Growth Increment Patterns In Roots Of Perennial Forbs In The Loess Plateau,China

Soil erosion is serious in the Loess Plateau and the ecological environment is very fragile.Since the implementation of the project of returning farmland to forests and grasses in China,herbaceous plants have played an important role in accelerating the carbon cycle,protecting the environment,and conserving water and soil.However,in recent years,some herb species have experienced serious recession.The decline of Medicago sativa L.in arid regions is particularly serious,which growth period is short,and growth decline occurs after the sixth year of growth.The physiological mechanism of its occurrence has not been discussed yet.Recent studies have demonstrated that many forb species develop growth rings in the roots that represent annual growth increments,which are likely to reflect the variations in growth conditions,life history strategies,and climate response sensitivity of the species.However,there are few studies related to growth rings for herbaceous species in the Loess Plateau,China.Our research was to analyze herb-chronological data in the Loess Plateau to determine the annual growth variation and life history strategies of perennial forb species under water shortage conditions.In our research,the 113 main roots of 16 perennial forb species at 13 different precipitation sites in the Loess Plateau were collected.Cross-sections(10-15 ?m thick)were produced from the sample roots using a sledge microtome.Annual growth rings in the main roots were identified and measured by differentiating between earlywood and latewood in the secondary xylem.Maximum xylem diameter,an indication of hydraulic conductance efficiency,was also measured under a microscope.The changing trends in annual growth rings and maximum xylem diameters with increasing age were identified.The results indicated that most of the forb species(87.5%,14 species)in the Loess Plateau showed clearly demarcated growth rings in the root xylem that were most likely annual rings.The average age of sampled forb species was about 7 years.The width of annual growth rings of all species demonstrated a linearly decreasing trend with age,and the maximum decrease range was 92.7-221.6 ?m,which was mainly ascribed to the climate warming and drying and the gradual decline in soil water availability over time.With increasing age,the changing trends in xylem diameter could be roughly classified into three types:The xylem diameter of native forb species(Potentilla spp.,Oxytropis racemosa,Cynoglossum divaricatum,etc)tended to increase monotonically over time,which the maximum change range is 6.9-21.2?m and the growth is sustainable.;by contrast,the xylem diameter of planted forb species(Medicago sativa,Sophora flavescens,and Glycyrrhiza uralensis)tended to decrease monotonically over time,which the maximum change range is 19.1-25.0?m and the growth tend to decline;additionally,the xylem diameter of Limonium bicolor(distributed from the northwestern part of the Loess Plateau to semi-desert regions)increased at first and later decreased,resulting in a curvilinear growth trajectory over time,which the change range is 13.6-17.9?m,and indicated the tradeoff strategy between hydraulic conductance efficiency and safety for forbs under extremely dry conditions.The annual ring samples of Medicago sativa L.and Potentilla recta were collected at four precipitation gradient sites in the Loess Plateau,where the weather data were collected.The annual ring width and vessel size of Medicago sativa L.and Potentilla recta were analyzed with age and modern characteristics of climate change.The results showed that the annual ring width of Medicago sativa L.and Potentilla recta tended to decrease with the increasing of age,which was more significant where the precipitation was lower.In Linfen area(precipitation amount:550mm),the average annual ring width of Medicago sativa and Potentilla recta was respectively 191.6 pm and 164.3 ?m,but in Yanchi(350 mm)were 76.2 ?m and 63.5 ?m,This is due to the fact that plants are experiencing more severe water stress in areas where the precipitation is small,resulting in inequacy water for growth and a significant slowdown in the root growth rate.In addition,the root vessel size of Medicago sativa L.decreased significantly with age,and the downward trend was more significant in the sample with a smaller amount of precipitation.The root vessel size of the Potentilla recta increased significantly with age.In Linfen area(precipitation amount:550mm),the root vessel average annual changes of Medicago sativa and Potentilla recta was respectively 0.40?m and 0.61 ?m,but in Yanchi(350 mm)were 1.51?m and 2.10?m,This indicates that under water stress,Medicago sativa L.root vessel decrease so that they can not meet their own growing water demand,showing a declining trend,and the recession time is earlier in the places where water stress is severe.Potentilla recta can increase the vessel to improve the efficiency of water transport for meet the ever-increasing water demand with age.Our results presented substantial evidence of the presence of growth rings in the secondary xylem of main roots in perennial forbs of the Loess Plateau,and analyzed the differentiated life history strategies of those species,and explored the growth mechanism of planted species and natural species in order to provide a scientific basis for the ecological restoration system and vegetation construction in the area.