Non-native plant species climb mountains in a different way to native species

Invasive plants typically start in lowlands, yet they’re capable of spreading uphill. Yet higher elevations tend to be colder than the lowlands, and have thinner soils. The changing conditions pose different challenges to a plant. To see how plants tackle those challenges, Paul Kühn and colleagues have been examining plants on Tenerife along an elevational gradient ranging from 55 to 1925 metres above sea level. They found that while the native plants showed intraspecific variability (variation of traits within a species), the non-native plants did not. So how were they also climbing in elevation.

“While most terrestrial ecosystems are affected by biological invasions, mountains represent a rare exception in that only recently non-native plant species have been documented to expand there, and only a small proportion of these become dominant,” write Kühn and colleagues. For this reason invasive species in mountain environments have been less studied than in lowlands.

The team put down the lack of invasion to a combination of circumstances. The first being that getting propagules (seeds or spores) uphill is difficult. The other is that the plants in higher elevations resist the interlopers. Yet they note that increased human activity will help transfer plants to higher ground. With transport to new locations, invasive species should be capable of gaining a toehold in higher habitats.

Kühn and colleagues proposed three hypotheses, one that as plants rise in elevation, they should adopt more conservative growth strategies, but that non-native species will be less conservative, compared to native species at the same altitude. The next was that non-native species should have a wider range of intraspecific variation than the native species. Finally, they proposed that non-native species will cover a larger trait-space than native species.

To test their hypothesis, the team surveyed site on Tenerife. Tenerife, in the Canary Islands is dominated by the volcano of Mount Teide. It rises to a height of around 3,700 metres, and has a variety of vegetation on its slope. “The natural vegetation along the southern slope begins with coastal and thermophilous scrub in the areas up to 1000 m a.s.l., transitions to forests of Canary pine (Pinus canariensis) up to 2000 m a.s.l. which are followed by high mountain scrub on the central plateau of Las Cañadas from 2000 to 2500 m a.s.l. The alpine regions close to the summit are only inhabited by a few specialized plant species,” write Kühn and colleagues.

Working with the MIREN (Mountain Invasion REsearch Network) project, the team surveyed a number of plots parallel to roads leading up the mountain. To make sure they were comparing like with like, the scientists surveyed the plants when they were flowering or fruiting. So the survey started at low elevations and worked its way up the slope. This was to ensure that the survey was measuring fully-developed plants, and not baking delayed development of higher plants into the results.

The results didn’t always support the hypotheses proposed by the botanists. For example, while there was a trend to more conservative growth in higher habitats, this was not clear-cut, said Kühn and colleagues. “Interestingly, although the native species studied here did not reach such high elevations as the non-native species, their trait shift towards a more conservative growth strategy was more pronounced compared to the non-native species, which did not show a clear response to elevation. Therefore, our results only partly support our hypothesis, namely that both native and non-native species display trait changes along the elevational gradient, but not that the magnitude of trait changes is similar.”

One reason for the difference might be due to how the plants arrived at the survey sites, said the team. The non-native plants are thought to be recent colonists, and so a mixture of plants that seeded there and new arrivals from lower down. This might have caused the relative lack of adaptation in the survey plants. In contrast, the native plants are assumed to have naturally colonised the sites, and so have already been environmentally filtered for their traits.

The team say this same re-colonisation process would explain why traits varied more at lower altitudes for non-native species, compared to the native plants.

However, when it came to the hypothesis that the non-native plants were simply more variable, this proved not to be the case. In fact the analysis showed that the non-native plants were only covering a fraction of the strategies that the native plants used. This result came as a surprise.

“There is little evidence that intraspecific trait variability is associated with the success of non-native species to spread towards higher elevations. Rather, it seems that being able to express a certain set of traits is more useful for the successful upwards spread of non-native species than having larger intraspecific trait variability than native species, similar to the conclusions Murphy et al. (2016) drew in their study,” write the authors.

The results show how non-native invasive species could be moving in to areas that have, until now, been refuges for biodiversity. But the results could also show some hope in protecting areas, the team conclude. “Along elevational gradients, environmental filters gradually restrict the functional suitability of non-native species. This might make it increasingly difficult for non-native species to propagate through the different vegetation types along the gradient and bridge the gap between abruptly changing habitat types, for example crossing the treeline or percolating away from the road into natural plant communities.”