Study On The Influence Mechanism Of Light Reallocation On Woody Species Regeneration In Maolan Karst Forests

In karst areas,the soil is barren,the rock exposure rate is high,and the water and soil resources are easy to be lost.Seasonal water shortage often occurs,resulting in particularly fragile habitats.Karst forest often has some problems,such as simple stand structure,poor regeneration of dominant tree species,and even some forests can’t be renewed normally.These questions give rise to obvious vegetation degradation.Forest gaps are common in karst forests and can promote forest regeneration.However,the shrub layer plants,that exist before forest gaps formation,made the light in the forest gap extremely complex,but the effect of shrub shading on seedling regeneration in the gap has not been reported.Consequently,in this study,we analyzed the effects of light reallocation in the gap on species diversity and functional trait indexes of seedlings in the evergreen deciduous broad-leaved mixed forest in Maolan National Nature Reserve.We aim to clarify the internal regeneration mechanism of karst forest gap and provide a theoretical basis for the natural regeneration and positive succession of Karst degraded forest.The main results are as follows:(1)Effects of shrub shading on light allocation in seedling layers under different forest gaps in Karst forests: There was a very significant positive correlation between and openness and light characteristic parameters of the shrub layer in different gap sizes(P < 0.01),and there was a significant difference in different gap sizes(P < 0.05).There was a very significant positive correlation between openness of the shrub layer and light characteristic parameters of the seedling layer in different gap sizes(P < 0.01),and there was no significant difference in different gap sizes.The light environment of shrub layer is significantly different from that of seedling layer in different gap sizes,and the light environment parameters of shrub layer are significantly higher than seedling layer(P < 0.05).The openness of shrub layer(shrub shading)accounted for 62.6% of trans direct radiation,91.5% of trans diffuse radiation and 83.3% of trans total radiation.The shrub layer reallocated the light in the gap,which was a main factor affecting the light environment of the seedling layer under the gap in Karst forests.(2)Effects of light reallocation on woody species diversity of seedling in Karst forests gaps: 174 woody species seedlings belonging to 124 genera and 68 families were investigated in 15 forest gaps,and the number of species in different forest gaps was different.Nandina domestica,which had positive regeneration responses to gap,were the dominant species in gaps.The gaps with larger area had higher woody species diversity.The accumulating explanatory rate of gap characteristics,topographic factors,karst microhabitat and seedling layer light environments was 81.7%.Among which,the explanatory rate of light environment in seedling layer was 47.5%,the topographic factor was 29.5%,the gap characteristics was 18.8%,and the karst microhabitat was 5.3%.The results indicated the light environment in seedling layer is the main driving factor affecting seedling species diversity.(3)Effects of light reallocation on functional properties of woody species seedlings in Karst forests gap: with the increase of shrub shading percentage in the forest gaps,the seedlings tend to allocate biomass to the aboveground part to maintain normal physiological activities or growth.Moderate shading can promote the accumulation of non-structural carbon content in seedlings in the gaps.With the increase of shrub shading,N,P and K were enriched in seedling leaves.At the same time,the growth restriction of seedlings was changed from controlled by n to controlled by P.To sum up,seedlings in Karst forests gap adopt a survival strategy with resource conservative.In conclusion,the light obtained by the seedlings in the gaps are reallocated by the shrub layer plants.The reallocated light in the gaps that really affect the species composition and diversity of the seedlings.Meanwhile,it also has a significant impact on the growth and renewal strategy of the seedlings.Therefore,the light reallocation through the shrub layer in the forest gap play an important role in the regeneration of woody species seedlings.Therefore,we can artificially form forest gaps of different sizes or appropriately cut down the shrub layer plants in the forest gap to promote the renewal of degraded karst forest.