| Forest microclimate was the basis of revealing forest ecosystem function and assessing environmental benefit of forests. It not only provided fundamental materials for forest ecological research, but also providing evidences for effects of global climate change on forest ecosystems and its response. This paper aimed at tropical mountain rainforest in Jianfengling, Hainan Island. On the base of forest community traits, we choose typical plant community (primary forest and secondary forest) as under canopy microclimate monitoring site, and also the nearby barren land as open space microclimate monitoring site. The high temporal resolution data in 2008 were used to analyze microclimate space-time changes and effects of tropical mountain rainforest. The main results were as follows:(1) Overall microclimate characteristics of tropical mountain rainforestIn 2008, annual mean air temperature(Ta) was 18.8℃, annual maximum air temperature 28.0℃, annual minimum air temperature 4.1℃, annual precipitation(P) 2500mm, annual relative humidity(RH)91.0%, annual mean wind speed(WS) 0.50m/s, annual mean vapor pressure(VP) 2.0kpa, annual mean soil temperature(Ts)19.5℃, annual mean soil moisture (Ms) 19.5%, annual solar radiation(Rs) 4600～5000MJ, annual net radiation(Rn) 2800～3200MJ.(2) Space-time changes of microclimate in tropical mountain rainforestDiurnal variance of air temperature showed upside down U shape, and the minimum value occured in 6:00-8:00, the maximum value appeared near to 14:00. Annual changes trends were para-curve, that is the minimum value was in February and December, and the maximum value was in July, also the change extends in dry seasons were much fierce than that in wet seasons. The diurnal difference in dry seasons was higher than that in wet seasons, and the maximum value was in the period between dry and wet seasons. Air temperature in the canopy showed temperature inversion, and the lowest temperature center was in the appearance of primary forest canopy at night, furthermore, the highest temperature center was 10m far from canopy at daytime.Diurnal variance of air relative humidity revealed inverse trends to air temperature- V shape, that is to say, the highest value in noon and the lowest in early morning and night. Annual changes were double dump trends, that is to say, the highest was in February and October the lowest in May. At noon, relative humidity was the highest in the 12m height of primary forest canopy. During a day, the canopy humidity decreased with height increase. In the primary forest, monthly mean RH decreased with height increase from August to December; while in the secondary forest, monthly mean RH decreased with height from April to December.Diurnal variance of air vapor pressure showed sine fluctuation. The daily peak value of vapor pressure appeared the sunrise time (6:00-7:00), and the daily valley value appeared around afternoon. The annual changes of vapor pressure were the same as air temperature, showing the single-peaked trends. During a day, the vapor pressure in the canopy decreased with height, daily valley value appeared around canopy surface. In the morning, the low-pressure formed in the canopy surface, and at afternoon, the high-pressure formed in the forest ground. There were no relationship between monthly mean vapor pressure and the height of forest canopy.Diurnal variance of mean soil temperature showed sine fluctuation. The daily peak value of soil temperature appeared 18:00-19:00, and the daily valley value appeared around 8:00-9:00. The daytime was the process of warming, and nighttime was the process of cooling. The annual changes of soil temperature were the same as air temperature. In the primary forest, diurnal variance of soil temperature within the depth of 50cm was obvious, whereas in the secondary forest, the critical depth was 30cm. Monthly mean soil temperature increased with the depth.Diurnal variance of mean soil moisture did not showed obvious trends, and the annual changes of soil temperature were the same as air humidity. The daily ranges were much lower than that of relative humidity. In the primary forest, the soil moisture with depth of 30cm is the highest of profile of 100cm, whereas in the secondary forest, soil moisture increased with the depth. There were no relationship between monthly mean soil moisture and the height of soil depth.Diurnal variance of wind speed revealed showed upside down U shape, and the annual changes showed single-peaked trend. The monthly peak of wind speed appeared in June, and the valley value appeared in January and December. The wind speed in the canopy increased with height, and the increasing rate in the daytime was higher than that of nighttime. Diurnal variances of solar radiation, net radiation and photosynthetic active radiation were the same as air temperature, and the daily peak appeared 12:00, while at night the solar radiation and PAR were near to zero, net radiation value was minus. Annual changes showed single-peaked trend, monthly peak appeared in April, and monthly valley value was in February.(3) Comparison of microclimatic ecological effects in and out of forest There were significant differences among microclimate in and out of forest. Air temperature, soil temperature and wind speed in the forest were much higher than that of open field, and relative humidity in the forest was much lower than that of open field. There was no significant differences about solar radiation, relative humidity and vapor pressure between primary forest and secondary forest (p>0.05), while soil temperature, wind speed and PAR in the primary forest were much lower than that of secondary forest, and net radiation of canopy in the primary forest was much higher than that of secondary forest. All these evidences illustrated that there was ecological effects of cooling and increasing humidity in the tropical mountain forest, and the effects of energy utilization and wind proof in the primary were superior to secondary forest. It reflected that tropical forests played an important role in the regulating microclimate.(4) Regression analysis and principle component analysis of microclimatic factorsThere were significant linear correlations among the climatic factors in the forest, except for wind speed and soil moisture. Air temperature, vapor pressure and soil temperature have significant positive correlations (R2>0.80), and there were significant linear correlations among the radiation groups. Furthermore, using the regression analysis of microclimatic elements between inside and outside of forest , we learn that there is a significant correlation between inside and outside of forest, meanwhile, the correlation of air temperature, soil temperature and wind speed in the secondary forest were the most significant (R2>0.80).Multiple linear regressions demonstrated that radiation had a significant negative correlation with air relative humidity and wind speed, and also had a significant positive correlation with air temperature. Soil heat flux had a significant negative correlation with soil moisture and temperature, and also had significant positive correlation with air temperature and relative humidity. The fitting effects of multiple linear regressions in the primary better than that of secondary forest. The good regression models were about net radiation and soil heat flux (Decision coefficient R2>0.90).Principle component analysis showed that tree growth in the primary forest depended on three environmental factors which were heat, light and water (Cumulative contribution rate 89%), and tree growth in the secondary forest depended on two environmental factors which were heat and comprehensive factor about radiation and water(Cumulative contribution rate 84%). It will provide theoretical foundation for tropical forest preservation, sustainable management, restoration and forestation.
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