Vegetation Characteristic Of Evergreen-deciduous Broadleaved Forest Ecotone And Its Formation Mechanism, A Study In Anhui

Evergreen-deciduous broadleaved forest ecotone (EDFE) locates in the transition area from subtropics to warm temperate zone, eastern China, and was sensitive to climate change. Under long-time anthropical disturbance, natural vegetation survived in mountain areas with small remnant area. The controversy about which bioclimatic zone EDFE should belongs to has been existed for a long time, due to little know about its vegetation characteristic and formation mechanism. Evergreen broadleaved forest (EBF) gradually transits to deciduous broadleaved forest (DBF) from south to north, in Anhui province, which has been the hot spot for EDFE studying. In this research, we conducted a vegetation survey from south to north in Anhui, with collecting habitat data and plant traits data at the same time. The following researching questions were addressed:1) How do forest communities transit across EDFE at different landforms?2) What kinds of environment factors drive forests transition and how plant traits responded to those environment factors at community level?3) How do communities distribute in EDFE and why there are mixed evergreen-deciduous broadleaved forests?4) How to determine the boundary of EDFE and which bioclimatic zone should EDFE belongs to?There were different vegetation transition patterns for low elevation forests from south to north among landforms in Anhui. Comparing to forests on the upper slope, EBFs were replaced by DBFs at lower latitude on lower slope. This asymmetric transition patterns among landforms caused a wide range of EDFE. At plot level, richness decreasing rate of evergreen broadleaved woody plants were greater than increasing rate of deciduous ones with increasing latitude, which causing total species richness decreased from south to north.Climate factors, especially those relating to the cold stress or drought stress, were most correlated with community transition along latitude gradient. According to correlation, minimum monthly potential evapotranspiration (PETmin) has the strongest explanation, followed by mean temperature of the coldest quarter (MTCQ),coldness index (CI),annual actual evapotranspiration (AET) and precipitation of driest quarter (PDQ). Slope, aspect and total soil phosphorus content (SoilP) were orthogonal to climate factors, suggesting them relating to the distribution pattern along landform gradient.According to RLQ analysis, leaf nitrogen and phosphorus per unit leaf mass (Nmass,Pmass), leaf area (LA), leaf ash content (LAC) and leaf life-span (LL) were correlated with all kinds of environment factors. Wood density (WD) and leaf relative electrolyte leakage at0℃(ELO) were just correlated with environmental energy factors. Specific leaf area (SLA) and leaf dry matter content (LDMC) had no significant relationship with environment factors. All traits except LL were most correlated with climate factors relating to the cold stress. LL was most correlated with SoilP. In general, with increasing latitude, Nmass, Pmass, LA, LAC, WD increased and LL, LDMC decreased due to temperature dropping. Evergreen broadleaved trees such as Cyclobalanopsis glauca, Castanopsis sclerophylla, Lithocarpus glaber, Lithocarpus henryi, Cyclobalanopsis myrsinifolia, which can distribute in higher latitude or higher altitude, possessed higher Nmass, LDMC and lower LL, ELO.EDFE showed a mosaic transition pattern intermixed by EBF and DBF patches, which caused by hierarchy EBF-DBF transition at latitude, altitude and landform levels. Evergreen broadleaved species dominated on the lower slope at lower latitude and lower altitude. In contrast, deciduous broadleaved species dominated on the upper slope at higher latitude and higher altitude. Generally, EBF dominated by evergreen species such as Cyclobalanopsis glauca and Castanopsis sclerophylla, occurred at sites with greater water-energy condition. DBF dominated by deciduous species such as Quercus variabilis, occurred at sites with weak water-energy condition. Mixed forest dominated by Quercus variabilis and Cyclobalanopsis glauca occurred at transition area between EBF and DBF.The mixed forest dominated by Quercus variabilis and Cyclobalanopsis glauca was an outcome of hierarchy EBF-DBF transition at muti-levels. Deciduous broadleaved tree species dominated at the upper tree layer and evergreen broadleaved tree species dominated at the lower tree layer, which raised a vertical evergreen-deciduous mixing pattern. Because Quercus variabilis had greater height growth rate and maximum height than Cyclobalanopsis glauca and both of them could maintain population by seedlings and resprouting, respectively, the mixed forest could persist for hundreds of years without disturbances. When being destroyed, it would degrade to a forest dominated by Platycarya strobilacea, Dalbergia hupeana and Pinus massoniana.According to all the above results, the mosaic pattern intermixed by EBF and DBF patches was proposed as the criterion to determine boundary of EDFE. Following this criterion, the northern boundary of EDFE was the northern distribution limit of mesophytic EBFs such as Castanopsis eyrei forest, and the southern boundary of EDFE was the northern distribution limit of valley EBFs, such as Cyclobalanopsis glauca forest. Because climate, floristic composition and agriculture-forestry management strategies in EDFE were similar with those in subtropical EBF zone, EDFE could be considered as a sub region of EBF zone in eastern China.