Population Structures and Dynamics of Rhododendron Communities with Different Stages of Succession in Northwest Guizhou, China.

To explore the population structures and dynamics of Rhododendron shrub communities at different stages of succession in northwest Guizhou, China, this study examined the populations of Rhododendron annae and Rhododendron irroratum shrub with two different stages. A space-for-time substitution was employed to establish the diameter class/height structures, static life tables, and survival/mortality rate/disappearance rate curves of both Rhododendron populations with different orders of succession. Their structural and quantitative dynamics were analyzed, and their development trends were predicted. The results showed that, quantitatively, the populations of R. annae and R. irroratum in the two Rhododendron communities with different orders of succession were dominated by age classes one, two, and three as well as height classes i, ii, and iii. The number of Rhododendron plants at the three age classes and the three height classes accounted for 97.61-100% of the total. The quantitative dynamic indices of R. annae and R. irroratum were both greater than 0, with and without considering external interference. In terms of age class and height structures, both Rhododendron populations were expanding populations, presenting "inverted-J-shaped" and irregular pyramid patterns. There was a sufficient number of young individuals, but few or no old individuals. Both survival curves of the populations of R. annae and R. irroratum in the two Rhododendron communities with different orders of succession belonged to the Deevy-II type. In the late stage of succession, the mortality curves and disappearance curves of both Rhododendron populations in these communities presented a trend of increasing first and then decreasing with increasing age class. This result indicates that at each age class, R. annae and R. irroratum showed a trend of gradual increase after two, four, and six years. In brief, the populations of R. annae and R. irroratum have rich reserves of seedlings and saplings, but high mortality and disappearance rates. In this context, it is necessary to reduce human interference and implement targeted conservation measures to promote the natural renewal of Rhododendron populations.

Seasonal resource selection of free-ranging Zhongwei goats in the semi-arid grassland.

Goats rarely move and forage randomly. They tend to move in ways generally influenced by biotic and abiotic factors, respectively. However, few studies have explored the foraging behaviour of goats in the absence of predation and human disturbance. Based on step selection function modelling framework, Normalised Difference Vegetation Index, vegetation surveys, and Global Positioning System tracking of 124 free-ranging domestic adult male Zhongwei goats over one year (2016-2017) were used to assess how biotic and abiotic environmental factors affected their spatiotemporal distribution, and developed a conceptual model to represent the goats' trade-off between forage quantity and preference at different seasons, in the semi-arid grassland of Loess Plateau of 1 178 hectare. The results showed that spatial distributions of goats responded to spatiotemporal variation of biotic factors rather than abiotic factors of elevation, slope and solar radiation, which indicated that biotic factors were of priority to abiotic factors in the foraging process for the goats. According to the season changing, the goats positively used areas with higher forage quantity in the spring and winter, areas of higher forage quantity and preferred species in summer, and areas of abundance of preferred species in autumn. We developed a model to describe the phenomenon that the goats selected areas with higher preferred species only when the forage quantity was plentiful, otherwise they selected areas with higher forage quantity. Better understanding of the patterns and drivers of spatiotemporal distribution of the goats can improve our ability to predict foraging behaviour of livestock in heterogeneous environment and lead to better management practices and policies for the sustainability of these semi-arid landscapes and associated ecosystem services.

Effects of fertilization gradient on the production performance and nutritional quality of cultivated grasslands in karst areas.

Optimal fertilization is an important measure for managing cultivated grasslands, and a necessary means for maintaining the nutrient balance, yield, and quality of grassland ecosystems. This study aimed to explore the effects of organic fertilizers on the production performance and nutritional quality of cultivated grasslands in karst areas. Two types of monocultured cultivated grasslands (i.e., Medicago sativa and Dactylis glomerata) were employed as the research objects, and a randomized block design was adopted to investigate the effects of five fertilization gradients on the forage height, coverage, yield, and nutritional quality of the cultivated grasslands. According to the results, the plant height, coverage, and yield of M. sativa first presented an increasing trend, then decreased with increasing fertilization gradient, with a peak at 20-30 t/hm2 fertilization gradient. The height, coverage, and yield of D. glomerata increased gradually with increase in fertilization gradient, and peaked at 40 t/hm2. Meanwhile, the crude protein (CP) and ether extract (EE) contents of both grassland types displayed first presented an increasing trend, then decreased with increasing fertilization gradient, and peaked at 10-30 t/hm2 fertilization gradient. The neutral detergent fiber (NDF) and acidic detergent fiber (ADF) contents of M. sativa presented "N-shaped" and "M-shaped" change trends with increasing fertilization gradient, while those of D. glomerata showed "V-shaped" and "M-shaped" change trends, reaching minimum values under fertilization gradients of 30 and 20 t/hm2, respectively. Year, fertilization, and year × fertilization (Y×F) significantly affected the plant heights, coverages, dry/fresh weight ratios, and yields of M. sativa and D. glomerata. The contribution of coverage to the subordinate function of M. sativa was greatest at a fertilization gradient of 20 t/hm2. Meanwhile, the subordinate function values of the height and coverage of D. glomerata increased gradually with increasing fertilization gradient, but the difference in the subordinate function value of height was only 0.09%-0.18% under the fertilization gradient of 20-40 t/hm2. Evaluation of forage nutrition revealed 10-30 t/hm2 and 20-30 t/hm2 as the optimal organic fertilizer application rates for M. sativa and D. glomerata, respectively.

Grazing alters the relationships between species diversity and biomass during community succession in a semiarid grassland.

The relationships between biodiversity and ecosystem functions (BEF) are crucial for ecosystem management. However, little is known about how grazing affects BEF relationships in the context of ecological succession. Here, using a 5-year experiment in a semiarid grassland of the Loess Plateau, China, we mainly focused on how grazing affected the relationships between plant species diversity and aboveground biomass (AGB) and explored the underlying mechanisms behind the relationships. In addition, we compared the plant dynamics of community composition and structure under no-grazing and grazing treatments during succession. We found that the plant species diversity-AGB relationship shifted from a negative-linear pattern in no-grazing to a humped-back model in grazing during plant community succession, suggesting that grazing could regulate dominant species and alter the availability of light resources to suppress competitive exclusion during succession. In addition, changes in annual plants over time played crucial roles in the BEF relationships. The increase in annual Salsola collina in this study, which also alters multiple mechanisms of plant interaction, had a significant effect on the negative-linear relationship both with and without grazing. On average, compared to no-grazing treatment, grazing significantly decreased the plant community density (39.53 %), cover (16.97 %), height (7.85 %), and AGB (9.35 %), but increased plant diversity, including species richness and the Shannon-Wiener index, and especially dramatically enhanced the Shannon-Wiener index (ranging from 1.55 to 2.13). These results underline the close association between grazing and the dynamics of plant communities in semiarid grasslands during succession. In particular, our findings further reveal grazing-dependent relationships between diversity and AGB, which have significant implications for the management and biodiversity conservation measures of semiarid grassland ecosystems.

Karst grassland forage quality and its determinants in Guizhou Province of Southwest China.

Forage quality is a key property of grassland ecosystems. In this study, grassland forage qualities were measured at 373 sampling sites throughout Guizhou Province in the karst mountain region of Southwest China, and the factors affecting it were explored. The forage quality level of most plant species was categorized into four levels: (1) preferred forage species; (2) desirable forage species; (3) consumed but undesirable forage species; and (4) non-consumable or toxic forage species. High temperature and precipitation appeared to facilitate the growth of preferred forage species, but limited the growth of other plants. Increasing soil pH had a positive impact on the number and biomass of preferred forage plants, but a negative influence on other plants, especially non-consumable or toxic plants. Both GDP and population density had a positive correlation with the number and biomass of preferred forage species, while such correlations for other levels of forage species tended to be negative. Grazing could lead to a decrease in the preferred forage species. Therefore, it is suggested that by focusing on soil improvement in grassland and maintaining an appropriate grazing intensity, global warming and rapid economic growth in Guizhou Province will likely contribute to increase the forage quality of karst grasslands in Southwest China.

Contribution of Fine Roots to Soil Organic Carbon Accumulation in Different Desert Communities in the Sangong River Basin.

This study explored the relationship between soil organic carbon (SOC) and root distribution, with the aim of evaluating the carbon stocks and sequestration potential under five plant communities (Alhagi sparsifolia, Tamarix ramosissima, Reaumuria soongorica, Haloxylon ammodendron, and Phragmites communis) in an arid region, the Sangong River watershed desert ecosystem. Root biomass, ecological factors, and SOC in different layers of a 0-100 cm soil profile were investigated. The results demonstrated that almost all living fine root biomass (11.78-34.41 g/m2) and dead fine root biomass (5.64-15.45 g/m2) levels were highest in the 10-20 cm layer, except for the P. communis community, which showed the highest living and dead fine root biomass at a depth of 60-70 cm. Fine root biomass showed strong seasonal dynamics in the five communities from June to October. The biomass levels of the A. sparsifolia (138.31 g/m2) and H. ammodendron (229.73 g/m2) communities were highest in August, whereas those of the T. ramosissima (87.76 g/m2), R. soongorica (66.29 g/m2), and P. communis (148.31 g/m2) communities were highest in September. The SOC of the five communities displayed strong changes with increasing soil depth. The mean SOC value across all five communities was 77.36% at 0-30 cm. The highest SOC values of the A. sparsifolia (3.08 g/kg), T. ramosissima (2.35 g/kg), and R. soongorica (2.34 g/kg) communities were found in June, and the highest value of the H. ammodendron (2.25 and 2.31 g/kg, p > 0.05) community was found in June and September. The highest SOC values of the P. communis (1.88 g/kg) community were found in July. Fine root production and turnover rate were 50.67-486.92 g/m2/year and 1.25-1.98 times per year. The relationships among SOC, fine root biomass, and ecological factors (soil water content and soil bulk density) were significant for all five communities. Based on the results, higher soil water content and soil conductivity favored the decomposition of root litter and increased fine root turnover, thereby facilitating SOC formation. Higher pH and bulk density levels are not conducive to soil biological activity and SOC mineralization, leading to increased SOC levels in desert regions.