Effects of grazing on photosynthetic features and soil respiration of rangelands in the Tianshan Mountains of Northwest China

Grazing reduces plant sexual reproduction but increases asexual reproduction: A global meta-analysis

Grazing affects grasslands worldwide. However, the global patterns and general mechanisms of how grazing affects plant reproductive traits are poorly understood, especially in the context of different climates and grazing duration. We conducted a meta-analysis of 114 independent grazing studies worldwide that measured plant reproductive traits in grasslands. The results showed that the number of tillers of plant increased under grazing. Grazing did not affect the number of reproductive branches of forbs, but significantly reduced the number of reproductive branches of grasses. Grazing increased the number of vegetative branches of all plants and reduced the proportion of reproductive branches. Grazing significantly reduced the number of flowers in forbs. Seed yield in the two plant functional groups was reduced compared with no-grazing. Under grazing, the sexual reproduction of grasses decreased much more substantially than that of forbs. This may be due to biomass allocation pattern of grasses under grazing (i.e., belowground versus aboveground). Under grazing, plants tended to adopt rapid, low-input asexual reproduction rather than long-term, high-risk sexual reproduction. This study represents the first large-scale evaluation of plant reproductive trait responses under grazing and demonstrates that grazing inhibits sexual reproduction and promotes asexual reproduction. The effect of grazing on plant sexual reproduction was influenced by grazing intensity, mean annual precipitation, and grazing duration. These results will assist in the development of sustainable grazing management strategies to improve the balance between human welfare and grassland ecosystem health.

Grazing Intensity Alters Leaf and Spike Photosynthesis, Transpiration, and Related Parameters of Three Grass Species on an Alpine Steppe in the Qilian Mountains

The effect of grazing on leaf photosynthesis has been extensively studied. However, the influence of grazing on photosynthesis in other green tissues, especially spike, has remained poorly understood. This study investigated the impact of different grazing intensities (light grazing (LG), medium grazing (MG), and heavy grazing (HG)) on leaf and spike photosynthesis parameters and photosynthetic pigments of three grass species (Stipa purpurea, Achnatherum inebrians, and Leymus secalinus) on an alpine steppe in the Qilian Mountains. Grazing promoted leaf photosynthesis rate in S. purpurea and L. secalinus but reduced it in A. inebrians. Conversely, spike photosynthesis rate decreased in S. purpurea and L. secalinus under intense grazing, while there was no significant difference in spike photosynthesis rate in A. inebrians. The leaf and spike net photosynthetic rate (Pn) and transpiration rate (Tr) in S. purpurea were the greatest among the three species, while their organ temperatures were the lowest. On the other hand, grazing stimulated leaf chlorophyll biosynthesis in S. purpurea and L. secalinus but accelerated leaf chlorophyll degradation in A. inebrians. Furthermore, spike chlorophyll biosynthesis was inhibited in the three species under grazing, and only L. secalinus had the ability to recover from the impairment. Grazing had a positive effect on leaf photosynthesis parameters of S. purpurea and L. secalinus but a negative effect on those of A. inebrians. However, spike photosynthesis parameters were negatively influenced by grazing. Among the three species investigated, S. purpurea displayed the greatest ability for leaf and spike photosynthesis to withstand and acclimate to grazing stress. This study suggests that moderate grazing enhanced leaf photosynthetic capacity of S. purpurea and L. secalinus but reduced it in A. inebrians. However, spike photosynthetic capacity of three grass species decreased in response to grazing intensities.

Soil respiration and its Q10 response to various grazing systems of a typical steppe in Inner Mongolia, China

Background

As one of the important management practices of grassland ecosystems, grazing has fundamental effects on soil properties, vegetation, and soil microbes. Grazing can thus alter soil respiration (Rs) and the soil carbon cycle, yet its impacts and mechanisms remain unclear.

Methods

To explore the response of soil carbon flux and temperature sensitivity to different grazing systems, Rs, soil temperature (ST), and soil moisture (SM) were observed from December 2014 to September 2015 in a typical steppe of Inner Mongolia under three grazing systems: year-long grazing, rest-rotation grazing, and grazing exclusion. In addition, plant aboveground and root biomass, soil microbial biomass and community composition, and soil nutrients were measured during the pilot period.

Results

Soil respiration was significantly different among the three grazing systems. The average Rs was highest under rest-rotation grazing (1.26 μmol·m⁻²·s⁻¹), followed by grazing exclusion (0.98 μmol·m⁻²·s⁻¹) and year-long grazing (0.94 μmol·m⁻²·s⁻¹). Rs was closely associated with ST, SM, potential substrate and root, and soil microbe activity. The effects of grazing among two grazing systems had generality, but were different due to grazing intensity. The root biomass was stimulated by grazing, and the rest-rotation grazing system resulted in the highest Rs. Grazing led to decreases in aboveground and microbial biomass as well as the loss of soil total nitrogen and total phosphorus from the steppe ecosystem, which explained the negative effect of grazing on Rs in the year-long grazing system compared to the grazing exclusion system. The temperature sensitivity of Rs (Q₁₀) was higher in the rest-rotation and year-long grazing systems, likely due to the higher temperature sensitivity of rhizosphere respiration and higher “rhizosphere priming effect” in the promoted root biomass. The structural equation model analysis showed that while grazing inhibited Rs by reducing soil aeration porosity, ground biomass and SM, it increased Q₁₀ but had a lower effect than other factors. A better understanding of the effects of grazing on soil respiration has important practical implications.

Long-Term Overgrazing-Induced Memory Decreases Photosynthesis of Clonal Offspring in a Perennial Grassland Plant

Previous studies of transgenerational plasticity have demonstrated that long-term overgrazing experienced by Leymus chinensis, an ecologically dominant, rhizomatous grass species in eastern Eurasian temperate grassland, significantly affects its clonal growth in subsequent generations. However, there is a dearth of information on the reasons underlying this overgrazing-induced memory effect in plant morphological plasticity. We characterized the relationship between a dwarf phenotype and photosynthesis function decline of L. chinensis from the perspective of leaf photosynthesis by using both field measurement and rhizome buds culture cultivated in a greenhouse. Leaf photosynthetic functions (net photosynthetic rate, stomatal conductance, intercellular carbon dioxide concentration, and transpiration rate) were significantly decreased in smaller L. chinensis individuals that were induced to have a dwarf phenotype by being heavily grazed in the field. This decreased photosynthetic function was maintained a generation after greenhouse tests in which grazing was excluded. Both the response of L. chinensis morphological traits and photosynthetic functions in greenhouse were deceased relative to those in the field experiment. Further, there were significant decreases in leaf chlorophyll content and Rubisco enzyme activities of leaves between bud-cultured dwarf and non-dwarf L. chinensis in the greenhouse. Moreover, gene expression patterns showed that the bud-cultured dwarf L. chinensis significantly down-regulated (by 1.86- to 5.33-fold) a series of key genes that regulate photosynthetic efficiency, stomata opening, and chloroplast development compared with the non-dwarf L. chinensis. This is among the first studies revealing a linkage between long-term overgrazing affecting the transgenerational morphological plasticity of clonal plants and physiologically adaptive photosynthesis function. Overall, clonal transgenerational effects in L. chinensis phenotypic traits heavily involve photosynthetic plasticity.