The multivariate index of ecosystem multifunctionality.
To quantify the overall differences in ecosystem functions, we
calculated the multivariate index of multifunctionality of the average
change in ecosystem functions in the PTE-polluted habitats compared to
the natural habitats (Fig. 6). The
multivariate index of multifunctionality was best explained by the
climate-PTEs-topography interaction model (Supplementary Materials Table
9, R2 = 0.86). However, the patterns of dissimilarity
in the multivariate index of multifunctionality could not fully explain
the dissimilarity structure of the species communities (Fig. 1d and Fig.
6), indicating that the relationship between species composition and
ecosystem functioning in actual ecosystems was more complex than
expected. Across the elevation zones, the models showed that the changes
in ecosystem multifunctionality (i.e., the average changes in the
ecosystem functions of the PTE-polluted habitats compared with those of
the natural habitats) decreased with PTE intensity (Fig. 5b (1)).
However, as we took both climate and topographic factors into account,
the opposite trend of ecosystem multifunctionality with m-PTE intensity
(m-PTE is a complex variable taking all PTEs, topography, and climate
variables into account) was observed (Fig. 5b (2)). This finding
indicated that the changes in ecosystem multifunctionality would deviate
from the actual situations if climate factors and topographic variables
were not included as explanatory variables. In detail, in the four
studied climate zones of the Qilian Mountains, the impact of climate and
other factors on ecosystem multifunctionality was higher in PTE-polluted
habitats of the relatively humid mountain grasslands and alpine deserts
than in the arid low-elevation desert steppes (Fig. 5c). Moreover,
stronger changes were observed for the soil-mediated ESFs than for the
plant-mediated ESFs (Supplementary Materials Fig. 5 a-d), and a higher
change in the soil-mediated ESFs occurred in the lowland mine areas
(both desert and grassland) (Supplementary Materials Fig. 5 e, f).
———————-
Place Figs 5-6 here
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The results of our study demonstrate the importance of fully considering
climatic and topographic impacts while assessing the changes in plant
diversity caused by mining activities in different elevation zones of
mountain ranges. We highlighted that the presence of PTEs can unevenly
affect plant diversity trends along elevation gradients. First, compared
with natural habitats, the plant turnover in PTE-polluted habitats at
mid-elevation sites may be greater than that at sites in any other
climate zone of the Qilian Mountains. Indeed, at mid and low elevations,
species habitats have a higher spatial coincidence, with many species
cooccurring in narrow geographical
ranges38,32,33.
Moreover, extreme climatic conditions (e.g., hurricanes and heavy
precipitation) have a greater impact on the habitats of mid- and
low-elevation species, where plant richness is higher and more sensitive
to PTE sediments. Second, speciation increases in the habitat mosaic of
topographically complex
areas34,35,.
Interestingly, we found that in the mountains with very high peaks and
more rugged terrain, changes in the PTE deposition in the topsoil were
more likely to promote the upslope/downslope movement of rare species,
leading to their disruptive selection and
displacement39,45(Supplementary Material Fig. 9). Similar phenomena have occurred
historically in other
contexts32,36,
for instance, when vegetation belts moved upslope during warm and wet
interglacial periods, leading to the fragmentation of populations and
genetic divergence. As temperatures dropped again in glacial episodes,
vegetation belts moved downslope, forcing secondary contact of
populations and leading to founder
effects32,36.
Third, the succession mechanisms of plants showed significant
differences in different PTE-polluted habitats. In the mine areas of the
lowland grasslands, several toxic and harmful herbaceous plants (i.e.,Stellera chamaejasme , Potentilla multicaulis, andPedicularis Linn) occupied the niche of the PTE-polluted areas in
the humid mountain grasslands (Supplementary Materials Fig. 7 and Table
8). Species evenness showed greater changes, especially in the sites
containing more ultramafic rocks (such as AR, an asbestos plant, XTS,
and multimetal mines), where ultrahigh magnesium content and low
phosphorus availability act as powerful and selective plant
filters11. In contrast,
in the mine areas located in the arid mountain desert, the large and
medium shrubs restricted the dispersal of PTEs and protected the nearby
small shrubs and grasses with high fertility, leading to the
establishment of new plant communities. In general, compared with the
vulnerable ecosystems in low-elevation dry deserts and warm grasslands,
the mine areas located in alpine deserts and cold humid grasslands may
have a stronger resistance to disturbance and may possibly support more
rapid restoration processes.
Despite the lack of time-series data for the Qilian Mountains, the
”space-for-time-approach” could explain the changes in biodiversity and
ecosystem functions caused by mining
activities37. Based on
the long-term effect of mining activities over the broad elevation
gradients of the mountain range, the models we proposed demonstrated,
from a holistic perspective, the importance of climate change on the
impact of metal pollution on different spatial and temporal scales.
Although previous studies have considered the critical importance of
local adaptation along natural elevation gradients, they only explored
the potential effects of zinc (Zn) and climate change (20 and 24 ℃) on
certain species (Ischnura
elegans )23. Our
models, instead, took into account the majority of the potential drivers
(12 variables) that shape the contemporary patterns of biodiversity and
generated predictions, which we tested with independent data. More
particularly, the models showed that the impact on biodiversity and
ecosystem functions depends largely on the original climatic environment
and geological conditions. In vulnerable mining ecosystems, the
processes driven by climatic or other factors may present contrary
effects on species richness and vegetation cover, which may conceal the
actual succession effects and habitat expansion or contraction at
landscape level due to the presence of PTEs. From a long-term
perspective, at larger spatial scales (hundreds of kilometers)
encompassing a range of ecosystems and climate types, the importance of
topography declines, while broad-scale climatic variations become
substantially more
important38. Given that
most mining areas in the Qilian Mountains, as well as in other mountain
ranges, have been closed for only some decades and that the community
succession in PTE-polluted habitats cannot usually reach a stable level
after decades or even hundreds of
years39, our findings
have worrying implications for the impacts of global climate change. As
a consequence, we strongly suggest considering the role of global
climate change when developing management strategies for plant diversity
conservation in the mine areas of high-altitude mountain ranges.
Acknowledgements We thank the Land and Resources Bureau,
Environmental Protection Agency (EPA) (Qinghai and Gansu Province), and
the Qilian Mountain National Park authority for their support, and for
granting us access to the Qilian Mountain National Park; as well as all
of the companies who allowed us to work and helped to collect data at
mine areas of the Qilian Mountains. This work was supported by the
Second Tibetan Plateau Scientific Expedition and Research Program (STEP)
(Grant No. 2019QZKK1003), the Strategic Priority Research Program of
Chinese Academy of Sciences (Grant No. XDA20040301), the National
Natural Science Foundation of China (Grant No. 41890824), and CAS
Interdisciplinary Innovation Team-Global Change Hydrology.
Author contributions J.Z.L and H.W.L designed the concept for
ecological research of mine areas in Qilian Mountains. M.L.B. and L. H.
developed the mathematical theory. J.Z.L and A.G. processed the data of
plant diversity. J.Z.L, H.W.L., T.C.Y., P.P.T., S.S. F., Q.Y., Q.W.N.,
Y.Y.Y., C.Y., M.T., W.F., Y.X.X and F.P.Y. established study sites and
collected data. J.Z.L., S.S.F, M.T., Y.Y.Y., Q.W.N. and C.Y. conducted
the chemical analysis of the plant and soil samples. T.C.Y., P.P. T and
Q.Y. processed the topography and climate data of the Qilian Mountains.
All authors contributed to the subsequent drafts.
Competing interests The authors declare no competing interests.