3.1 AUC value
The AUC of the training and test sets of the distribution prediction model of the six mangrove species in Beibu Gulf ranged from 0.912 to 1 (Fig. 3). The AUC values of the training and test sets of the distribution prediction model for all mangrove species as a whole were 0.882, 0.869, respectively (Fig. 4). Based on the AUC of the test set, the simulation of the MaxEnt model for mangroves was deemed accurate. Thus, the model was highly reliable and could predict the distribution of the dominant mangrove species in Beibu Gulf of Guangxi.
3.2 Analysis of dominant environmental factors
Factors affecting mangrove habitat included bioclimate, topography, marine salt data, SST, substrate type, and land-use type. The contribution of variables based on interactions between different environmental variables (Hu et al. 2020; Liu et al. 2019).
We investigated the test results of different factors affecting mangrove distribution (Fig. 5) to ascertain the dominant factors affecting the distribution of specific species. The dominant factors affecting overall mangrove distribution in Beibu Gulf were elevation, wetland index, mean temperature of warmest quarter, and substrate type. (Fig. 5). Subsequently, we analyzed the factors affecting specific mangrove species distribution in Beibu Gulf.
We investigated the importance of various environmental factors on the distribution probability of each of the six selected mangrove species. The three environmental factors imperative to the geographical distribution of Aricennia marina were elevation, mean sea surface salinity in the coldest season and maximum temperature of warmest month (Fig. 6a), with their cumulative contribution rate accounting for 50.7% and the other 13 environmental factors accounting for 49.3% of its distribution (Fig. 7). Among the remaining factors, the total contribution of bioclimatic factors, topography, sea surface salinity, SST, substrate type, and land-use type accounted for 39.1, 34, 16.6, 1.2, 6.1, and 2.9%, respectively. Based on the contribution and importance of various environmental factors for predicting the distribution of Aricennia marina, elevation limited optimal planting areas, while the mean salinity of the sea surface in the coldest season reflected the salt preference of this species. The contributions of land-use type and SST to the distribution of Aricennia marina were relatively low.
Regarding Aegiceras corniculatum, the three environmental factors imperative to its geographical distribution were elevation, wetland index, and substrate type (Fig. 6b), with their cumulative contribution accounting for 41.7%, and the remaining 13 environmental factors accounting for 58.3% of its distribution. Among the remaining factors, the total contribution rate of bioclimate, topography, sea surface salinity, bioclimate, SST, substrate type, and land-use type accounted for 36.3, 39.6, 18.9, 0.1, 2.1, and 3.1%, respectively (Fig. 7). The contribution of marine salinity in predicting the distribution of Aegiceras corniculatum was greater than that for Aricennia marina, which indicates that Aegiceras corniculatum is more sensitive to salinity than Aricennia marina. The contribution of SST, substrate type, and land-use type to the distribution of Aegiceras corniculatum was relatively low.
For Kandelia obovata, the three environmental factors most important to its geographical distribution were elevation, substrate type, and wetland index (Fig. 6c), with a cumulative contribution percentage of 42.3%. Among the dominant influencing factors, elevation reflected the restrictions on the planting location of Kandelia obovata. In addition, possible this species were relatively sensitive to substrate type.
Regarding Bruguiera gymnorrhiza, the three environmental factors with the largest effect on its geographical distribution were maximum temperature of the warmest month, precipitation in the warmest quarter, and substrate type (Fig. 6d), accounting for 79.1% of the cumulative contribution. Among the influencing factors,
the total contribution rate of bioclimate, topography, sea surface salinity, SST, substrate type, and land-use type accounted for 76.9, 4.1, 0.7, 0.4, 15.8, and 1.2%, respectively. These results show that Bruguiera gymnorrhiza was sensitive to bioclimate and substrate type.
For Rhizophora stylosa, the three environmental factors that most affected its geographical distribution were precipitation of the warmest quarter, substrate type, and mean temperature of the warmest quarter (Fig. 6c), together accounting for 82.9% of the cumulative contribution. Among the factors influencing the distribution of this species, the contribution rate of bioclimate, topography, sea surface salinity, SST, substrate types, and land-use type accounted for 78.3, 6.6, 2.9, 0.4, 11.9, and 0%, respectively. These results indicate that the distribution of Rhizophora stylosa is sensitive to bioclimate.
Finally, for Acanthus ilicifolius, the three environmental factors with the greatest importance for its geographical distribution were substrate type, mean sea surface salinity in the warmest season, and wetland index (Fig. 6f), with their cumulative contribution representing 72.5%. The significance of substrate type reflects the substrate preference of Acanthus ilicifolius, while the significance of wetland index reflects the limitations of topographic conditions on Acanthus ilicifolius. Furthermore, the importance of mean sea surface salinity in the warmest season reflects the sensitivity of Acanthus ilicifolius to salinity, with too high salinity levels possibly affecting the species growth.
3.3 Range of environmental factors affecting mangrove habitat suitability
The variables representing the mangrove suitability factors in Beibu Gulf were selected based on the logistic mode of the MaxEnt model. Correlations between habitat suitability and environmental variables were analyzed using a probability distribution logic output value of 0.5 as the boundary. This enabled us to investigate the optimal thresholds for the main environmental variables affecting the six selected mangrove species (Table 2).
The suitable elevation threshold ranges for the growth of mangrove plants were determined. The minimum suitable elevation values of the plants are ordered from lowest to highest: Aricennia marina, Aegiceras corniculatum, Rhizophora stylosa, Kandelia obovata, Bruguiera gymnorrhiza, and Acanthus ilicifolius. The lowest elevation value suitable for the growth of Aricennia marina was − 0.84 m, whereas the lowest elevation value suitable for the growth of Acanthus ilicifolius was relatively high (Fig. 8).
Table 2 Thresholds of the dominant environmental factors affecting the distribution of six investigated mangrove species.
Order | Mangrove species | Three dominant environmental factors | Limitation |
1 | Aricennia marina | Elevation | -0.84–1.27 m |
Mean sea surface salinity in the coldest season | 16.41–25.31‰ |
Max. temperature of warmest month | 32.1–32.3°C |
2 | Aegiceras corniculatum | Elevation | -0.68–2.02 m |
Wetland index | 4.11–9.81 |
Substrate type | Mixed mudflat |
3 | Kandelia obovata | Elevation | -0.50–1.88 m |
Substrate type | Mixed mudflat |
Wetland index | 4.49–8.33 |
4 | Bruguiera gymnorrhiza | Max༎temperature of warmest month | 32.3–32.4°C |
Precipitation in the warmest quarter | 638–753 mm |
Substrate type | Mixed mudflat |
5 | Rhizophora stylosa | Precipitation in warmest quarter | 723–746 mm |
Substrate type | Mixed mudflat |
Mean temperature of warmest quarter | 28.7–28.9°C |
6 | Acanthus ilicifolius | Substrate type | Mixed mudflat |
Mean sea surface salinity in the warmest season | 3.39–7.37‰ |
Wetland index | > 5.28 |
A number of trends were observed for the wetland index (Fig. 9). Compared to other mangrove species, Aricennia marina had the lowest minimum wetland index, as it can grow in the low tide zone. In contrast, Acanthus ilicifolius had the largest minimum wetland index indicating that it can grow in the high tide zone.
Regarding the average salinity of the sea surface in the coldest season, Kandelia obovata had a relatively wide range (5.91–17.92‰; Fig. 10). According to the highest suitable value of mean sea surface salinity in the coldest season, the investigated species can be ordered from highest to lowest as follows: Rhizophora stylosa > Bruguiera gymnorrhiza > Aricennia marina > Aegiceras corniculatum > Kandelia obovata > Acanthus ilicifolius. Regarding the most suitable value of mean sea surface salinity in the warmest season, the species can be ordered from highest to the lowest as follows: Aricennia marina > Rhizophora stylosa > Kandelia obovata > Bruguiera gymnorrhiza > Aegiceras corniculatum > Acanthus ilicifolius. The most suitable range of mean sea surface salinity in the coldest season for Aricennia marina was 16.41–25.31‰, and the most suitable value was 24.27‰. The suitable range of the average sea surface salinity for Acanthus ilicifolius in the warmest season was 3.39–7.37‰, and the most suitable value was 4.07‰.
3.4 Suitable areas for mangroves in the Beibu Gulf of Guangxi
Based on the findings of Hu et al. (2020), we divided potential mangrove areas into four groups based on their suitability as follows: best (> 0.7), medium (0.7–0.5), low (0.2–0.5), and unsuitable (0–0.2). Based on the investigation of six mangrove species in Beibu Gulf as the overall input for the model (Fig. 11g), we obtained an optimal suitable area of 13,816 hm2. The areas of high fitness are located primarily in the Dandou Sea on the west side of the Shatian Peninsula in the southeast of Hepu County, Guangxi; Tieshan Port, Qinzhou Bay, and the Dafeng River in the center of the Guangxi coast; Fangcheng Bay on the west of the Guangxi coast; and the Shankou Mangrove National Nature Reserve. The mangroves are concentrated in the Shankou Mangrove National Nature Reserve, Beilun Hekou Mangrove National Nature Reserve, and Maoweihai Mangrove Autonomous Region Nature Reserve (Fig. 11g).
With regard to individual species, the size of the optimum suitable area for Aricennia marina was 10,341 hm2 (Fig. 11a, Table 3), with areas of high fitness primarily distributed in Tie Shan Gang, the Beihai Golden Bay mangrove reserve, and along the open coasts in the south of the Beihai National Wetland Park and Beilun Estuary Mangrove National Nature Reserve. The size of the optimal suitable area for Aegiceras corniculatum was 13,154 hm2 (Fig. 11b), with highly suitable areas primarily distributed in estuaries, such as Lianzhou Bay, the Maoweihai Mangrove Autonomous Region Nature Reserve, and the Dafeng River.
The optimal area for Kandelia obovata was 10,672 hm2 (Fig. 11c), with highly suitable areas distributed along Qinzhou Bay, the Dafeng River, and Beilun Estuary Mangrove National Nature Reserve. The size of the optimal area for Bruguiera gymnorrhiza was 2565 hm2 (Fig. 11d), with highest fitness areas for this species primarily distributed in the Dandou Sea area of the Shankou Mangrove Reserve, Yingluo Port, and Beilun Estuary Mangrove National Nature Reserve. The extent of the most suitable area for Rhizophora stylosa was 1158 hm2 (Fig. 11e), with optimal areas for this species distributed primarily in the Dandou Sea area and Yingluo Port of the Shankou Mangrove Reserve, but very few in other regions of the study area. The size of the most suitable area for Acanthus ilicifolius was 4054 hm2 (Fig. 11f), with areas with the highest fitness for this species primarily distributed in regions with low estuarine salinity in Lianzhou Bay and the Shankou Mangrove Reserve.
Table 3 Suitable mangrove areas.
Mangrove species | Optimal suitable area (hm2) | Medium suitable area (hm2) |
Aricennia marina | 10,341 | 39,875 |
Aegiceras corniculatum | 13,154 | 37,063 |
Kandelia obovata | 10,672 | 20,682 |
Bruguiera gymnorrhiza | 2565 | 4385 |
Rhizophora stylosa | 1158 | 3226 |
Acanthus ilicifolius | 4054 | 6949 |
Beibu Gulf contains three mangrove nature reserves, a National Wetland Park, with a total area of 15,794 hm2, and a mangrove area of 3977 hm2. The mangrove protection rate is 42.62%. Based on the analysis of the superposition of the distribution of protected areas and potentially suitable mangrove areas, 49.10% of the most suitable areas are included within protected areas (Fig. 12). Eight areas were identified as priority areas for mangrove protection and restoration. The vacant mangrove protection areas in Beibu Gulf are primarily distributed in Lianzhou Bay, along the Dafeng River, the East Bay of Fangchenggang, and Tieshan Harbor.