The effect of shrub clearing on the control of the fire-prone species Ulex parviflorus

Forest Ecology and Management 186 (2003) 47–59

The effect of shrub clearing on the control of the fire-prone species Ulex parviflorus M.J. Baezaa,*, J. Ravento´sb, A. Escarre´b, V.R. Vallejoa a

Fundacio´n CEAM, Parque Tecnolo´gico Paterna, C/Charles Darwin 14, 46980 Valencia, Spain b Departamento de Ecologı´a, Universidad de Alicante, Ap. 99, 03080 Alicante, Spain Received 29 April 2002; received in revised form 6 March 2003; accepted 22 April 2003

Abstract Vegetation clearing is a fuel control technique used to reduce the risk of fires in fire-prone shrublands (e.g. Ulex parviflorus (gorse) shrublands). Nonetheless, its efficacy can be undermined as much by the reproductive strategies of the different species as by the structural organization of their phytomass. In our research we tested the hypothesis that effective control of Ulex parviflorus could be limited by the vertical dead/live shoot distribution and that this distribution could affect posterior growth. We applied clearing as a fuel-control technique in three juvenile shrublands. For the following 4 years, we studied the effects of this clearing application by measuring several structural variables of the individuals involved. At the end of the study we compared the structure of treated individuals with that of untreated ones. The clearing treatment eliminated only 46% of the population. The large dependence shown between cutting height and dead shoot height in relation to treatment effectiveness (individual mortality) suggests that applying the clearing treatment below the dead shoot level rather than above it would increase effectiveness considerably. In response to the loss of phytomass, the individuals that survived the clearing treatment increased their weight by a factor of 5 in the second year. This would explain the rapid re-establishment of the relation between phytomass and basal diameter after 2 years. Significantly larger values of phytomass and basal diameter showed a compensatory growth by the fourth year. These results show that clearing is not effective in young shrublands since it generates Ulex parviflorus-dominated shrublands in a short period of time. Temporal changes in the vertical dead/live shoot distribution are seen as a key morphological variable in the effectiveness of using clearing to control this species. # 2003 Elsevier B.V. All rights reserved. Keywords: Shrub control; Clearing; Ulex parviflorus; Vertical phytomass distribution; Dead and green shoots; Structure dynamics

1. Introduction Fuel-control techniques to reduce the risk of intense fires consist mainly of the application of drastic perturbations (fire, clearing, herbicides, grazing, etc.) *

Corresponding author. Tel.: þ34-96-1318227; fax: þ34-96-1318190. E-mail address: [email protected] (M.J. Baeza).

on vegetation communities generically denominated as ‘‘undesirable’’. Paradoxically, Harper (1977) found that some of these species can only survive in perturbed environments. All natural systems experience perturbations with the potential to influence decisively the structure and dynamic of their vegetation communities (Armesto and Pickett, 1985). Troumbis (1985) stated that perturbations not only affect the phytomass and production

0378-1127/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0378-1127(03)00237-8

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M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

of an individual; they also have repercussions on the individual’s functioning and reproduction and, as an indirect effect, they alter both the biotic and the abiotic environment where they develop. Hillside cultivation by means of terracing, a characteristic feature of the Mediterranean Basin, can be considered a chronic, high-intensity perturbation that reduces the resilient capacity of the ecosystem (Tatoni, 1992). Posterior abandonment of these cultivations favours colonisation by obligate seeder species such as Ulex parviflorus, which are characterised by fuel accumulation (Baeza et al., 1998) and are thus highly sensitive to perturbations such as fire (Tatoni, 1992; Vallejo and Alloza, 1998; Pausas and Vallejo, 1999). After a perturbation and by means of secondary succession processes, Mediterranean ecosystems tend toward regeneration of the initial state (Trabaud and Lepart, 1980). Zedler (1977) considered two main types of plant reproductive strategies: species with obligate vegetative reproduction and species showing obligate sexual reproduction. A third group of species, denoted facultative resprouters (Keeley, 1986), combines both reproduction strategies. In Mediterranean vegetation, fire is an important functional agent. Many studies have analysed the effect of fire on the resprouting capacity of species showing obligate vegetative reproduction (Malanson and Trabaud, 1988; Moreno and Oechel, 1991a; Trabaud, 1991; Lloret and Lo´ pez-Soria, 1993), as well as on the regeneration by seedling recruitment in species of obligate sexual reproduction (Frazer and Davis, 1988; Thomas and Davis, 1989; Moreno and Oechel, 1991b; Roy and Sonie, 1992). Nevertheless, there are very few studies analysing Mediterranean shrubland regeneration in relation to other types of perturbations (Whelan, 1995) and, especially, in relation to fuel control management (Botelho, 1999). In terms of fuel control, clearing results in the partial or total loss of phytomass. Thus, in this sense its effect is similar to the impact produced by herbivory. In fact, most studies simulate this tissue loss as a livestock food source by means of different-intensity prunings of the plant (Armesto and Pickett, 1985; Augner et al., 1997). Although the effect of herbivores on plants, i.e. loss of phytomass, is generally negative, McNaughton (1983) has shown that plants can respond to herbivory with an increase in growth, producing the so-called overcompensation effect.

Plant responses to herbivory vary greatly. The effect of cutting (a simulation of herbivory) in the accumulated growth of a plant can be zero, positive or negative, depending on the availability of foliar area, meristems, stored nutrients, soil resources, and frequency and intensity of defoliation (Alward and Joern, 1993; Noy Meir, 1993; Trenbath, 1993). Clearly, the potential for regrowth and compensation depends critically on the moment in which the biomass loss is produced in relation to the phenological cycle of the plant (Silva and Ravento´ s, 1999). In general, the earlier the attack, the more possibilities for regrowth (Crawley, 1997). In shrublands under a high risk of fire, clearing and prescribed burning are the preferred management techniques for fuel control. There are, however, very few studies analysing the effect of these practices on post-fire and/or post-clearing survival and regeneration (Rundel et al., 1987; Bradstock and Myerscough, 1988). Based on the above-mentioned reproductive strategies, it could be hypothesised that in strict obligate seeder species like Ulex parviflorus, post-perturbation regeneration would be produced mainly by seed germination. Nevertheless, it is not known whether other regeneration mechanisms, such as vegetative growth, respond to the perturbation produced by clearing as a fuel control technique. The objectives of the present work were: (1) analyse the effect of clearing on the survival of Ulex parviflorus individuals, (2) characterise the individuals that survived the perturbation during a 4-year-period, and (3) compare the structure of Ulex parviflorus after the fuel-control treatment.

2. Materials and methods 2.1. Study area and experimental design The three sites (Ban˜ eres, Confrides and Castell) are located between the provinces of Alicante and Valencia in eastern Spain (Fig. 1). The sites range between 450 and 900 m above sea level. The mean annual rainfall ranges between 500 and 600 mm and the mean annual temperatures between 13.8 and 14.5 8C. Bedrock is marl and soils are Calcaric cambisol (FAO, 1988). The sites were dry cropland until the 1950s. When the three areas were burned in 1991, they were covered with mature pine trees (Pinus halepensis).

M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

49

Castellón

Portugal

Spain

Valencia Region

Bañeres

Valencia

Castell

Confrides

Alicante

Fig. 1. Map of the three different study areas in eastern Spain (Valencia region. Ban˜ eres site: 388430 N, 08390 W; Confrides site: 388420 N, 08180 W; Castell site: 388390 N, 0890 W).

At the beginning of the present study (1994), the vegetation on the three sites consisted of 3-year-old shrubs, with Ulex parviflorus as the dominant species in the post-fire regeneration (Fig. 2). In the spring of 1994, a plot of land measuring 33 m  33 m was selected and cleared on each of the three sites in the study. A second plot (control) was also selected on each site to which no treatment was applied; this can be seen in the experimental unit (Fig. 2). These areas were close to each other in order to avoid any variability due to environmental factors. Thus, we had three replicates per treatment. The silvicultural activity to control fuel and reduce the risk of fire was applied once only, in the spring of Experimental unit (x3 repliques) Pinus halepensis Forest Burned (1991)

33 m

Gorse shrubland Clearing (1994)

Gorse shrubland Control

33 m

Fig. 2. Scheme and dimensions of the experimental unit applied per site (Ban˜ eres, Confrides and Castell).

1994; it consisted of a clearing treatment carried out mechanically by means of a scrub-clearing machine with a vertical-axle chain drive. The cutting height of the scrub-clearing machine was set between 10 and 15 cm of the upper soil level depending on the topography of the plot. 2.2. Prior characterisation Ulex parviflorus accumulates dead fuel in its aerial parts. This dead fuel is formed by spines, small twigs and fine stems, all of which we will refer to as dead shoots. In the young state, a large proportion of the phytomass in the lower strata dies, and the live biomass (green shoots) is localised in the upper part of the plant (Fig. 3). Thus, it is possible to clearly differentiate two strata in the vertical distribution of the phytomass. A month prior to application of the clearing treatment, we analysed 30 individuals per replicate, measuring their plant height and basal diameter at surface level; they were then taken to the laboratory, ovendried at 80 8C for 24 h and later weighed. The height reached by the dead shoots was considered a relevant variable because it influences the higher or lower availability of photosynthetic phytomass. Both variables taken together may influence the mortality of individuals. Thus, we decided to make a more detailed sampling by also measuring dead shoot height in 306, 195 and 131 individuals in Ban˜ eres, Confrides and Castell, respectively.

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M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

Fig. 3. Vertical distribution of phytomass and structural variables in a young individual (3 years old) of Ulex parviflorus.

2.3. Effect of clearing on individual mortality One year after treatment application and after one growth period, we made a preliminary evaluation of the effect of the treatment on individual mortality. This was done as a preventive measure since the response this species might show to the clearing treatment was unknown, and the possibility existed that, as a consequence of the clearing, apparently dead individuals might resprout or apparent survivors might die shortly afterwards. In May 1995, this preliminary evaluation was effected in 10 randomly distributed 1 m2 sub-plots per replicate. The number of live and dead individuals on each sub-plot were recorded. Our initial hypothesis was that the higher or lower cutting height of the scrub-clearing machine would have direct effects (mortality) and/or indirect effects (compensatory growth) on Ulex parviflorus as a result of the loss of photosynthesising phytomass. This hypothesis was later tested in 180 individuals (60 per replicate), of which 90 were live individuals and 90 were dead individuals. The variables measured were cutting height and basal diameter at surface level.

were made during this period: May 1995, June 1996 and June 1998, i.e. at 1, 2 and 4 years from the treatment application. In each sampling 90 individuals were randomly selected (30 per replicate), making sure that all the individuals had been submitted to the clearing treatment. Basal diameter was measured at surface level. Plant height was considered to be from surface level to the top of the plant. All the plants were taken to the laboratory, oven-dried at 80 8C for 24 h and then weighed. 2.5. Effect of the treatment (clearing versus control) on the structure of the individual To evaluate the null hypothesis that the treatment had no effect on the community in the medium term (4 years), we analysed different structural variables in the individuals. Control plants were collected from the untreated plots. In June of 1998, 180 plants were randomly selected, 30 per treatment and replicate, and we measured plant height, dead shoot height and basal diameter at surface soil. All the plants were taken to the laboratory, oven-dried at 80 8C for 24 h and then weighed.

2.4. Growth dynamics in individuals that survived the clearing

2.6. Statistical analysis

To study the structural dynamics of the treated individuals we analysed a total of 270 individuals during the 4 years of the experiment. Three samplings

Using one-factor ANOVA, we compared the effect of the clearing on live and dead individuals in relation to cutting height and basal diameter. Using the

M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

Chi-square test of independence (Ott, 1988), we analysed the relation between cutting height and two levels—one above and the other below the dead shoots—to see if this variable conditioned survival. The null hypothesis of independence between the two factors suggests that the individual’s post-treatment survival is independent of whether it is cut at a height that is higher or lower than the average height of the individual’s pre-treatment dead shoots. The evolution in plant height, basal diameter and phytomass of the control and the cleared plants was analysed by means of an orthogonal design. We used a two-factor ANOVA with a fixed factor (time) and a random factor (site). This design is more suitable than repeated measures analysis in our case because each time we made measurements we used different individuals (Underwood, 1997). Repeated measures analysis is a way to minimize variance variability when measurements are always taken on the same individuals. Finally, to assess the effect of the treatment at the end of the experiment we used the above-mentioned variables plus dead shoot height to compare the untreated (7-year-old) control individuals with the treated individuals (4 years old after treatment) by means of one-factor ANOVA. Prior to the ANOVA comparisons, a logarithmic transformation (natural logarithm) was applied to the structural variables (continuous) to homogenise variances. The subsequent comparison of averages was made by the LSD test. Variance homogenisation was attained in the majority of the comparisons analysed. Their evaluation was performed with the Cochran test. If the variables did not present a normal distribution we applied the Kruskal–Wallis test.

3. Results 3.1. Effect of clearing on individual mortality Based on a total of 315 individuals, which represents an initial average density of 10.5 individuals per m2, the vegetation clearing eliminated 46.1% of the population, leaving a high percentage of live individuals (53.9%). To see if there were any structural differences between live and dead individuals 1 year after the clearing treatment application, we measured the basal diameter in both types of individuals and

51

Table 1 Structural characteristics at 1 year after clearing treatment application

Basal diameter (cm) Cutting height (cm)

Live

Dead

0.60  0.18 a 17.95  7.13 a

0.54  0.10 a 9.26  3.59 b

Mean  standard error (n ¼ 90). Different letters between live and dead individuals indicate significant differences (P < 0:01, test LSD).

found no significant differences between live and dead individuals (F1;4 ¼ 0:23; P ¼ 0:65) (Table 1). Later, we tested the hypothesis that cutting height is a key element in individual mortality. This variable affected individual mortality to a highly significant degree (F1;4 ¼ 60:7; P ¼ 0:001), on the order of two times higher in live individuals than in dead ones (Table 1). Cutting height can be interpreted as representing different degrees of intensity in the perturbation; thus, there is an inverse relation between cutting height and plant mortality, and this is clearly related to the vertical biomass-necromass distribution in Ulex parviflorus. This is synthesised in Fig. 4, which shows the distribution of live/dead individuals according to the cutting height of each individual on the three sites studied, as well as the mean height of dead shoots previous to the treatment. The mean dead shoot height showed no significant differences between the areas studied (F1;4 ¼ 0:62; P ¼ 0:59). Confrides is outstanding with 11:18  4:68 cm (95% confidence interval: [12.11, 10.24]). Ban˜ eres and Castell show very similar values: 8:98  4:45 cm (95% confidence interval: [9.47, 8.39]) and 8:07  4:12 cm (95% confidence interval: [8.79, 7.35]), respectively (Fig. 4). Cutting height values in live individuals showed greater heterogeneity as a possible consequence of the mechanical application and the topographic characteristics of the plots. But in spite of this greater heterogeneity, the analyses showed no significant differences between sites (w2 ¼ 4:3; d:f: ¼ 3; P ¼ 0:11) (Ba~neres ¼ 16:46  6:33, Confrides ¼ 19:074:93 and Castell ¼ 17:27  8:86). The w2 test confirmed the large interdependence of the two factors analysed (cutting height and dead shoot height) with a high level of significance (Fig. 4 and Table 2). In the three areas studied, we rejected the null hypothesis of independence of

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M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

Fig. 4. Cutting height of each of the Ulex parviflorus live individuals (white circles, n ¼ 30) and dead individuals (black circles, n ¼ 30). The discontinuous horizontal line indicates the mean dead shoot height previous to treatment: Ba~ neres ¼ 8:98  4:45 cm, n ¼ 306; Confrides ¼ 11:18  4:68 cm, n ¼ 195; Castell ¼ 8:07  4:12 cm, n ¼ 131. Results of the Chi-square test of independence (see Table 2).

factors. We thus concluded that mortality in Ulex parviflorus plants depends on applying the clearing treatment below the dead shoot level, or in other words, the probability of Ulex parviflorus survival is high if the clearing treatment is applied above the dead shoot level.

3.2. Growth dynamics in survivors of the clearing treatment The plant height of the individuals increased in a sustainable way with time after an initial decrease due to the treatment. This increase in plant height with

M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59 Table 2 Results of the independence test of w2 for H0: independence of the factors clearing height and dead shoot height in the mortality of Ulex parviflorus plants for the three different experimental sites

Ban˜ eres Confrides Castell

19.46 21.43 11.73

w2a

d.f.

10.8 10.8 10.8

1 1 1

Sig. 0.001 0.001 0.001

Plant height (A) 100

80

Plant height (cm)

w2exp

53

Clearing 60

40

20

Table 3 Results of a two-way variance analysis to assess the effects of site and time on plant height, phytomass and basal diameter before and 1, 2 and 4 years after the clearing application in individuals that survived the treatment Degrees of freedom

(A) Plant height Time (T) Site (S) ST Error

3 2 6 348

Mean square 38.896 0.296 0.159 0.099

F-ratio

0

1

2

3

4

5

4

5

Years after treatment

Phytomass (B) 500

P 400

244.981 1.866 1.591

0.000 0.234 0.149

Phytomass (g)

Sources of variation

0

Clearing 300

200

100

3 2 6 348

(C) Basal diameter Time (T) 3 Site (S) 2 ST 6 Error 348

182.779 1.236 3.818 0.625

47.874 0.324 6.111

0.000 0.735 0.000

0 0

1

2

3

Years after treatment

Basal diameter (C) 3

1.718 0.016 0.065 0.0068

26.33 0.246 9.558

0.001 0.790 0.000

Bold type indicates significant differences (P < 0:01).

time showed no differences between sites (Table 3A, Fig. 5A). The behaviour of phytomass (Table 3B, Fig. 5B) and basal diameter (Table 3C, Fig. 5C) presented a somewhat complex dynamic with time. These variables showed a significant interaction between time and site. This interaction was due mainly to the post-treatment performance of individuals on site 1 (Ban˜ eres). On this site, the clearing treatment eliminated more individuals than on the other two sites, and the regeneration followed a different dynamic. At the end of the experiment, we found fewer but bigger individuals here. This indicates differences between the three sites with relation to the phytomass and basal diameter distribution of individuals over time (in Ban˜ eres the distribution of the individuals ranged

Basal diameter (cm)

(B) Phytomass Time (T) Site (S) ST Error

2

Clearing

1

0 0

1

2

3

4

5

Years after treatment

Fig. 5. Temporal dynamic (mean  S:E:) in plant height, phytomass and basal diameter previous to treatment and at 1, 2 and 4 years after treatment in individuals that survived the clearing treatment.

from 85 to 1270 g with a mean of 451:5  296:5 g, whereas in Confrides the individuals ranged from 32 to 466 g, with a mean of 195  122:5 g, and in Castells from 58 to 686 g with a mean of 260  160:4 g). The relation between basal diameter and phytomass decreased as a result of the clearing application.

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M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59 8

8

Before Treatment

7

1 year

6

r=0.93

Ln phytomass (g)

Ln phytomass (g)

6

7

5 4 3 2 1

r=0.72

5 4 3 2 1

0

0

-1

-1 -3

-2

-1

0

1

2

-3

-2

Ln Basal diameter (cm)

0

1

2

1

2

Ln Basal diameter (cm)

8

8

2 years

7 6

4 years

7 6

r=0.82

Ln phytomass (g)

Ln phytomass (g)

-1

5 4 3 2 1

r=0.89

5 4 3 2 1

0

0

-1

-1 -3

-2

-1 Ln

0

1

Basal diameter (cm)

2

-3

-2

-1

0

Ln Basal diameter (cm)

Fig. 6. Relations between basal diameter and phytomass (on logarithm scales) previous to treatment and at 1, 2 and 4 years post-treatment, in individuals that survived the clearing treatment. All the regressions are highly significant (P < 0:001). Results of the t paired test: pre-treatment  1 year t ¼ 3:35, d:f: ¼ 164; P ¼ 0:001; 1 year  2 years t ¼ 2:63; d:f: ¼ 180; P ¼ 0:01; 1 year  4 years t ¼ 3:22; d:f: ¼ 183; P ¼ 0:01; pre-treatment  2 years t ¼ 0:12; d:f: ¼ 150; ns; pre-treatment  4 years t ¼ 0:07; d:f: ¼ 155; ns; 2 years  4 years t ¼ 0:17; d:f: ¼ 169; ns.

This decrease could be observed until the year following the treatment (Fig. 6). From the second post-treatment year on, the relation between these two variables was re-established. In the comparative regression-slope analysis (paired t-test, Fig. 6) between the different years, only the regression slope for the first post-treatment year shows significant differences with respect to the other years. Slope comparisons before the treatment and after 2 and 4 years presented no significant differences. 3.3. Treatment effect (clearing versus control) on plant structure At 4 years after the treatment, some differences in structural response were observed between the

cleared and the control plants. The structural variables analysed presented two clearly differentiated patterns: on the one hand, the relation between plant height and dead shoot height, and on the other hand, the relation between basal diameter and phytomass (Fig. 7). Plant height showed clearly significant differences between the two treatments (F1;4 ¼ 46:4; P ¼ 0:002). Individuals not subjected to treatment (control) presented higher values than cleared individuals. Dead shoot height followed the same pattern as plant height, though the differences showed a lower level of significance (F1;4 ¼ 40:9; P ¼ 0:003). However, this relation between dead shoot height and plant height showed a decreasing tendency between treatments (61.56–41.64%).

M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

120

Plant height (cm)

100

b

80

60

40 20

Height dead shoots (cm)

80

a

a 60

b 40

20

0

0

Control

Control

Clearing

Clearing

Treatment

Treatment 500

3

400

a

2

a

1

Phytomass (g)

Basal diameter (cm)

55

a

300

a 200

100

0

0

Control

Control

Clearing

Clearing

Treatment

Treatment

Fig. 7. Mean values and standard error of the structural variables analysed in Ulex parviflorus plants at 4 years after application for the clearing and control treatments. Different letters indicate significant differences (P < 0:05) between both treatments.

The pattern was different for basal diameter and phytomass (Fig. 7). There was no significant difference between treated individuals and control individuals with respect to basal diameter (F1;5 ¼ 4:3; P ¼ 0:107). The same is true for phytomass: it was a little higher in the plants undergoing clearing than in the control plants. Likewise, the phytomass of the control plants showed no significant differences in comparison with the cleared plants (F1;4 ¼ 2:1; P ¼ 0:224). The contrasting behaviour of some of the structural variables analysed, e.g. plant height is greater in untreated plants while phytomass is larger in cleared plants, translates into a different phytomass distribution (Fig. 8). In this sense, we can find significant

differences between treatments with respect to apparent density (F1;179 ¼ 86:9; P < 0:000), with the cleared plants showing much higher values.

4. Discussion After a fire, strict reseeder species regenerate solely by seed germination whereas resprouter species regenerate mainly by vegetative growth (Keeley, 1986; Naveh, 1990). Although Ulex parviflorus is an obligate seeder, our research indicates that when it is subjected to a perturbation such as clearing (partial loss of its aerial biomass) its aerial parts show regeneration by vegetative growth.

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M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

Fig. 8. Averages and standard error of the apparent fuel density at 4 years after the clearing and control treatment. Different letters indicate significant differences (P < 0:001) between both treatments.

The clearing treatment produced considerable changes in the aerial structure of the plants. One year after treatment, there was a significant reduction in both the plant height and the aerial phytomass of the individuals tested. This coincides with the general perception that perturbations (clearing, burning or grazing) result in an initial reduction in the dimensions of the plants affected (Papatheodorou et al., 1993; Silva and Ravento´ s, 1999). Moreover, approximately 50% of the population did not survive the perturbation. With respect to basal diameter, no significant differences were found between dead or live individuals, showing that they were not structurally different before the clearing treatment. The cutting height and thus the different degree of perturbation showed clearly significant differences between live and dead individuals. The great dependence found between cutting height and dead shoot height could explain the mortality of these individuals. That is to say that the mortality of these individuals depended on the vertical distribution of the phytomass. In 9-year-old mature communities, the effectiveness of using the clearing treatment to eliminate individuals was 100% (Baeza, 2001). The same result was seen in senescent, 17-year-old individuals cleared to make a fire break (personal observation). Studies made by

Baeza (2001) in shrublands of different ages (3, 9 and 17 years old) showed that in Ulex parviflorus the dead shoot height shows a high correlation with the age of the individual (r 2 ¼ 0:85; P < 0:01; n ¼ 151). Therefore, it is easy to predict the shrubland age that would be most effective for applying treatments to control this species. Faraco (1998) observed that Cytisus eriocarpus failed to resprout after a natural fire in a mature 17-year-old shrubland, whereas juvenile (4-year-old) individuals removed by means of silviculture treatments did resprout. In their work on grasses, Silva and Ravento´ s (1999) observed that the loss of aerial phytomass had the greatest effect on species with aboveground meristems (Andropogon semiberbis) and the least effect on species with rhizomes and deep lateral meristems (Leptocoryphium lanatum). Our results indicate that Ulex parviflorus either is not provided with or is incapable of forming adventitious leaf buds in its dead shoot fraction or its lignified structures (e.g. stalks); thus, it is highly vulnerable to perturbations produced at these levels. Guerrero (1998) observed that the species that resprout after perturbations have enlarged roots for accumulating nutritional reserves, enabling them to survive a perturbation. This phenomenon has also been studied

M.J. Baeza et al. / Forest Ecology and Management 186 (2003) 47–59

exhaustively in resprouting shrublands with a high fire risk in Australia (Pate et al., 1990). Species incapable of resprouting from the roots (Cistaceae) show strategies similar to Ulex parviflorus, i.e. they are strict reseeders with a limited capacity of the root to accumulate nutrients (Guerrero, 1998). In the individuals that survived the clearing treatment, an obvious result was the loss of live photosynthetic tissues and the modification in the relations among the different components of the phytomass. As a consequence of this, a significant reduction in the basal diameter/ phytomass relationship was observed 1 year after the treatment. Nevertheless, the remaining live individuals responded to the treatment by increasing their weight five-fold during the second year (and to a lesser extent in the following years), thus explaining the reestablishment of the initial basal diameter/phytomass relation in the short period of 2 years. The considerable increase in phytomass after the perturbation must be the result of an increase in the rate of photosynthesis of the surviving photosynthetic phytomass (Coley et al., 1985; Oesterheld and McNaughton, 1991) due to a rapid restructuring in response to the new conditions created by the perturbation. Our results indicate that strict reseeder species like Ulex parviflorus show compensatory vegetative growth in response to perturbations that produce the loss of part of their aerial phytomass. At 4 years after the treatment, this compensatory response was observed in the phytomass and the basal diameter. Nevertheless, the compensatory effect was not observed in the plant height or in the dead shoot height. With respect to the aerial fraction, the clearing caused significant changes in plant structure and in related cover conditions (85% vegetation cover before clearing versus 9% 1 year after clearing); these changes are related to a larger availability of radiation incident on the individual plants and thus to a reduction in the intraspecific competition for light (McNaughton, 1984; Tilman, 1988; Gerry and Wilson, 1995), resulting in a compensatory growth in basal diameter and aerial biomass. In relation to the plant height of the cleared individuals, this is possibly due to the fact that the problem was not with light availability but with photosynthetic phytomass production (phyllodes and fine photosynthesising branches) (McNaughton, 1984; Milchunas et al., 1988). In addition, the dead shoot height was lower in the cleared individuals with the result that the cleared individuals

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showed a 20% decrease in the relation between plant height and dead shoot height in comparison with the control individuals. Another aspect contributing to the non-observation of compensation with respect to plant height was that the control individuals grew in high density populations and to gain height had to invest in branch production since light is a limiting resource. In previous studies (Baeza, 2001) it was observed that in very competitive habitats (mature gorselands) the largest biomass growth is produced in branches larger than 0.5 cm in basal diameter, i.e. in branches that facilitate the access of limiting resources such as light. Tilman (1988) suggested that plant height is the most important determinant in the competition for light availability. As a result of the changes in the growth rates of their different structures, the cleared individuals showed a more compact phytomass, formed mainly by green shoots. This compactness is the result of the presence of individuals of greater weight, but smaller size than the control plants. This change in the apparent density of the cleared individuals may be due to morphological adjustments (morphological plasticity) (Alward and Joern, 1993) by means of which Ulex parviflorus can modify its production towards the photosynthesising structures it lacks as a result of the clearing. Results obtained by Baeza et al. (1998) show that this fuel compactness decreases with shrubland age, causing a greater aeration of the fuel and increasing the risk of subsequent fires. Both clearing and prescribed burning are highly efficacious fuel control treatments that substantially reduce the fuel load available for future fires. Most evaluations of these techniques are limited to analysing their effectiveness in eliminating fuel. We propose that their effectiveness should also be analysed from the point of view of the processes set in motion to regenerate the original shrubland. Prescribed burnings in gorseland (Baeza, 2001), even at low intensities, eliminate practically 100% of the individuals of the community; however, germination of seeds in the soil facilitates the recruitment of new individuals capable of regenerating the gorse stand. Moreover, applying clearing techniques to young gorse stands leads to phenomena of compensatory growth by which the gorse stand is regenerated. Nevertheless, the changes in the biomass-necromass relation which take place throughout the growth of the plant are crucial to its

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mortality. Thus, maximum effectiveness in controlling Ulex parviflorus occurs in advanced stages of its development, when the dead shoot height is above the cutting height of the clearing machine.

Acknowledgements We thank two anonymous reviewers for their valuable comments on the manuscript. This research was supported by the Regional Ministry of the Environment (Generalitat Valenciana) and Bancaja. We thank J. Scheiding for the English translation of the text, and T. Jimenez and B. Molla for collaboration in field sampling.

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