SLF botanists and volunteers recheck Graubünden's 1930s flora one hectare at a time

A resurvey is only as good as its baseline. This one goes back to the early 1930s, when the botanists Josias Braun-Blanquet and Eduard Rübel documented the flora of Graubünden ^[2]. Together, the 1,104 one-hectare squares cover about 1,104 hectares, or roughly 11 square kilometers ^[17]. With each square checked one to three times, that comes to between 1,104 and 3,312 plot visits in all ^[18]. "We can see where 2,500 plant species used to occur and where they're found today," said Christian Rixen, the SLF botanist who co-leads the project ^[4][16].

The comparison is designed to find three things: the elevations where species grew almost 100 years ago, which of them are shifting upward, and whether incoming competitors are displacing them lower down ^[12]. The third question decides how an upward record should be read. A species that gains sites at its upper edge and keeps its lower ones has expanded its range. One that also loses the lower sites has moved.

Disappearance is the hardest of these changes to confirm from plot surveys. Rixen visited a plot below the Jatzhorn, at 2,400 to 2,500 meters ^[6]. Two of his notable finds there were species that had gone unrecorded and then turned up: mountain buttercup after three years, and an alpine sedge that he said "hasn't been observed here for 13 years" ^[5]. Plant diversity varies a lot from square to square, depending on the mix of grass, rock, scree and stream on each ^[13]. With at most three checks per square ^[3], a plant missing from a list may still be growing on the slope.

Trees give the clearest sign of movement uphill. Besides the spruce, Rixen had found three larches at 2,450 meters ^[8]. He puts the treeline rise at about 400 meters in 50 years and blames increasingly warm summers ^[9]. Then he added: "But the trees are lagging behind, which is why it's interesting when we find individual specimens further up" ^[10]. The two statements only fit together if the 400 meters is where the climate now allows trees to grow, with the trees themselves still catching up. On that reading, the small spruce at 2,520 meters ^[7] is one of the trees catching up. Only later visits can show whether it survives.

Rixen also said warming does not help every species. Many plants need to build up a set amount of warmth over the year. In a hot summer like this one, he said, they cannot use all of it, and for them autumn starts earlier ^[11].

The thing this doesn't tell you is how much of any change is due to climate. Almost a century separates the two surveys ^[1]. Other things on a hillside can change in that time, grazing among them: on the morning of the visit, cows were grazing further down the slope ^[15]. Survey methods have changed too. Rixen, project coordinator Ingrid Jansen and the volunteers log plants in a smartphone app, and the InfoFlora app helps less expert participants identify species ^[14]. The account does not describe how the project corrects for differences in search effort between the 1930s botanists and today's teams. Until species-by-species elevation changes are published for all 1,104 squares, the case for climate-driven movement rests on Rixen's treeline figure and what he has seen on individual plots ^[9].