Introduction
In recent decades, human activities have caused a significant loss of
global biodiversity due to their impact on climate and
vegetation1,2.
In some “hotspots”, vegetation diversity variations have led to major
changes in ecosystem function, stability, and
serviceability3,4.
The establishment of global conservation priorities and an understanding
of the alteration processes in different environments have become
necessary to focus the limited economic resources on those areas with
the highest protection value and that are most threatened by
environmental
changes5,6.
As demonstrated by several studies, long-term metal mining and
processing have disturbed mountain ecosystems and altered the succession
of plant communities in many regions. For example, in the mountains of
Eastern Europe (Alps)7,
North America (Appalachian
Mountains)8, and South
Africa9, as well as in
other mineral-rich mountain ranges, extensive mining has triggered the
spread of potentially toxic elements (PTEs, such as Cd, Zn, Cu, Pb, Ni,
Hg, and Cr), which profoundly alter the local environment for the growth
of plant
species10,11,12.
However, some researchers argued that the effect of PTEs on vegetation
succession was shown to affect plant species richness within limited
small
areas12,13.
Other studies on habitats and abiotic factors have demonstrated that
regional level variations (e.g., climate patterns and topographic
heterogeneity) may lead to greater changes in community succession
processes14-16. For
example, the effects of PTE transport can overlap with rapidly changing
climatic conditions along elevation gradients, causing variations in
plant diversity and ecosystem
function17,18and, eventually, concealing the effects of PTEs on
succession19,20,21.
Understanding the potential additive and interactive effects of climate,
topography, and PTEs on biodiversity and ecosystem functions (ESFs) at
multiple spatial scales is therefore crucial to comprehending the
ecological consequences of abandoned mines
worldwide10,20,22.
Although some authors have been highlighted the influence of PTEs
transport in small-scale experiments or in study regions with narrow
climatic
gradients8,9,23,
most mining ecosystems still lack long-term biophysical and ecological
data sets for assessments of possible ecosystem responses. Ideally, the
prediction of climate-related ecological changes requires the
simultaneous consideration of multiple climate drivers and the
assessment of the mid- to long-term temporal changes to capture the
complex, interacting
responses24.
“Space-for-time” substitutions, also known as ergodic gradient
studies, allows for quantitative predictions to be realized in the
absence of long-term data sets or a detailed mechanistic knowledge of
possible responses to climatic
changes25,26.
This approach uses multiple sites across environmental gradients (i.e.,
the elevation gradient) to predict the time trajectory in ecological
changes, which is considered to be causally correlated with the changes
across the gradient24.
Studies employing this approach have been applied in paleoenvironmental
investigations of
pollen27, altitude
changes in bird
distribution28 and
biodiversity changes in land use29.
The objective of this study was to quantify the combined impact of
climate, mountain topography, and PTEs on plant biodiversity and
ecosystem functions through a broad survey of the multiregional mining
sites of the Qilian Mountain metallogenic belt in the northeastern
Tibetan Plateau (China) (Supplementary materials Fig. 1, Table 1).
Within an elevation gradient rising from 700 to 5,808 m above sea level
(a.s.l.) and an extension of approximately 800 km, the Qilian Mountains
are a metal-rich mountain range that covers several major natural
ecosystems of temperate regions, from low-elevation desert to mountain,
mountain shrub-steppe, and alpine grassland, including high-elevation
desert ecosystems. Based on the “space-for-time” method, a multimodel
framework was proposed to assess the statistical distribution of
ecological responses arising from exposure to one or more metal
stressors along different elevation gradients (1500-3600 m a.s.l) and
determine which elevation zone (or climate condition) has habitats that
are more vulnerable to PTE threats. The importance of a series of
environmental covariates correlated with ecological risks, PTE
transport, and species richness patterns was quantified. For instance,
elevation and ecological risk caused by PTE transport were found to be
significantly correlated in all the elevation gradients (r = 0.68).
Climate variables such as the mean annual precipitation (MAP, r =
-0.92), mean annual temperature (MAT, r = 0.79), transpiration rate
(ET0, r = 0.45), and wind speed (win10, r = -0.83) were found to be
correlated with elevation. Among the topographic factors, both slope (r
= 0.41) and aspect (r = 0.40) were correlated with the distribution of
the ecological risk related to mines. Afterward, we selected four
variables to characterize the intensity of PTEs (ecological risk index,
mining years, closure period, and the relative proportion of mining
area/plant/factory in the surrounding landscape) and calculated their
normalized indicators (PTE intensity). Finally, we compared the
statistical support of generalized additive models (GAMs), including the
individual and interactive effects of PTEs, climate, and topography, on
biodiversity and ecosystem functions.