Seasonal patterns of above- and belowground production
The growing season started earlier in the spring and lasted considerably longer in the autumn below ground than above ground in alpine grassland. Vegetation coverage showed a distinct peak in the middle of summer, while the root elongation intensity maximum was even more pronounced at the beginning of the season. Dynamics in seasonal belowground and aboveground production, such as relative productivity rates, were not synchronous, and peaks were found in other regions (Ernakovich et al., 2014; Blume-Werry et al., 2015; Cao et al., 2020). A loss of greenness with senescence correlated with a reduction in photosynthetic activity. Accordingly, root growth and turnover could be explained by the functional plasticity in response to changing environmental conditions (Warren et al., 2014; Hong et al., 2017). Higher specific root length in the nongrowing season (October-April) than in the growing season (May-September) has been proven (Cao et al., 2020). The phenomenon that fine roots take up nutrients and grow in early spring also corresponds to our own observations. Similarly, root elongation intensity during late summer and early autumn does not return to early summer rates in forest ecosystems (J. D. Joslin, 2001). One possible explanation for this is that carbon allocation to sugars and starch of fine roots reflects their role in structural support and storage in late summer (Keel et al., 2012).
Grassland primary production would be underestimated if above- and below-ground part at the same time, due to the pattern and timing of root production and high turnover rate. We found that grassland and tundra were less coupled in habitats with leaf and root production than forests were (Steinaker and Wilson, 2008). Root observations in our study were mainly of fine roots (<2 mm), which strongly influences both the seasonal potential for soil resources and water acquisition and the plant fluxes of root carbon into soil pools (Mccormack et al., 2014). Conceptually, the response of fine roots in inherent relative growth rate to short-term changes was sensitive to root traits and morphology (Iversen et al., 2014). Under the current climatic change, fine root responses among different ecosystem communities and whether these communities become more uniform should be understood in the future (Chen et al., 2018).
Abiotic parameters influencing shoot and root growth
Grassland primary production has been reported to be related to climate parameters such as temperature, but in our study, there was a significant positive correlation between soil moisture, not air temperature or precipitation, and canopy greenness from May to August. Soil temperature even offsets the leaf growth rate at the beginning of the season. Herbaceous plant leaf traits are affected by the key controls of soil in semiarid regions (Zhang et al., 2022). In contrast, the temperature of grassland roots and leaves increased significantly with soil temperature (Steinaker and Wilson, 2008). Therefore, warmer winters were not found to induce a later green-up. This may be due to the different effects of temperature on leaves, roots, and soil in alpine ecosystems, all of which are affected by the different key environmental factors. Continued increases in temperature enhanced precipitation transpiration and reduced soil moisture from May to July. For the alpine steppe, there was less soil moisture supply than in the alpine meadow, and the alpine steppe was more sensitive to water but not to temperature (Xu et al., 2016).
The cumulative precipitation of the early season had a lag effect on root traits, so it was not necessary for plants to increase their root length to capture more water as the growing season precipitation increased (Zhang et al., 2019). Soil water availability in the surface soil layer is generally more affected by evaporation than that in the deeper soil. Soil moisture in the subsurface layer (10-30 cm) was more closely correlated with root length. The layer effects were obvious at high altitudes or latitudes in grassland (Cao et al., 2020). Therefore, root biomass which is located in the upper soil was also significantly correlated with cumulative precipitation. A previous study showed that the relative productivity rate, renewal time, and contribution to total belowground biomass were higher at the subsurface (5-15 cm) than at the surface layer (5 cm), which experiences freeze−thaw events in spring (Garcia-Pausas et al., 2010).
Microclimate conditions due to freeze−thaw cycles
On the one hand, increases in the frequency and high temperatures of late-winter warming events are likely to trigger early episodes of soil thaw, possibly reducing the capacity of plants to take up nutrients in early spring (Chen et al., 2015). On the other hand, our results presented the storage effect of soil in late autumn, providing water to roots in early spring, and the effect of soil-vegetation-atmosphere has been proven using observations, numerical experiments, and models (Yang and Wang, 2019). The degradation of permafrost was presented as manifested by ground surface thaw settlement (Sun et al., 2022). The manipulation of freeze−thaw cycles enhanced the release of nutrients, enabling roots to acquire resources to maintain life (Henry, 2007; Song et al., 2017).
In alpine ecosystems, decreased water availability from earlier snow melt and warmer summer temperatures lead to earlier senescence, indicating divergent responses to climate change, particularly in the spring and fall shoulder seasons (Ernakovich et al., 2014). This agreement with our results may be due to the important role of the freeze−thaw process. Climate change alters the seasonal patterns of temperature and precipitation, including the soil microclimate conditions. The ways in which species and communities respond to the changes in the seasonal timing and synchrony of events, or seasonality (Warren et al., 2014), are central to understanding and predicting responses in community structure and composition to global changes.