Contrasting strategies to cope with drought by invasive and endemic ...

Biodivers Conserv (2007) 16:2123–2136 DOI 10.1007/s10531-006-9131-9 ORIGINAL PAPER

Contrasting strategies to cope with drought by invasive and endemic species of Lantana in Galapagos Jesu´s M. Castillo Æ Pablo Leira-Doce Æ Jorge Carrioń-Tacuri Æ Edison Munõz-Guacho Æ Aı´da Arroyo-Solı´s Æ Guillermo Curado Æ David Doblas Æ Alfredo E. Rubio-Casal Æ ´ lvarez-Lo´pez Æ Susana Redondo-Go´mez Æ Antonio A. A Regina Berjano Æ Giovanny Guerrero Æ Alfonso De Cires Æ Enrique Figueroa Æ Alan Tye Received: 17 May 2006 / Accepted: 25 September 2006 / Published online: 27 October 2006 Springer Science+Business Media B.V. 2006

Abstract This study compares how Lantana camara, an invasive species, and L. peduncularis, an autochthonous one, cope with drought in Galapagos. Soil surface temperature was the abiotic environmental parameter that best explained variations in photosynthetic stress. Higher soil surface temperatures were recorded in the lowlands and in rain-shadow areas, which were also the driest areas. L. peduncularis, with a shallow root system, behaved as a drought-tolerant species, showing lower relative growth rates, which decreased with leaf water content and higher photosynthetic stress levels in the lowlands and in a northwest rain-shadow area in comparison with higher and wetter locations. Its basal and maximal fluorescences decreased at lower altitudes, reflecting the recorded drops in chlorophyll concentration. In contrast, L. camara with a deep root system behaved as a drought-avoiding species, showing leaf and relative water contents higher than 55% and avoiding permanent damage to its photosynthetic apparatus even in the driest area where it showed very low chlorophyll content. Its relative growth rate decreased more in dry areas in comparison to wetter zones than did that of L. peduncularis, even though it had greater water content. Furthermore, L. camara showed higher water contents, growth rate, and lower photosynthetic stress levels than L. peduncularis in the arid lowlands. Thus, L. peduncularis maintained lower maximum quantum efficiency of photosystem II photochemistry (Fv/Fm) than L. camara even at sunrise, due to higher basal fluorescence values with similar maximal fluorescence, which indicated J. M. Castillo (&) Æ P. Leira-Doce Æ A. Arroyo-Solı´s Æ G. Curado Æ D. Doblas Æ ´ lvarez-Lo´pez Æ S. Redondo-Go´mez Æ R. Berjano Æ A. E. Rubio-Casal Æ A. A. A A. De Cires Æ E. Figueroa Departamento de Biologıá Vegetal y Ecologıá, Facultad de Biologıá, Universidad de Sevilla, Apartado 1095, 41080 Sevilla, Spain e-mail: [email protected] J. Carrioń-Tacuri Æ E. Munõz-Guacho Æ G. Guerrero Universidad Central de Ecuador, Ciudad Universitaria, Quito, Ecuador A. Tye Charles Darwin Research Station, Puerto Ayora, Isla Santa Cruz, Gala´pagos, AP 17-01-3891, Quito, Ecuador

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permanent damage to PSII reaction centres. Our results help to explain the success and limitations of L. camara in the invasion of arid and sub-arid environments. Keywords Chlorophyll fluorescence Æ Invasion Æ Photosynthetic stress Æ Soil temperature Æ Lantana camara Æ Lantana peduncularis

Introduction Island ecosystems are particularly vulnerable to biological invasions, which have become their main conservation problem (Brockie et al. 1988), and the Galapagos Archipelago is no exception (Schofield 1989). Many introduced plant species are recognised as having an adverse effect on native vegetation in Galapagos (Tye 2001). Most of these invasions are in the humid highlands, where introductions for agriculture have been more frequent and conditions more favourable than in the semiarid lowlands (Tye et al. 2002). However, semiarid lowlands also suffer important invasions, among which Lantana camara L. (Verbenaceae) is one of the main invaders (Lawesson and Ortiz 1990). Lantana camara, a woody, strongly branching shrub from tropical America, is considered to be among the world’s ten worst weeds, invading more than 60 countries worldwide (Sharma et al. 2005). It is invading four inhabited islands in Galapagos, occupying thousands of hectares mostly in the semiarid lowlands, where it displaces native vegetation and in places forms impenetrable thickets (Eliasson 1982; Cruz et al. 1986; Mauchamp et al. 1998). In some areas, L. camara grows together with the Galapagos endemic Lantana peduncularis Anderss. (Hamann 2004). This situation offers an unusual opportunity to analyse from an ecophysiological point of view how these closely related species deal with drought, the most important stress factor in the semiarid lowlands of Galapagos (Hamann 2001, 2004). No previous study has analysed ecophysiological responses to drought in plants of the Galapagos. The main aim of this study was to compare how L. camara and L. peduncularis cope with drought in Galapagos. We analysed variations in the root system distribution, water status, growth, photosynthetic pigments and stress level of both species in relation to environmental variation, by means of topographical transects across environmental gradients (Castillo et al. 2000). Photosynthetic stress was measured by chlorophyll fluorescence, which corresponds to variation in the photosynthetic function in response to environmental change (Maxwell and Johnson 2000). Our results contribute to an understanding of the physiological characteristics of invasive species in arid ecosystems as well as their mechanisms of invasion.

Methods Study sites The Galapagos are volcanic islands located in the Pacific Ocean, approximately 1,000 km west of Ecuador, on the equator. Our study was carried out on two islands: Santa Cruz (9010¢05¢¢–9032¢55¢¢ W; 028¢43¢¢–046¢23¢¢ S) and Floreana (9021¢07¢¢– 9030¢37¢¢ W; 113¢04¢¢–121¢30¢¢ S) during the dry season, from August to November 2005 (Fig. 1).

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Fig. 1 Map of Galapagos archipelago, indicating the location (w) on Santa Cruz and Floreana islands of the three studied topographical gradients (1, 2 and 3) and of the site where diurnal cycles were carried out with both Lantana species growing together (4)

From January to June, daytime maximum temperature averages 29C and mean temperatures are between 25C and 26C (Ziegler 1995), and intermittent rains may fall although most days are sunny. From July to December temperatures are lower (18-26 C) and there is less rain in the lowlands although a mist layer, known locally as ‘garua’, more frequently occurs during this season, with cloudy days more frequent than sunny days (Alpert 1963; Wiggins and Porter 1971). The vegetation is strongly zoned by altitude, since most rainfall is orographic (Alpert 1963; Wiggins and Porter 1971). The littoral zone includes mangroves and coastal dunes. Semiarid lowlands, where L. peduncularis and L. camara grow together, are characterised by endemic tree cacti of Opuntia and Jasminocereus and a shrub layer or by open woodland. The transition zone is characterised by closed

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mixed forest, where Lantana species are present also. The humid zone above this does not support Lantana species. Exploratory fieldwork allowed us to identify suitable study sites. Two topographical gradients were chosen in the southeast and north of Santa Cruz (10–260 m and 400–600 m above sea level, respectively), where both species were present (L. peduncularis frequent and L. camara scarce), and one in northwest Floreana (130–420 m), where L. camara had invaded thousand of hectares and L. peduncularis was also present. In addition, we studied diurnal cycles of fluorescence in southeast Santa Cruz, at 25 m a.s.l. close to Puerto Ayora, where both Lantana species grew together (Fig. 1). Root system distribution Root systems of both Lantana species (n = 3 plants for each species) were explored by digging manually using a shovel around isolated individuals to a depth of 30–40 cm until the bedrock appeared and the root system was free of soil. Any deep roots penetrating into the bedrock were noted. Abiotic environmental factors Measurements of abiotic environmental factors were carried out in parallel with ecophysiological and growth measurements. Height above sea level was measured using a GPS (model eTrex Vista Garmin) with its altimeter adjusted to zero at sea level in the field with 3 m accuracy. Photosynthetic photon flux density (PPFD in lmol m–2 s–1) was recorded with a portable photometer (Licor-189). Bare soil surface temperature (C) was measured with a non-contact portable recorder (Autopro, Raytek, Santa Cruz, USA) (n = 10 measurements per site). Air temperature (C) and relative humidity (%) were measured at 1.5 m above soil surface with a portable thermo-hygrometer (Elka FTM-10) and data logging system (Escort, New Zealand) every 5 min. Wind speed (m s–1) was measured with an anemometer (Martin Marten TA-6000, Barcelona) at five heights above soil surface (0.5, 1.0, 1.5, 2.0 and 2.5 m, n = 20) throughout the southeast elevational gradient on Santa Cruz. Chlorophyll fluorescence Chlorophyll a fluorescence was measured in the youngest fully developed leaves on plants along the three topographical gradients described above during the middle part of the day (11–14 h solar time) on frequent cloudy days with a mean PPFD of 1052 ± 66 lE m2 s–1 (21 plants for L. peduncularis and 16 plants for L. camara; 4–6 leaves per plant) during September 2005 (points 1, 2 and 3 in Fig. 1). Fluorescence was recorded also in diurnal cycles every 3 h from sunrise to sunset at the site close to Puerto Ayora (15 plants for L. peduncularis and 10 plants for L. camara; n = 2–3 leaves per plant) on sunny and cloudy days during August and September 2005 (point 4 in Fig. 1). Light- and dark-adapted fluorescence parameters were measured using a portable modulated fluorimeter (FMS-2, Hansatech Instrument Ltd., England). Leaves were dark-adapted for 30 min, using leaf-clips. The minimal fluorescence level in the dark-adapted state (F0) was measured using a modulated pulse ( 0.05). On the other hand, relative humidity was between 25% and 90%, increasing with elevation (P < 0.0001, n = 60). Frequent variations over 5-min intervals were recorded in air temperature (maximum ca. 2C), and relative humidity (maximum ca. 18%). Temperature of bare soil surface during the middle part of the day was independent of elevation when all the measurements made along all three gradients were included in the analysis (P > 0.05), varying between 26C and 76C. However, temperature of soil surface increased at lower elevations in southeast Santa Cruz (r = –0.80, P < 0.0001, n = 32) and northwest Floreana (r = –0.85, P < 0.0001, n = 38). Soil surface temperature at a given altitude was always higher in northwest Floreana than in southeast Santa Cruz (Fig. 2). In southeast Santa Cruz, temperature of bare soil surface during the middle part of the day was independent of wind speed, which was lower at higher altitudes (r = –0.38, P < 0.05, n = 31), varying from 1 m s–1 to 7 m s–1. Air temperature increased with soil surface temperature when all data were included in the analysis (r = 0.49, P < 0.0001, n = 60). However, air temperature was independent of soil surface temperature and elevation in north and southeast Santa Cruz. Chlorophyll fluorescence parameters for both Lantana species, such as Fv/Fm, Fv, Fm and FPSII, increased with elevation when the topographical gradients were

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Bare soil temperature (οC)

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Altitude (m) Fig. 2 Relationship between temperature of bare soil surface at midday (n = 10) and altitude above sea level on three topographical gradients in Galapagos Islands (•, Southeast Santa Cruz Island; D, Northwest Floreana Island; n, North Santa Cruz Island). Data are mean ± SE. Regression equations: Southeast Santa Cruz Island (—): y = 49.67 – 0.08x; Northwest Floreana Island (ÆÆÆÆÆ) y = 76.71 – 0.10x

analysed independently, but these correlations disappeared when all data from different locations were analysed together. Furthermore, every fluorescence parameter was independent of atmospheric conditions (wind speed, air temperature and air relative humidity) for both Lantana species when grouping records from different orientation and elevations together. Fv/Fm and FPSII for L. peduncularis were lower at higher soil temperature when grouping data from different orientations and elevations (r = –0.68, P < 0.001, n = 21; r = –0.73, P < 0.0005, n = 21, respectively) (Fig. 3). This decrease in Fv/Fm as soil temperature increased was due to a decrease in Fv, since Fm dropped more than F0; both parameters were also negatively correlated with soil temperature (P < 0.01). NPQ for L. peduncularis increased with higher soil temperature during the middle part of the day (r = –0.50, P < 0.05, n = 21), and was correlated negatively with Fv/Fm and FPSII (P < 0.005). In contrast, Fv/Fm, FPSII and the other fluorescence parameters in L. camara were independent of soil surface temperature during the middle part of the day. L. camara showed higher Fv/Fm (ca. 0.800) and FPSII (ca. 0.650) than L. peduncularis at soil temperatures higher than 30C. On bare soils with temperature between 25C and 30C both Lantana species showed similar photochemical efficiencies (Fig. 3). In southeast Santa Cruz, RGR, RWC and LWC of apical leaves increased with elevation for both Lantana species (P < 0.05), except RWC for L. peduncularis. The variations in RWC and LWC with elevation were higher in L. peduncularis than in L. camara. RGR, RWC and LWC were higher in L. camara than in L. peduncularis at every elevation and RGR decreased with LWC for L. peduncularis (r = 0.77, P < 0.005, n = 7) (Fig. 4). Chlorophyll (a + b) and carotenoid (c + x) concentrations increased and Chl a + b/C (c + x) ratio decreased with elevation for L. peduncularis (P < 0.05) (Table 1).

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Fig. 3 Relationships between temperature of bare soil surface at midday (C; n = 10) and (a) potential photochemical efficiency (Fv/Fm; n = 4–6 leaves per plant) and (b) quantum efficiency of PSII (FPSII; n = 4–6 leaves per plant) for Lantana peduncularis (n = 21 plants; s) and L. camara (n = 16 plants; •) Data are mean ± SE. Regression equations: L. camara (—), Fv/Fm: y = 0.95 – 6.26x; FPSII: y = 0.92 – 9.84x

Diurnal cycles During the diurnal cycles monitored at 25 m a.s.l. in southeast Santa Cruz, maximum wind speeds were recorded during the middle part of the day (ca. 5.0 m s–1), being lower at dawn and sunset (ca. 2–3 m s–1). Air temperature increased from dawn (ca. 20C) to midday (ca. 30–35C), decreasing until sunset (ca. 25C). Relative humidity showed inverse diurnal variations, with maximum at dawn (ca. 80%) and minimum at midday (ca. 48%). Bare soil surface temperature showed similar oscillations, starting at dawn at the same values as air temperature (ca. 20C) and increasing to ca. 55C at midday (Fig. 5). Lantana camara showed higher Fv/Fm and FPSII than L. peduncularis from dawn to sunset in the dry season at 25 m a.s.l. in southeast Santa Cruz. A dynamic photoinhibition manifested as a gradual decrease of Fv/Fm at higher PPFD was

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Altitude (m) Fig. 4 Relationships between (a) relative growth rates (RGR, n = 10), (b) relative water content (RWC) and (c) leaf water content (LWC) of apical leaves of Lantana peduncularis (s) and L. camara (•) with altitude in southeast Santa Cruz Island. Data are mean ± SE. Regression equations: RGR: L. peduncularis (ÆÆÆÆÆ), y = 4.99 + 9.7x; L. camara (—), y =–0.14 + 7.20x. RWC: L. camara (—), y = 55.72 + 0.10x. LWC: L. peduncularis (ÆÆÆÆÆ), y = 48.42 + 0.09x; L. camara (—), y = 54.59 + 0.10x

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Table 1 Chlorophyll a (Chl a), chlorophyll b (Chl b), carotenoids (Cx + c) contents (mg g–1 DW), and Chl a + b/Cx + c ratio of the youngest fully developed leaf during the middle part of the day throughout an elevational transect (31–217 m above sea level) on southeast Santa Cruz for Lantana peduncularis and at the site close to Puerto Ayora for L. peduncularis and L. camara in Galapagos Islands Chl a L. peduncularis at elevational 217 m transect on SE Santa Cruz 160 m 148 m (m a.s.l.) 60 m 49 m 31 m Puerto Ayora

1.70 2.17 1.52 1.06 0.85 0.08

± ± ± ± ± ±

Chl b 0.25 0.78 0.37 0.61 0.29 0.02

0.68 0.83 0.53 0.35 0.27 0.04

± ± ± ± ± ±

Cx + c 0.05 0.30 0.14 0.23 0.09 0.01

0.20 0.28 0.26 0.27 0.18 0.03

± ± ± ± ± ±

Chl/c 0.06 14.00 ± 3.95 0.05 10.05 ± 2.92 0.03 7.85 ± 1.69 0.19 4.70 ± 1.50 0.06 6.20 ± 0.17 0.01 3.90 ± 1.08

L. camara 0.25 ± 0.12 0.10 ± 0.04 0.07 ± 0.03 L. peduncularis 0.72 ± 0.06 0.25 ± 0.06 0.21 ± 0.01

4.66 ± 0.92 4.60 ± 0.49

Data are mean ± SE (n = 3)

recorded in both species. Fv/Fm oscillated between 0.639 ± 0.016 and 0.768 ± 0.015 in L. peduncularis and between 0.730 ± 0.016 and 0.848 ± 0.005 in L. camara. Fv was similar in both species, decreasing during the middle part of the day, whereas F0 kept constant diurnal values, being higher in L. peduncularis (ca. 120 relative units of fluorescence) than in L. camara (ca. 80 relative units of fluorescence) (Fig. 5). Although NPQ values were similar in both species at dawn, L. camara developed higher NPQ than L. peduncularis during the day, with maximum differences around midday (t-test, P < 0.05). Photochemical quenching was higher in L. camara at dawn and sunset (t-test, P < 0.05) and these differences disappeared around midday, where qP decreased in both species (Fig. 5). LWC and RWC at midday were a little higher in L. camara (57 ± 0% and 47 ± 2%, respectively) than in L. peduncularis (54 ± 2% and 37 ± 6%, respectively). Fv/Fm was lower on cloudy days (PPFD 500–800 lE m–2 s–1) than on sunny days (PPFD between 1100 lE m–2 s–1 and 2100 lE m–2 s–1) during the middle part of the day and this difference was higher in L. peduncularis (–40.2% from 0.733 ± 0.010 to 0.653 ± 0.017) than in L. camara (–30.7%, from 0.792 ± 0.005 to 0.757 ± 0.008). This drop was due to a decrease in Fv, resulting from a greater decrease in Fm than in F0. The decreases in FPSII and qP on sunny days compared to cloudy days were also higher in L. peduncularis (FPSII: –48.5%, from 0.608 ± 0.012 to 0.313 ± 0.015; qP: –30.7%, from 0.88 ± 0.01 to 0.61 ± 0.05) than in L. camara (FPSII: –39.6%, from 0.614 ± 0.011 to 0.371 ± 0.014; qP: –11.1%, from 0.90 ± 0.01 to 0.80 ± 0.04). Finally, NPQ increased more in L. camara (+206.6%, from 0.61 ± 0.04 to 1.87 ± 0.15) than in L. peduncularis (+166.0%, from 0.50 ± 0.03 to 1.33 ± 0.12) from cloudy to sunny days. L. peduncularis showed higher Chl a, Chl b and Cx + c than L. camara (t-test, P < 0.05) (Table 1).

Discussion Among the invaders of the arid lowlands of Galapagos, Lantana camara is one of the most important (Lawesson and Ortiz 1990), where it grows throughout topographical gradients together with the closely related endemic L. peduncularis. These species

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Fig. 5 Diurnal variation in (a) potential photochemical efficiency (Fv/Fm), (b) quantum efficiency of PSII (FPSII), (c) basal fluorescence (F0, in relative units of fluorescence), (d) photochemical quenching (qP), (e) variable fluorescence (Fv, in relative units of fluorescence), and (f) non-photochemical quenching (NPQ) for Lantana peduncularis (s) and L. camara (•), and (g) photosynthetic photon flux density (PPFD in lE m2 s–1; n) and (h) bare soil surface temperature (in C; m) at 25 m above sea level on Santa Cruz Island. Data are mean ± SE

deal with drought by very different strategies: L. peduncularis has a shallow root system and behaves as a drought-tolerant species, whereas L. camara shows a deep root system, behaving as a drought-avoiding species. Drought varies in Galapagos depending on both orientation and elevation above sea level (Wiggins and Porter 1971). However, elevation was not a good indicator of plant stress during the dry season, since plant stress responses were independent of it when areas with different orientations were analysed together. On other hand, variations in the immediate atmospheric environment (characterised in this study by wind speed, air temperature and relative humidity) also did not explain plant stress, due to different behaviour in different areas. Areas more exposed to the dominant southeast winds experienced faster changing atmospheric conditions, with smaller variations with elevation than in the northwest rain-shadow areas, where relative

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humidity and air temperature varied with elevation. In this context, soil surface temperature was the abiotic environmental parameter among all those recorded that best explained the variations in photosynthetic stress. Higher soil surface temperatures were recorded in the lowlands close to the coast and in rain-shadow areas, which were also the driest areas. Lantana peduncularis, with a shallow root system, showed lower RGR, which decreased with LWC and higher photosynthetic stress levels during the middle part of the day in the lowlands and in a northwest rain-shadow area in comparison with higher and wetter locations. F0 and Fm decreased for L. peduncularis at lower altitudes, reflecting the recorded drops in chlorophylls concentrations. Although L. peduncularis dealt with drought by increasing dissipation of excess excitation energy by heat emission, reflected in higher NPQ and lower chlorophylls/carotenoids ratios, this photoprotection was not enough to avoid PSII damage, and the decrease in Fm was higher than in F0, which provoked a drop in Fv/Fm due to a deactivation of PSII reaction centres (Maxwell and Johnson 2000). The high stress levels recorded for L. peduncularis agree with its high mortality rates during prolonged droughts in Galapagos (Hamann 2004). In contrast, fluorescence parameters in L. camara, with deep roots and LWC and RWC always higher than 55%, were independent of variations in the abiotic environment due to orientation and elevation. Lantana camara was able to avoid permanent damage to its photosynthetic apparatus even in the driest areas where it grew, showing very low chlorophyll concentrations. These results indicated that the reduction of chlorophylls did not result from severe photoinhibitory damage but, instead, it may be an adaptive response against the adverse conditions of the Galapagos dry season (Kyparissis et al. 1995). The RGR of L. camara decreased more in dry areas in comparison to wetter zones than did that of L. peduncularis, even though it had greater water content, which is a typical response for a droughtavoiding species exposed to water shortage (Ferrio et al. 2003; Van Staalduinen and Anten 2005). Furthermore, L. camara showed higher LWC, RWC and RGR, and lower photosynthetic stress levels (higher Fv/Fm, FPSII and qP) than L. peduncularis in the arid lowlands with soil surface temperatures higher than 40C around midday during the frequent cloudy days of the dry season. Thus, L. peduncularis maintained lower Fv/Fm than L. camara even at sunrise, due to higher F0 values with similar Fm, which indicated permanent damage to PSII reaction centres. This may be related to the superior capacity of L. camara to increase thermal energy dissipation during the middle part of the day when radiation levels were higher, reflected in higher NPQ (Maxwell and Jonson 2000). These differences were recorded also at midday during sunny days, when L. peduncularis showed higher stress levels than L. camara and a lower capacity to acclimate to high radiation intensities, reflected in higher photoinhibition levels. In higher, moist areas with soil surface temperatures between 25C and 40C, both species showed similar ecophysiological responses, but L. camara had higher RGR. Previous studies have also described the existence of different functional types (drought-tolerant and drought-avoiding species) within one genus, relating these contrasted strategies with the morphology of the root system and photosynthetic stress levels (Castillo et al. 2002), with hydraulic conductance and vulnerability to embolism (Nardini and Tyree 1999; Engelbrecht et al. 2000; Lemoine et al. 2001) or with the control of stomatal conductance, leaf water potential and photosynthetic carbon fixation (Dickson and Tomlinson 1996; Nardini et al. 1999).

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Our results help to explain the success and limitations of L. camara in the invasion of arid and sub-arid environments. Adult individuals have deep roots that may allow them to reach the water table, and low leaf water structural requirements. Moreover, their photosynthetic apparatus accommodates to high radiation levels even during drought and they are able to live in soils at high temperatures (Tsiotsiopoulou et al. 2003). These capabilities favour this alien species in comparison with the endemic L. peduncularis. However, L. camara suffered a significant decrease in RGR, LWC and RWC in drier areas, probably due to its inability to reach very low water table levels, since drought-avoiding species try to maintain constant RWC (Nardini et al. 1999). L. camara did not invade very dry areas with soil surface temperature around midday higher than 60C during cloudy days of the dry season, although L. peduncularis was present there. The lower distribution limit of L. camara was ca. 25 m a.s.l. in the southeast and ca. 130 m in the northwest rain shadow, where average annual rainfall is less than 500 mm (Wiggins and Porter 1971). As in our study, areas with a mean rainfall less than 600 mm year–1 have been identified as the limit to L. camara invasion in Australian forests (Fensham 1996). This could reflect a limitation by water stress when it has not yet developed a deep root system during its establishment period, identified as one of the most sensible stages in the life cycle of Galapagos plants (Hamann 2004). The capacity to develop a deep root system at an early stage has been identified as an important trait for the survival (Chapin 1995) and competitive ability of aliens (Lopez-Zamora et al. 2004). Acknowledgements We thank the Consejerıá de la Presidencia de la Junta de Andalucıá for their approval of the International Cooperation Project AI60/04, and the Galapagos National Park for their collaboration. Thanks to Raquel and Cristina for helping in the lava fields.

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