Báo cáo khoa học: "Growth response of holm oak (Quercus ilex L) to commercial thinning in the Montseny mountains"

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Original article Growth response of holm oak (Quercus ilex L) to commercial thinning in the Montseny mountains (NE Spain) X F Rodà Mayor Centre de Recerca Ecològica i Aplicaciones Forestals (CREAF), Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain August 1992; accepted 6 January 1993) 17 (Received Summary — Growth responses of holm oak (Quercus ilex) to commercial thinning were studied in the Montseny Biosphere Reserve (northeast Spain), where selection thinning for firewood production is currently the dominant form of management in holm oak forests. Thinning significantly increased mean stem diameter increment by 83% over that of unthinned plots during the 6-9-yr period after thinning, and by 48% from 9-12 yr after thinning. Absolute diameter increment was positively corre- lated with initial diameter at 1.30 m from the ground (dbh) both in thinned and unthinned plots. Thin- ning increased growth in large trees more than in smaller trees. Relative diameter growth was nega- tively correlated with initial dbh. It is concluded that individual holm oak stems in previously coppiced stands respond vigourously to thinning, and still do so 6-9 yr after thinning. The growth response di- minishes 9-12 yr after thinning due to canopy closure. However, absolute rates of stand growth, as well as basal area and stem biomass increments, were unaffected by thinning during these time in- tervals, an example of density compensation. canopy closure I Quercus ilex = holm oak I selection thinning I stand growth / tree growth Résumé — Effet d’une éclaircie commerciale sur la croissance d’un chêne vert (Quercus ilex L) dans les montagnes du Montseny (NE de l’Espagne). L’effet d’une éclaircie commerciale sur la croissance du chêne vert (Quercus ilex) a été étudié dans la réserve de la Biosphère du Montse- ny (NE Espagne). Dans cette région, l’éclaircie sélective pour la production du bois de chauffage est la forme la plus commune de gestion des forêts. L’éclaircie a augmenté l’accroissement de diamètre des tiges de 83% par rapport aux placettes non éclaircies entre 6 et 9 ans et de 48% entre 9 et 12 ans après le traitement. L’accroissement absolu de diamètre est corrélé positivement avec le dia- mètre initial à 1,30 m. Les gros arbres ont davantage augmenté leur croissance que les petits. L’ac- croissement relatif en diamètre est corrélé négativement avec le diamètre initial à 1,30 m. On peut conclure que les tiges du chêne vert dans le taillis étudié ont une réponse vigoureuse à l’éclaircie et que cette réponse se prolonge encore 6 à 9 ans après. L’effet sur la croissance diminue 9 à 12 ans après l’éclaircie par suite de la fermeture du couvert végétal. Cependant, les taux absolus d’accrois- sement du peuplement, ainsi que la croissance de la surface terrière et de la biomasse des tiges, ne sont pas affectés par l’éclaircie pendant ces intervalles de temps, ce qui constitue un exemple de compensation de la densité. chêne vert / éclaircie sélective / fermeture de la couverture végétale / Quercus ilex = accroissement du peuplement / croissance des tiges
INTRODUCTION to selection thinning for firewood, a silvicul- tural method that was previously practised only to a limited extent. This important Selection thinning is a standard silvicul- management change is widespread in tural practice that has been successful in holm oak forests in the region of relatively many forest types for sustained timber pro- high rainfall in northeast Spain. Usually duction in uneven-aged stands (Boudru, about one-third to one-half of the canopy 1989). Additionally, thinning can be used trees are cut at intervals from 18-25 yr, to favour tree regeneration, improve the changing the stand to an uneven-aged environmental conditions for wildlife, modi- stand. There is very little quantitative infor- fy the likelihood and impact of disturban- mation on the effects of such change, ei- ces, or create spatial patterns of communi- ther on tree growth and forest production ty types and species richness (Johnson or on its ecological consequences. and Krinard, 1983; Frankling and Forman, The purpose of this paper is to report re- 1987). sults on tree and stand growth after a com- Thinning increases the availability of mercial thinning of a holm oak stand, in the light, water and nutrients to the remaining 6-12-yr interval after thinning. trees. As a result, tree growth is usually in- creased after thinning. Growth responses to thinning have been modeled to provide MATERIAL AND METHODS increased knowledge to be applied in fo- restry (Hibbs and Bentley, 1984; Piennar and Shiver, 1984; Whyte and Wollons, Study site 1990). Thinning effects on tree growth are usually studied in terms of stem diameter increment, height growth, and canopy ex- This study was carried out within the Torrent de pansion of the remaining trees (Hamilton, la Mina catchment at La Castanya Biological Station (41 ° 46’ N, 2° 21’ E) in the Montseny 1981; Ducrey, 1988; Baldwin et al, 1989; mountains, a natural park and biosphere re- Bouchon et al, 1989; Cutter et al, 1991), serve in northeast Spain. The lower half of this but effects on production of stump re- 200-ha catchment is covered by a dense holm sprouts (Ducrey and Boisserie, 1992; Re- oak forest where biomass, primary production tana et al, 1992) and epicormic sprouts and nutrient cycling have been extensively in- (Paysen et al, 1991) have been studied as vestigated (Ferrés et al, 1984; Escarré et al, well. Growth responses to thinning are rel- 1987; Avila and Rodà, 1988; Caritat and Terra- das, 1990; Mayor, 1990; Rodà et al, 1990; Can- atively well known in many coniferous adell and Rodà, 1991; Bonilla and Rodà, 1992; (Hamilton, 1981; Baldwin et al, 1989; Mayor and Rodà, 1992). Climate is subhumid Whyte and Woollons, 1990) and decidu- Mediterranean with a mean annual precipitation ous broad-leaved species (Bouchon et al, of 870 mm. The bedrock is a metamorphic phyl- 1989; Cutter et al, 1991). lite and soils are rather shallow, sandy-loam A peculiar situation arises in extensive dystric xerochrepts with a high stone content. Slopes are very steep (mean 34°). Holm oak is tracts of Mediterranean hardwood forests virtually the only tree species in the tree layer. that were intensively coppiced in the past The understory is sparse. Most of this forest has for charcoal production, resulting in high not been disturbed since the end of charcoal density even-aged stands of relatively production in the 1950s. The present stand small stump resprouts. After abandonment structure is dominated by multi-stemmed trees of charcoal production in the 1950s, many originating from stump resprouting, though sin- private owners shifted in the early 1970s gle-stemmed trees are also common.
dbh are likely to change as a result of thinning. Field measurements Conversely, for stem biomass the slow rates of growth displayed by holm oak makes unlikely For this study we took advantage of a commer- that allometric relations with dbh change to any cial thinning carried out in 1979 by the private significant extent during the first 12 yr after thin- owner of a sector of the east-facing slope of the ning. Stem biomass increment was determined catchment, at an altitude of 900 m. Estimated the difference between biomass at initial and as mean annual temperature at this topographic final dates for the periods 1985-1988, 1988- position is 11-12°C. In late June 1985, 4 repli- 1991 and 1985-1991. cate plots were laid out within the thinned area, and 3 control plots in an adjacent unthinned area. Since the thinning was commercial instead Statistical analysis of experimental, thinned and unthinned plots could not be interspersed. However, the thinned and unthinned plots were very close together, thinning on stem diameter growth Effects of had the same slope aspect and steepness and the whole study period were tested rates over similar soil. Aerial photographs taken in 1978 by a t-test, using the arithmetic mean diameter before thinning confirmed that the forest was growth rate of each plot, and by an analysis of quite homogeneous. covariance (ANCOVA) of individual growth rates Circular plots with an area of 154 m were 2 using initial stem diameter (dbh) as a covariate. used. When the plots were laid out, dbh (diame- Time-dependence of tree and stand growth ter at 1.30 m from the ground) was measured rates were tested by repeated measures analy- for all living stems forming the tree layer (dbh ≥ sis of variance. To guard against the effect of 5 cm). All stems were permanently numbered autocorrelation in the dependent variable(s), and a line was painted on the exact point along Greenhouse-Geisser and Hunyh-Feldt epsilon the stem where diameter was measured. This estimates were used to correct the P-values. In greatly increased the accuracy of stem diameter no case did these corrections affect the result of increments determined from repeated measure- the analyses and are not reported here. Analy- ments. Stem diameters were remeasured in July ses were performed with the SuperANOVA sta- 1988 and July 1991. Diameter increment (over tistical package (Abacus Concepts, 1989). bark) for each stem during each period of 3 or 6 During the study period, 9 out of 230 tallied yr (1985-1988, 1988-1991, and 1985-1991) holm oak stems developed cracks or bumps at was determined from difference in diameter at the point of diameter measurement, preventing both dates. Stem biomass (wood plus bark of a meaningful reading of their diameter incre- the trunk and branches down to 5 cm in diame- ments. These stems were not taken into ac- ter) for each stem was estimated for each date count in analyses involving stem growth rates. through an allometric regression on dbh derived When considering stand growth rates (basal for this holm oak forest. From the several availa- area and biomass increments), diameter of ble regressions (Canadell et al, 1988), that for these 9 stems at the dates of interest were esti- trees 4-7 m in height was used, since height of mated by linear regression of final dbh on initial most stems was within this range. The regres- dbh. sion was: RESULTS where SB is stem biomass (kg dry weight), and Stand structure dbh is in cm. We preferred to estimate stem bio- mass instead of total aboveground biomass be- cause, as here defined, it is the component of At the start of the study, ie 6 yr after thin- the tree utilized for firewood, and because total ning, density and basal area of the tree biomass includes the biomass of fine branches expected, significantly high- layer were, as and leaves. The latter components are rather in unthinned than in thinned plots. Mean dynamic and their allometric relationships with er
was 2 837 ± 348 (SE) stems·ha -1 density in unthinned plots, and 1 608 ± 77 (SE) -1 stems·ha in thinned plots (t = 4.0, df = 5, P= 0.01).Mean basal area was 28.2 ± 4.5 (SE) m in unthinned plots, and 11.3 -1 ·ha 2 ± 1.0 (SE) m in thinned plots (t -1 ·ha 2 = 3.8, df 5, P=0.013). Mortality from 6-12 = yr after thinning was very low. Summing over all plots, only 4 out of 230 initial stems died during this 6-yr period. This yielded a mean annual mortality rate of 0.3%. Ingrowth to the tree layer (dbh ≥ 5 cm) is also very limited in unthinned holm oak plots in this area (Mayor and Rodà, unpublished data) because virtually all stems with dbh < 5 cm are suppressed stems having no or negligible diameter growth. Stump sprouts were abundant in the thinned plots but none of these had reached a dbh of 5 cm even 12 yr after thinning. Therefore, as mortality and in- growth were negligible, stem density of the tree layer measured 6 yr after thinning should be nearly the same as that just af- ter thinning. In this way we can estimate that this commercial thinning removed 43% of the holm oak stems having a dbh > 5 cm. This thinning intensity is common for thinnings undertaken by private owners at Montseny. The same computation cannot be applied to estimate the percentage of basal area removed, since basal areas must have changed during the first 6 yr af- Tree growth ter thinning. However, it must be noted than thinning intensity was higher in terms of basal area removed than it was in num- Absolute stem diameter increment ber of stems, because thinning was more intense in the larger size classes, as is Mean absolute stem diameter increment commonly the case at Montseny. This can during the interval from 6-12 yr after thin- be deduced from the higher quadratic ning was 0.83 mm·yr (±0.05 SE, n 3) -1 = mean diameter still detectable 6 yr after for unthinned plots, and 1.43 mm·yr-1 thinning in unthinned plots (11.2 cm) than (±0.04 SE, n 4) for thinned plots. The ef- = in thinned plots (9.7 cm). As a result of the fect of thinning was highly significant (t = size-selective thinning, stems with a dbh > 9.7, df = 5, P=0.0002). 15 cm accounted for 15% of the number of stems in the unthinned plots but only 3% The time-dependence of the above ef- in the thinned plots (fig 1). fect can be addressed by analyzing separ-
ately data for the period 1985-1988 (6- initial dbh (x, in for trees in cm) were un- 9 yr after thinning) and 1988-1991 (9-12 thinned plots: yr after thinning), as shown in table I. A re- peated measures analysis of variance was used to evaluate significance of differenc- es through time and those due to treat- ment (thinned versus unthinned). Both and for trees in thinned plots: time and treatment had a significant effect (P 0.002 in both cases). No interaction = between treatment and time was found. Stem diameter increments were higher for thinned than for unthinned plots, and were higher during the first period (1985-1988) An ANCOVA to test whether was run than the second (1988-1991) for both thinning still had a significant effect on di- thinned and unthinned plots (table I). Thin- ameter growth after discounting the effect ning increased mean stem diameter incre- of initial dbh, and whether there was a sig- ment by 83% over that of unthinned plots nificant interaction between thinning and during the period 6-9 yr after thinning, and initial dbh. The full ANCOVA model includ- by 48% from 9-12 yr after thinning. ed terms for treatment (thinned or un- Absolute increments (mm·yr in stem ) -1 thinned), initial dbh as covariate, and the diameter of individual holm oaks during the interaction between both. This full model interval from 6-12 yr after thinning were gave a significant effect of dbh (F 1.213 = weakly but positively and significantly cor- 28.0, P 0.0001),as expected from the = related with initial stem diameter, both in above regressions; a non-significant effect thinned and unthinned plots (P of treatment (F 0.98, P 0.32), and 0.0002, 1.213 = = = r = 0.38 and P 0.0001, r= 0.34, respec- a doubtfully significant interaction (F 1.213 = = tively). Thus, large trees showed on aver- 2.5, P 0.12). The ANCOVA was then re- = age higher absolute growth rates than peated deleting the non-significant treat- smaller ones. Linear regressions between ment term, with the result that not only the stem diameter increment (y, mm·yr and ) -1 initial dbh but also the interaction between
them. The repeated measures analysis of thinning and dbh became highly significant variance gave significant effects for treat- (F = 53.8, P= 0.0001). This means 1.214 ment and time (P < 0.002 in both cases), that thinning increased absolute diameter and for their interaction (P= 0.026). The in- growth rates more in larger trees than in teraction arose because during the first pe- smaller ones: mean diameter increments riod (1985-1988) relative diameter incre- where 138% higher in thinned than in un- ment was much higher in thinned than in thinned plots for trees of dbh 11-15 cm, unthinned plots while this difference de- and 98% higher for trees of dbh 5-8 cm. creased in the second period: mean rela- tive diameter increment was 108% higher Relative stem diameter growth in thinned than in unthinned plots during 6-9 yr after thinning, but only 47% higher growth rates in stem diameter Relative during 9-12 yr after thinning (table I). computed for individual stems divid- were ing the annualised absolute increment (mm yr in a given period by the stem ) -1 Stand growth diameter at the start of the period, and ex- pressing the result as a percentage. Mean increment relative diameter increments during the in- Basal area terval from 6-12 yr after thinning were During the interval from 6-12 yr after thin- 0.87% yr and 1.64% yr in unthinned , -1 -1 ning, mean basal area of the tree layer in- and thinned plots, respectively (table I). As creased in the unthinned plots from 28.2 to opposed to absolute diameter increments, 30.2 m (table II). Mean basal area in -1 ·ha 2 relative diameter growth rates during the the unthinned plots increased from 11.3- interval from 6-12 yr after thinning were 13.4 m Mean annual basal area in- . -1 ·ha 2 weakly but negatively and significantly cor- crement was 0.33 and 0.35 m in ·yr -1 ·ha 2 related with initial stem diameter (P = unthinned and thinned plots, respectively 0.025, r -0.20 for unthinned plots, and = P= 0.016, r = -0.25 for thinned plots). The (table II). corresponding linear regressions between As before, a repeated measures analy- relative diameter growth rates over this 6- sis of variance was used with absolute yr period (y, % yr and initial dbh (x, cm) ) -1 and, separately, relative basal area incre- were, for trees in unthinned plots: ments as dependent variables. The latter was calculated dividing the absolute basal area increment of each plot by the basal area at the start of the considered period, and expressing the result as a percentage (table III). For absolute increments, neither and for trees in thinned plots: thinning, time, nor their interaction were significant (P> 0.29 in all cases). For rela- tive increments, both thinning and time 0.0006 and P 0.02, were significant (P = = respectively), while the interaction between The ANCOVA gave significant effects of them was marginally significant (P = 0.056). Relative basal area increment had 1.213 12.1, P 0.0006) both thinning (F = = 1.213 (F 10.7, P == 0.001), to be higher in thinned plots, as we found, and initial dbh = since absolute basal area growth was not without significant interaction between
the above increments slightly underesti- production since some stem mate stem mortality occurred during this period. A repeated measures analysis of vari- ance with absolute and, separately, rela- tive stem biomass increments (the latter calculated as explained for the relative ba- sal area increment) as dependent vari- ables yielded the same results as de- scribed for basal area growth. This is no surprise since basal area is a function of squared dbhs, and stem biomass is an al- lometric function of dbh raised to an expo- nent of 2.04 (see Methods). affected by thinning whilst initial basal area much reduced by it. was DISCUSSION Stem biomass increment In 18 plots of closed holm oak forest span- most of the topographic variation with- ning During the interval from 6-12 yr after thin- in the Torrent de la Mina catchment, the ning, mean stem biomass in unthinned plots increased from 72.0-77.3 t·ha -1 mean diameter increment during 1985- 1988 was 0.87 mm·yr (Mayor, 1990). -1 (table II), while that of thinned plots in- Our results for the unthinned plots are very creased from 28.5 to 34.0 t·ha Mean in- . -1 similar: 1.06 mm·yr for the same period, -1 crements in stem biomass were 0.88 and 0.91 t·ha yr for unthinned and thinned -1 -1 and 0.83 mm·yr for the whole 6-yr per- -1 iod. Similar growth rates (1.05 mm·yr ) -1 plots, respectively. It should be noted that
uptake of water and nutrients from a larger found in a lowland, unthinned holm were root system, are probably involved in this oak coppice on calcareous bedrock in southern France (Ducrey and Toth, 1992), response. where mean precipitation is slightly higher Growth response to thinning was very than at Montseny (1 000 mm·yr In con- ). -1 6 to 9 yr after strong in the interval from trast, holm oak diameter increments were and declined in the period 9-12 thinning, much smaller (0.27 mm·yr in the Prades ) -1 yr after thinning. Using dendrochronologi- mountains (120 km southwest of Mont- cal methods, Cutter et al (1991) found that seny) probably due to the lower rainfall Quercus vetulina (a deciduous oak) and very high stand density (Mayor and showed increased growth responses to Rodà, submitted). thinning until 10-12 yr after thinning, Holm oak at Montseny showed a posi- growth rates felling then to pre-thinning tive growth response to thinning, as evi- values. In our case, the reduced growth re- denced by enhanced growth rates for stem sponse 9-12 yr after thinning can be linked diameter, and for relative increments of to canopy closure around this time. Inspec- basal area and stem biomass. For all tion of thinned plots 12 yr after thinning re- these variables thinning increased growth vealed that canopy closure was almost rates around 2-fold. Mean diameter incre- complete. ment in thinned plots was 1.43 mm·yr . -1 Effects of thinning on tree growth are Similar results were found by Ducrey and best conceptualized by considering thin- Toth (1992) in a holm oak coppice where a ning as a man-made disturbance that re- moderate thinning treatment with a reduc- duces the stand density and increases the tion in basal area of 40-45% yielded a availability of resources for the remaining mean diameter increment of 1.50 mm·yr . -1 trees. Increased availability of space, light, The commercial thinning we studied re- water and nutrients implies a decrease in duced stem density by 43%, and reduction competition between trees. Thinning re- in basal area must have been greater. Re- leases previously occupied space; this, to- tana et al (1992) found a mean basal area gether with increased light reaching the reduction of (67% ± 5 SE) for holm oak crowns of the remaining trees, allows for stands in another Montseny site. However, crown expansion through shoot elongation most forest owners at Montseny do not and growth of lateral shoots. These gener- conduct thinning on a quantitative basis, al response patterns hold both for trees de- and thinning intensity can change from rived from seed or from resprouting. one owner to another and from year to Holm oaks in thinned stands at Mont- year. seny show relatively fast rates of canopy Holm oak responded to thinning differ- expansion in the few first years after thin- ently according to tree size. In absolute ning (Mayor and Rodà, unpublished data). terms, growth of large stems was stimulat- Wider and denser crowns result in a higher ed by thinning more than that of smaller leaf area of each individual stem after thin- trees. Large trees probably have a greater ning, thus increasing the light interception capacity for resource acquisition, and are capacity of the tree. Interestingly, Hamilton thus more able to taken advantage of the (1981) found that in thinned stands where increase in resource availability that takes crowns had been experimentally reduced, place after thinning, and to eventually use the observed growth response was less these resources for growth. More specifi- than expected for the same thinning inten- cally, a higher capacity for canopy expan- and sity without crown reduction. Water and vigorous branches, higher sion, more
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