Effects Of Canopy Gap Size On Growth And Spatial Patterns Of Regeneration Of Pinus Tabulaeformis And Larix Principis-rupprechtii

Chinese pine(Pinus tabulaeformis Carr.,hereafter YS)and Larix principis-rupprechtii Mayr.(hereafter LYS)are the main afforestation species in north China.They play an important role in wood production,improving ecological environment and other environmental protection.However,large area of plantation planted in 1960-1970s by government is facing low forest stability and ecological service functions,and parts of these plantaions have matured or near maturity.Therefore,how to restore and rebuild sustainable forest ecosystems have become an important issue needs to be addressed urgently.Under the background of near natural forest management,creating gap has been considered to be a silviculture measure that can maintain the stability of stand structure and ensure the success of tree regeneration,and ultimately to achieve continuous forest coverage.Therefore,to gain more insight into the influence of canopy gap(hereafter gap)size on the growth and spatial patterns of YS and LYS regeneration in this study,5 gap size classes were separately set up in YS plantation(CK(100 m2,under canopy),L-Ⅰ(0.75 H),L-Ⅱ(1.00 H),L-Ⅲ(1.25H),L-Ⅳ(1.50 H);H represented the mean canopy height)and YS-LYS mixed plantation(CK(100 m2,under canopy),L-I(20-50 m2),L-II(50-100 m2),L-Ⅲ(100-250 m2),L-Ⅳ(>250 m2)).The main results and conclusions in this study as follows:1.Effects of canopy gap size on growth and spatial patterns of regeneration in Pinus tabulaeformis plantation(1)Five tree and four shrub species were found in gaps.All other species combined accounted for only 5.12%of the total regeneration.For the species diversity patterns of woody plants,the results showed that there were no changes between CK and gap.and among gap sizes,which indicated that creating gaps in YS plantation cannot change the species diversity patterns in a short period of time.(2)For the regeneration established before(REBG)and after(REAG)gap creation,gap size and gap section have no effect on regeneration density,but significantly influenced their growth with a significant increasing trend along the gradient of gap size.Significantly taller and shorter regeneration were generally found in L-Ⅲ and CK,respectively.The tallest regeneration occurred in section B,where the average height was more one times than that of the shortest regeneration growing in CK.(3)All gap size classes showed approximately normal age,height and ground diameter distribution with a longer tail toward larger individuals for increased gap sizes,which indicated that larger size of gaps were conducive to a wide range of regeneration size coexistence.The vacancies may occur in size distribution in the future results from a small number of small size regeneration.(4)The results of annual height and ground diameter growth of REBG and REAG showed that though height and ground diameter of regeneration positively responded to gaps,these responses of REBG were not immediate and presented a time lag of 2-4 years.Similarly,for REAG,significant differences first occurred within years 2-4 after germination in the same growing season among gap size classes.(5)The maximum density of REAG occurred most often on the northeastern edges of gaps with a diameter of less than 1.5 H,whereas the highest density of REBG appeared in the center and on the southeastern edges of gaps with a diameter equal 0.75 H,and the northeastern edges of gaps with a diameter of equal 1.5 H.These results suggested that REAG preferred microsites under the closed canopy and at the gap periphery,whereas REBG favored the sun-exposed gap centers.The tallest REAG and REBG were found at approximately the same locations as the highest densities.Our exploratory analysis revealed that gap expansion along the northeastern borders will likely be necessary to implement canopy gap closure and successful recruitment in the study area.(6)Clustered distributions exhibited greater frequency for REAG than REBG among gap size classes.The spatial patterns of REAG and REBG were mostly random and there were no spatial associations between these two age classes of regeneration within individual gaps.These findings likely be resulted from the differences in the light requirements between them and insufficient environmental heterogeneity among gap size classes.2.Effects of canopy gap size on growth and spatial patterns of regeneration in Pinus tabulaeformis-Larix principis-rupprechtii mixed plantation(1)Twenty-one woody plant species included eleven tree and ten shrub species were found under the canopy and in the gaps.The percentages of YS and LYS regeneration in the different gap sizes were the two largest,accounted for 5.23-43.79%and 50.96-83.08%,respectively.For the species diversity patterns of woody plants,there were significant differences between CK and gap,between L-IV and others,whereas no changes were found among L-Ⅰ,L-Ⅱ and L-Ⅲ.These findings indicated that creating a certain size of gaps could change the species diversity patterns of woody plants in YS-LYS mixed plantation.(2)Gap sizes have significant effect on density and growth of YS and LYS regeneration.The density of YS in L-I(0.89 tree m-2)was significantly larger than that of other gap size classes.The density of LYS has no difference between L-I(2.19 tree m-2)and L-IV(1.81 tree m-2),but significant larger than that of CK(0.84 tree m-2).The growth indexes of YS and LYS increased with the increase in the gap size,of which the maximum and minimum growth indexes were occurred in L-IV and CK,respectively.There exist significant differences in regeneration growth indexes among gap sections although there were no differences in density.The largest and smallest growth indexes of YS and LYS presented in section B and CK,respectively.(3)The maximum density of YS occurred most often on section B and C toward the southwest of gaps with size less than 250 m2,and on section D toward the northeast and northwest of L-Ⅳ,which suggested that YS tended to distribute in the shade area with the increase of gap size.The age and height of YS generally located in a well-lit area,whereas their ground diameter occurred most often toward the northwest of gaps.Although suitable areas for LYS density showed an increasing trend with the increase of gap size,the highest density appeared towards northeast and southeast within gaps.The maximum age of LYS occurred most often on section B and C,whereas the highest height and ground diameter tended to appear in areas with adequate illumination with increasing gap size.(4)The relationships of’ density and growth of regeneration to distance to gap center and corresponding azimuth were examined.The results were almost consistent with those in(3)though it was convenient to find out the specific locations suitable for regeneration distribution and growth in this part.(5)The spatial patterns of YS were mostly random across all scales(50.98-97.06%),and the spatial patterns of LYS were mostly aggregation across all scales(68.63-86.67%),with the intensity of the aggregated distribution increased first and then decreased along the gradient of research scale.Across all scales,there were no spatial associations between YS and LYS within individual gaps.(6)No significant spatial associations were detected for the growth of YS among gap size classes,except negative correlation presented in CK for age distribution(6.67%)and positive correlation occurred in L-IV for the growth of crown(2.78%).For the growth of LYS,either positive or negative correlation was detected within gaps of size less than 250 m2.However,positive and negative correlation were simultaneously detected in L-IV,and negative correlation gradually transitioned to neutral and then presented as positive correlation with the increase of research scales,which suggested that the growth of LYS in L-IV was mutually exclusive on the small scales and promoted each other on the large scales.