22

Silva Balcanica, 14(1)/2013

PECULIARITIES IN GROWTH AND VOLUME INCREMENT OF DOUGLAS-FIR PLANTATIONS IN THE REGION OF CHEPINO,

NORTH-WEST RHODOPES

Angel Ferezliev, Alexander Delkov Forest Research Institute – Sofia Bulgarian Academy of Sciences

Abstract

The course and regularities in growth and volume increment of pure and mixed Douglas-fir dendrocoenoses were determined with the help of total stem analysis of sample stems selected in height groups according to the method of vertical profiles. In contrast to other inventory indices, both differ according to total course of curves and to moments of peak values. The influence of climate on the course of volume increment curves of sample trees was followed. It is expressed through the incre-ment minimums and maximums according to this inventory index in different years of the trees’ growth in height groups in investigated sample plantations in mountain massif Chepino, North-West Rhodopes. The current volume increment in most trees increases in the beginning and reaches its maximum value in the end of the inves-tigation period. Peak occurrence of the average volume increment is reported for most of the trees in the end of investigation, when it is still early for highest average productivity of the tree according to inventory index. The peak value of average volume increment is at higher levels of moisture indices towards temperature ones for the height groups from the upper part of the canopy and in different proportions towards average temperatures and precipitations during the active period. The occur-rence of the maximum of average increment gives orientation for the most rational from economic point of view utilization of trees. Obtained information, generalizing maximums and minimums of volume increment and knowing of regularities in its changing give the possibility to make conclusions for the current increment. The to-tal course of curves, characterizing the volume increment in climate changes condi-tions in the past, together with this one determined according to height and diameter, could be used for modelling and choice of optimal economic approach in Douglas-fir stands in North-West Rhodopes.

Key words: Douglas-fir plantations, growth, current volume increment, aver-age volume increment, height groups, summer air temperatures index, annual pre-cipitations index

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INTRODUCTION

There are insufficient investigations on the growth and volume increment of Douglas-fir plantations established in various places in Bulgaria (Sirakov, 1952; Petrov, Prokopiev, 1962; Zlatanov et al., 1981; Kanev, 1982; Petkova, 1987, 1989; Popov, 1991, 2006). Studies in the region of Western Rhodopes (without stem analy-ses) were carried out by Popov et al. (2005, 2006).

Studies on the growth of Pseudotsuga menziesii (Mirb.) Franco in North-West Rhodopes were carried out in young plantations (Tabakov, 1976, 1981) and, dur-ing last years, in middle-aged stands as well (Ferezliev, Tsakov, 2007; Ferezliev, 2009; Ferezliev, Tsakov, 2011, 2012; Ferezliev, 2012). For the stands in this part of the Rhodopes there is an absence of investigations on determination of volume and following the peculiarities in growth and increment according to this inventory index. The aims of this investigation are part and continuation of efforts to establish a fundament of normative-inventory basis of this tree species and are directed to de-termination of volume increments – current (Zcurr) and average (Zav) and peculiarities in growth and increment curve through stem analysis on sample stems – average in height groups.

The investigated course of volume increment (expressed through maximum and minimum values of increment) is compared to climatic behaviour (tempera-tures and precipitations), corresponding with specifics in growth of trees according to height groups. Average representatives according to height groups were selected for analysis, which indicated highest levels in the course of growth process accord-ing to volume.

MATERIAL AND METHODS

Object of investigations are one pure and three mixed (with Scots pine, birch and Dahurian larch) medium-aged Douglas-fir plantations. According to the forest-vegetation regions of Bulgaria (Zahariev et al., 1979), the plantations are situated in sub-region Western Rhodopes of the Thracian forest-vegetation region and Middle-mountain belt of beech and coniferous forests.

Four sample parcels were established in investigated plantations. The first and second ones are in the areas Varbitsa and Byala voda within the range of State Forest Enterprise ‘Alabak’. Three sample plots were established in these parcels from 1250 up to 1400 m a.s.l. in Douglas-fir plantations mixed with Scots pine and birch at the age of 41 to 44. Pure Douglas-fir plantations are 47-year-old. This is the third parcel, formed in the area Petrovo dere, State Forest Enterprise ‘Rakitovo’, with two sample plots at 1100 m. The last parcel, including one sample plot, is in mixed Douglas-fir/Dahurian larch plantation at the age of 51 at 1400 m in Training-and-Education Forest Enterprise ‘Yundola’. The silvicultural and inventory characteristics of the sample plots is shown in Table 1.

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Tabl

e 1.

Silv

icul

tura

l and

inve

ntor

y ch

arac

teris

tics o

f sam

ple

plot

s

Ex-

peri-

men

-ta

l pa

r-ce

l (E

P)

Loca

tion

Sam

-pl

e pl

ot

(SP)

Alti

-tu

de

a.s.l

. (m

)

Ex

-po

si-

tion

Site

Slop

e(˚

)A

ge(y

ears

)A

rea

(ha)

Com

posi

tion

Num

ber o

f tre

es

Ps

. m

en-

zies

ii

Pi-

nus

syl-

ves-

tris

B

et-

ula

pen-

dula

Po

pu-

lus

trem

-ul

a

Fa-

gus

syl-

vati- ca

Larix

da

hu-

rica

for

SP

1

SFE

Ala

bak,

Va

rbits

a ar

ea

112

50

S –

slop

e up

per

part

Т-II

-2

С2

(75)

1641

0.1

Ps. m

enzi

esii-

8;

B. p

endu

la-1

; P.

sylv

estr

is-1

195

2116

--

-23

2

2

SFE

Ala

bak,

B

yala

vo

da a

rea

2

1400

E –

slop

e up

per

part

Т-II

-2

С-2

,3

(76)

10-

1544

0.12

Ps. m

enzi

esii-

8 P.

sylv

estri

s-2

B. p

endu

la

P. tr

emul

a F.

sylv

atic

a

101

433

12

-15

0

30.

14

Ps. m

enzi

esii-

9;

P. sy

lves

tris-

1;B

. pen

dula

;P.

trem

ula;

F.

sylv

atic

a

119

131

71

-14

1

3

SFE

Rak

itovo

, Pe

trovo

de

re a

rea

411

00N

–

slop

e lo

wer

pa

rt

Т-II

-2

C-2

(7

1)

10-

1547

0.1

Ps. m

enzi

esii-

1010

5-

--

--

105

511

000.

1Ps

. men

zies

ii-10

105

--

--

-10

5

46

1400

S –

sl

ope

low

er

part

Т-II

-2

1451

0.22

Ps.

men

zies

ii-8;

L.

dah

uric

a-1;

P. sy

lves

tris-

110

917

--

-47

173

TEFS

Yu

ndol

aС

2 (7

5)

25

Table 2. Course of volume increment of analysed stems

Experimental parcel/sample

plot

age-А, to

reachup

re la tive height

(Н)

Stem vol-ume

Volume incre-ment - m³

Experimental parcel/sample

plot

age-А, to

reach up

relative height

(Н)

Stem vol-ume

Volume incre-ment - m³

№ of analysed stem

years m³ cur-rent

Zcurr

average Zav

№ of analysed stem

years m³ cur-rent

Zcurr

average Zav

1 2 3 4 5 1 2 3 4 5EP1SP1163

I height group

10 0.0018 0.0004 0.0002 EP2SP286

I height group

10 0.0089 0.0017 0.000915 0.0254 0.0047 0.0017 15 0.0353 0.0053 0.002420 0.0732 0.0096 0.0037 20 0.0882 0.0106 0.004425 0.1499 0.0153 0.0060 25 0.1619 0.0147 0.006530 0.2450 0.0190 0.0082 30 0.2707 0.0218 0.009035 0.3868 0.0284 0.0111 35 0.4070 0.0273 0.011640 0.6321 0.0491 0.0158 40 0.5699 0.0326 0.0142

EP1SP1153

II height group

10 0.0051 0.0010 0.0005 EP2SP287

II height group

10 0.0053 0.0010 0.000515 0.0304 0.0050 0.0020 15 0.0216 0.0032 0.001420 0.0764 0.0092 0.0038 20 0.0646 0.0086 0.003225 0.1401 0.0127 0.0056 25 0.1262 0.0123 0.005030 0.2192 0.0158 0.0073 30 0.2038 0.0155 0.006835 0.3351 0.0232 0.0096 35 0.2935 0.0179 0.008440 0.5090 0.0348 0.0127 40 0.4022 0.0217 0.0101

EP1SP1210

III height group

10 0.0014 0.0003 0.0001 EP2SP2105

III height group

10 0.0061 0.0011 0.000615 0.0097 0.0017 0.0006 15 0.0200 0.0028 0.001320 0.0303 0.0041 0.0015 20 0.0459 0.0052 0.002325 0.0710 0.0081 0.0028 25 0.0810 0.0070 0.003230 0.1200 0.0098 0.0040 30 0.1400 0.0118 0.004735 0.1825 0.0125 0.0052 35 0.2138 0.0148 0.006140 0.2537 0.0142 0.0063 40 0.3053 0.0183 0.0076

EP1SP1175

IV height group

10 0.0010 0.0002 0.0001 EP2SP2133

IV height group EP2SP2133

10 0.0029 0.0005 0.000315 0.0038 0.0005 0.0003 15 0.0133 0.0021 0.000920 0.0160 0.0025 0.0008 20 0.0392 0.0052 0.002025 0.0451 0.0058 0.0018 25 0.0714 0.0065 0.002930 0.0801 0.0070 0.0027 30 0.1042 0.0066 0.003535 0.1154 0.0071 0.0033 35 0.1350 0.0062 0.003940 0.1483 0.0066 0.0037 40 0.1689 0.0068 0.0042

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1 2 3 4 5 1 2 3 4 5EP2SP325

I height group

10 0.0123 0.0023 0.0012 EP3SP418

II height group

10 0.0040 0.0008 0.000415 0.0540 0.0083 0.0036 15 0.0280 0.0048 0.001920 0.1438 0.0180 0.0072 20 0.0804 0.0105 0.004025 0.2817 0.0276 0.0113 25 0.1470 0.0133 0.005930 0.4776 0.0392 0.0159 30 0.2277 0.0161 0.007635 0.7183 0.0482 0.0205 35 0.3170 0.0179 0.009140 1.0385 0.0640 0.0260 40 0.4374 0.0241 0.0109

45 0.5756 0.0276 0.0128EP2SP342

II height group

10 0.0083 0.0015 0.0008 EP3SP463

III height group

10 0.0088 0.0017 0.000915 0.0344 0.0052 0.0023 15 0.0351 0.0053 0.002320 0.0984 0.0128 0.0049 20 0.0789 0.0088 0.003925 0.1951 0.0193 0.0078 25 0.1336 0.0109 0.005330 0.3378 0.0286 0.0113 30 0.2070 0.0147 0.006935 0.5126 0.0350 0.0146 35 0.2816 0.0149 0.008040 0.7268 0.0428 0.0182 40 0.3744 0.0186 0.0094

45 0.4734 0.0198 0.0105EP2SP351

III height group

10 0.0101 0.0019 0.0010 EP3SP445

IV height group

10 0.0055 0.0010 0.000515 0.0442 0.0068 0.0029 15 0.0243 0.0038 0.001620 0.1009 0.0113 0.0050 20 0.0682 0.0088 0.003425 0.1862 0.0171 0.0074 25 0.1196 0.0103 0.004830 0.2825 0.0193 0.0094 30 0.1701 0.0101 0.005735 0.3890 0.0213 0.0111 35 0.2003 0.0060 0.005740 0.5040 0.0230 0.0126 40 0.2260 0.0051 0.0057

45 0.2529 0.0054 0.0056EP2SP3119

IV height group

10 0.0071 0.0013 0.0007 EP3SP571

I height group

10 0.0077 0.0015 0.000815 0.0244 0.0035 0.0016 15 0.0619 0.0108 0.004120 0.0530 0.0057 0.0027 20 0.1804 0.0237 0.009025 0.0901 0.0074 0.0036 25 0.3259 0.0291 0.013030 0.1220 0.0064 0.0041 30 0.5040 0.0356 0.016835 0.1693 0.0095 0.0048 35 0.6655 0.0323 0.019040 0.2345 0.0131 0.0059 40 0.9109 0.0491 0.0228

45 1.2808 0.0740 0.0285

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1 2 3 4 5 1 2 3 4 5EP3SP4

7I height group

10 0.0176 0.0034 0.0018 EP3SP584

II height group

10 0.0098 0.0019 0.001015 0.1012 0.0167 0.0067 15 0.0461 0.0073 0.003120 0.2439 0.0285 0.0122 20 0.1141 0.0136 0.005725 0.3923 0.0297 0.0157 25 0.2069 0.0186 0.008330 0.5348 0.0285 0.0178 30 0.3366 0.0259 0.011235 0.6271 0.0184 0.0179 35 0.4821 0.0291 0.013840 0.7893 0.0325 0.0197 40 0.6832 0.0402 0.017145 0.9195 0.0260 0.0204 45 0.9647 0.0563 0.0214

EP3SP535

III height group

10 0.0057 0.0011 0.0006 EP4SP6146

II height group

10 0.0014 0.0003 0.000115 0.0365 0.0062 0.0024 15 0.0188 0.0035 0.001320 0.0924 0.0112 0.0046 20 0.0781 0.0119 0.003925 0.1581 0.0131 0.0063 25 0.1719 0.0188 0.006930 0.2351 0.0154 0.0078 30 0.3080 0.0272 0.010335 0.3002 0.0130 0.0086 35 0.4456 0.0275 0.012740 0.3966 0.0193 0.0099 40 0.5698 0.0248 0.014245 0.5391 0.0285 0.0120 45 0.7675 0.0395 0.0171

50 1.0111 0.0487 0.0202EP3SP549

IV height group

10 0.0055 0.0011 0.0005 EP4SP6117

III height group

10 0.0033 0.0007 0.000315 0.0295 0.0048 0.0020 15 0.0288 0.0051 0.001920 0.0682 0.0077 0.0034 20 0.0795 0.0101 0.004025 0.1124 0.0089 0.0045 25 0.1389 0.0119 0.005630 0.1604 0.0096 0.0053 30 0.2093 0.0141 0.007035 0.1963 0.0072 0.0056 35 0.3138 0.0209 0.009040 0.2361 0.0080 0.0059 40 0.4282 0.0229 0.010745 0.2856 0.0099 0.0063 45 0.6045 0.0353 0.0134

50 0.8729 0.0537 0.0175EP4SP683

I height group

10 0.0069 0.0014 0.0007 EP4SP687

IV height group

10 0.0035 0.0007 0.000315 0.0626 0.0111 0.0042 15 0.0423 0.0078 0.002820 0.1795 0.0234 0.0090 20 0.1126 0.0141 0.005625 0.3518 0.0345 0.0141 25 0.2123 0.0199 0.008530 0.5685 0.0433 0.0190 30 0.3187 0.0213 0.010635 0.8204 0.0504 0.0234 35 0.4127 0.0188 0.011840 1.0729 0.0505 0.0268 40 0.4677 0.0110 0.011745 1.3070 0.0468 0.0290 45 0.5212 0.0107 0.011650 1.5084 0.0403 0.0302 50 0.5553 0.0068 0.0111

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The method of vertical profiles was applied in these investigations (Аssman, 1954; Kennel, 1965; Krastanov, 1978; Tsakov, 2000). For this purpose, measure-ments of heights and diameters of all trees in each sample plot were done in tree spe-cies, as well as disposition in accordance height groups according to the percentage of the quantity of tree stand’s dominating height (Hd). Totally 906 trees were investi-gated, from then 734 – Douglas-fir (81.0%), 94 – Scots pine (10.4%), 47 – Dahurian larch (5.2%), 20 – birch (2.2%), 8 – aspen (0.9%) and 3 – beech (0.3%).

Douglas fir trees are disposed in four relative groups (І height group – over 95% from Hd; ІІ – 90–95%; ІІІ – 80–90%; ІV – 50–80%), and volume increment was monitored (Table 2). The course of current and average increment according to this inventory index was followed, as well as climate influence on 5 representative stems

Fig. 2. Climate indices change for summer air temperatures and annual precipitations in the region of MS Yundola

Fig. 1. Climate indices change for summer air temperatures and annual precipitations in the region of MS Velingrad

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from height groups (N 71, 83, 84, 117 and 87) – with best indices in the course of growth process according to volume.

The data about temperatures and precipitations was collected in the meteo-rological stations in Velingrad and Yundola. The changes in the temperature and precipitation regime during the growing season, shown on Fig. 1 and 2, are based on indices of summer air temperatures (It) and annual precipitation amounts (Iw). (Ferezliev, 2012). These indices are obtained as follows: for certain year average temperatures in the growing season from May to September (Ti) and precipitation amount for the whole year (Wi) are divided by average values of these quantities (Tav and Wav) for the relevant period.

RESULTS AND DISCUSSION

The results by Petkova (1989) were confirmed that a common feature of vol-ume increment of Douglas-fir plantations is its progressive increasing together with the age and considerable acceleration after 10-th year of age. Comparison with most accelerated volume increment of Douglas-fir in Central Balkan Range (current vol-ume increment for 5 years almost 4 (3.3) times) proves the advantage of investigated sample stems in North-West Rhodopes where accelerated increment is 11.8 times (from 0,0004 to 0,0047 m³) for Douglas-fir N163 – I height group of SP1. For the others analysed trees this increment is within the range from 2.5 (for N105 – III group in SP2 and N175 – IV group in SP1) to 11.7 times (N146 – II height group in SP6).

In contrast to the recorded more considerable volume increment in Douglas-fir plantations from 15 to 20 years of age in the mountain part of the Balkan Range, detention is calculated in investigated plantations in the same period (from 1.6 times for sample trees N119 – IV height group in SP3 and N49 – IV height group in SP5 to 5 times for Douglas-fir N175 – IV group in SP1). For the most of the others sample stems the increment is increased from 1.7 to 2.7 times.

The following specific features are observed within the course of current and average volume increment:

Current increment increases faster in the beginning, reaching up highest value in the end of investigated period for sample tree N71 – I height group (0.0740 m3) and lowest – 0.0054 m3 for N45 (IV height group). Lowest Zv

curr is recorded at the age of 10 for N175 – IV height group – 0.0002 m3.

The average increment in the beginning has lower values than the current one. In the end of the analysed period it reaches the maximum for N83 (I height group, SP6) – 0.0302 m3 and minimum for N175 (IV height group) – 0.0037 m3. Zv

av is low-est in 10th year of age for N210 and 175 – III and IV height groups and for N146 – II height group – 0.0001 m3.

The course of both kinds of increment in investigated periods is various and generalizes the following peculiarities:

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Current volume increment increases continuously from the beginning to the end of investigated period for 13 trees (N163, 153, 210, 86, 87 – SP2, 105, 25, 42, 51, 18, 63, 84 and 117) and reaches up its peaks in the end of the period. For another six sample stems – N119, 133, 71, 35, 49 and 146 peak values of Zv

curr are in the end of investigation but at more uneven course of the increment curve (there is single de-creasing of Zv

curr at the age of 30, 35 and 40 – then follow increase and peak again in the end of investigation). For 5 Douglas-fir trees the maximum in the current volume increment is recorded earlier – at the age of 25 for N45; at 30 – for N87 – SP6, at 35 – N175 and at 40 – for N7. All analysed Douglas-fir trees have one peak of their current volume increment.

The average volume increment grows more evenly than the current one. For most trees (22) the maximum values were recorded in the end of investigation. Only for N45 and 87 – SP6 (representatives of IV height groups), Zv

av is reached a peak earlier – at the age of 30 and 35, respectively.

The age of quantitative maturity is between 35 and 40 only for N45 and 87 – SP6. For the others of the trees the crossing of curves of both increments has not yet occurred by the moment of investigation.

The course of volume increment was also followed according to climate changes in years of growth of trees in height groups.

For I height group, the course of current volume increment of Douglas-fir N71 was recorded. This increment has the highest peak for the group – 0.0740 m3 – reached at the age of 45 in 2007. With the exception of single decrease between 30th and 35th (1992-1997), in the rest of the time of investigated period this increment constantly accumulates, especially in the last 10-year interval of investigation (1997-2007), when it increases more than twice.

For the investigated height group, the average volume increment is highest for Douglas-fir N83 in its 50th year in 2008. In the beginning the values of Zv

av have constantly grown – from 0.0007 m3 to 0.0302 m3 (maximum in the end of investi-gated period). The approximate correlation according to the volume increment is kept compared to the beginning of investigation – here with over 43 times more in its end.

Climatic influence is followed through calculated indices of temperature and precipitation regime in the peak and minimum years in the values of both increments for the investigated inventory index. In peak years, the following correlations be-tween precipitation (Iw) and temperature (It) indices are present in Zv values:

The maximum value of current volume increment (Zvcurr-max) is in 2007

(Fig.1), when Iw is higher than It, and both indices are with values over than the aver-age ones in the growing period, i.e. 1< It< Iw.

The obtained Zvav-max in 2008 cannot be discussed due to the absence of

data about temperatures and precipitations in that year from Meteorological Station Yundola. The minimum of volume increment (Zv

curr-min) is in 1972, at Iw >1 and It<1, and again decrease is observed in 1997 at relatively high temperatures close to

31

the average ones (It>1) and summer precipitations in much smaller quantity (Iw<1). Similar correlation between indices (It>1 and Iw<1) is observed for the minimal value of Zv

av-min in 1968 (Fig. 2).For II height group highest level of current volume increment has Douglas-fir

N84 – 0.0563 m3, which was accumulated at the 45th year (in 2007). This peak is with 23.9% lower than the maximum value reached by the previous height group.

Zvcurr has been increasing to the mentioned peak value from the beginning of

investigation period and this increase is more clearly expressed from 1997 to 2007. Totally for the investigation period the current volume increment for this tree has increased its value almost 30 times.

The highest average volume increment is again for Douglas-fir N84. The max-imum (0.0214 m3) is reached again at the age of 45 in 2007. From the beginning until this age, the values of Zv

av have increased – from 0.0010 m3 at the age of 10 to 21 times higher average volume increment by the end of investigation.

Reached maximums and minimums in both increments according to investi-gated inventory indices of investigated sample trees are identical according to time periods in which they were accumulated. In same conditions in 2007 and 1972 (as well as for I height group), maximums are accumulated (Zv

curr-max and Zvav-max)

and, respectively, minimums are expressed (Zvcurr-min and Zv

av-min).The course of current volume increment of Douglas-fir N117 was followed for

III height group – 0.0537 m3 – at the age of 50 in 2008. This peak is with 4.6% lower than the maximum value of this increment reached at the previous height group. This value of Zv

curr has increased from the beginning of the investigated time period up to 40 years of age and shows obvious progress in the last 10-year period – 1998-2008 and particularly in 2003-2008. Totally for the investigation period, the current vol-ume increment for the studied tree has increased its value nearly 77 times.

The same sample tree (N117) has the highest value of average volume incre-ment. The maximum of Zv

av (0.0175 m3) has occurred at the age of 50 in 2008. From the beginning until this age, values of Zv

av have increased constantly – from 0.0003 m3 at the age of 10, and by the end of investigation the average volume increment has been 58 times higher – the most significant growth for this increment until now.

The maximums (Zvcurr-max and Zv

av-max) are reached in 2008 and under the influence of the previous 10-year period (1998-2008). No comment will follow due to missing data about temperatures and precipitations for this year (Fig.2). The mini-mums are for the conditions in 1968, valid for the minimal average volume incre-ment for I height group.

For IV height group, the maximum value for the current volume increment is recorded for sample tree N87 (SP6) – 0.0213 m3, but in contrast to its behaviour in the previously analysed cases, this is achieved not by the end of the investigation period but quite earlier – at the age of 30 in 1988. Besides, this recorded peak is with lowest value (2.5 times lower than the maximum value of this increment reached up in the previous height group). Zv

curr has been increasing to the mentioned peak value

32

from the beginning until the age of 25 with a slight delay in the last 5-year period before reaching up its maximum value. From there, until the end of investigation, the current volume increment begins to decrease, especially in the period between 35 and 40 years of age, i.e. 1993-1998, and reaches 0.0068 m3 in 2008. Totally for the investigation period, the current volume increment for the studied tree has increased its value just about 9.7 times in the end.

Again, like the previous two height groups, the tree with highest current incre-ment (N87 – SP6) also has highest peak according to average volume increment. The maximum (0.0118 m3) has occurred at the age of 35 in 1993 and this is the lowest value of peaks of this increment recorded until now. From the beginning until this age, Zv

av constantly increases – from 0.0003 m3 at the age of 10, after its peak value it decreases and the average volume increment is 37 times higher by the end of in-vestigation (0.0111 m3).

Fig. 2 shows the conditions, under which minimums and maximums of sam-ple tree increments are expressed. The reached maximum value in current volume increment Zv

curr-max in 1988 is at correlation between the two indices Iw<It<1, after successive drop of precipitation index after 1976 and its minimal value for this time interval during the mentioned year. In 1993, the maximum of Zv

av is at Iw < It <1 – one of the lowest levels (after 1994) of this index for the entire time period from 1935 to 1999.

Second minimum (Zvcurr-min) was recorded in 2008, after the lowest value at

the age of 10 in 1968. Due to the absence of data about the climate in this year in MS ‘Yundola’, conditions in the period 1993-1998 will be taken into account, when strong decrease of current volume increment was observed. This happened under strong fluctuation in the values of It (under and above average ones) and less for Iw (predominantly increasing), but always under average levels. The biggest difference in correlations between two indices is in 1994, when It has a level quite over the average one and Iw –the lowest for the observed yearly interval.

CONCLUSIONS

On the basis of carried out investigation, the following conclusions and rec-ommendations could be drawn:

The current volume increment for most of the trees increases in the beginning and reaches up its peak value in the end of the investigation period.

The growth of the current volume increment from 10th to 15th year of age is more accelerated, compared to investigations carried out until now in other regions of the country and, in contrast to them, significantly decreases in the next 5-year time period.

For the most of the trees, the maximum of average volume increment is re-corded by the end of investigation, when the moment of highest average productivity of the tree according to the studied inventory index has not come yet.

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The peak value for the average volume increment is at Iw>It for the height groups from the upper part of the canopy and they are on various proportion towards the average temperatures and precipitations during the active period. At reverse cor-relation and Iw<It<1, the maximal value for Zv

av has been reached up – in IV height group.

Used height groups, bound by dominating height and indices of temperature and precipitation regime, allowed to apply a new approach in characterization of the volume increment in middle-aged Douglas-fir plantations in this region of the Rhodopes.

Accumulated information, generalizing maximums and minimums of volume increment and knowledge of regularities in its changing, make possible to draw con-clusions about the current increment using the average one, which is important for carrying out of silvicultural activities in forests, for determination of future trees, etc. The occurrence of the maximum of average volume increment gives the idea about most rational from economic point of view utilization of trees.

The availability of information about the general course of curves, character-izing the volume increment course, together with this one according to thickness and height through stem analysis (Ferezliev, 2012; Ferezliev, Tsakov, 2012) in climate change conditions in the past, can be used for more optimal choice of economic ap-proach in the management of Douglas-fir plantations.

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E-mail: obig@abv.bg