Plant water sources are controlled by climate type and root depth

Recently, the research group of researcher Chen Hongsong of the Institute of Subtropical Agroecology, Chinese Academy of Sciences found that unlike the characteristics of transpiration water age (reflecting the time source of plant water use) of plants in non-karst key zones, the average transpiration age of typical plants in karst key zones is the smallest (less than 30 days), which is related to the rapid hydrological process of complex geotechnical structure mastering in karst key zones.

Through the comparison of different climate zones, the research group found that in the relatively humid climate zone, plant transpiration mainly depends on the water source replenished by the precipitation of the month (accounting for about 60% of the annual water source). In relatively arid climates, plant transpiration relies heavily on water sources recharged from precipitation in previous months/seasons. The results were recently published in Water Resources Research.

Conceptual diagram of plant transpiration age estimation model. Photo courtesy of interviewee

Compared with the non-karst key zone with thicker soil layer, the water storage capacity in the same depth range is obviously low, coupled with strong water permeability and high risk of karst drought, which seriously restricts the growth and sustainable recovery of vegetation in this area. Therefore, finding the source of plant water use is the key to understanding the adaptation mechanism of plants in the key karst zone to the environment of “more rocky, less soil and less water”.

Water source is the core of plant water adaptation mechanism, different from the spatial source of plant water, the temporal source of plant water is to indicate when plants use precipitation to replenish water sources, which can comprehensively reflect the adaptation mechanism of plants to precipitation changes. Especially in the context of global climate change, it is unclear how changes in precipitation patterns affect plant water use patterns and their adaptation mechanisms.   

Previous research by Chen Hongsong’s group showed that the depth of water sources directly affects the water potential regulation and drought vulnerability of karst lithophytes. However, due to the challenge of complex underground rock and soil structure and water source sampling in the key karst zone, it is difficult to accurately quantify the spatial source of plant water utilization. From the perspective of the time source of plant water use, a model to quantify the water retention time of plant root zone was proposed, and it was found that the diversion phenomenon of precipitation in plant root zone was widely present in different key zones and climatic environments.

In order to further study the adaptation mechanism of plants to the complex geotechnical structure and rapid hydrological process in the key karst zone, Chen Hongsong’s research group estimated the proportion of water replenished by precipitation (new water) in the root zone of plants by using stable isotope tracing technology based on the ratio of new and old water in the root zone, and calculated the seasonal variation of plant transpiration water age by piecewise linear mixed water age model based on the ratio of new and old water in the root zone. The seasonal dynamics and influencing mechanisms of transpiration age of typical plants under different climate types were comparatively analyzed.

This study reveals that different key zone structures regulate precipitation infiltration, recharge and retention time by affecting plant root depth distribution and root zone water storage capacity, which in turn affects the transpiration age and water adaptation mechanism of plants. Studies have shown that climate type is the main factor affecting the average water age of plant transpiration, but it is also regulated by root depth. In addition, the influence of evaporation on the stable isotope of the root water source was considered, and the correction problem of the evaporation effect in the study of the water source of stable isotope tracer plants was solved.

The study also indicates that plants in the key karst zone have a unique water adaptation mechanism to the rapid hydrological processes dominated by complex geotechnical structures, but plants with this adaptation strategy may face severe drought stress and die under the condition of no precipitation replenishment for a long time.

The research was supported by the National Natural Science Foundation of China, the Youth Fund and other projects. (Source: China Science News, Wang Haohao, Luo Zidong)

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