Study of Seasonal Stem Radial Growth of Populus euphratica in the Lower Reaches of the Heihe River

Using the point-dendrometer,the stem radial growth of Populus euphratica,a dominant tree species of the desert riparian forest in the lower reaches of the Heihe River,an inland river in Northwest China,was measured in the growing season in the years of 2009-1010.The seasonal rhythm of stem radial growth of P.euphratica and its relationship to environment factors was analyzed coupling with the synchronous instrumental records of air temperature and groundwater level.According to the stem radial growth characteristics,the whole diurnal cycle can be assorted to three types:(A) the stem diameter expands over the last maximum when it start to contract(ΔR+),(B) the stem diameter's expansion can not reach the last maximum(ΔR-) and(C) the stem diameter expands continually from the last maximum and the contraction phase absents in the daily cycle(ΔR++).In the growth season(from May to early October),the cumulative stem radial growth shows "S" style.The annual total growth is about 2.89±0.91 mm and similar to the average ring width of P.euphratica.The whole period can be subdivided into three phases.The Phase 1 shows slow growth rate,0.46~9.17 μm·d-1,before later May,the Phase 2 is the highest growth period with the rate of 29.90-50.62 μm.day-1 from the later May to early August,and then the growth rate decreases to-0.23~2.78 μm·d-1 in the Phase 3 from early August to the end of growth season.In the three phases,the ΔR+ type accounts for 63.64%,85.51% and 48.61%,respectively.It can be sure that the period from later-May to early August is the main period for the growth ring formation.Although both air temperature and groundwater level have significant correlation to the stem radius growth in this period,the groundwater level should be the substantial factor.Therefore,for the dendrochronological study,the tree ring of P.euphratica can be used for reconstructing the regional water regime process,such as river flow and groundwater level changes.For the management of desert riparian forest,keeping the suit groundwater level in the spring and summer,with some frequent spring floods,is the precondition for the tree's health growth and maintaining suitable forest stand structure.