The Ecological Adaptability To Light Of Plant Functional Groups In Different Successional Stages Of A Tropical Montane Rain Forest On Hainan Island, China

The tropical rain forest in Hainan Island of China is one of the most important forestecosystems that have the most complex structure and the richest biodiversity in China andpossesses the significant values of conservation and research. The tropical montane rain forestcovers the mountainous area froan 700 to 1300 m altitude, which has the broadest distributionarea. However, the excessive deforestation and unreasonable land use have led to the largereduction of the area of primary forest and formed the degraded ecosystems of differentsuccessional stages. The studies on the physical environments and the ecological adaptabilityof the plants in different successional stages can increase our knowledge of the successionalmechanism of forest ecosystem and is important to restore the ecological function andbiodiversity conservation of the rain forest on Hainan Island.The ecological adaptability to light of the representative plants in different successionalstages were measured by the field and controlled experiments. These plants were divided intodifferent functional groups basing on their physiological and morphological characteristics.And the ecological adaptive modes and functions were studies. The main research results wereas follows.1. In the early successional stage, environmental factors changed greatly and therepresentative plants had different adaptive manners. Thysanolaena maxima possessed strongerwater use ability, whose adaptation was mainly through the morphological architectureregulation strategy (by rolling of each leaf and shading of different leaves on the plant).Miscanthus floridulus had greater water regulation ability and its adaptation was mainlythrough the physiological regulation strategy (by higher photosynthetic capacity and water useefficiency (WUE)). However, Melastoma candidum and Melastoma sanguineum integrated themorphological architecture and physiological strategies (by high Pn and leaf self-shading).According to the ecophysiological characteristics and adaptation modes, the plants in the earlysuccussional stage of the tropical rain forest on Hainan Island can be categorized into threefunctional groups: (1) physiological adaptation group; (2) morphological adaptation group;(3) physiological and morphological integrated adaptation group.2. The morphological, physiological and growth properties of seedlings of five dominanttropical montane rain forest tree species under four different relative irradiances (RI 100% (high light), 50% (moderate light), 30% (low light) and 10% (weak light)) were studied bythe pot experiments from June 2005 to March 2006. The six species exhibited differentmorphological adaptation modes to the variation of light intensities. Within each light regime,the investments of Endospermum chinense and Phoebe hungmoensis were lower in leaf andhigher in stem. Elaeocarpus dubius and Acmena acuminatissima had higher investments in leafand lower in stem. However, Parakmeria lotungensis had the highest root investment. On thephysiological adaptation, E. chinense and P. hungmoensis had higher maximum rate ofphotosynthesis (A_max) than E. dubius and P. lotungensis. In growth properties, in relativegrowth rate (RGR_M) A. acuminatissima was the highest, E. chinense, P. hungmoensis and E.dubius were intermediate, and P. lotungensis was the lowest. Furthermore, comparing in termsof morphological plasticity, E. ehinense was the highest, P hungmoensis and A. rostrata weremoderate, while E. dubius and P. lotungensis were the lowest. By integrating the three species'ecophysiological characteristics, light adaptation modes and principle component analysis ofmorphological plasticity, we recognized E. chinense and P. hungmoensis should belong to thepioneer functional group, E. dubius and A. acuminatissima should belong to mid-successionalfunctional group, and P. lotungensis should belong to shade- tolerant climax functional group.3. There were different dynamic changes on morphological indexes of plant functionalgroups in different light regimes during the growth period. During the period, pioneerfunctional group increased quickly in the rainy season and not significantly in the dry seasonon total leaf area and crown area. For mid- successional functional group, LN, TLA and CAincreased linearly. However, for climax functional group showed little changes in the aboveindexes. In leaf area index, pioneer functional group decreased gradually, but for mid-successional and climax functional group, the changes were not distinct. Crown area (CA) forpioneer and mid-successional functional groups were the smallest in high or moderate light,but for climax functional group it was in weak light. In high light, leaf area index (LAI) wasthe lowest for pioneer functional group, but was the maximum for mid-successional and climaxfuctional groups.4. The physiological characteristics of photosynthesis under different light regimes weremeasured every two months. Within each light regime, all species decreased distinctly inAugust on net photosynthesis (Pn), transpiration rate (E) and water use efficiency (WUE)under each light regime. However, stomatal conductance (g_s) for all species showed littlechanges from June to December and an apparent increase in March of the next year. Pioneer functional group had the highest Pn in moderate light. Pn which was close among high,moderate and low light was higher than weak light for mid-successional group. However,climax functional group possessed the highest Pn in low light. Comparing the climaxfunctional group, pioneer functional group exhibited higher photosynthetic capacity, but waslower in water use ability. And mid- successional functional group was close to the pioneerfunctional group in the above indexes.5. The seedlings of plant functional groups in different successional stages had distinctgrowth patterns during the experiment period. Relative growth rate of pioneer functional groupdecreased quickly in the rainy season, but no significant changes and close zero in the dryseason. Mid-successional functional group showed large variety in RGR, and had obviousgrowth in the dry season. However, RGR for climax functional group changed little and waslowest. Within each light regime during the period, pioneer functional group had higher RGRthan climax P. lotungensis. And in the dry season, mid-successional functional group in RGRwas higher than other species. The results of the regression equations indicated RGR weresignificant correlative with Pn and the increment of leaf area in different light regimes forpioneer functional group. But for mid- successional functional group, RGR was not correlatedwith Pn and the increment of leaf area. However, climax functional group's RGR showedsignificant correlation with Pn and the leaf area increment in high and moderate light, but wasrelated with the leaf increment in the low and weak light.6. Diurnal changes of photosynthetic parameters under clear days were monitored in therainy and dry season, respectively. In the rainy season within each light regime, all functionalgroups' diurnal changes in Pn, g_s and E decreased gradually as time increased, whereas WUEchanged little. In the dry season, the diurnal changes of all functional groups in Pn and g_sdecreased obviously at 12:00 under high and moderate light, but not in low and weak light. Ereached a maximum at 12:00 and than decline. However, WUE changed little during the day.Among different light regimes, Pn of all functional groups changed not significantly in therainy season, and WUE were the highest in weak light. However, Pn were the greatest inmoderate light, but WUE showed no apparent changes. Within each light regime, pioneerfunctional group had the higher mean daily values of Pn, g_s, and E than climax functionalgroup both in the rainy and dry season, whereas mid-successional group was closer withpioneer functional group in the above indexes. But for the mean daily values of WUE, pioneerfunctional groups were near the climax functional group, whereas mid-successional functional group was the lowest.The mean daily value of Pn for pioneer and climax functional groups showed nosignificant variations between in rainy and in dry season, but was lower for mid-successionalfunctional group in rainy than in dry season. Pioneer functional group had higher WUE in dryseason than in rainy season, whereas mid-successional and climax functional groups differednot significantly, which indicated pioneer functional group had stronger water regulativeability.7. Diurnal changes of gas exchange were made in clear and overcast days of plantfunctional groups in different successioanl stages. In the clear days, Pn and g_s of all speciesdecreased obviously at the noon, and then increased in the afternoon in high and moderate light,whereas they were not significantly in low and weak light. E reached a maximum at 12:00 andthan decline. However, WUE changed little during the day. In the overcast days, diurnalchanges of Pn of functional groups were double unimodal pattern, and the peak values were12:00 and 16:00, respectively, whereas it decreased gradually. For g_s and E, all functionalgroups reduced stepwise in the overcast day, but WUE altered not obviously. The mean dailyvalue of Pn for pioneer functional group was higher in clear than in overcast days under highlight. Mid-successional group showed higher mean daily value of Pn in clear than in overcastdays under all light regimes. However, for climax functional group, the mean daily value of Pnwas higher under moderate and weak light in clear than in overcast days. For the mean dailyvalue of WUE, pioneer functional group was lower under high light in clear than in overcastdays, whereas it was not significant for mid-successional and climax functional group, whichindicated that pioneer functional group had stronger water regulative ability. In other lightregimes, all functional groups did not changed obviously.