|其他摘要||Tarim River basin, the most complete preservation and concentrated distribution area of Populus euphratica in China, occupies an extremely important position in ecology of Central Asia desert area. The characteristics of breeding, ecological pattern and process of P.euphratica has been fundamentally changed by the impact of climate change and human activities. P.euphratica forest along the riverbank and seedlings distribution relate to the floodplain were mentioned in a lot of literature, but the details are vague. The effect of flood disturbance on occurrence and development of P. euphratica is poorly understood. There are few scholars pay attention to the effect of flooding on regeneration and community succession of P. euphratica forest. In older to investigate the effect of flooding on seed dispersal, implantation, germination, growth and survival of P.euphratica, control experiments were carried out. Through long-term field observations, the concept model of P. euphratica seedlings was established. This study can preliminarily clarify recruitment mechanism of P. euphratic, and also can provide a theoretical basis for recovery of P. euphratica forest recession in downstream of Tarim River.
(1) The dispersal of the P. euphratica seed began in middle July, and ended in late September. Most of the populous seed distributed between end of August and mid-Sepetember. In spatial, the density of the seed is decreased with the distance increased. The highest seed density of populous is located around 20 m away from P. euphratic. The seed dispersal time of P. euphratic is consistence with the time of flooding.
(2) The seed of P. euphratica is a type of short lifetime seed. The viability of seeds under full light and shading conditions was 7d and 40d respectively. The seeds of P.euphratica germinated rapidly. The temperature range for germination was from 5℃ to 40℃. The seed wasn’t sensitive to light. It could germinate under all light conditions. However, the light showed significant influence on the radicle growth. When the solution water potential was -2.04Mpa, or a salt concentration of 2.3%, P.euphratica seeds could hardly germinate.
(3) The decreased of groundwater table is the main reason for the death of the P. euphratica seeding. The experiment showed that the best groundwater decreased speed for P. euphratica seeding growth is 1-2cm/day. The energy was distributed to growth of root priority, and the growth rate was 2.5mm/d with water table decline.
(4) The inundation inhibited the development of the populous seedling, and changed the biomass allocation pattern. More accumulated biomass was distributed to stem and leaf, and less biomass was distributed to root after inundation. P. euphratica would not grow under long period of inundation, but after water was drainage, the populous would recovery to growth again. The survival rate of P. euphratica seedlings is 100% under long time period inundation. For adaption to the inundation condition, the lenticel hypertrophy occurred at stem, which indicate that the plant is a water resistant species. P. euphratica appears to be a flood-tolerant tree species.
(5) Flooded seedlings showed a tendency for reduction of net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (E) and intercellular CO2 concentration (Ci). This pattern also occurred in the Fv/Fm, Fv'/Fm', ΦPSII and qP parameters. A decline in net photosynthetic rate subjected to flooding not only depends on stomatal factors but also on non-stomatal ones. Stomatal closure is induced by the decline of stomatal conductance, and also by decline in the content of chlorophyll and phyotosynthetic electron transport activity. Flood resulted in an increase in the soluble sugar and MDA content. Under adversity stresses, plants start their own defense systems, which reduce the level of stress injury and maintain normal plant growth.
(6) The inundation would change the particle size composition of soil on the floodplain, increase the soil density and the soil water capacity. At the same time, the inundation would increase the soil water content and nutrient; reduce the salt content in the soil. So the flood could provide suitable condition for P. euphratica seeding. The distribution of P. euphratica seeding is powered by force of gravity, wind and water.
(7) P. euphratica seedlings become established on bare, moist and newly deposited sediment floodplain after flood recession. Flood inundation, scouring and deposition are the reasons for the mass mortality of P. euphratica seedlings.
(8) The conceptual model has been developed to describe P. euphratica seedling safe recruitment. The key ecological drivers of P. euphratica seedling recruitment include three components: site hydrology, seed release timing, and seedling tolerance to arid and flood. Because these species have short lifetime seeds, dispersal timing controls when and where on the riverbanks seeds terminate. Recuitment typically occurs as floodwaters recede. The ability to resist drought and flooding is critical for long term survival. If only coupling the relationship between the three forms, seedling recruitment can occur. So it will take time for the successful recruitment of P. euphratica needs times.|