US Forest Service and Puerto Rico Natural Resource vehicles block the entrance of the closed El Yunque National Forest damaged by the passing of Hurricane Maria. (AFP photo)

When I visited El Yunque National Forest in Puerto Rico, six months after Hurricane Maria, the roadside debris still hadn't been removed. Power to many rural areas hadn't been restored. Many people had left the island to escape these conditions, and a study published in the New England Journal of Medicine estimates that several thousand people may have died because of the storm.

But if you want to study what happens when hurricanes hit tropical forests -- which they are doing with increasing frequency -- then El Yunque is where you want to be. The forest here was hit hard by Hurricane Hugo in 1989, had damage from Georges in 1998, was side-swiped by Irma in early September 2017 and then floored two weeks later by Maria and her 225-km-per-hour winds.

These forests have evolved with hurricanes, so they are remarkably resilient. Since the 1940s, foresters then ecologists and biogeochemists at El Yunque have been staking out their study plots, delineating them with surveyors' pin flags and white PVC pipe. The forest is further festooned with mesh baskets to measure how many seeds and leaves fall from trees and plants, flagging tape to mark study areas that are monitored regularly, small solar collectors to run equipment, and instruments for recording data, including soil nutrients, temperature, humidity, tree diameters, and outputs of methane and nitrous oxide.

Forest ecologist Jess Zimmerman is its lead principal investigator of the Luquillo research site, and director of the El Verde field station -- one of two in the forest. Right after Hurricane Maria hit, while the rest of the island was reeling, the scientists were in a race against time to get back to their measurements.

Some of their equipment was buried by leaves and branches. The roof to the research building had been blown off, and trees were leaning on other buildings. After assessing the damage, Mr Zimmerman led a crew out to the forest. They used hand saws and grunt labour to open the trails to the study plots. They worked urgently, as a number of the experiments relied on timely data collection. The staff measures nitrate and potassium levels in the soil water every two weeks, and understanding Maria's effects on these nutrients was crucial. "It's samples like that you don't want to lose the rhythm to in an event like this," Mr Zimmerman explained.

The most striking thing about the forest in those first days was its new colour: brown. If you gazed up at a once verdant hillside, you might think it had been burnt -- all the green leaves were gone.

No mature tree, anywhere, can withstand 225-km-per-hour winds. Along with the leaves, something has to give, and it will be the weakest part of the structure: the roots, the trunk, or the crown. In temperate forests where hurricanes are a rarity most large trees are uprooted because the roots are the weak link. In the tropical forest at El Yunque, the roots held, some of the trunks were snapped, but nearly all of the crowns blew apart. The vast majority of the trees remained upright, even though they'd lost their limbs, branches, and leaves. It was as if someone had replaced trees with telephone poles.

Six months after the hurricane, the green had returned. The telephone poles had already transformed into bottle brushes -- their green leaves growing like bristles from the upper stem. Unlike mainland forests, a tropical forest has no dormant season, so the palm, the tabonuco, the ausubo, and all the other trees began sprouting branches and leaves almost immediately. The trees had no time to waste, because they continue to respire and the only way to replenish the storeroom of carbohydrates is through photosynthesis -- which requires leaves.

It's safe to assume that many of these trees stood through San Felipe II in 1928, the most recent hurricane comparable in strength to Maria. Their survival strategy is sacrificing their crowns, like sailors reefing their sails when the wind gets too rough. Any species less windfirm has been knocked off the mountain by now.

While the canopy's greening was spectacular, it was no match for what was happening on the ground, where a sea of verdant jade was flooding in around us. Mr Zimmerman pointed out the pioneer tree species soaking up the sun. These species cannot grow in the deep shade of a rainforest, but when a hurricane blows the canopy away, they seize the day. Seeds that were deposited in the soil decades ago suddenly have enough sunlight to germinate. They do so by the thousands. One of these pioneers, the cecropia, gathers light with a leaf so huge it could function as an umbrella.

"We're seeing a lot of herbs that we haven't seen," Mr Zimmerman said, pointing to heliconia, a wild plantain. "There are huge patches of these heliconia, and they were virtually non-existent before the storm… In two or three years there will be tons of shrubs in the understory. This is their time to grow, to flower, and to produce fruit to fill up the seed banks and to wait for the next hurricane. This is their opportunity to go."

But they are racing against the clock. Can any of the new trees grow fast enough to make it to the canopy? The 30% canopy cover of today will return once again to a full canopy, leading to light levels near the ground low enough to make photographers weep. And the pioneer species will indeed die young, starved of sunlight, though they will have made deposits in the seed bank.

The ecological concept of resilience is surprisingly straightforward. It is a measure of how long it takes for a system to return to pre-disturbance conditions after an event like a hurricane. The forests of El Yunque have shown that within 25 years, they can return to pre-hurricane conditions. Only time will tell if Puerto Rico's political, social, and economic systems will make that kind of recovery. ©2018 Zocalo Public Square