The Effects Of Defoliation On Abouvegoudn Growth And Fine Roots Dynamics In Fraxinus Mandschurica And Larix Gemelinii Seedlings

Fine root(diameter?2 mm)is not only the most important vegetative organ of terrestrial plants,but also plays an important role in carbon and nutrient cycles in individual and forest ecosystem level.Defoliation by insects may lead to significant changes in tree growth and physiological metabolism.However,how fine root dynamics responding to defoliation is still poorly understood.Manchurian ash(Fraxinus mandschurica)and Dahurian larch(Larix gmelinii)seedlings,with different biomass allocation and height growth strategy,were used to investigate the effects of different levels of artificial defoliation(0%(control),40%,80%leaf area removal in summer and 40%leaf area removal in autumn)on the growth of aboveground(height and stem collar diameter)and belowground(production,mortality,and longevity of fine roots).Minirhizotron approach was employed to determine the seasonal dynamics of fine roots in two years,and seedling height and stem collar diameter were measured concurrently.The results are as follows:(1)The effects of defoliation(summer and autumn)on the growth of ash and larch seedlings had significantly different.Defoliation in summer reduced the growth of seedling height and stem collar diameter in both species,with a stronger effect on the stem collar diameter.Effects on aboveground growth by defoliation were enhanced by increasing defoliation intensity in both species.Compared with the control seedlings,the height and collar diameter of ash were reduced by 3.3%to 12.1%and 5.7%to 23.1%in the end of growing season in 2013,respectively,but the height and collar diameter of larch were only slightly reduced(<12%),while all' treatments had no difference in 2014.The effects of defoliation in autumn on aboveground growth in both species had no difference in 2013,while aboveground growth reduced in 2014.Effect on aboveground growth in ash by autumn defoliation was larger than lash,especially stem collar diameter.(2)Effects of defoliation(summer and autumn)on fine roots production in both species had difference.Defoliation in summer significantly reduced fine roots numbers and length growth,especially significant effect on ash(p<0.05).Defoliation in summer did not change season pattern of fine roots production with numbers,but delayed fine root length production peak in 2013.Compared with ash,defoliation led to fine root production(numbers and length)peak disappear in larch.Defoliation in autumn had no significant effects on fine root production with numbers and length in ash and larch.(3)Effects of defoliation(summer and autumn)on fine roots mortality in both species had difference.Defoliation in summer significantly reduced fine roots production in ash(numbers and length,p<0.05).Fine roots mortality(numbers and length)and the relative of fine roots mortality rate were no significantly different among different levels of defoliation in 2013,while effects on these indicators by defoliation were weakened by increasing defoliation intensity in 2014.80%defoliation in larch reduced fine roots mortality,compared with before defoliation,fine roots total mortality with 40%and 80%defoliation after defoliation were 2.5 and 1.7 times respectively,while control was 2.2 times.Defoliation in autumn had no significant effects on fine roots mortality with numbers in both species,but led to fine roots mortality with length increase in larch,while had little effects on ash.(4)Defoliation in summer significantly reduced fine root standing crops in both species(p<0.001),in which the relative growth of fine root standing crops(the maximum crops after defoliation minus the current crops at the date of defoliation,then divided by the current crops at the date of defoliation)was reduced by increasing defoliation intensity.Defoliation in autumn significantly reduced fine root number and length of standing crops(p<0.05)in larch,but had no effect on ash.(5)Fine root longevity in two species exist obvious difference.Mean root longevity of control seedlings in ash,which initiated before and after defoliation in summer,was respectively 412±4 days and 36±63 days.Ash had longer mean root longevity than larch which was respectively 307±17 days and 262±6 days.Effects on mean root longevity initiated in 2013 by defoliation in summer were decreased by increasing in roots in both species.Cox proportional hazards regression analysis showed year and season of root birth and order had significant effects on fine root longevity in two species(Various factors hadp<0.01),when controlling for other covariates(factors),but defoliation in summer had no significant influence.While defoliation in autumn significantly reduced fine root longevity(particularly mean root longevity)in ash,but had no significant effects on larch.In sum,our results showed:(1)Defoliation in summer imposed significant effects on aboveground growth(especially stem collar diameter)in ash and belowground growth(particularly root production)in larch.Defoliation in autumn had stronger effects on aboveground growth(especially stem collar diameter)in ash than larch,but had no significant effects on fine root production and mortality in two species.(2)Defoliation in summer imposed no significant effects on fine root longevity in two species,while defoliation in autumn decreased significantly fine root longevity in ash(particularly mean root longevity),but had no influence on larch.So the effects of defoliation in summer and autumn on aboveground growth and fine roots dynamics in ash had much stronger than larch,and ash was more sensitive to defoliation.