Plant biomass allocation and driving factors of grassland revegetation in a Qinghai-Tibetan Plateau chronosequence

Biomass allocation is a key factor in understanding how ecosystems respond to changing environmental conditions. The role of soil chemistry in the above- and belowground plant biomass allocation in restoring grassland is still incompletely characterized. Consequently, it has led to two competing hypotheses for biomass allocation: optimal partitioning, where the plants allocate biomass preferentially to optimize resource use; and the isometric hypothesis, which postulates that biomass allocation between roots and shoots is fixed. Here we tested these hypotheses over a chronosequence of alpine grasslandsion undergoing restoration in the Qinghai-Tibetan Plateau, these range from severely degraded to those with 18 years of revegetation with an intact grassland (as a reference). A high proportion of biomass was allocated to the roots in the revegetated grasslands, and more biomass to shoots in the degraded and intact grasslands. The grasslands gradually decreased their root to shoot ratio as revegetation continued, with the lowest value in year 18 of revegetation. Our results showed that aboveground biomass (AGB) was increased by available phosphorus (P), soil moisture, and negatively related to bulk density, while belowground biomass (BGB) was positively impacted by total P and negatively by nitrate nitrogen (N). The trade-off between them was positively associated with available P and nitrate-N, and soil nutrient availability is more linked to increased AGB relative to BGB. Our study indicates that biomass allocation is highly variable during the revegetation period from degraded grassland, and is linked with soil properties, thus supporting the optimal partitioning hypothesis.</p

Effects of 5-Year Nitrogen Addition on Species Composition and Diversity of an Alpine Steppe Plant Community on Qinghai-Tibetan Plateau

The N deposition rate is notably increased in China, especially in the Qinghai-Tibetan Plateau (QTP). How plants respond to the projected N deposition on the alpine steppe is still in debate. In this study, to investigate the effects of N deposition on the plant community of the alpine steppe, we simulated N deposition at six different N addition rate levels (0, 8, 24, 40, 56, 72 kg N ha&minus;1 y&minus;1) from 2015 to 2019. Species composition and diversity were investigated as the assessment indices. The results showed that the importance value of grasses significantly increased with the increase of the N addition rate, while that of forbs significantly decreased. A high N addition rate (72 kg N ha&minus;1 y&minus;1) induced species composition change, making Leymus secalinus become the most dominant species within the entire plant community. Compared with the control (without N addition), species richness, Shannon&ndash;Weiner diversity, Simpson dominance and Pielou Evenness were significantly reduced under a high N addition rate. The changes of plant diversity in the alpine steppe were closely correlated with dynamics of soil nutrients, especially total carbon (TC), total phosphorus (TP) and ammonia nitrogen (NH4-N). Our findings suggested that a high N deposition rate (72 kg N ha&minus;1 y&minus;1) could significantly change plant composition and reduce the diversity of the alpine steppe, though they were less affected by low N deposition rates at present. With the increase of the N deposition rate, plant composition and diversity of the alpine steppe may be negatively affected in the future. In addition, Leymus secalinus is more competitive than other species with an N deposition rate increase. Soil C, soil P and soil NH4-N variation induced by N deposition might play a key role in regulating changes in plant composition and diversity in the alpine steppe. In addition, longer term field investigation needs to be carried out to testify to this phenomenon with the increase of N deposition in the future

Impacts of Short-Term Grazing Intensity on the Plant Diversity and Ecosystem Function of Alpine Steppe on the Qinghai&ndash;Tibetan Plateau

Livestock grazing is the primary land use of grasslands worldwide. Grazing has been asserted to alter grassland ecosystem functions, such as productivity, nutrient cycling, and biodiversity conservation. However, few studies have focused on the impact of grazing intensity on the ecosystem multifunctionality (EMF) of alpine grasslands. We conducted a field experiment of manipulating sheep grazing intensity effects on alpine steppe by surveying plant community characteristics and ecosystem functions. Our results showed that plant community composition was altered with increasing grazing intensity, and the dominant species shifted from grasses and sedges to forbs. EMF was the highest under no grazing (CK) and the lowest under heavy grazing (HG), but there was insignificant difference between CK and HG. HG significantly decreased some indicators that reflected nutrient cycling functions, such as soil available nitrogen, plant leaf nitrogen (PN) and phosphorus content (PP). Furthermore, plant diversity had strong correlations with SOC, total nitrogen (TN), and PN. The results could provide scientific bases for biodiversity conservation and sustainable grazing management of alpine steppe

Target species rather than plant community tell the success of ecological restoration for degraded alpine meadows

Ecological conservation and restoration projects in Three-river Headwater Region (TRHR) have been implemented to respond to the serious degradation of alpine meadow ecosystem. Compared with the substantial amount of studies on the recovery of plant communities, soil quality and ecosystem services, few studies have focused on the restoration performance of target species. Therefore, we conducted this study to analyze the dynamics and underlying mechanisms of species richness and aboveground biomass of plant communities and target species. The results showed that restoration actions (grassland cultivation and fencing) did not significantly improve the community aboveground biomass of the degraded alpine meadows, while the actions did significantly promote community species richness, target species richness and target species aboveground biomass. Community species richness (20%-45%) was significantly impacted by the restoration strategies, and community aboveground biomass (31%-53%) was impacted by the mean annual precipitation and mean annual temperature. The species richness (57%) and aboveground biomass (63%) of the target species were directly and indirectly impacted by restoration actions, climatic factors, biotic factors and soil factors. The implications of this study stress that target species should be highlighted in assessing the restoration success of degraded alpine meadows. Integrating climatic factors, target species and other biotic and abiotic indicators could help us better understand and evaluate restoration actions for degraded alpine meadows in the TRHR or other similar regions worldwide

Short-term grazing changed temporal productivity stability of alpine grassland on Qinghai-Tibetan Plateau via response species richness and functional groups asynchrony

The stability of grassland is pivotal in maintaining the grassland functioning and services. However, the knowledge about the consequences of grazing on stability of grassland productivity is limited. Based on a 3-year manipulated experiment with two grazing regimes (i.e., continuous grazing and rotational grazing) along four levels of grazing-intensity (0, 4, 6, and 8 sheep per hectare) on the Qinghai-Tibetan Plateau, we researched the responses of temporal stability of plant community and functional groups of alpine grasslands to different grazing management regimes. We found that grazing regimes, intensity and their interaction affected the temporal stability of aboveground net primary productivity of alpine grassland (ANPP), and intermediate rotational grazing significantly reduced the temporal stability of ANPP, whereas increased the stability of productivity of functional group of forbs. The response species to continuous grazing and rotational grazing are different, for grasses and forbs respectively. Rotational grazing and intermediate continuous grazing increased the diversity of response species. Structural equation modeling further revealed that functional groups asynchrony stabilized the productivity of the alpine grassland ecosystem under grazing in the Qinghai-Tibetan Plateau. These results showed that the rotational grazing and intermediate continuous grazing can maintain the productivity stability, and highlighted importance of regulating the relationships among functional groups for promoting the stability of alpine grassland under grazing management