Seed Dispersal Process And Natural Regeneration Pattern Of Constructive Species In The Pine-oak Forests Of The Qinling Mountains, China

Pine–oak mixed forests and mosaic pure forests of Pinus tabulaeformis, Quercus alienavar. acuteserrata and Pinus armandii are common in the Qinling Mountains forest ecosystemat the altitude of800–2300m. Pure forests of P. tabulaeformis, Q. aliena var. acuteserrataand their mixed forests are the prominent communities in the Qinling Mountains.Regeneration potential of the major woody plants is supposed to affect the stability andfunction of the forest ecosystems. Therefore, it is needed to investigate the naturalregeneration process of pine-oak mixed forest in the Qinling Mountains, to answer thefollowing questions:1) whether seedling regeneration of the constructive species have thesame advantages as their mother trees in the pine-oak mixed forest belt?2) which factorsfacilitate or limit the regeneration and death of pioneer species?3) why oak species usuallyexhibit greater regeneration capacity than Pinus species in the pine-dominated stands andpine–oak mixed forests?4) what roles do animals play in the seed dispersal processes ofconstructive species? The answers to these questions will help us reveal the mechanisms ofspecies coexistence, regeneration and community succession and will provide valuableinformation for supporting the development of effective forest management and restorationplans.In the present study, we determined the woody plant composition and analyzed theseedling regeneration characteristics and diameter class structure of the constructive speciesof the secondary forest (oak stands, pine–oak stands, and pine stands) in the pine oak forestbelt in the middle of south slope in the Qinling Mountains. We monitored the seed rain andseed bank, and tracked the seed dispersal of Q. aliena var. acuteserrata and P. tabulaeformisusing coded plastic tags in the middle, eastern and western Qinling Mountains, to elucidatethe successional processes in the mixed forests. We revealed the seed dispersal processes andmechanisms of the constructive species under gap disturbance in the pine-oak mixed forests.We also tested the hypothesis whether the litter coverage shows contrasting protective effectson seeds denpendent on tree species. We studied the effects of the seed quality and the ratio ofnoninfested to insect-infested acorns on seed dispersal and seedling establishment to elucidate the ecological implications of insect infestations for the natural regeneration of oaks, andanalyzed the influence of Q. aliena var. acuteserrata acorn quantity on the seed dispersal ofthe constructive species in the pine-oak mixed forests. We further clarified the regenerationrules of constructive species under different conditions, including stand density, slope aspect,slope position, and altitude. The results are as follows:1. Species composition and regeneration characteristics of woody plant seedlings inthe pine-oak mixed forest beltsRegeneration characteristics of main woody plant seedlings in the pine-oak mixed forestswere conducted in the Qinling Mountains. The results showed that Q. aliena var. acuteserrataseedlings had higher abundance than pine seedlings in the P. tabulaeformis forests. Bycontrast, P. tabulaeformis seedlings are rare in the oak forests in the mosaic distributionregions between the Q. aliena var. acuteserrata and P. tabulaeformis forests. Seedling densitywas significantly higher than sapling density in Q. aliena var. acuteserrata; however, nosignificant difference was found in P. tabulaeformis. The diameter structure of Q. aliena var.acuteserrata population in the pine-oak mixed forests showed a reverse J-shape distributionfrom small size classes to large size classes. The population of P. tabulaeformis appeared tobe normal distribution in the pine-oak mixed forests, i.e., the individuals of large size classwere relatively stable but saplings were rare in the pine-oak mixed forests. The results suggestthat the factors affecting the trasition from seedlings to samplings did not affect the overallpopulation regeneration of Q. aliena var. acuteserrata. Q. aliena var. acuteserrata populationwill maintain the reverse J-shape regeneration patterns without large-scale interference and isexpected to replace the P. tabulaeformis to be the first dominant species. The fact that maturetrees lack corresponding saplings in the conpies of P. tabulaeformis and P. armandii is lesslikely to support the community stability in the long run.2. Rodent-mediated acorn dispersal promotes oak invasion of pine forestsP. tabulaeformis is considered to be a pioneer species during succession and it is usuallyinvaded by late successional Q. aliena var. acuteserrata. However, the mechanism thatunderlies its invasion remains unclear. In the present study, we investigated the seed rain andseed bank, and tracked the seed dispersal of Q. aliena var. acuteserrata and P. tabulaeformisusing coded plastic tags in the middle, eastern and western Qinling Mountains to elucidate thesuccessional process in mixed forests. Our results indicated that Apodemus draco, A.peninsulae, and Sciurotamias davidianus were the main seed dispersers and predators in theQinling Mountains. The seeds of both Q. aliena var. acuteserrata and P. tabulaeformis wereharvested rapidly, and there were no differences in the seed removal rates of the two species.Most of the P. tabulaeformis seeds were eaten in situ, whereas the acorns of Q. aliena var. acuteserrata were more likely to be scatter-hoarded in all plots. P. tabulaeformis seeds werehighly unlikely to be dispersed into the Q. aliena var. acuteserrata forests. By contrast, over50%of the released acorns were transported into the P. tabulaeformis stands where theyestablished several seedlings. Based on the coupled recruitment patterns of Q. aliena var.acuteserrata and P. tabulaeformis, we suggest that the seed dispersal by small rodentsfacilitates the seed invasion of oak into pine forest, but not vice versa.3. Seed predation patterns favor the regeneration of dominant species in forest gapscompared with the understory in an oak-pine mixed forestRecent studies have demonstrated the higher likelihood of regeneration in forest gapscompared with the understory for the dominant species in pine-oak mixed forest. Here, wetested whether rodent seed predation or dispersal was beneficial for gap regeneration. Wetracked the seed predation and dispersal of Q. aliena var. acuteserrata and P. armandii usingcoded plastic tags in the forest understory close to gaps. Our results demonstrated that theproportions of primary buried seeds of both species were significantly more abundant in theforest understory compared with gaps. After seed caching, however, significantly lowerproportions of the seeds of both species survived in the forest understory compared with gapsduring the30-day observation period. The final survival proportions of the seeds cached in theforest understory were lower than those cached in the gaps the next spring, which indicatedthat small rodents rarely retrieved scatter-hoarded seeds from forest gaps. Our findingssuggest that rodent seed predation patterns may benefit the regeneration of the dominantspecies in gaps compared with the understory in a pine-oak mixed forest. In the study area,reforestation usually involves planting seedlings but direct sowing in forest gaps may be analternative means of accelerating forest recovery and successional processes.4. Litter serves as seed sanctuary for small-seeded tree species but not large-seededtree species in the Qinling Mountains, ChinaAlthough it is well accepted that litter may cover seeds and thereby benefit their survival,little is known about whether litter contributes differentially to seed survival of plant specieswith contrasting seed sizes. In this study, we examined the effect of different levels of littercoverage on seed predation and removal/burial of small-seeded P. tabulaeformis andlarge-seeded P. armandii and Q. aliena var. acuteserrata in the Qinling Mountains, China.Our results showed that litter failed to well protect P. armandii and Q. aliena var.acuteserrata seeds from being removed by small rodents. By contrast, litter provided a betterrefuge for seed survival of P. tabulaeformis. At high level of litter coverage, less seeds P.tabulaeformis were found to be eaten in situ. However, seeds of P. armandii and Q. aliena var.acuteserrata were more likely to be eaten in situ at high level of litter coverage. Therefore, we propose that litter coverage act as a sanctuary for seeds of small-seeded P. tabulaeformis butnot large-seeded P. armandii and Q. aliena var. acuteserrata in the Qinling Mountains.5. Effect of relative abundance of Q. aliena var. acuteserrata acorns on seeddispersal of three tree species in the Qinling Mountains, ChinaThe effects of Q. aliena var. acuteserrata acorn quantity and rodent density on seeddispersal of three tree species (P. tabulaeformis, Q. aliena var. acuteserrata and P. armandii)were investigated in Huoditang Forest in autumn of2012and2013. The results showedyearly fluctuation in rodent density, with the trap success in2013(40.0%) being higher thanthat in2012(29.3%). Q. aliena var. acuteserrata acorn quantity differed significantlybetween the two years (5.75±0.81acorns·m-2vs30.58±4.95acorns·m-2), with the relativeseed abundance (per-capita rodent) in2013(14.38) being significant lower than that in2012(104.37). Seed dispersal rates of P. tabulaeformis, Q. aliena var. acuteserrata and P. armandiiwere significantly reduced at higher per-capita seed abundance. The proportion ofscatter-hoarded Q. aliena var. acuteserrata and P. armandii were significantly increased athigher per-capita seed abundance. In2012, rodents tended to transfer Q. aliena var.acuteserrata and P. armandii seeds at further distance. The relative seed abundance of Q.aliena var. acuteserrata might be a key factor determining the seed dispersal of the other treespecies in the study area.6. Effects of insect infestation on Q. aliena var. acuteserrata acorn dispersal in theQinling Mountains, ChinaThe nutritious acorns produced by oak trees (Quercus spp.) are attacked frequently byinsects that feed on seed reserves. However, previous studies have partly considered theeffects of insect infestations on animal seed dispersal and subsequent plant regeneration. Inthis study, we tested whether the effects of insect infestations on seed dispersal werefrequency dependent (i.e., ratio of noninfested to infested acorns). We tracked the seeddispersal of plastic-tagged noninfested and insect-infested Q. aliena var. acuteserrata acornswith different frequencies in the Qinling Mountains in central China. Our resultsdemonstrated that noninfested acorns were removed more rapidly, regardless of theinfestation rates. Insect-infested acorns were also more likely to be eaten in situ, whereasnoninfested acorns were more likely to be scatter-hoarded independent of infestation rates.Moreover, noninfested acorns were dispersed further and had higher survival rates thaninsect-infested acorns. Our findings suggest that the seed dispersal advantages of noninfestedacorns compared with infested acorns was not frequency dependent, thereby indicating aconsistently higher dispersal likelihood of noninfested acorns.7. Effects of environmental factors on seedling regeneration of constructive species in a pine-oak mixed forest in the Qinling Mountains, ChinaOur results showed that the stand density, slope aspect, slope position, and altitude hadsignificant effects on the seedling and sapling densities in pine-oak mixed forest. The seedlingand sapling densities of three tree species increased with the stand density (850trees hm-2to1525trees hm-2) whereas the sapling density was significantly lower in stands (1900treeshm-2). The seedling and sapling densities of Q. aliena var. acuteserrata and P. tabuliformis,respectively, were significantly increased at altitudes of1283m to1665m, whereas theseedling and sapling density decreased significantly1835m. The seedling and sapling densityof P. armandii were significantly higher at altitudes of1283m to1835m. The sapling densityof P. armandii and Q. aliena var. acuteserrata decreased gradually from the southwest(20–75°) in sunny conditions whereas it increased on the shady slopes to the northeast (40°).The seedling densities of three tree species and sapling density of P. tabuliformis increasedgradually from the southwest (20–75°) in sunny conditions and on the shady slopes to thenortheast (40°). The seedling densities of Q. aliena var. acuteserrata on the lower slopes weremuch higher than the middle and upper slopes, whereas the sapling density did not differ. Theseedling and sapling density of P. armandii and P. tabuliformis, respectively, weresignificantly higher at slopes of lower to upper. Thus, enclosing stands on middle altitudes,shady slopes at a moderate density are expected to promote the natural regeneration ofpine-oak mixed forest.