timber plantations would be a top priority, as would habitat restoration near remaining G. keule stands. But these aren’t the options discussed in a recent paper by Oxford University plant biologist David Boshier and his colleague Tonya Lander, of the National Institute of Agronomic Research in France.
In a paper published by Current Biology, the researchers make a counter-intuitive case for pine plantations. If it weren’t for these non-native industrial forests, G. keule populations might not be as fragmented and reliant upon long-distance pollination. Yet when comparing clear cuts, small farms, and pine plantations, the latter may offer an unexpected ecological function—according to the findings, pine plantations increase the likelihood of G. keule pollination simply because pollinators pass through them quickly.
“We found that pollinators travelled with the highest probability through plantation, the lowest through clearfell, and with moderate [probability] through agriculture and native forest,” says Lander.
The findings are based on a survey of a ten-by-eight kilometer area in central Chile. The researchers collected over 1,000 seeds from 144 G. keule trees and then used DNA microsatellites to determine paternity.
Microsatellites contain a repeating DNA sequence (like CACACA), and the number of repeats often varies within a population. An individual might inherit three repeats from their mother and six on the homologous chromosome from their father. A different individual might inherit four and seven repeats, respectively. This is why microsatellites can be used to trace lineage.
The researchers collected seeds from the mother trees, and after microsatellite techniques identified the most likely fathers, a line was drawn on a map between the parent trees, representing the shortest distance the pollinator could have traveled. The proportion of the line that crossed over pine plantation as opposed to agriculture and clear cuts was then calculated. It turns out that G. keule trees separated by pine plantations are 26.8 times more likely to be pollinated than those separated by clear cuts. They are 1.4 times more likely to be pollinated than trees sandwiching farmland.
Lander and Boshier devised what they call the “Circe Principle” as an explanation for these unexpected results. It’s not surprising that pollinators avoid clear cuts, but why do they travel through pine forest as opposed to flower-filled farms?
Much like Odysseus was lured away from his epic journey by the enchanting food at Circe’s table, the researchers speculate that farms distract insects. After the pollinators fly in, they may never want to leave. Pine forests, in contrast, contain fewer stopping points, facilitating long-distance pollination—a potential boost for a fragmented species like G. keule.
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“If there are not enough individuals in a given population to breed effectively, then long distance pollination between populations may be beneficial,” says Lander.
As to improving G. keule’s status, Lander and her colleagues suggest reducing the size of clear cuts. They also propose converting clear cuts to agriculture—a strategy with a controversial precedent. While agroforestry techniques might incorporate G. keule, a few native trees in a shelterbelt or home garden can’t replace fully functional forestland. And as logging in the US has demonstrated, the farms and vineyards planted in clear cuts can pave the way for housing within a few generations—all sources of revenue for logging companies that over-harvest terrain.
Instead of clear cutting, it might be more sustainable to selectively harvest. The authors don’t consider this option in their paper, but Lander says, “it makes sense that pollinators might use areas of a selectively harvested plantation in the same way that they use a plantation, so they would be more likely to travel through it than through clearfells.”
As to which management practice would bear the most fruit, reducing the fragmentation of native G. keule forests through restoration and conservation could minimize the need for long-distance pollination. This could also benefit pollinators, which prefer to forage and nest in a central place, and might be taxed by a trek through pine forest. The researchers don’t say how much G. keule pollination is carried out by each species, and pine plantations may not be permeable to all pollinators alike. In this light, the findings might be species specific—at least when it comes to the connectivity created by pine forests.
Depending on the preferences of different pollinators, it may be that corridors of intact native habitat offer a comparative advantage. The authors suggest pollinators travel with a lower probability through native forest, but this relates to a different question: do successful fertilizations occur when the beginning and ending legs of the journey—which take place in native forest—are shorter or longer? In either case, the pollinator must fly between two disconnected G. keule populations. The researchers did not survey continuous, un-fragmented native habitat, but this type of landscape may support more robust populations of G. keule and its pollinators.
Despite these uncertainties, the findings shed new light on the corridor design process. Many studies classify land as habitat or non-habitat. For example, when a city encroaches on a mountainside, the habitat up top contrasts with the pavement below. This can isolate populations that once interbred, and bridging these islands is a driving motivation for urban green corridors.
Yet Lander’s findings show that not all landscapes can be classified as habitat or non-habitat, and some of the grey areas may perform specific functions. The trick is anticipating the unique services offered by a patch of open space. For example, when trying to bring pollinators to fragmented G. keule stands, a pine plantation would be more beneficial than a clear cut. If the goal is to create foraging habitat for declining pollinators, corridors might be more successful if they include farms.
While the study does not show that farms and plantations have an advantage over corridors of intact native forest, the findings support the need for a broader vision of connectivity—one that considers the role of diverse landscapes along with the continuity of open space.