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Effects of post-fire silviculture practices on Pachyrhinus squamosus defoliation levels and growth of Pinus halepensis Mill.
Forest Ecology and Management 167 (2002) 185±194
Effects of post-®re silviculture practices on Pachyrhinus squamosus defoliation levels and growth of Pinus halepensis Mill. Ana GonzaÂlez-Ochoa*, Jorge de las Heras Departamento de ProduccioÂn Vegetal y TecnologõÂa Agraria, Universidad de Castilla-La Mancha, ETSIA, Campus Universitario s/n, 02071 Albacete, Spain Received 31 January 2001; received in revised form 3 July 2001; accepted 20 July 2001
Abstract Pinus halepensis Mill. (Aleppo pine) forms dense and extensive stands after frequent ®res in SE Spain. Due to the high densities reached by this species after ®re, thinning is necessary. Two great ®res occurred in SE Spain in August 1994 and natural regeneration stands of P. halepensis were managed in 1999 using several silvicultural treatments (thinning, pruning and scrubbing). The objective was to improve the stands quality and accelerate pine canopy growth. However, since a few years, damages produced by several defoliator species from Pachyrhinus genus (Coleoptera, Curculionidae) have been detected in young treated P. halepensis stands in Spain. To test the intensity of the P. squamosus attack, two post-®re localities submitted to different treatments were selected (Yeste and Calasparra, in SE Spain) corresponding to different climate conditions (dry and semiarid, respectively). Results showed that the most intense defoliation level was recorded in Yeste. Beetles preferentially ate needles from the previous growing season in both localities. Signi®cant differences for defoliation percentage depending on silvicultural treatments were also recorded. Nutritional state of the needles for nitrogen, phosphorus and potassium was similar in both localities showing some relationship with silvicultural treatments. In general, the more intense the silvicultural treatment, the higher defoliation percentage was recorded. Growth pattern was similar in both study sites, reaching bring higher where treatments were more intense. Reduced needle and shoot growth was registered in Calasparra relative to Yeste. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Wild ®re; SE Spain; Defoliator; Aleppo pine; Curculionidae
1. Introduction Pinus halepensis Mill. occurs in a large variety of climatic conditions in the Iberian Peninsula, throughout its Mediterranean Basin distribution (Blanco et al., 1998). This species has been subjected the effects of *
Corresponding author. Tel.: 34-9-6759-9200; fax: 34-9-6759-9238. E-mail address: [email protected] (A. GonzaÂlez-Ochoa).
®re along the centuries and has developed adaptations in response to this perturbance, such as massive releasing of seeds (Saracino et al., 1997). If the frequency of ®res is low, P. halepensis forms high density stands due to its serotinous cones (BarbeÂro et al., 1998). The density reached is sometimes so high that it is imperative to reduce it by silvicultural practices. In the last few years, intense infestations have been recorded of Pachyrhinus sp. species (Coleoptera, Curculionidae), on both planted (del
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 7 1 8 - 6
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A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
Pozo, Personal Communication) and naturally-regenerated P. halepensis seedlings after ®re and silvicultural practices (Lapesa, 2000). Pachyrhinus sp. occurs in the Mediterranean Basin and central Europe primarily. In Spain there are several species of this genus, attacking pine forests of all ages, but with a preference for young stands. The likelihood of a pest infestation depends on several factors. Environmental factors are generally the most important in¯uencing insect reproduction. Other important factors are, abundance of dead wood in the forest, weakening of trees, reduction in natural enemies and ®nally, a wrongly scheduled silvicultural practices (Romanyk and Kadahia, 1992). Some of these factors may be responsible for the Pachyrhinus sp. outbreaks that have become usual in young P. halepensis forests in Spain. An infestation of Pachyrhinus squamosus Kiesenwetter appeared 1 year after silvicultural practices were applied to two different 5-year-old P. halepensis forests in SE Spain. The objectives of this study were to evaluate the pest infestation from different points of view: (i) to determine if there are differences in the damage level depending on the age of the pine needles, (ii) to discover if the intensity of the damage is related to the silvicultural practices, (iii) to ®nd any possible relationship between the intensity of the damage and tree morphological variables, and (iv) to relate the intensity of the infestation to climate conditions and nutritive level of the needles. Finally, we related the growth response of the trees to beetle defoliation and silvicultural practices.
2. Materials and methods 2.1. Study sites Two large ®res occurred in mature P. halepensis forests in August 1994 in SE Spain. Total surface burnt was about 44,000 ha, in two different provinces (Albacete and Murcia). In each burnt zone a locality with high regeneration density was selected: Yeste (province of Albacete) and Calasparra (province of Murcia). General characteristics of the sites are shown in Table 1. After ®re, natural regeneration took place in both of them, reaching a very high density of seedlings: 4,722 trees/ha (2;134 S:D:) with a medium height of 105 cm in Yeste and 45,000 trees/ha (20,400 S:D:) with a medium height of 51 cm in Calasparra in year 1999. Experimental plots were established in each study site in order to conduct different silvicultural treatments (thinning, scrubbing and pruning and different combinations between them) shown in Table 2. In Yeste a total of 27 plots were installed in which seven different treatments were conducted (three replicates of every treatment). Besides, six more plots were left as controls. In Calasparra, 21 plots were established and six different treatments were conducted (three replicates for each treatment) and three more plots as controls. The plot size was 15 m 10 m. Due to the high attack intensity and the proximity of the control to the treated plots, three additional plots were installed in Yeste 1 km away from the experimental block. Two meteorological stations were installed in 1999 in both study sites.
Table 1 Characteristics of the localities and sampling dates since study of P. squamosus outbreak in P. halepensis in Spain
Location Tree age (years) Annual precipitation (mm) Annual average temperature (8C) Soil texture class pH N (%) P (mg/l) K (mg/l) Silvicultural treatments date Defoliation sampling date Needles and shoots sampling date
Yeste
Calasparra
28200 W; 388220 S 6 530 13.1 Sandy 8.6 0.45 3.60 200.10 15 July 1999 4 May 2000 2 June 2000; 6 October 2000
18380 W; 388160 S 6 290 16.5 Sandy loam 8.7 0.19 3.43 215.24 1 August 2000 19 May 2000 19 June 2000; 28 October 2000
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194 Table 2 Silvicultural practices applied to ®re-regenerated P. halepensis in two study sites Treatments
Silvicultural practice
T S TS TP SP TSP C T2 S P C2
Thinning (1600 trees/ha) Scrubbing Thinning (1600 trees/ha) and scrubbing Thinning (1600 trees/ha) and pruning Scrubbing and pruning Thinning (1600 trees/ha), scrubbing and pruning Control (inside experimental block) Thinning (800 trees/ha), scrubbing and pruninga Control outside experimental blocka
a
Practice not performed in Calasparra.
In spring 2000 the two study sites sustained infestations of P. squamosus. Climate conditions before and after the outbreak are shown in Fig. 1. Average temperature during the previous 16 months before the outbreak was 14 8C in Yeste and 15.7 8C in Calasparra. During this period total annual rainfall was of 612 mm in Yeste and 361 mm in Calasparra.
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2.2. Infestation sampling Sampling was conducted in May 2000 to evaluate the intensity of the infestation at every study site. Needles from ®ve trees in each plot were randomly taken (15 trees per treatment). Two samples of needles of 20 g each were taken: one of the youngest needles (from the previous growing season, i.e. 1999) and another of older needles. This was done to test if there was a signi®cant difference in damage level depending on age of the needles. Immediately, chlorophyll a, and b, and total in the needles were determined spectrophotometrically in 100% dimethylformamide extracts, using the equations of Inskeep and Bloom (1985). Water content of the needles was also measured by calculating the difference between fresh and dry weight (storing samples 24 h at 105 8C). Finally, damaged needles were counted (needles bitten by the insect) to obtain the defoliation percentage. In this way, when any reference to ``defoliation percentage'' is given, data are not a visual estimation but a totally objective parameter. Defoliation percentage,
Fig. 1. Monthly precipitation (mm) and average temperature (8C) in the study sites: (A) Yeste; (B) Calasparra. Arrow means P. squamosus outbreak.
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
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chlorophyll a and b, total chlorophyll, and water content were used to determine if there were signi®cant differences between young and old needles damage. To test the damage level depended on the type of silvicultural practices, defoliation percentage was used. Finally, we looked for relationships between defoliation and tree size, total height (H, cm), diameter at the middle height (D1, mm) and stem diameter at 30 cm from the stem base of the tree (D2, mm) were measured, for all the sample trees. Due to the differences in climate conditions, the two study localities were considered separately. However, descriptive comparisons were done taking into account the climate conditions of 1999 and 2000 and nutrient level of the needles. To evaluate the response of pine growth after the defoliation, needles and shoots from the 2000 growing year were measured twice: immediately after infestation collapse (June 2000) and 6 months later (October 2000).
At the beginning of the infestation, samples were taken from the current year needles, from three trees per plot in both localities, to determine foliar macronutrients. Total nitrogen and total phosphorous were determined with standard micro-Kjeldahl techniques (Jones and Case, 1990). Total potassium was obtained using plasma emission spectroscopy (DeBolt, 1980). Data obtained are shown in Table 3. 2.3. Statistical methods For all statistical tests, data were transformed when necessary using the log or arcsine transformation, to meet the assumptions of normality and homoscedasticity. Both tables and ®gures present the original nontransformed data and standard error of the mean (S.E.). To test for differences in percentage of defoliation, chlorophyll and water content between 1-yearold needles and the oldest needles, a two samples
Table 3 Total nitrogen, phosphorus and potassium content in P. halepensis needles (mean S:E:) in two study sites in Spain during a P. squamosus outbreak (April 2000) Treatments
N (%)
P (g/kg)
Yeste T S TS TP SP TSP C T2 S P
1.1 0.9 1.1 1.2 1.1 1.4 0.9 1.7
0.3 0.1 0.0 0.0 0.0 0.2 0.1 0.1
Calasparra
Yeste
1.1 0.9 1.1 1.2 1.1 1.2 0.9
0.8 0.5 1.0 0.9 0.7 0.9 0.6 1.1
0.0 0.1 0.1 0.1 0.2 0.0 0.1
K (g/kg)
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Calasparra
Yeste
0.9 0.5 0.8 0.7 0.7 0.8 0.7
2.8 2.0 2.4 2.5 2.4 2.6 2.3 3.6
0.0 0.0 0.0 0.0 0.0 0.0 0.0
Calasparra 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
3.6 2.6 3.9 2.9 2.7 3.3 2.8
0.0 0.0 0.1 0.0 0.0 0.0 0.0
Table 4 Defoliation percentage, chlorophyll a and b, and total chlorophyll, and water content of P. halepensis needles in two study sites (mean S:E:), immediately after the P. squamosus attack (May 2000)a Yeste
Defoliation percentage (%) Chlorophyll a (mg/g fresh needles) Chlorophyll b (mg/g fresh needles) Total chlorophyll (mg/g fresh needles) Water content (%) a
Calasparra
Young needles
Old needles
Young needles
Old needles
79.7 0.4 0.1 0.6 56.1
36.7 0.6 0.2 0.8 43.0
40.5 0.4 0.2 0.6 40.3
25.7 2.0 b 0.5 0.0 a 0.2 0.0 a 0.7 0.0 a 33 1.9 b
2.3 0.0 0.0 0.1 1a
a a a a
2.6 0.0 0.0 0.0 1.3
b b b b b
Means within a row followed by different letters differ signi®cantly at P < 0:05 at every site.
2.3 0.0 0.0 0.0 2.4
a a a a a
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
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Fig. 2. Defoliation percentage (mean S:E:) of young needles recorded in the study plots in Yeste and in Calasparra. Bars with the same letter are not signi®cantly different at P < 0:05.
comparison analyze was used. A one-way ANOVA was used to test for the differences in defoliation level and growth differences due to the different silvicultural treatments. Later, Fisher's least signi®cant difference (LSD) procedure was used to discriminate among the means. Finally, to determine the relationships between tree defoliation and
morphological parameters, we used a regression model. Defoliation percentage was used as dependent variable; independent variables were: tree height (cm), diameter at the middle height (mm) and diameter at 30 cm from the base height (mm). All statistical analysis were conducted using a critical P-value of 0.05.
Fig. 3. Defoliation percentage (mean S:E:) of old needles recorded in the study plots in Yeste and in Calasparra. Bars with the same letter are not signi®cantly different at P < 0:05.
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T Yeste Needles (cm) Shoots (cm) Calasparra Needles (cm) Shoots (cm) a
S
TS
TP
SP
TSP
C
May October May October
2.57 5.99 10.18 14.34
0.06 0.22 0.45 0.55
b bc c d
1.71 5.25 8.85 12.86
0.06 0.11 0.34 0.48
a b b b
2.67 6.78 10.16 14.15
0.09 0.22 0.34 0.48
b 3.02 c 6.05 c 9.78 cd 12.98
0.08 0.21 0.25 0.37
c 2.79 bc 6.62 bc 10.53 bc 14.45
0.08 0.16 0.34 0.37
b c c d
3.25 6.30 10.31 13.8
0.12 0.23 0.25 0.48
cd c c bcd
May October May October
1.56 3.96 10.05 6.59
0.08 0.23 0.49 0.39
b c cd b
0.71 3.29 7.96 3.71
0.12 0.18 0.33 0.18
a b b a
1.69 3.75 9.22 6.57
0.15 0.18 0.46 0.50
b bc c b
0.15 0.26 0.48 0.51
c d d c
0.19 0.21 0.48 0.44
b 3.06 bc 5.52 bc 12.9 bc 10.35
0.15 0.20 0.44 0.40
d d e d
Means within a row followed by different letters differ signi®cantly at P < 0:05.
2.35 5.11 10.8 8.08
1.81 3.85 8.98 7.23
T2 S P
1.73 4.37 7.63 10.52
0.04 0.37 0.17 0.26
a a a a
0.58 2.60 6.56 3.59
0.07 0.16 0.38 0.26
a a a a
3.46 8.11 13.25 18.99
0.08 0.18 0.50 0.47
d d d e
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
Table 5 Needle and shoot growth (mean S:E:) of P. halepensis immediately after a P. squamosus infestation (May) and 6 months later (October)a
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
3. Results 3.1. Damage level in relation to needles age Defoliation percentage and water content of P. halepensis were signi®cantly higher in young than in old needles, in both localities (Table 4). For chlorophyll a b, and total chlorophyll content, recorded values were signi®cantly higher in the older than in the younger needles in Yeste; no signi®cant differences in chlorophyll content were registered in Calasparra. 3.2. Defoliation level in relation to silvicultural practices For young needles (Fig. 2) ANOVA revealed signi®cant differences in defoliation percentage between plots subjected to different silvicultural practices in Yeste (F 4:40; d:f: 134; P < 0:001). However, no signi®cant differences were found in Calasparra (F 0:62; d:f: 104; P > 0:05). In Yeste three homogeneous groups were identi®ed. One group included only control plots with the lowest damage. The intermediate damage group included the S, T2 S P, T, T P and S P treatments. The third group, with the highest level of damage, included T S P and T S trees. The highest percentage of defoliation was of 95 2%, while the lowest was of 68 4% for the control plot. In plots located out of the experimental block (C2) of silvicultural practices defoliation percentage was of 2%. In Calasparra there were no signi®cant differences in damage by silvicultural treatment. The highest level of damage was obtained in the T treatment (46 7%), while the lowest corresponded to the T S treatment (29 7%). Defoliation percentages for old needles are shown in Fig. 3. Signi®cant differences were registered among silvicultural treatments. Defoliation ranged from 41 6 to 23 3% in Yeste and from 34 6 to 13 3% in Calasparra. In Yeste three damage groups were detected: one including C, T, S, T S, T P and S P treatments. The second group included T, S, T S, T P, S P and T2 S P treatments. The third group was T S P treatment with the highest level of damage. In Calasparra four defoliation groups were identi®ed: the ®rst (with the lowest damage) T S, C,
191
T P; the second with C, T P, S P, T S P; the third with T P, S P, T S P, S and the last group with S P, T S P, S and T, with the maximum damage. 3.3. Relation between defoliation percentage and morphological parameters A regression model related defoliation percentage (D) of the young needles (1999) to total height of the tree (H), diameter at mid-height (D2) and at 30 cm height from the soil. To avoid possible effects of silvicultural treatments in the ®nding relation only trees in the control plots (C) were considered. In Yeste, statistically signi®cant models are the followings: arcsine D 0:01 64:94 0:52H 1:87D2 (R2 0:23; P < 0:05) and arcsine D 0:01 64:77 0:52H 0:082D1 1:87D2 (R2 0:23; P < 0:05). In Calasparra no signi®cant model were found. 3.4. Tree growth after defoliation ANOVA tests were used to determine differences in growth of needles and shoots of trees in Yeste and Calasparra depending on silvicultural treatments. Those were checked at two different times: immediately after the beetle infestation (June 2000) and after the dry season (October 2000). Values are shown in Table 5. We observed signi®cant differences in growth due to the silvicultural practices. In Yeste the most intense treatments, T S P and T2 S P, produced the highest needle and shoot growth. On the contrary, control plots showed the lowest growth for needles and shoots at the two different dates. In Calasparra growth of needles and shoots followed a similar pattern: T S P produced the maximum values for growth and needles and shoots of control trees (C) presented the lowest growth. Needles and shoots growth in Yeste was always larger than in Calasparra. 4. Discussion A similar feeding preference of P. squamosus beetles was detected in the two different study sites.
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P. squamosus prefers P. halepensis needles growing the previous season over older needles. These needles have high water and protein contents but with low concentrations of tannins relative to old needles (Feeny, 1970). Differences for chlorophyll a and b, and total chlorophyll between young and old needles were recorded in Yeste but not in Calasparra. This could be due to the different intensity of the attack in the two sites which is greater at Yeste than at Calasparra. It is known that photosynthesis rate is reduced by stress conditions (Mehy et al., 1994). Mechanical injury of leaves could affect the production of chlorophyll in old and young needles. In Yeste, silvicultural practices could be a trigger factor of P. squamosus outbreak. The highest level of defoliation was registered where treatments were more intense. This joined to the fact that plots with the lowest density of seedling were more attacked, could indicate that the performance of the insect is related to the amount of light reaching the foliage. Other Curculionidae species such as Hylobius abietis develop great populations in the sunny exposure È rlander et al., 2000). For Dendrocto(Dajoz, 1998; O nus frontalis (one of the more serious bark beetle pest occurring in pine forests in southern USA), lighting is recognized to be an important factor in its ecology (Blanche et al., 1983). At Calasparra the outbreak pattern for the outbreak was similar, but the attack level much lower than at Yeste. A possible explanation for this may be differences in climate conditions. Outbreaks of defoliating insects often follow periods of drought or heavy rain (White, 1969). In Yeste total rainfall during the previous 16 months before the outbreak was higher (662 mm) than the total amount at Calasparra for the same period (362 mm). Difference in the intensity of attack could be due to differential nutritional state of the needles in the two study sites. It has been shown that stressed and nitrogen-enriched trees are more suitable to be attacked by phytofagus insects than the normal trees (White, 1974). In this sense, Lymantria dispar survive and develop best when fed tissue with high nitrogen concentrations (Joseph et al., 1993) but, on the contrary, Panolis ¯ammea abundance is not strongly in¯uenced by the nitrogen content of its host plants (Watt, 1989). Foliage of P. halepensis in Yeste has a
similar level of nitrogen, phosphorus and potassium than that at Calasparra. Also, nutritional values of the forest seem to be related more to the silvicultural practices to which pines were subjected, than to the quality site. In this sense, the more intense the treatment (T S P) the higher the nitrogen level. Several ecological factors can induce tree stress and mobilization of the nitrogenous compounds (White, 1984). Silvicultural practices may be one of the factors that increase nutrient intake in vegetation (Daniel et al., 1979). This could have occurred in the more intensively treated plots: silvicultural treatments (i.e. thinning, scrubbing and pruning) probably resulting in tree stress and, as a consequence, in an increment in the nitrogen concentration of the needles. In Yeste, regression models showed a positive relationship between diameter and the defoliation percentage. On the contrary, the relationship between defoliation percentage and height was negative. This was a predictable result as it was detected visually in the forest that shorter trees sustained a more intense attack. In Calasparra no relationship between defoliation percentage and diameter or height of the trees was obtained. The abundance of an insect (A) depends on the biotic potential of the insect (B) and the resistance of the site (R: meaning all the factors contributing to reduce the multiplication of an insect). The relationship is A B=R. Usually there is a balance in the forest so that A remains below critical values but if site resistance is reduced, the abundance of the insect will be increased (Romanyk and Kadahia, 1992). In this study resistance of the site was modi®ed by silvicultural practices. Consequently, the biotic equilibrium existed between the insect and site was broken leading to P. squamosus outbreak. Besides, intensive management practices may disrupt the balance between common insects pests and their natural enemies triggering outbreaks (Nowak and Berisford, 2000). Thinning causes a sudden but temporary change in microclimate and tree physiology (Bartos and Amman, 1989). Later, trees develop in a more healthy way and become less susceptible to pest attack (Matson et al., 1987). However, during the ®rst months after thinning, some adverse effects such as reduced tree growth, increased pests susceptibility or extreme microclimate conditions may occur (Nebeker
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
et al., 1985). It has been suggested that thinned trees divert carbohydrates to growth instead of the production of defensive chemicals (Lorio, 1986). Thus, the immediate response to silvicultural practices may lead to greater susceptibility to P. squamosus outbreak in P. halepensis treated plots. Another factor to take into account to reduce damage of the forest by P. squamosus, is the time of the year in which treatments are conducted. If treatments are conducted at the end of the summer, September±October may have time to recuperate their equilibrium after thinning stress. Defoliation also weakness trees which become more susceptible for the attack by other biotic or abiotic factors, for example, to Scolytidae. Mortality in P. halepensis due to Scolytidae in Israel is lower in forest thinned between April and September than if the forest is thinned between October and March (Mendel et al., 1992). This period coincides with the optimal timing for treatment proposed in this work, to reduce P. squamosus damage. In spite of the intensity of the attack (in several plots in Yeste almost 100% of the young needles were attacked) trees continued growing growth immediately after beetles had gone. The growth response of needles and buds was similar and it was related to the silvicultural practices. The longest needle growth was registered in the more intense treatments. Needles and shoots reached a higher length in Yeste than in Calasparra probably due to a quality better site for P. halepensis in Yeste. 5. Conclusion The two study areas, Yeste and Calasparra, yielded similar results with regards to the feeding preferences of P. squamosus and the causes of the outbreak. Trees subjected to intensive silvicultural practices sustained the highest defoliation percentage and produced the longest needle and shoot growth. Pest attack was more intense in Yeste than in Calasparra, possibly due to different climate conditions between sites which affected the P. squamosus outbreak. Naturally-regenerated P. halepensis stands with high densities developed after ®re in SE Spain must be thinned to reduce ®nal density. These silvicultural practices could lead to a Pachyrhinus sp. outbreak,
193
resulting in defoliation. Knowledge of the conditions leading to the outbreak, i.e. soil type, climate conditions, time of thinning and time of expected attack, should improve the general schedule of forest management. So, more research should be conducted to determine where and when carrying out silvicultural practices should be conducted in young P. halepensis stands, what ®nal densities get and how to manage it. Acknowledgements We thank to Dr. Zvi Mendel for his assistance and useful advice and Dr. M.A. Zarazaga for the help on beetle determination. We are grateful to the Regional Forestry Service Delegation of Albacete (Junta de Comunidades de Castilla-La Mancha) and Murcia for providing the research site. We also thank the following people for technical help and ®eld data collection: E. Simarro, A. Atienza, P. Artime, C. PeÂrez, J.J. Soler, A. TuÂnez and J.A. Ochoa. This research was supported through funds provided by the FEDER Programme (UE): 1FD97-0441. References BarbeÁro, M., Loisel, R., QueÂzel, P., Richardson, D.M., Romane, F., 1998. Pines of the Mediterranean Basin. In: Richardson, D.M. (Ed.), Ecology and Biogeography of Pinus. Cambridge University Press, Cambridge, pp. 153±170. Bartos, D.L., Amman, G.D., 1989. Microclimate: an alternative to tree vigor as a basis for mountain beetle infestations. Res. Pap. US Dep. Agric. For. Serv. INT-400. Blanche, C.A., Hodges, J.D., Nebeker, T.E., Moehring, D.M., 1983. Southern pine beetle: the host dimension. Miss. Agric. For. Exp. Stan. Bull. 917, 29. Blanco, E., Casado, M.A., Costa, M., Escribano, R., GarcõÂa, M., GeÂnova, M., GoÂmez, A., GoÂmez, F., Moreno, J.C., Morla, C., Regato, P., Sainz, H., 1998., Los bosques ibeÂricos. Planeta, Barcelona. Dajoz, R., 1998. Les insectes et la foret. Technique & Documentation, Paris. Daniel, P.W., Helms, U.E., Baker, F.S., 1979. Principles of Silviculture. McGraw-Hill, New York. DeBolt, D.C., 1980. Multielement emission spectroscopic analysis of plant tissue using DG argon plasma source. J. Assoc. Off. Anal. Chem. 63, 802±805. Feeny, P., 1970. Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillar. Ecology 51, 656±681.
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Inskeep, W.P., Bloom, P.R., 1985. Extinction coef®cients of chlorophyll a and b in N,N-dimethylformamide and 80% acetone. Plant Physiol. 77, 483±485. Jones Jr., J.B., Case, V.W., 1990. Sampling, handling, and analysing plant tissue samples. In: Westerman, R.L. (Ed.), Soil Testing and Plant Analysis, 3rd Edition. Soil Science Society of America, Madison, WI, pp. 389±427. Joseph, G., Kelsey, R.G., Moldenke, A.F., Miller, J.C., Berry, R.E., Wernz, J.G., 1993. Effects of nitrogen and Douglas-®r allelochemicals on development of the gypsy moth, Lymantria dispar. J. Chem. Ecol. 19 (6), 1245±1263. Lapesa, F., 2000. Clareo mecanizado en un regenerado natural de Pinus halepensis, en el monte de Utilidad PuÂblica no. 1 ``Rodanas'' en EÂpila, Zaragoza. Foresta 10, 28±33. Lorio Jr., P.L., 1986. Growth-differentiation balance: a basis for understanding southern pine beetle±tree interactions. For. Ecol. Manage. 14, 259±273. Matson, P.A., Hain, F.P., Mawby, W., 1987. Indices of tree susceptibility to bark beetles vary with silvicultural treatment in a loblolly pine plantation. For. Ecol. Manage. 22, 107±118. Mehy, M., Olioso, A., Trabaud, L., 1994. Chlorophyll ¯uorescence as a tool for management of plant resources. Remote Sens. Environ. 47, 2±9. Mendel, A., Boneh, O., Riov, J., 1992. Some foundations for the application of aggregation pheromone to control pine bark beetles in Israel. J. Appl. Entomol. 114, 217±227. Nebeker, T.E., Hodges, J.D., Harr, B.K., Moering, D.M., 1985. Thinning practice in southern pines with pest management
recommendations. Tech. Bull. US Dep. Agric. For. Ser. 1703. Nowak, J.T., Berisford, C.W., 2000. Effects of intensive forest management practices on insect infestation levels and loblolly pine growth. J. Econ. Entomol. 93 (2), 336±341. È rlander, G., Nordlander, G., Wallertz, K., Nordenhem, H., 2000. O Feeding in the crowns of Scots pine trees by the pine weevil Hylobius abietis. Scand. J. For. Res. 15, 194±201. Romanyk, N., Kadahia, D., 1992. Plagas de insectos en las masas forestales espanÄolas. Ministerio de Agricultura, Pesca y AlimentacioÂn. ICONA, Madrid. Saracino, A., Pacella, R., Leone, V., Borghetti, M., 1997. Seed dispersal and changing seed characteristics in a Pinus halepensis Mill., forest after ®re. Plant Ecol. 130, 13±19. Watt, A.D., 1989. The chemical composition of pine foliage in relation to the population dynamics of the pine beauty moth, Panolis ¯ammea, in Scotland. Oecologia 78, 251± 258. White, T.C.R., 1969. An index to measure weather-induced stress of trees associated with outbreaks of psyllids in Australia. Ecology 50, 905±909. White, T.C.R., 1974. A hypothesis to explain outbreaks of looper caterpillars, with special reference to populations of Selidosema suavis in a plantation of Pinus radiata in New Zealand. Oecologia 16, 279±301. White, T.C.R., 1984. The abundance of invertebrate herbivores in relation to the availability of nitrogen in stressed food plants. Oecologia 63, 90±105.
Effects of post-®re silviculture practices on Pachyrhinus squamosus defoliation levels and growth of Pinus halepensis Mill. Ana GonzaÂlez-Ochoa*, Jorge de las Heras Departamento de ProduccioÂn Vegetal y TecnologõÂa Agraria, Universidad de Castilla-La Mancha, ETSIA, Campus Universitario s/n, 02071 Albacete, Spain Received 31 January 2001; received in revised form 3 July 2001; accepted 20 July 2001
Abstract Pinus halepensis Mill. (Aleppo pine) forms dense and extensive stands after frequent ®res in SE Spain. Due to the high densities reached by this species after ®re, thinning is necessary. Two great ®res occurred in SE Spain in August 1994 and natural regeneration stands of P. halepensis were managed in 1999 using several silvicultural treatments (thinning, pruning and scrubbing). The objective was to improve the stands quality and accelerate pine canopy growth. However, since a few years, damages produced by several defoliator species from Pachyrhinus genus (Coleoptera, Curculionidae) have been detected in young treated P. halepensis stands in Spain. To test the intensity of the P. squamosus attack, two post-®re localities submitted to different treatments were selected (Yeste and Calasparra, in SE Spain) corresponding to different climate conditions (dry and semiarid, respectively). Results showed that the most intense defoliation level was recorded in Yeste. Beetles preferentially ate needles from the previous growing season in both localities. Signi®cant differences for defoliation percentage depending on silvicultural treatments were also recorded. Nutritional state of the needles for nitrogen, phosphorus and potassium was similar in both localities showing some relationship with silvicultural treatments. In general, the more intense the silvicultural treatment, the higher defoliation percentage was recorded. Growth pattern was similar in both study sites, reaching bring higher where treatments were more intense. Reduced needle and shoot growth was registered in Calasparra relative to Yeste. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Wild ®re; SE Spain; Defoliator; Aleppo pine; Curculionidae
1. Introduction Pinus halepensis Mill. occurs in a large variety of climatic conditions in the Iberian Peninsula, throughout its Mediterranean Basin distribution (Blanco et al., 1998). This species has been subjected the effects of *
Corresponding author. Tel.: 34-9-6759-9200; fax: 34-9-6759-9238. E-mail address: [email protected] (A. GonzaÂlez-Ochoa).
®re along the centuries and has developed adaptations in response to this perturbance, such as massive releasing of seeds (Saracino et al., 1997). If the frequency of ®res is low, P. halepensis forms high density stands due to its serotinous cones (BarbeÂro et al., 1998). The density reached is sometimes so high that it is imperative to reduce it by silvicultural practices. In the last few years, intense infestations have been recorded of Pachyrhinus sp. species (Coleoptera, Curculionidae), on both planted (del
0378-1127/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 8 - 1 1 2 7 ( 0 1 ) 0 0 7 1 8 - 6
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Pozo, Personal Communication) and naturally-regenerated P. halepensis seedlings after ®re and silvicultural practices (Lapesa, 2000). Pachyrhinus sp. occurs in the Mediterranean Basin and central Europe primarily. In Spain there are several species of this genus, attacking pine forests of all ages, but with a preference for young stands. The likelihood of a pest infestation depends on several factors. Environmental factors are generally the most important in¯uencing insect reproduction. Other important factors are, abundance of dead wood in the forest, weakening of trees, reduction in natural enemies and ®nally, a wrongly scheduled silvicultural practices (Romanyk and Kadahia, 1992). Some of these factors may be responsible for the Pachyrhinus sp. outbreaks that have become usual in young P. halepensis forests in Spain. An infestation of Pachyrhinus squamosus Kiesenwetter appeared 1 year after silvicultural practices were applied to two different 5-year-old P. halepensis forests in SE Spain. The objectives of this study were to evaluate the pest infestation from different points of view: (i) to determine if there are differences in the damage level depending on the age of the pine needles, (ii) to discover if the intensity of the damage is related to the silvicultural practices, (iii) to ®nd any possible relationship between the intensity of the damage and tree morphological variables, and (iv) to relate the intensity of the infestation to climate conditions and nutritive level of the needles. Finally, we related the growth response of the trees to beetle defoliation and silvicultural practices.
2. Materials and methods 2.1. Study sites Two large ®res occurred in mature P. halepensis forests in August 1994 in SE Spain. Total surface burnt was about 44,000 ha, in two different provinces (Albacete and Murcia). In each burnt zone a locality with high regeneration density was selected: Yeste (province of Albacete) and Calasparra (province of Murcia). General characteristics of the sites are shown in Table 1. After ®re, natural regeneration took place in both of them, reaching a very high density of seedlings: 4,722 trees/ha (2;134 S:D:) with a medium height of 105 cm in Yeste and 45,000 trees/ha (20,400 S:D:) with a medium height of 51 cm in Calasparra in year 1999. Experimental plots were established in each study site in order to conduct different silvicultural treatments (thinning, scrubbing and pruning and different combinations between them) shown in Table 2. In Yeste a total of 27 plots were installed in which seven different treatments were conducted (three replicates of every treatment). Besides, six more plots were left as controls. In Calasparra, 21 plots were established and six different treatments were conducted (three replicates for each treatment) and three more plots as controls. The plot size was 15 m 10 m. Due to the high attack intensity and the proximity of the control to the treated plots, three additional plots were installed in Yeste 1 km away from the experimental block. Two meteorological stations were installed in 1999 in both study sites.
Table 1 Characteristics of the localities and sampling dates since study of P. squamosus outbreak in P. halepensis in Spain
Location Tree age (years) Annual precipitation (mm) Annual average temperature (8C) Soil texture class pH N (%) P (mg/l) K (mg/l) Silvicultural treatments date Defoliation sampling date Needles and shoots sampling date
Yeste
Calasparra
28200 W; 388220 S 6 530 13.1 Sandy 8.6 0.45 3.60 200.10 15 July 1999 4 May 2000 2 June 2000; 6 October 2000
18380 W; 388160 S 6 290 16.5 Sandy loam 8.7 0.19 3.43 215.24 1 August 2000 19 May 2000 19 June 2000; 28 October 2000
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194 Table 2 Silvicultural practices applied to ®re-regenerated P. halepensis in two study sites Treatments
Silvicultural practice
T S TS TP SP TSP C T2 S P C2
Thinning (1600 trees/ha) Scrubbing Thinning (1600 trees/ha) and scrubbing Thinning (1600 trees/ha) and pruning Scrubbing and pruning Thinning (1600 trees/ha), scrubbing and pruning Control (inside experimental block) Thinning (800 trees/ha), scrubbing and pruninga Control outside experimental blocka
a
Practice not performed in Calasparra.
In spring 2000 the two study sites sustained infestations of P. squamosus. Climate conditions before and after the outbreak are shown in Fig. 1. Average temperature during the previous 16 months before the outbreak was 14 8C in Yeste and 15.7 8C in Calasparra. During this period total annual rainfall was of 612 mm in Yeste and 361 mm in Calasparra.
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2.2. Infestation sampling Sampling was conducted in May 2000 to evaluate the intensity of the infestation at every study site. Needles from ®ve trees in each plot were randomly taken (15 trees per treatment). Two samples of needles of 20 g each were taken: one of the youngest needles (from the previous growing season, i.e. 1999) and another of older needles. This was done to test if there was a signi®cant difference in damage level depending on age of the needles. Immediately, chlorophyll a, and b, and total in the needles were determined spectrophotometrically in 100% dimethylformamide extracts, using the equations of Inskeep and Bloom (1985). Water content of the needles was also measured by calculating the difference between fresh and dry weight (storing samples 24 h at 105 8C). Finally, damaged needles were counted (needles bitten by the insect) to obtain the defoliation percentage. In this way, when any reference to ``defoliation percentage'' is given, data are not a visual estimation but a totally objective parameter. Defoliation percentage,
Fig. 1. Monthly precipitation (mm) and average temperature (8C) in the study sites: (A) Yeste; (B) Calasparra. Arrow means P. squamosus outbreak.
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chlorophyll a and b, total chlorophyll, and water content were used to determine if there were signi®cant differences between young and old needles damage. To test the damage level depended on the type of silvicultural practices, defoliation percentage was used. Finally, we looked for relationships between defoliation and tree size, total height (H, cm), diameter at the middle height (D1, mm) and stem diameter at 30 cm from the stem base of the tree (D2, mm) were measured, for all the sample trees. Due to the differences in climate conditions, the two study localities were considered separately. However, descriptive comparisons were done taking into account the climate conditions of 1999 and 2000 and nutrient level of the needles. To evaluate the response of pine growth after the defoliation, needles and shoots from the 2000 growing year were measured twice: immediately after infestation collapse (June 2000) and 6 months later (October 2000).
At the beginning of the infestation, samples were taken from the current year needles, from three trees per plot in both localities, to determine foliar macronutrients. Total nitrogen and total phosphorous were determined with standard micro-Kjeldahl techniques (Jones and Case, 1990). Total potassium was obtained using plasma emission spectroscopy (DeBolt, 1980). Data obtained are shown in Table 3. 2.3. Statistical methods For all statistical tests, data were transformed when necessary using the log or arcsine transformation, to meet the assumptions of normality and homoscedasticity. Both tables and ®gures present the original nontransformed data and standard error of the mean (S.E.). To test for differences in percentage of defoliation, chlorophyll and water content between 1-yearold needles and the oldest needles, a two samples
Table 3 Total nitrogen, phosphorus and potassium content in P. halepensis needles (mean S:E:) in two study sites in Spain during a P. squamosus outbreak (April 2000) Treatments
N (%)
P (g/kg)
Yeste T S TS TP SP TSP C T2 S P
1.1 0.9 1.1 1.2 1.1 1.4 0.9 1.7
0.3 0.1 0.0 0.0 0.0 0.2 0.1 0.1
Calasparra
Yeste
1.1 0.9 1.1 1.2 1.1 1.2 0.9
0.8 0.5 1.0 0.9 0.7 0.9 0.6 1.1
0.0 0.1 0.1 0.1 0.2 0.0 0.1
K (g/kg)
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
Calasparra
Yeste
0.9 0.5 0.8 0.7 0.7 0.8 0.7
2.8 2.0 2.4 2.5 2.4 2.6 2.3 3.6
0.0 0.0 0.0 0.0 0.0 0.0 0.0
Calasparra 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0
3.6 2.6 3.9 2.9 2.7 3.3 2.8
0.0 0.0 0.1 0.0 0.0 0.0 0.0
Table 4 Defoliation percentage, chlorophyll a and b, and total chlorophyll, and water content of P. halepensis needles in two study sites (mean S:E:), immediately after the P. squamosus attack (May 2000)a Yeste
Defoliation percentage (%) Chlorophyll a (mg/g fresh needles) Chlorophyll b (mg/g fresh needles) Total chlorophyll (mg/g fresh needles) Water content (%) a
Calasparra
Young needles
Old needles
Young needles
Old needles
79.7 0.4 0.1 0.6 56.1
36.7 0.6 0.2 0.8 43.0
40.5 0.4 0.2 0.6 40.3
25.7 2.0 b 0.5 0.0 a 0.2 0.0 a 0.7 0.0 a 33 1.9 b
2.3 0.0 0.0 0.1 1a
a a a a
2.6 0.0 0.0 0.0 1.3
b b b b b
Means within a row followed by different letters differ signi®cantly at P < 0:05 at every site.
2.3 0.0 0.0 0.0 2.4
a a a a a
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Fig. 2. Defoliation percentage (mean S:E:) of young needles recorded in the study plots in Yeste and in Calasparra. Bars with the same letter are not signi®cantly different at P < 0:05.
comparison analyze was used. A one-way ANOVA was used to test for the differences in defoliation level and growth differences due to the different silvicultural treatments. Later, Fisher's least signi®cant difference (LSD) procedure was used to discriminate among the means. Finally, to determine the relationships between tree defoliation and
morphological parameters, we used a regression model. Defoliation percentage was used as dependent variable; independent variables were: tree height (cm), diameter at the middle height (mm) and diameter at 30 cm from the base height (mm). All statistical analysis were conducted using a critical P-value of 0.05.
Fig. 3. Defoliation percentage (mean S:E:) of old needles recorded in the study plots in Yeste and in Calasparra. Bars with the same letter are not signi®cantly different at P < 0:05.
190
T Yeste Needles (cm) Shoots (cm) Calasparra Needles (cm) Shoots (cm) a
S
TS
TP
SP
TSP
C
May October May October
2.57 5.99 10.18 14.34
0.06 0.22 0.45 0.55
b bc c d
1.71 5.25 8.85 12.86
0.06 0.11 0.34 0.48
a b b b
2.67 6.78 10.16 14.15
0.09 0.22 0.34 0.48
b 3.02 c 6.05 c 9.78 cd 12.98
0.08 0.21 0.25 0.37
c 2.79 bc 6.62 bc 10.53 bc 14.45
0.08 0.16 0.34 0.37
b c c d
3.25 6.30 10.31 13.8
0.12 0.23 0.25 0.48
cd c c bcd
May October May October
1.56 3.96 10.05 6.59
0.08 0.23 0.49 0.39
b c cd b
0.71 3.29 7.96 3.71
0.12 0.18 0.33 0.18
a b b a
1.69 3.75 9.22 6.57
0.15 0.18 0.46 0.50
b bc c b
0.15 0.26 0.48 0.51
c d d c
0.19 0.21 0.48 0.44
b 3.06 bc 5.52 bc 12.9 bc 10.35
0.15 0.20 0.44 0.40
d d e d
Means within a row followed by different letters differ signi®cantly at P < 0:05.
2.35 5.11 10.8 8.08
1.81 3.85 8.98 7.23
T2 S P
1.73 4.37 7.63 10.52
0.04 0.37 0.17 0.26
a a a a
0.58 2.60 6.56 3.59
0.07 0.16 0.38 0.26
a a a a
3.46 8.11 13.25 18.99
0.08 0.18 0.50 0.47
d d d e
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
Table 5 Needle and shoot growth (mean S:E:) of P. halepensis immediately after a P. squamosus infestation (May) and 6 months later (October)a
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3. Results 3.1. Damage level in relation to needles age Defoliation percentage and water content of P. halepensis were signi®cantly higher in young than in old needles, in both localities (Table 4). For chlorophyll a b, and total chlorophyll content, recorded values were signi®cantly higher in the older than in the younger needles in Yeste; no signi®cant differences in chlorophyll content were registered in Calasparra. 3.2. Defoliation level in relation to silvicultural practices For young needles (Fig. 2) ANOVA revealed signi®cant differences in defoliation percentage between plots subjected to different silvicultural practices in Yeste (F 4:40; d:f: 134; P < 0:001). However, no signi®cant differences were found in Calasparra (F 0:62; d:f: 104; P > 0:05). In Yeste three homogeneous groups were identi®ed. One group included only control plots with the lowest damage. The intermediate damage group included the S, T2 S P, T, T P and S P treatments. The third group, with the highest level of damage, included T S P and T S trees. The highest percentage of defoliation was of 95 2%, while the lowest was of 68 4% for the control plot. In plots located out of the experimental block (C2) of silvicultural practices defoliation percentage was of 2%. In Calasparra there were no signi®cant differences in damage by silvicultural treatment. The highest level of damage was obtained in the T treatment (46 7%), while the lowest corresponded to the T S treatment (29 7%). Defoliation percentages for old needles are shown in Fig. 3. Signi®cant differences were registered among silvicultural treatments. Defoliation ranged from 41 6 to 23 3% in Yeste and from 34 6 to 13 3% in Calasparra. In Yeste three damage groups were detected: one including C, T, S, T S, T P and S P treatments. The second group included T, S, T S, T P, S P and T2 S P treatments. The third group was T S P treatment with the highest level of damage. In Calasparra four defoliation groups were identi®ed: the ®rst (with the lowest damage) T S, C,
191
T P; the second with C, T P, S P, T S P; the third with T P, S P, T S P, S and the last group with S P, T S P, S and T, with the maximum damage. 3.3. Relation between defoliation percentage and morphological parameters A regression model related defoliation percentage (D) of the young needles (1999) to total height of the tree (H), diameter at mid-height (D2) and at 30 cm height from the soil. To avoid possible effects of silvicultural treatments in the ®nding relation only trees in the control plots (C) were considered. In Yeste, statistically signi®cant models are the followings: arcsine D 0:01 64:94 0:52H 1:87D2 (R2 0:23; P < 0:05) and arcsine D 0:01 64:77 0:52H 0:082D1 1:87D2 (R2 0:23; P < 0:05). In Calasparra no signi®cant model were found. 3.4. Tree growth after defoliation ANOVA tests were used to determine differences in growth of needles and shoots of trees in Yeste and Calasparra depending on silvicultural treatments. Those were checked at two different times: immediately after the beetle infestation (June 2000) and after the dry season (October 2000). Values are shown in Table 5. We observed signi®cant differences in growth due to the silvicultural practices. In Yeste the most intense treatments, T S P and T2 S P, produced the highest needle and shoot growth. On the contrary, control plots showed the lowest growth for needles and shoots at the two different dates. In Calasparra growth of needles and shoots followed a similar pattern: T S P produced the maximum values for growth and needles and shoots of control trees (C) presented the lowest growth. Needles and shoots growth in Yeste was always larger than in Calasparra. 4. Discussion A similar feeding preference of P. squamosus beetles was detected in the two different study sites.
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P. squamosus prefers P. halepensis needles growing the previous season over older needles. These needles have high water and protein contents but with low concentrations of tannins relative to old needles (Feeny, 1970). Differences for chlorophyll a and b, and total chlorophyll between young and old needles were recorded in Yeste but not in Calasparra. This could be due to the different intensity of the attack in the two sites which is greater at Yeste than at Calasparra. It is known that photosynthesis rate is reduced by stress conditions (Mehy et al., 1994). Mechanical injury of leaves could affect the production of chlorophyll in old and young needles. In Yeste, silvicultural practices could be a trigger factor of P. squamosus outbreak. The highest level of defoliation was registered where treatments were more intense. This joined to the fact that plots with the lowest density of seedling were more attacked, could indicate that the performance of the insect is related to the amount of light reaching the foliage. Other Curculionidae species such as Hylobius abietis develop great populations in the sunny exposure È rlander et al., 2000). For Dendrocto(Dajoz, 1998; O nus frontalis (one of the more serious bark beetle pest occurring in pine forests in southern USA), lighting is recognized to be an important factor in its ecology (Blanche et al., 1983). At Calasparra the outbreak pattern for the outbreak was similar, but the attack level much lower than at Yeste. A possible explanation for this may be differences in climate conditions. Outbreaks of defoliating insects often follow periods of drought or heavy rain (White, 1969). In Yeste total rainfall during the previous 16 months before the outbreak was higher (662 mm) than the total amount at Calasparra for the same period (362 mm). Difference in the intensity of attack could be due to differential nutritional state of the needles in the two study sites. It has been shown that stressed and nitrogen-enriched trees are more suitable to be attacked by phytofagus insects than the normal trees (White, 1974). In this sense, Lymantria dispar survive and develop best when fed tissue with high nitrogen concentrations (Joseph et al., 1993) but, on the contrary, Panolis ¯ammea abundance is not strongly in¯uenced by the nitrogen content of its host plants (Watt, 1989). Foliage of P. halepensis in Yeste has a
similar level of nitrogen, phosphorus and potassium than that at Calasparra. Also, nutritional values of the forest seem to be related more to the silvicultural practices to which pines were subjected, than to the quality site. In this sense, the more intense the treatment (T S P) the higher the nitrogen level. Several ecological factors can induce tree stress and mobilization of the nitrogenous compounds (White, 1984). Silvicultural practices may be one of the factors that increase nutrient intake in vegetation (Daniel et al., 1979). This could have occurred in the more intensively treated plots: silvicultural treatments (i.e. thinning, scrubbing and pruning) probably resulting in tree stress and, as a consequence, in an increment in the nitrogen concentration of the needles. In Yeste, regression models showed a positive relationship between diameter and the defoliation percentage. On the contrary, the relationship between defoliation percentage and height was negative. This was a predictable result as it was detected visually in the forest that shorter trees sustained a more intense attack. In Calasparra no relationship between defoliation percentage and diameter or height of the trees was obtained. The abundance of an insect (A) depends on the biotic potential of the insect (B) and the resistance of the site (R: meaning all the factors contributing to reduce the multiplication of an insect). The relationship is A B=R. Usually there is a balance in the forest so that A remains below critical values but if site resistance is reduced, the abundance of the insect will be increased (Romanyk and Kadahia, 1992). In this study resistance of the site was modi®ed by silvicultural practices. Consequently, the biotic equilibrium existed between the insect and site was broken leading to P. squamosus outbreak. Besides, intensive management practices may disrupt the balance between common insects pests and their natural enemies triggering outbreaks (Nowak and Berisford, 2000). Thinning causes a sudden but temporary change in microclimate and tree physiology (Bartos and Amman, 1989). Later, trees develop in a more healthy way and become less susceptible to pest attack (Matson et al., 1987). However, during the ®rst months after thinning, some adverse effects such as reduced tree growth, increased pests susceptibility or extreme microclimate conditions may occur (Nebeker
A. GonzaÂlez-Ochoa, J. de las Heras / Forest Ecology and Management 167 (2002) 185±194
et al., 1985). It has been suggested that thinned trees divert carbohydrates to growth instead of the production of defensive chemicals (Lorio, 1986). Thus, the immediate response to silvicultural practices may lead to greater susceptibility to P. squamosus outbreak in P. halepensis treated plots. Another factor to take into account to reduce damage of the forest by P. squamosus, is the time of the year in which treatments are conducted. If treatments are conducted at the end of the summer, September±October may have time to recuperate their equilibrium after thinning stress. Defoliation also weakness trees which become more susceptible for the attack by other biotic or abiotic factors, for example, to Scolytidae. Mortality in P. halepensis due to Scolytidae in Israel is lower in forest thinned between April and September than if the forest is thinned between October and March (Mendel et al., 1992). This period coincides with the optimal timing for treatment proposed in this work, to reduce P. squamosus damage. In spite of the intensity of the attack (in several plots in Yeste almost 100% of the young needles were attacked) trees continued growing growth immediately after beetles had gone. The growth response of needles and buds was similar and it was related to the silvicultural practices. The longest needle growth was registered in the more intense treatments. Needles and shoots reached a higher length in Yeste than in Calasparra probably due to a quality better site for P. halepensis in Yeste. 5. Conclusion The two study areas, Yeste and Calasparra, yielded similar results with regards to the feeding preferences of P. squamosus and the causes of the outbreak. Trees subjected to intensive silvicultural practices sustained the highest defoliation percentage and produced the longest needle and shoot growth. Pest attack was more intense in Yeste than in Calasparra, possibly due to different climate conditions between sites which affected the P. squamosus outbreak. Naturally-regenerated P. halepensis stands with high densities developed after ®re in SE Spain must be thinned to reduce ®nal density. These silvicultural practices could lead to a Pachyrhinus sp. outbreak,
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resulting in defoliation. Knowledge of the conditions leading to the outbreak, i.e. soil type, climate conditions, time of thinning and time of expected attack, should improve the general schedule of forest management. So, more research should be conducted to determine where and when carrying out silvicultural practices should be conducted in young P. halepensis stands, what ®nal densities get and how to manage it. Acknowledgements We thank to Dr. Zvi Mendel for his assistance and useful advice and Dr. M.A. Zarazaga for the help on beetle determination. We are grateful to the Regional Forestry Service Delegation of Albacete (Junta de Comunidades de Castilla-La Mancha) and Murcia for providing the research site. We also thank the following people for technical help and ®eld data collection: E. Simarro, A. Atienza, P. Artime, C. PeÂrez, J.J. Soler, A. TuÂnez and J.A. Ochoa. This research was supported through funds provided by the FEDER Programme (UE): 1FD97-0441. References BarbeÁro, M., Loisel, R., QueÂzel, P., Richardson, D.M., Romane, F., 1998. Pines of the Mediterranean Basin. In: Richardson, D.M. (Ed.), Ecology and Biogeography of Pinus. Cambridge University Press, Cambridge, pp. 153±170. Bartos, D.L., Amman, G.D., 1989. Microclimate: an alternative to tree vigor as a basis for mountain beetle infestations. Res. Pap. US Dep. Agric. For. Serv. INT-400. Blanche, C.A., Hodges, J.D., Nebeker, T.E., Moehring, D.M., 1983. Southern pine beetle: the host dimension. Miss. Agric. For. Exp. Stan. Bull. 917, 29. Blanco, E., Casado, M.A., Costa, M., Escribano, R., GarcõÂa, M., GeÂnova, M., GoÂmez, A., GoÂmez, F., Moreno, J.C., Morla, C., Regato, P., Sainz, H., 1998., Los bosques ibeÂricos. Planeta, Barcelona. Dajoz, R., 1998. Les insectes et la foret. Technique & Documentation, Paris. Daniel, P.W., Helms, U.E., Baker, F.S., 1979. Principles of Silviculture. McGraw-Hill, New York. DeBolt, D.C., 1980. Multielement emission spectroscopic analysis of plant tissue using DG argon plasma source. J. Assoc. Off. Anal. Chem. 63, 802±805. Feeny, P., 1970. Seasonal changes in oak leaf tannins and nutrients as a cause of spring feeding by winter moth caterpillar. Ecology 51, 656±681.
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