|其他摘要||Climate change models generally agree that sustained temperature increase will happen on the global surface, and shifts in precipitation will result in higher drought intensity and frequency in the near future. Under this climate change background, changes in vegetation will be driven by drought globally. Evidence of drought-induced tree mortality has been reported in recent years in many regions of the world，especially in arid and semi-arid ecosystems. Lacking understanding of the plant mortality mechanisms by drought will hamper the predictions of the vegetation dynamic under climate change background. There are two mainstream hypotheses explaining the plant mortality mechanisms: hydraulic failure and carbon starvation, proposed base on the water and carbon balance response of plants to drought, respectively. Time sequence of water and carbon associated variations and interaction of the two during drought ultimately determines the cause of death.
The Gurbantonggut Desert is the second largest desert of China. Haloxylon ammodendron, as the constructive species in Gurbantonggut Desert, has fortissimo barren tolerance, drought resistance and fine sand-fixation features that is of great significance to maintain desert ecosystem stability. In recent years, however, with the influence of climate change and human activities, both the distribution areas and coverage of H. ammodendron in the south rim of Gurbantonggut Desert are decreasing. When summer drought comes, H. ammodendron seedlings mortality increased with the reduced soil moisture availability, which will hamper the establishment, recruitment and distribution of this species. Knowing water and carbon dynamics in response to drought of H. ammodendron seedlings in the establishment period is helpful to understand the mortality mechanism. In addition, young seedlings are more likely subjected to water deficiency compared to adult individuals due to the lack of fully developed root system. Consequently, the biomass allocation patterns and the role of the root in regulating survival for H. ammodendron seedlings need to be further investigated.
In the current study, a greenhouse experiment was performed with first-year seedlings of H. ammodendron. Physiological and morphological traits were monitored throughout the drought to confirm their time sequence of occurrence. Also, the survival time for above- and belowground parts were quantified respectively, and the importance of root physiology and morphology in individual survival was discussed. Additionally, the response of biomass allocation pattern to drought was examined and its role in recovery after re-watering was analyzed. At last, a preliminary study was conducted to understand the kinematic growth characteristics of H. ammodendron seedlings radicle.
The results showed that: (1) the quantified survival duration showed that the survival time of the seedling root system (died at 70 d of drought) was double the survival time for the shoot (died at 35 d). Difference in survival time between compartments resulted from sustained root respiration supported by increased non-structural carbohydrates ratio in the root under drought. Meawhile, the time sequence of variations in water and carbon associated physiological indicators was determined in the process of drought to death, and the trade-offs and strategy between water and carbon safty were discussed. (2) Hydraulic disfunction killed the aboveground parts by limiting the availability of stored non-structural carbohydrates, which emphasized the interaction and interdependence of the two mechanisms of hydraulic failure and carbon starvation; after the death of the aboveground parts, the live root contributed to resprouting following drought, and the resprout capacity relies on the carbohydrates that preferentially invested to root. (3) Proposed the mortality criterion for this species: the individual ‘real’ die when both occurrence in aboveground tissues and root gas exchang arrive almost zero as well as the resprout capacity get to zero. This criterion builds the boundary between ‘stress’ and ‘mortality’, and sets the premise for mortality mechanism study. (4) Biomass of H. ammodendron seedlings was preferentially allocated to roots along a fixed allometric trajectory, irrespective of water availability. In prolonged stressful environments, fixed biomass allocation patterns may reduce plasticity costs while increase fitness. The fixed biomass allocation of H. ammodendron seedlings contributes to recovery after re-watering. (5) At the beginning of H. ammodendron germination, the root region that has the highest growth rate locates farthest from the root tip. The elongation was not observed in the region that 2 mm away from the root tip all the time. And maximum curvature usually occurred in the regions that near the root tip, forming two centers and had changed positions with the growth of radicle. Elongation growth and bending growth alternates in time, little or no overlap in spatial.
The study suggests that the desert shrub H. ammodendron has an "intrinsic" root-protect strategy: sacrificing aboveground parts during drought contributes to reduction of carbon demand, while preferentially invest to belowground parts to ensure the survival of root. Meanwhile, the stored root non-structural carbohydrates are the energy sources for resprouting. Accumulation of biomass formed large roots, which can maintain water supply - demand balance through effective morphological adjustment in prolonged drought of desert. Hence, the survival of H. ammodendron seedlings in such environment is both regulated by physiological and morphological adjustments, as well as water and carbon interactions.|