Spatial And Seasonal Variations Of Leaf Area Index (LAI) In Subtropical Secondary Forests Related To Floristic Composition And Stand Characters

The canopy is important interface connecting carbon (C) flux, water and energy exchanges between forests and atmosphere. Leaf area index (LAI) is an important parameter applied in the ecological process models to simulate production and hydrological cycle in forest ecosystems. However, the LAI data in Chinese subtropical forests is lacking, spatial heterogeneity of LAI and its controlling factors have not been fully understood in the stand scale. We established a permanent plot for each of three forests (ie:i.e.90 m x 190 m irregular plot for P. massoniana-L. glaber mixed forest,100 m x 100 m plot for Choerospondias axillaris deciduous forest, and 100 m×100 m plot for L. glaber-Cyclobalanopsis glauca evergreen broadleaved forest). Each plot was divided into 10 m x 10 m subplots. And the hemipherical photographs were taken at the centre of each subplot of the three forests using SY-S01A LAI measuring instrument throughout four measurement seasons, ie. in April, July and October in 2014 and January in 2015. The ratio of woody to total area (a), the clumping index (ΩE) and effective LAI values were calculated for the accurate LAI. Spatial heterogeneity of LAI in different measurement seasons were analysed by using geostatistics method. The generalised additive models (GAMs) were used to establish the relationship between LAI and stand factors and analyse the effect of stand factors on LAI variations. Our results showed as follows:(1) The ratio of woody to total area (a) in the three forests showed different seasonal variation patterns. The mean a values in C. axillaris forests were between 0.04-0.15, and exhibited a unimodal pattern of seasonal variations, with the maximum mean a value (0.15) occurring in January and the minimum mean a value (0.04) in July which was generally consistent with the amount of leaf litter collected by litter tap. The mean a values in P. massoniana-L. glaber forest were between 0.06-0.09, with the maximum mean a value (0.09) occurring in October and the minimum mean a value (0.06) in January. The mean a values in L. glaber-C. glauca forest were between 0.05-0.15, with the maximum mean a value (0.15) occurring in April and the minimum mean a value (0.05) in July. Mean ΩE values in the three forests were between 0.84-0.92.(2) Interactive effects of measurement seasons and forest types on LAI were significant (p<0.01), LAI differed significantly among forests in the same measurement seasons (p<0.05), and LAI difference was significant among measurement seasons for the same forest types (p<0.05). The C. axillaris forest exhibited a unimodal pattern of seasonal variations, with the maximum mean LAI value (3.11±1.18) occurring in July and the minimum mean LAI value (1.28±0.44) in January. In the P. massoniana-L. glaber forest and the L. glaber-C. glauca forest, the minimum mean LAI value appeared in January, and LAI values in the other three measurement seasons were relatively stable, with the maximum mean LAI value both occurring in October and the minimum mean LAI value appearing in January. In the three forests, LAI in the P. massoniana-L. glaber forest had low variations (CV:27.5%-38.3%), while LAI in the L. glaber-C. glauca forest had the highest variations (CV:33.1%-59.6%). The seasonal variations of LAI in three forests were consistent with plant phenology.(3) LAI values measured in January in the three forests and also in April, July and October in the L. glaber-C. glauca forest exhibited strong spatial autocorrelations at short distance. These LAI data were best fitted with gaussian model or exponential model (r2> 0.50), the range were between 13.80 m and 27.00 m. LAI values measured in April, July and October in P. massoniana-L. glaber forest and the C. axillaris forest exhibited weak spatial autocorrelations at short distance. These LAI data were best fitted with gaussian model, linear model or spherical model (r2<0.50), the range were between 11.70 m and 92.69 m. Kriging was used to analyse the spatial distribution of LAI in the three forests. Obvious patch distribution pattern of LAI values occurred in three forests during the non-growing period and this pattern gradually dwindled in the growing season.(4) We chose the measurement seasons in which the LAI values were the minimum and maximum, respectively, to analyse the effect of stand factors on LAI variations. The LAI values were different among three forest types in January. And LAI measured in January tended to increase as BA increased and showed a positive nonlinear relationship with crown width and a negative nonlinear relationship with crown coverage. The LAI values tended to decrease as the proportion of deciduous species to total stand BA increased. The LAI values were different among three forest types in July. And LAI measured in July tended to increase as species richness increased and showed a positive nonlinear relationship with crown coverage and a negative nonlinear relationship with stem number.Our study provided LAI data in subtropical secondary forests among different seasons, and analysed the spatial heterogeneity of LAI and the effect of stand factors on LAI, the results provided the scientific basis for sampling strategies to accurate LAI estimates and simulating the forest growth and hydrological process in subtropical forests.