**3. Results**

#### *3.1. Modern Plant Species Richness Changes along the Elevational Gradient*

We observed an increasing richness of modern plant species from low-elevation areas to mid-elevation mountainous areas (Figure 2). Regarding the oasis and the downstream desert area from 200 to 700 m a.s.l., plant species richness was limited, ranging from 4 to 17 (Figure 2). The range of 200 to 400 m a.s.l. belongs to the oasis area, with a relatively high number of species; therefore, the observed plant species greatly fluctuated (Figure 2). From 700 m a.s.l. to the middle elevation, plant species richness showed an increasing trend, with a peak occurring in the forest belt at 1700 m a.s.l. (Figure 2). According to our field survey, the wet-preferring species *Picea schrenkiana* Fisch. & C.A.Mey. was dominating the forest belt, and the richness of the forest belt was mainly caused by the species richness of the understory, which can reach nearly 40 species (Figure 2).

**Figure 2.** The richness of modern plant species along the elevational gradient. The orange line represents the linear regression line.

#### *3.2. Surface Pollen Composition*

The surface pollen taxa within the range of 250–3500 m on the northern slope of the Tien Mountains were analyzed according to their richness changes along the elevational gradient (Figure 3). *Salix*, *Ephedra*, Caryophyllaceae, Asteraceae, *Thalictrum*, Ranunculaceae, and Rosaceae peaked in the range of 3500–2700 m. *Picea*, Poaceae, and Ranunculaceae reached a peak in the range of 2700–1700 m. *Betula*, *Artemisia*, *Salix*, *Tamarix*, *Ephedra*, *Thalictrum*, and Rosaceae peaked between 1700 and 700 m. *Ulmus*, Amaranthaceae, *Nitraria*, and *Tamarix* reached a peak below 700 m.

**Figure 3.** The richness of surface pollen taxa along the elevational gradient.

These taxa were further divided into the following four vegetation belts (Table 1). Alpine and sub-alpine meadows mainly included *Salix*, *Ephedra*, Caryophyllaceae, Asteraceae, *Thalictrum*, Ranunculaceae, and Rosaceae. These taxa can endure cold climates in the alpine zone. Conifer forest mainly included *Picea*, Poaceae and Ranunculaceae, where *Picea schrenkiana* was the dominant species favored by the wet climatic conditions. The typical steppe mainly contained *Betula*, *Artemisia*, *Salix*, *Tamarix*, *Ephedra*, *Thalictrum*, and Rosaceae. *Betula fruticosa* Pallas was located in the desert belt in front of the mountain, and is categorized as azonal vegetation. Desert vegetation contained *Ulmus*, Amaranthaceae, *Nitraria* and *Tamarix*. *Ulmus pumila* L. was distributed in the low-elevation river valleys, also belonging to the azonal vegetation classification. Amaranthaceae, *Nitraria*, and *Tamarix* are typically drought-tolerant species. Based on our division criteria, the richness of fossil pollen in the studied sedimentary site probably originated from different vegetation belts, which is soon discussed.



Moreover, in the modern ecosystem, the Shannon-Wiener index and the Simpson index consistently indicated the lowest pollen taxa diversity in the forest belt, while the highest pollen taxa diversity was in the alpine and sub-alpine meadows belt, followed by the typical steppe belt, and the desert vegetation belt (Table 2). According to the result of the Shannon-Wiener index, in the upper (2800–2600 m) and lower (1800–1600 m) boundaries and the center of the forest belt (2400–2100 m), the pollen taxa

diversities were 1.49, 1.65, and 1.21, respectively (Table 2). According to the result of the Simpson index, in the upper (2800–2600 m) and lower (1800–1600 m) boundaries and the center of the forest belt (2400–2100 m), the pollen taxa diversities were 0.63, 0.70, and 0.50, respectively (Table 2). Based on the results of the surface pollen analyses, we inferred that the pollen taxa diversity of the forest belt was generally low in the Tien Mountains, which was related to the very low pollen evenness due to the absolute dominance of *Picea* pollen in the forest belt (varying from 39% to 93%, with a mean of 67%) (Figures 2 and 3). Therefore, we inferred that the forest belt with very low pollen taxa diversity might affect the changes in pollen taxa diversity observed by the deposition sites during the Holocene, even though the forest belt had a larger plant species number than the belts of the typical steppe and desert vegetation (Figure 3).


**Table 2.** Vegetation belt and the pollen taxa diversity in the modern ecosystem.

*3.3. Pollen Taxa Diversity Change around Sayram Lake during the Holocene*

During the Holocene, belts of alpine and sub-alpine meadows, conifer forest and desert vegetation contributed to the pollen taxa diversity around Sayram Lake, while the belt of the typical steppe did not contribute, according to the regression relationships between pollen taxa diversity indicated by the Shannon-Wiener index and pollen richness from different vegetation belts (Figure 4). The linear regression showed that the conifer forest belt contributed the most to the pollen taxa diversity around Sayram Lake through the Holocene (*R*<sup>2</sup> = 0.40, *p* < 0.01), while contributions from the belts of alpine and sub-alpine meadows and desert vegetation were relatively small (*R*<sup>2</sup> = 0.25, *p* < 0.01 and *R*<sup>2</sup> = 0.37, *p* < 0.01, respectively) (Figure 4). This further suggests that pollen taxa diversity around Sayram Lake can reflect the forest belt change during the Holocene.

**Figure 4.** *Cont.*

**Figure 4.** Linear relationships between the observed pollen percentage from different vegetation belts and the Shannon-Wiener index. Notably, the horizontal axis represents the sum of the percentage of pollen taxa derived from different vegetation belts (from a to d) observed in the sedimentary sites (specific taxa contained in each vegetation belt are listed in Table 1). Panel (**A**) represents Sayram Lake, and panel (**B**) represents Aibi Lake.

During the warm and dry periods of the early Holocene, from 12,000 to 8000 years before present (yr BP) (Figure 5a,b), the conifer forest belt moved up, and the belts of alpine and sub-alpine meadows and typical steppe moved up as well (Figure 6a), because the observed pollen taxa diversity indicated by the Shannon-Wiener index around Sayram Lake was very high, reaching a mean of 1.56 (Figure 5c), which was close to the diversity index (1.65) of the lower boundary of the modern forest belt (Table 2). Moreover, the observed pollen taxa diversity indicated by the Simpson index around Sayram Lake was also very high, reaching a mean of 0.69 (Figure 5d), which was close to the diversity index (0.70) of the lower boundary of the modern forest belt (Table 2). Therefore, we infer that Sayram Lake was probably at the lower boundary of the conifer forest during the early Holocene, thus the forest-steppe ecotone moved upward accordingly, and was close to Sayram Lake.

**Figure 5.** Changes in pollen taxa diversity in the Tien Mountains and the Holocene climate. (**a**) Holocene temperature change for 30–90◦ N [29,30]. (**b**) Holocene moisture changes in arid central Asia [16]. Gray lines represent mean levels of pollen taxa diversity index during different periods around Sayram Lake and Aibi Lake (**<sup>c</sup>**–**f**). The yellow area represents the middle Holocene with low pollen taxa diversity.

During the middle Holocene, from 8000 to 4000 yr BP (Figure 5a,b), when the climate was warm and wet compared to the last stage, the conifer forest belt expanded and dominated, while the belts of alpine and sub-alpine meadows and the typical steppe were conversely compressed (Figure 6b), as the observed pollen taxa diversity indicated by the Shannon-Wiener index around Sayram Lake was very low, reaching a mean of 1.22 (Figure 5c), which was close to the diversity index (1.21) in the center of the modern forest belt (Table 2). In addition, the observed pollen taxa diversity indicated by the Simpson index around Sayram Lake was also very low, reaching a mean of 0.51 (Figure 5d), which was close to the diversity index (0.50) in the center of the modern forest belt (Table 2). Therefore, Sayram Lake was probably in the center of the forest belt during the middle Holocene, and the forest-steppe ecotone probably moved downward accordingly (Figure 6b).

As the late Holocene climate became cold and wet from 4000 yr BP onwards (Figure 5a,b), the conifer forest belt moved downward, and the belts of alpine and sub-alpine meadows and the typical steppe moved down as well (Figure 6c), because the observed pollen taxa diversity indicated by the Shannon-Wiener index around Sayram Lake was relatively high, reaching a mean of 1.36 (Figure 5c), which was close to the diversity index (1.49) of the upper boundary of the modern forest belt (Table 2). In addition, the observed pollen taxa diversity indicated by the Simpson index around Sayram Lake was also relatively high, reaching a mean of 0.65 (Figure 5d), which was close to the diversity index (0.63) of the upper boundary of the modern forest belt (Table 2). Therefore, we infer that Sayram Lake was probably at the upper boundary of the conifer forest during the late Holocene, and the forest-steppe ecotone moved further down and away from Sayram Lake (Figure 6c).

#### *3.4. Pollen Taxa Diversity Change Around Aibi Lake during the Holocene*

During the Holocene, the belts of alpine and sub-alpine meadows, conifer forest and typical steppe all contributed to the pollen taxa diversity around Aibi Lake, as the relationships between pollen taxa diversity indicated by the Shannon-Wiener index and the pollen richness from these belts were all positive (Figure 4). Notably, the belt of the conifer forest contributed the most to the pollen taxa diversity around Aibi Lake, according to the result of our linear regression (*R*<sup>2</sup> = 0.13, *p* < 0.05), followed by the alpine and sub-alpine meadows belts (*R*<sup>2</sup> = 0.12, *p* < 0.05) and the typical steppe belt (*R*<sup>2</sup> = 0.09, *p* < 0.05) (Figure 4). This further suggests that pollen taxa diversity around Aibi Lake can reflect the forest belt change during the Holocene.

During the warm and dry periods of the early Holocene from 12,000 to 8000 yr BP (Figure 5a,b), the conifer forest belt moved upward (Figure 6a), and the forest belt contributed less to the low-elevation deposition site of Aibi Lake and no longer dominated its diversity change, as the observed pollen taxa diversity was high, with a value of 1.35 indicated by the Shannon–Wiener index (Figure 5e). As such, we infer that the forest-steppe ecotone moved upward accordingly.

During the middle Holocene from 8000 to 4000 yr BP, when the climate was warm and wet compared to the last stage (Figure 5a,b), the conifer forest belt expanded and dominated (Figure 6b), and the forest belt heavily affected the low-elevation deposition site of Aibi Lake, as the observed pollen taxa diversity was low, with a value of 1.30 indicated by the Shannon-Wiener index, and a value of 0.62 indicated by the Simpson index (Figure 5e,f). Therefore, we infer that the forest-steppe ecotone moved downward accordingly.

As the late Holocene climate became cold and wet from 4000 yr BP onwards (Figure 5a,b), the conifer forest belt moved downward, but its area became smaller with low contribution compared to the last stage, because the observed pollen taxa diversity was high, with a value of 1.46 indicated by the Shannon-Wiener index, and a value of 0.64 indicated by the Simpson index (Figure 5e,f). Therefore, we infer that the forest-steppe ecotone moved further down accordingly.

#### *3.5. Comparison between the Two Lakes*

The regression relationships between the pollen taxa diversity calculated using the Shannon-Wiener index and the pollen richness from di fferent vegetation belts showed that pollen taxa diversities around Sayram Lake and Aibi Lake can reflect the forest belt change during the Holocene. When the forest belt expanded, *Picea* pollen dominated, leading to a decrease in the degree of species evenness in the conifer forest belt, because the pollen taxa diversity observed at the two sites was very low.

In general, the pollen taxa diversities around Sayram Lake and Aibi Lake had the same pattern of change in response to the Holocene climate change (Figure 5). In the process of coping with the warm and dry climate during the early Holocene, the forest belt moved upward (Figures 5 and 6a), as Sayram Lake had a high observed pollen taxa diversity (Figures 5 and 6a). In addition, Aibi Lake's pollen taxa diversity was also high, with less contribution from the upward forest belt (Figures 5 and 6a). Therefore, the forest-steppe ecotone moved upward accordingly during the early Holocene. In response to the warm and wet climate during the middle Holocene, the forest belt widened (Figure 6b), as Sayram Lake had very low observed pollen taxa diversity (Figures 5 and 6b). In addition, the *Picea schrenkiana*-dominated forest belt heavily influenced the low-elevation deposition site of Aibi Lake, as the observed pollen taxa diversity was low (Figures 5 and 6b). Hence the forest-steppe ecotone moved downward accordingly during the middle Holocene. In response to the cold and wet climate during the late Holocene, the conifer forest belt moved further downward (Figure 6c), as Sayram Lake had a high observed pollen taxa diversity (Figures 5 and 6c). In addition, Aibi Lake had a high observed pollen taxa diversity, with less contribution from the reduced forest belt (Figures 5 and 6c). Thus, the forest-steppe ecotone moved further down accordingly during the late Holocene.
