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Measured from space, 2019–2025
How tall Romania's forests are, measured by a laser on the Space Station
Between 2019 and 2025, a NASA laser on the International Space Station took 1,449,120 valid summer measurements of Romania's dense forest. Every dot on the map is one of them, coloured by canopy height. The 5 km square with the tallest forest it measured lies in the Tarcău mountains, typically 39.8 m tall.
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How a laser measures the height of a forest
NASA's GEDI instrument was mounted on the International Space Station in December 2018. It has three lasers, each firing 242 pulses a second, along eight tracks 600 m apart. Each pulse lights a circle about 25 m wide on the ground, and the circles follow one another every 60 m along the track.
The light comes back first from the tops of the crowns, then from the foliage below and finally from the ground. The shape of that echo gives the height below which 98% of the returned light lies: the canopy height in that circle (the rh98 metric). On the map, zoomed in here over the Tarcău mountains, every circle is drawn at its true size.
From March 2023 to April 2024 the instrument was kept in storage elsewhere on the outside of the station, so the summer of 2023 is missing. What remains is 1,449,120 valid measurements of dense forest from 567 summer passes of the station over Romania.
On slopes, the laser reads the forest taller
On a slope the ground echo spreads out: the upper edge of the circle is higher than the lower edge, and part of the difference goes into the height read off the echo. Across all dense forest, typical height is 20.8 m on nearly flat ground and 31.0 m on slopes steeper than 30°. Part of that gap is real, because mountain forests are taller. But it remains within the same broadleaf forest at 600–1,000 m: 23.1 m on flat ground, 30.2 m on slopes steeper than 30°. How much is geometry and how much is taller trees, this data cannot separate.
So the height comparisons in the next two chapters, and the values for places, use only the 824,627 measurements on slopes of 15° or less. On the map, measurements on steeper slopes are grey dots.
Romania's tallest measured forests are in the Tarcău mountains
Dividing the country into squares about 5 km across and keeping only squares with at least 30 measurements from at least 5 different passes leaves 2,068 squares. Four of the top five, with typical heights of 38.2–39.8 m, are in the Tarcău mountains around the communes of Tașca and Tarcău in Neamț County; the third is near Mălini, in the Stânișoara mountains of Suceava. All five lie at 890–1,130 m, read from a surface model that includes the crowns.
None of these five squares lies in a national park, a nature park or a Natura 2000 site. The nearest virgin or quasi-virgin forest in the national catalogue is 12–19 km away. The ranking sees only forest on slopes of 15° or less; forest on steep mountainsides cannot enter it.
| Rank | Square of about 5 km, near | Typical height | Altitude | Measurements | Passes |
|---|---|---|---|---|---|
| 1 | Tașca (Neamț) | 39.8 m | 1,040 m | 483 | 6 |
| 2 | Tarcău (Neamț) | 39.0 m | 1,060 m | 454 | 8 |
| 3 | Mălini (Suceava) | 38.6 m | 910 m | 361 | 7 |
| 4 | Tarcău (Neamț) | 38.6 m | 1,130 m | 509 | 7 |
| 5 | Tarcău (Neamț) | 38.2 m | 890 m | 286 | 5 |
| 6 | Bușteni (Prahova) | 37.5 m | 1,020 m | 149 | 5 |
| 7 | Lunca Cernii de Jos (Hunedoara) | 37.2 m | 1,010 m | 94 | 5 |
| 8 | Sinaia (Prahova) | 37.0 m | 1,020 m | 128 | 5 |
| 9 | Tarcău (Neamț) | 36.9 m | 1,090 m | 156 | 7 |
| 10 | Toplița (Harghita) | 36.7 m | 1,040 m | 361 | 7 |
Virgin and quasi-virgin forests stand taller in every altitude band, from 400 to 1,600 m
Romania's national catalogue of virgin and quasi-virgin forests lists 445 complexes (groups of forest stands) with coordinates. Comparing only broadleaf forest on slopes of 15° or less, within the same altitude band, the forest inside the complexes is taller in every band: by 4.5 to 7.1 m counting every measurement, and by 2.4 to 7.6 m if each complex with at least 5 measurements counts once (10–35 complexes per band). Re-running the first comparison 500 times with complexes drawn at random, in 95% of the re-runs the gap stays above 1.4 m in every band.
The catalogue gives each complex a central point and an area, with no outline. In the calculation each complex is a circle of the same area around its point, so the edges are approximate. The rest of the forest comes from the whole country, not from the same valleys. The laser measures how tall the trees are; it cannot tell their age.
The same spot, measured twice
Sometimes two passes at least six months apart hit the same spot, with the centres of their circles no more than 10 m apart. All slopes count here: on the same spot, the slope is the same at both passes. 4,170 such spots in dense forest show how far a single measurement can be trusted. Where the global forest-loss map (Hansen) records no loss, half the pairs differ by 3.5 m or less, and 95% fall between −18.6 and +13.6 m.
In 71 spots, forest at least 15 m tall had dropped below 5 m by the second pass. Only 17 of them also appear on the Hansen map. The others may be felling or windthrow the map missed, or bad echoes. None of the 221 pairs measured less than six months apart dropped like this, but they are too few to show how often a bad echo looks like a felling.
The other way round, in the 123 spots where Hansen records loss in the years of the two passes, the laser found a typical height of 23.3 m at the second pass. The loss can lie a few tens of metres from the circle, or come later in the same year.
On the map, the orange dots are the 54 drops that do not appear on the Hansen map, and the green ones the 17 that do.