Chương VII. ĐỊNH CỠ VÒNG NĂM

Chng VII. NH C VNG NM 7.1. GII THIU

B rng vng nm trn thn cy g mc nhng lp a xu c bin ng rt ln t nm ny qua nm khc. S bin ng ca b rng vng nm l mt hm s ca (hay c xc nh bi) kh hu v cc yu t khc ca mi trng.

PGS. TS. Ng. Van Them

iu xy ra l v, kh hu thay i s ko theo s thay i ca cc yu t phi kh hu. n lt mnh, s thay i ca cc yu t phi kh hu li nh hng n tng trng ca cy g.


Mc quan h gia b rng vng nm vi kh hu ty thuc vo:Bin sinh thi ca loi;Thi gian cy b phi ra cc yu t gii hn;Phm vi bin ng ca cc yu t gii hn c nh hng n tng trng


Chng 7 xem xt hai vn sau y:Nhng th tc xc nh bin ng b rng vng nm chun ha trong mi quan h vi cc yu t kh hu.Phng php xc nh mi quan h gia tng trng vi iu kin kh hu.


7.2. TH TC NH C VNG NMChng ta c th hnh dung nhng bin ng trong b rng vng nm ging nh bin ng cao ca ct thy ngn trong nhit k.


Theo , nhng bin ng trong b rng vng nm c xem nh mt dng c o c khong cch u n ca thi gian hoc bin ng ca cc bin mi trng theo thi gian.


V th, so snh nhng bin ng ca b rng vng nm vi nhng iu kin mi trng c th bit c mc thay i ca bin mi trng.


Bin s y c nh ngha l mt c tnh hoc mt yu t nhn cc gi tr khc nhau khi hon cnh thay i.Trong trng hp b rng vng nm, cc ch s tng trng l cc gi tr ca bin s tng trng trn cy g. Trong trng hp kh hu, lng ma v nhit khng kh trung bnh ca cc thng trong nm l nhng gi tr ca cc bin s kh hu.


Nhng yu t kh hu thay i s tc ng v gy ra s thay i b rng vng nm. Phm vi thay i ca b rng vng nm (rng hay hp) ty thuc vo cng v thi gian tc ng ca cc yu t kh hu.


Do , ngi ta gi phm vi thay i ca yu t kh hu c cy g ghi li trn b rng vng nm l s nh c (Calibration).Kt qa nh c o c bin ng vng nm tng ln hoc gim xung ty theo tp hp bin kh hu dng c lng vng nm.


Mc ch ca nh cXc nh s bin i v mt thng k ca bin ng b rng nm khi thay i bin kh hu.Xc nh cc bin kh hu da trn phn hi ca b rng vng nm.


Nhng th tc nh c b rng vng nmPhng php n gin nht l s dng mt bin hu ch xc nh bn cht ca mi quan h. Mt s phng php phc tp hn l s dng cc thng k xc nh nhng mi quan h da trn nhiu bin s. Trong trng hp ny, bt u t nhiu bin, th tc thng k s chn la nhng bin c ngha v loi b dn nhng bin km ngha.


Tt c th tc nh c bao gm mt s m hnh m t hoc xc nh mi lin h gia qu trnh sinh hc vi kh hu.S lng m hnh ch ra chng 5.Chng 7 c nhim v lm n gin ha cc m hnh v nh lng chng.


7.2.1. M hnh sinh hcPhn A ca hnh 7.1a l mt biu dng m t tm tt nhim v pht trin cc m hnh sinh hc.


Bc 1. Gi nh m hnhBc 2. Kho st nhng chng c sinh hcBc 3. M hnh c ph hp vi nhng chng c sinh hc?


Trc ht, chng ta gi nh mt s mi lin h sinh kh hu (bc 1). Chng hn: B rng vng nm ph thuc vo m t, cn m t li ph thuc vo lng ma ca mt s thng trong nm.B rng vng nm ph thuc vo s gi m nhit m bo cho quang hp ca cy...


Nh nghin cu c th gi nh mt m hnh c th no . K n, kho st s liu v kim nh s liu bng thc nghim hoc bng nhng chng c thu thp thc a (hnh 7.1a, bc 1, 2 v 3).Nu thiu nhng thng tin sinh hc cho mi quan h, nh nghin cu c th chn nhiu s liu trc khi kim nh m hnh khc. Nu thng tin sinh hc khng ph hp vi m hnh, nh nghin cu cn xem xt li gi nh, chnh sa li m hnh v kim nh li mi lin h mi (bc 2 v 3).


Khi thng tin ph hp vi m hnh sinh hc gi nh, nh nghin cu s s dng m hnh ny xy dng m hnh thng k dng cho vic kim nh hay nh c (phn B, hnh 7.1b).


Bc 8. c lng c ph hp vi s liu thc t hay khng?Bc 4. Xy dng m hnh thng kBc 5. Tm hm phn hi thch hpBc 6. Tnh nhng c lng t hm d onBc 7. So snh nhng c lng vi nhng gi tr thc


7.2.2. M hnh thng k

M hnh thng k c xy dng nhm mc ch m t nhng c trng ca m hnh sinh hc.

M hnh thng k l mt dng n gin ho mi quan h gia tng trng vi nhng bin kh hu.


Khi thay nhng bin tng trng v kh hu vo m hnh, chng ta s nhn c nhng c lng (hnh 7.1b, bc 4, 5, 6, 7). Nu m hnh thng k ging vi mi quan h thc t, th nhng bin ng trong nhng gi tr tnh ton t m hnh (s o l thuyt hay c lng) s ging vi nhng gi tr mu (s o thc t).


Trong v d mi quan h gia tng trng vi lng ma, chng ta c th gi nh mi quan h tuyn tnh gia tng trng v tng lng ma trong nhng thng no . Trong v d s dng nhit v tng trng, nhit o c trm kh tng c so snh vi s o tng trng.


hai m hnh trn, lng ma v nhit hng thng c tp hp theo nm v so snh vi s o tng trng hng nm.


Nhng gi tr b rng vng nm c tnh ton t m hnh thng k l nhng c lng ca tng trng.Trong nhng v d ny, ti liu kh tng l bin quan trc (bin c lp hay bin gii thch), cn tng trng thc l bin d on hay bin ph thuc (hnh 7.1b, bc 6, 7 v 8). Trong thng k, thut ng bin quan trc v c lng c s dng biu th tng ng s liu thng k u vo v kt qu thng k u ra.


Bin d on l s liu thc t m nhng c lng em so snh. Nhng sai khc (chnh lch) gia nhng gi tr d on thc t v nhng gi tr c lng c gi l nhng sai lch.Nhng sai lch m t bin ng ca h thng v nhng du hiu khng c a vo mi quan h thng k.


Nhng h s ca m hnh thng k kt hp vi cc bin quan trc to ra nhng c lng c gi l hm phn hi hoc hm chuyn i (hnh 7.1b, bc 5). Hm phn hi biu th nhng nh hng ca kh hu n tng trng cy g.


Nhng h s ca hm phn hi kt hp vi ch tiu kh hu c dng c lng tng trng. Nhng h s ca hm phn hi phn nh cy g phn hi li kh hu nh th no.


Khc vi hm phn hi, hm chuyn i biu din bin ng kh hu theo bin ng ca cc chui nin i vng nm.Trong hm chuyn i, s liu kh hu l bin s d on, cn nin i vng nm l bin s quan trc (bin c lp hay bin gii thch). y nhng ghi nhn tng trng ca cy g (thng qua b rng vng nm) c chuyn thnh nhng d liu kh hu.


Hm chuyn i cng bao gm mt tp hp h s khc nhau c s dng c lng kh hu t nhng gi tr b rng vng nm.Nhng h s ca hm chuyn i kt hp vi ch s vng nm c dng c lng kh hu.


Mt s lu V kh hu ca mt nm nht nh c nh hng n tng trng ca cy g sau mt s nm; do hm chuyn i c th c xy dng t mt s ch s vng nm sau khi kh hu xut hin. V chui vng nm c th c hin tng t tng quan; do hm chuyn i cng c th bao gm mt s ch s vng nm ca nhng nm trc khi kh hu xut hin. Th tc xy dng hm chuyn i cng c thc hin bng phn tch hi quy a bin ging nh hm phn hi. Hm chuyn i c th n gin, nhng cng c th rt phc tp. iu ty thuc vo s lng nin i c s dng xy dng hm chuyn i.


hnh 7.2 dn mt v d m t bin ng ca nhng ch s vng nm thc t loi thng ba l (Pinus keysia) v nhng ch s vng nm d on t hm phn hi. v d ny, ch s vng nm d on c c lng t mi quan h gia bin ng ch s b rng vng nm (Y) vi bin ng ch s thy nhit thng 9 (X) Lt.


Chui gi tr d on Y = 0,821 + 0,179*X vi r = 0,54.Chui gi tr thc t


bc 6, chng ta s dng hm phn hi c lng bin d on. Kt qu nhn c em so snh vi gi tr thc t (bc 7).


Khi s liu thc t v s liu d on ph hp vi nhau, th th m t quan h gia chng l mt ng thng i qua gc to . Nu iu va ni khng tho mn (hnh 7.1b, bc 8), th vic nh c b bc b. Khi m hnh b bc b, chng ta cn phi xem xt li m hnh sinh hc, m hnh thng k hoc c hai. Khi gi tr c lng ca bin d on ph hp vi s liu thc t, th bc k tip l kim tra (phn C, hnh 7.1c, bc 9).


7.2.3. Kim traKim tra thu c bng vic s dng s liu t phn tch nh c. Nhng h s ca hm nh c c p dng vo mt tp hp s liu d on cho mt khong thi gian khc nhau, cho mt trm kh tng, hoc cho nhng cy m t tnh mi lin h nh c.


S liu s dng trong nh c u tin c gi l mt tp hp ph thuc, cn s liu s dng kim tra l mt tp hp c lp (s liu khng tham gia lp hm d on).


Vic kim tra c tin hnh nhm xc nh nhng c lng nhn c t tp hp bin quan trc c lp c ging vi tp hp bin d on c lp hay khng. Th tc kim tra c th c lp li nhiu ln. iu ty thuc vo s liu d on c ph hp vi s liu kim tra c lp hay khng (hnh 7.1c, bc 11).


Nu mi quan h gia bin quan trc c lp v bin d on c lp ph hp vi nhau, th vic nh c c chp nhn.n y chng ta c th s dng mi lin h gia b rng vng nm vi cc yu t kh hu khi phc kh hu qu kh hoc gii thch tng trng (hnh 7.1c, bc 12).


7.3. VAI TR CA THNG K V KCH THC MU

Vic nh c thnh cng hay thp bi ph thuc rt ln vo vic chn m hnh thng k v kch thc mu.


vic nh c thnh cng theo mong i, m hnh thng k phi c xc lp trn nhng tp hp bin c quan h cht ch vi nhau.


Mt s lu B rng vng nm c quan h khng ch vi nhng yu t kh hu, m cn vi nhng yu t phi kh hu.B rng vng nm cng khng n nh, m thay i ty thuc vo cy mu, tui cy v v tr ca cy trong lp a. V th, vic thu mu vng nm xc nh mi lin h vi nhng bin kh hu l vic lm rt quan trng.


(3) Khi bin ng vng nm nhng cy mu ging nhau, th gi tr trung bnh ca vng nm l c ngha.

Ngc li, khi bin ng vng nm nhng cy mu khng ging nhau, th gi tr trung bnh ca vng nm c tnh t nhiu cy g l khng c ngha.


(4) Theo l thuyt thng k, mu ln lm gim bin ng ca gi tr trung bnh.

Mc d vy, nhiu nghin cu trong kh hu thc vt ch ra rng, vic gia tng kch thc mu khng phi lc no cng c li.

Chng hn, khi tng kch thc mu t 5 cy ln 10 cy, th tn hiu s gia tng v bin ng s gim nhiu hn so vi vic tng kch thc mu t 15 ln 20 cy.


(5) Trong cc nghin cu kh hu thc vt, iu quan trng l chn c nhng cy mu phn nh r bin ng ca kh hu.

t c mc ch ny, s lng cy mu c th ch l mt hoc mt s t cy.


7.4. CHN NHNG M HNH THNG KM hnh thng k cho cc hm phn hi c th c xy dng t s liu ca bin kh hu trung bnh thng hoc trung bnh ma (mt s thng).


Nhiu nghin cu cho thy, b rng vng nm phn ng khng r rt vi nhng bin kh hu trung bnh nm. Mi trng s ca hm phn hi nhn vi ti liu kh hu, kt qa tp hp li thu c cc c lng tng trng. Nu hm phn hi ph hp vi thc t, th kt qa s m rng cho nhng nm khc c iu kin kh hu tng t. i vi nhng trng hp m cc iu kin kh hu gn vi gi tr trung bnh, kt qa tng trng nhn c s gn vi gi tr trung bnh.


Chng ta c th xy dng rt nhiu m hnh thng k iu din mi quan h gia tng trng (Y) vi cc bin kh hu (Xi).V d:Y ma hng thng;Y - nhit hng thng;Y - cn bng nc hng thng;Y - s ngy trong ma sinh trng;Y tp hp nhiu yu t kim sot tng trng


Tuy nhin, thng qua kim nh thng k chng ta cn phi chn c nhng m hnh hu ch nht. Th no l mt m hnh thng k hu ch nht? l m hnh phn nh y mi quan h gia tng trng vi cc bin kh hu.


S thnh cng ca m hnh ph thuc vo nhng yu t no?Trnh nhn thc ca nh nghin cu.Chin lc thu mu. Ni chung, trc ht nh nghin cu c th bt u t nhng m hnh n gin, sau tin dn n nhng m hnh phc tp hn.


Chng VII. NH C VNG NM Ti sao nh nghin cu cn phi thn trng hay phi linh hot trong vic chn m hnh thng k din t mi quan h gia tng trng vi cc yu t kim sot tng trng (kh hu, phi kh hu)? Tr li:


Tng trng ca cy g ph thuc vo rt nhiu yu t khc nhau, trong bao gm c yu t kh hu ln cc yu t phi kh hu. Sinh trng ca mt loi cy g c th ph thuc rt cht ch vo iu kin kh hu ca thng ny, nhng ph thuc rt yu vo iu kin kh hu ca thng khc.


i khi tng trng ca nm ny ph thuc rt cht ch vo iu kin ca mt s thng no , nhng tng trng ca nm khc li ph thuc vo iu kin kh hu ca cc thng khc.


Nhng loi cy g khc nhau mc trong cng mt iu kin kh hu c phn ng vi cc yu t mi trng (kh hu, phi kh hu) khng ging nhau. Ngay trong cng mt loi cy, nhng c th mc nhng a hnh v t khc nhau cng phn ng khc nhau vi kh hu vng.


7.5. TNH A DNG TRONG VIC CHN BIN SKt qa ca mt m hnh din t mi quan h gia tng trng vi cc yu t kh hu ph thuc vo vic chn khong thi gian trong nm. Nu chn cc khong thi gian khc nhau, th m hnh s nhn cc kt qa khc nhau.


V d:Da vo s liu quan trc sn lng vi ca 11 v (t 1970 1980) Trm nghin cu Vi Ph H ca V Cng Hu (1980), Nguyn Vn Thm (2002) xy dng 14 m hnh biu din mi quan h gia ch s sn lng vi vi cc ch s kh hu (bng 7.1 v 7.2).


7.6. K THUT A BIN S

7.6.1. Gii thiu

chng 5 ch ra rng, tng trng ca cy g chu nh hng ca rt nhiu nhn t khc nhau (kh hu v phi kh hu).


Mt nhn t sinh thi no nh hng n tng trng ca cy g cng thay i theo thi gian. iu xy ra l do s thay i nhu cu sng ca cy v do nh hng phi hp ca nhiu yu t mi trng khc nhau.


Chng hn, nh hng ca nc v ma ma n cy g khc vi ma kh, v ma kh nhit khng kh cao hn. Mt khc, nh hng ca yu t kh hu ny cng ph thuc vo yu t kh hu khc trong nm. Chng hn, nhit khng kh thay i ty theo lng ma cao hay thp.


Do , nh nghin cu cn phi chn la nhng bin hu ch nht a vo m hnh biu th mi quan h gia tng trng vi nhng yu t kh hu.


Bng cch no c th tm c nhng bin hu ch nht a vo m hnh?Tr li:S dng k thut tng quan v thng k a bin.


Th tc phn tch tng quan v thng k a binTrc ht, nh nghin cu c th bt u t vic xy dng (phn tch) ma trn tng quan n.


Tip theo, s dng th tc loi b dn tng bin, kt qa s nhn c nhng bin kh hu phn nh chnh xc mi quan h vi tng trng.


Ni chung, vic tnh ton hi quy a bin l mt th tc rt phc tp. Ngy nay th tc phn tch hi quy v tng quan a bin c th c thc hin rt d dng nh nhng phn mm thng k nh Statgraphics Plus Version 3.0 5.1, SPSS, MINITAB


7.6.2. Tng quan v hi quy a binM hnh thng k gi nh mi quan h gia tng trng vng nm vi cc bin kh hu c dng:


Thc hnh phn tch tng quan v hi quy tuyn tnh a yu tCc bc phn tch tng quan v hi quy tuyn tnh a yu t bng phn mm thng k SPSS 10.0-14.0.


Bc 1. Thm d mi quan h n gia Y vi cc Xi v xc nh s b nhng bin Xi a vo m hnh.


Mc ch ca bc 1

Xc nh tng quan: Y - Xi;Xc nh tng quan: Xi - Xi.


bc 1, nhng h s tng quan cao gia bin Y vi Xi s c chn, bi v chng c nh hng ln n kt qu phn tch hi quy v m hnh d on.


Bc 2. Phn tch hi quy tuyn tnh bi.Mc ch ca bc 2 Tnh cc h s hi quy ring phn Kim nh quan h tuyn tnh Y - Xi Xc nh cc h s hi quyKim nh cng tuyn tnh.


Bc 3. Xy dng m hnh hi quy tuyn tnh bi v chn m hnh ph hp Mc ch ca bc 3Phn tch nhng thng s thng k ca cc bin c a vo m hnh hi quy.Phn tch nhng thng s thng k ca cc bin khng c a vo m hnh hi quy.Xc nh s lng bin d on ti u.


Bc 4. Xc nh phng php chn binPhng php loi b dn tng bin (Backward selection).Phng php a dn tng bin vo (Forward Selection).Phng php chn bin tng bc (Stepwise selection).


lm r k thut phn tch hi quy v tng quan a bin, bng 7.3 7.9 v hnh 7.3 dn v d phn tch nh hng ca lng ma v nhit t thng 6 n thng 8 n sinh trng ca loi thng Pinus sylvestris khu vc Varonhez thuc nc Nga.


T kt qu phn tch hi quy tng quan tng bc (bng 7.6 7.9), chng ta nhn c nhng m hnh thng k sau y:


7.7. XY DNG M HNH CHUYN IMc tiu c bn ca kh hu thc vt l khi phc li kh hu qu kh t mi quan h gia tng trng ca cy g vi cc yu t kh hu.


Cch thc khi phc li kh hu qu kh l coi nhng bin ng ca tham s kh hu tng ng vi nhng bin ng ca b rng vng nm. t c mc ch ny, trc ht tm mi lin h gia b rng vng nm vi cc yu t kh hu.


Bc tip theo l xy dng cc hm chuyn i biu din mi quan h gia nhng bin kh hu vi ch s vng nm.Sau s dng nhng m hnh chuyn i suy din nhng yu t kh hu t cc ch s vng nm tng ng vi nhng thi gian khc nhau.


Nhng iu cn lu chnh xc ca vic suy din nhng bin kh hu trong qu kh ph thuc cht ch vo vic chn chui vng nm. suy din chnh xc cc yu t kh hu qu kh, yu cu t ra l nhng bin ng ca chui vng nm c chn xy dng hm chuyn i phi tng ng vi nhng bin ng ca chui vng nm c s dng suy din bin kh hu. iu va ni c kim nh thng qua phn tch h s tng ng hoc kim nh du.


V d:T s liu ca bng 7.3 v thng qua thut ton thng k cho thy, ch s lng ma v nhit ca thng 6 8 (Y) khu vc Varonhez (Nga) c th c suy din t ch s sinh trng (ST) ca loi thng Pinus sylvestris nh sau:


Ht chng VII


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Chương VII. ĐỊNH CỠ VÒNG NĂM