Temperate Steppes Thrive Under Mower’s Blade

Is all human interference in ecosystems bad? In a study published in Functional Ecology, Zhang and colleagues investigated the effect of mowing on ecosystem stability. Probing how grassland environments react over both short-term (4 years) and long-term (16 years) periods, they looked at mowing impacts at varying heights on the temperate steppes of Inner Mongolia. Surprisingly, the team discovered that increased mowing frequency and shorter stubble height boosted the stability of these grassland ecosystems. They found that the physical damage caused by mowing could kickstart the plants’ defence systems, leading to greater diversity and stability in the plant populations.

The term ecosystem stability refers to the capability of an ecosystem to withstand shocks or disturbances, maintain its functional processes, and continue to support biodiversity. The research points towards the importance of plant defence traits in preserving the stability of mowed ecosystems. When plants feel stressed or threatened, they respond by activating physical or chemical lines of defence. For instance, the scientists found that the abundance and diversity of plant traits such as lignin, flavonoid, and phenol increase in response to mowing. Lignin helps provide plants with their structural integrity, while flavonoids and phenols assist in plant defence mechanisms against pests and diseases.

With mowing, plant species, like the forb species Artemisia frigida and Potentilla acaulis, display higher lignin concentrations. These plants not only stand tall against the physical stress of mowing but also prove adept at thriving in the cut-short communities. These findings align with previous research that showed metabolite concentrations in trees increased following defoliation, leading to greater stability of the ecosystems they belonged to.

Zhang and colleagues researched a temperate steppe, a grassy plain, in Inner Mongolia, China, to understand how biodiversity affects ecosystem stability. This area is characterized by its semi-arid climate, with an average temperature of 2.9°C and precipitation mainly occurring from May to October.

The researchers focused their study on three perennial (long-lasting) grasses: Stipa krylovii, Leymus chinensis, and Cleistogenes squarrosa, along with two perennial forbs, Artemisia frigida and Potentilla acaulis. These species accounted for around 65% of the overall plant biomass in the area during the experiment, which ran from 2016 to 2019.

The team conducted this research at the Research Station of the Institute of Botany, Chinese Academy of Sciences. Here, mowing experiments were conducted annually during August, which is typically when biomass is at its peak. They experimented with various stubble heights after mowing and compared their effects in both “short-term mowing”, initiated in 2015 and “long-term mowing”, begun in 2003. The researchers then gauged the impact of these activities on the stability of the ecosystem over time.

Each year, from 2016 to 2019, the researchers sampled plant above-ground biomass by removing all plants at the soil surface level. This allowed them to estimate the Above-ground Net Primary Productivity (ANPP), a measure of the quantity of energy that the plants of an ecosystem make available for other organisms.

In addition, the researchers determined ecosystem stability, species richness (the number of different species in a given area) and asynchrony (the idea that fluctuations in different species may cancel each other out, leading to a stable total biomass) as part of the study. 

They also collected fresh leaf samples to examine flavonoid and phenolic compounds: these are key chemical substances plants use to defend themselves from threats. Additionally, other plant defence traits were statistically analyzed to understand their impact on ecosystem stability.

The results show that both short-term and long-term mowing decrease the expected annual net primary production (ANPP) of a plant community. In layman’s terms, ANPP is essentially the plant world’s version of the economy – it measures the ‘output’ a plant community generates in a given year – it’s a great indicator of health. The decrease in primary productivity due to mowing patterns is amplified further when the height of remaining vegetation after mowing (stubble height) is decreased.

Interestingly, the duration of mowing has different effects on native plant species or ‘biodiversity. Continuous, or long-term, mowing has been discovered to increase the biodiversity of plant species within the community significantly. This, in turn, promotes ecosystem stability – how well the ecosystem can maintain and recover its functions in the face of disturbances. Contrary to the conventional wisdom that human interference damages ecosystems, this work suggests that certain interventions, like mowing in this case, might actually fortify them. Zhang and colleagues write: 

Our results support our first hypothesis that mowing with lower stubble height tended to increase ecosystem stability due to lower inter-annual variation in ANPP, thereby increasing the temporal stability of plant community productivity. As expected in our second hypothesis, an increase in mowing duration increased ecosystem stability because mowing duration significantly decreased the standard deviation (σ) of ANPP. Further analyses revealed that plant defence traits are important predictors of ecosystem stability, which supports our third hypothesis. Our results that plant defence traits are involved in maintaining mowing-induced ecosystem stability greatly expand the theory that higher species richness, asynchrony, population and dominant species stability contribute to greater ecosystem stability.

Zhang et al. 2023

However, the specifics matter. It’s not indiscriminate mowing the researchers endorse, but rather a sustainable mowing regimen of 10-cm stubble height for the temperate grasslands of Inner Mongolia, a conclusion drawn from field mowing experiments conducted over short-term and long-term periods. While mowing may work on the steppe, at that height it’s not time to give up on No Mow May yet.

READ THE ARTICLE
Zhang, L., Bai, W., Zhang, Y., Lambers, H. and Zhang, W.-H. (2023) “Ecosystem stability is determined by plant defence functional traits and population stability under mowing in a semi‐arid temperate steppe,” Functional Ecology. Available at: https://doi.org/10.1111/1365-2435.14401.