Transpriational Water Consumption And Its Influencing Factors In Mangroves Of Intertidal Zone

Mangroves grow in coastal habitats affected by tide,and have important ecological function and biological resources.Due to the influence of salty habitats,the water potential of mangroves are generally low.The water use of mangroves is conservative,however,their water transport efficiency is not unusually low.The water management of mangroves is adapted to intertidal environments.However,little is currently known regarding the stand water use(S)and regulatory mechanisms of mangrove forests over the long term and at stand level,particularly in the eastern group of mangroves.In this study,we focused on a typical native mangrove forest in Wenchang,Hainan Province,China,in which the sap flow density(SFD)of six tree species,including Bruguiera sexangula,B.s.var.rhymchopetala,Excoecaria agallocha,Rhizophora apiculata,Sonneratia alba and Xylocarpus granatum,was monitored for 2 years using thermal dissipation probe(TDP)sensors.Specifically,we examined the characteristics of SFD in mangroves,including the relationship between SFD and environmental factors and the radial variation in sap flow characteristics.We also estimated the response characteristics of S to environmental factors across species and annual variation in the S of mangrove forests between 2017 and 2018.The main findings are as follows:(1)We identified the clear and frequent short-time fluctuations in mangrove SFD,and there was no overlap of the daily peak of SFD with either the peak of photosynthetically active radiation(PAR)or vapor pressure deficit(VPD).Bimodal daily SFD patterns were common in all six mangrove species and were more evident in spring and summer,with interspecific differences.Moreover,nocturnal sap flow was observed in all six species.We observed a significant seasonal variation in sap flow,with the annual variation trend being essentially consistent with that of air temperature and evaporation(EVA).During the rainy season(summer and autumn),SFD was significantly higher than that during the dry season(winter and spring),and the annual minimum SFD was observed in winter.(2)We found that the SFD of mangroves was significantly positively correlated with PAR and VPD,although the influence of tidal movements on SFD appears to be smaller.The correlation between SFD and environmental factors differed between seasons,being higher in the rainy season,lower in the dry season,and lowest in winter.Furthermore,we detected a significant positive correlation between Max SFD and tree diameter at breast height(DBH)(R²=0.99,P<0.001),with SFD being more sensitive to VPD at higher Max SFD values.(3)During the dry season,the correlation between SFD and VPD was higher than that between SFD and PAR in B.s.var.Rhymchopetala and Excoecaria agallocha,indicating that these two species do not close their stomata in response to an increase in water stress,and that their water regulation strategy tends to be anisohydric.The sensitivity of the hourly mean crown conductance(Gc)in response to changes in VPD(m _C)was determined from the slope of the relationship between Gc and ln VPD(-d Gc/dln VPD),and we found that the m _Cvalues of all six mangrove species were strongly correlated with the reference conductance at 1 k Pa VPD(G_Cref).If the slope value of m _Cand G_Crefare not significantly different from the proposed theoretical value of 0.6,a species can be considered isohydric;otherwise,the species is assumed to be anisohydric.Based on this classification,we can deduce that B.sexangula,Rhizophora apiculata and Sonneratia alba are isohydric,whereas B.s.var.rhymchopetala,E.agallocha and Xylocarpus granatum are anisohydric.(4)Radial attenuation of the SFD of B.s.var.rhymchopetala and E.agallocha was observed to be of a typical uniform descending type,whereas that of X.granatum was found to be a non-typical Gauss type,both of which are consistent with the SFD radial characteristics of trees with diffuse-porous wood.If radial changes in SFD are not considered,the whole-tree daily water use of B.s.var.rhymchopetala would be overestimated by between19%and 25%,E.agallocha by between 15%and 21%and X.granatum by between 24%and65%,and the S value of the mangrove forest as a whole would be overestimated by between5.5%and 7.5%.(5)Among trees with a similar DBH,E.agallocha was characterized by the highest water use per tree(F),followed in descending order by S.alba,B.s.var.rhymchopetala,B.sexangula,R.apiculata,and X.granatum.Considering the radial changes in SFD,the daily water use of B.sexangula(DBH=9.6-15.8 cm)was between 1.02 and 16.07 kg d^-1,B.s.var.rhymchopetala(DBH=9.2-17.1 cm)was between 1.88 and 16.84 kg d^-1,E.agallocha(DBH=7.9-12.9 cm)was between 2.88 and 21.68 kg d^-1,R.apiculata(DBH=9.4-18.6 cm)was between 0.70 and 15.55 kg d^-1,S.alba(DBH=7.6-17.5 cm)was between 1.05 and 22.41 kg d^-1,X.granatum(DBH=7.3-20.5 cm)was between 1.01 and 8.79 kg d^-1.And on average,the daily water use of all six species was significantly lower in winter than in the other three seasons.On the basis of similar DBH,the maximum daily water consumption of mangroves measured in the present study was found to higher than that of mangroves investigated at other study sites.(6)The F of mangroves was significantly positively correlated with DBH(R²=0.81?0.89;P<0.05),and on the basis of the relationship between F and DBH,we developed a model of F to estimate the water use of trees without TDP measurements in sample plots.Taking into consideration the influence of radial changes in SFD,we obtained S values of298.19 to 357.27 mm,and 269.19 to 321.12 mm for mangrove forests at the two sample plots in 2017 and 2018,respectively.The annual rainfall in 2017 and 2018 was 1688.3 mm and1431.5 mm,respectively.Although the F value of the mangrove vegetation examined in the present study was relatively high,the basal area of our study site was significantly lower than those reported for other mangrove forests in different regions,thereby accounting for the lower S value we obtained.However,when assessed in terms of an equal basal area,we found that the S value obtained in the present study is comparable to that of tropical forest in Xishuangbanna,Yunnan,China,which lies on a similar latitude.On an annual scale,S shows a close association with EVA,although this accounts for only 20%to 30%of EVA.In spring and winter,the daily pattern of S was observed to be highly consistent with that of temperature,and rainfall was found to have a pronounced influence on mangrove forest S,with the daily S being significantly reduced under conditions of intense daytime rainfall.In contrast,we identified no obvious effect of tides on the S of mangrove vegetation.(7)In this study,we systematically measured annual changes in the SFD and stand water use of six species in a mangrove forest of the eastern group,which indicated that the S of mangroves was not unusual lower than other tropical forest,and that the selected mangrove species are characterized by divergent water regulation strategies.The results of this study will be useful for investigations of the carbon and water cycles of mangrove ecosystems and will contribute to the protection and management of mangrove forests.