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In high-mountain habitats, summer frosts have negative consequences for plant fitness, therefore high-mountain plants have developed mechanisms of avoidance and tolerance to cope with freezing temperatures. Various hypotheses have been proposed to explain the prevalence of one freezing-resistance mechanism over another, focusing on thermal conditions without a consensus. We hypothesize that the prevalence of a freezing-resistance mechanism depends on the life-history strategy of the species, and is probably the consequence of a trade-off between growth/reproduction and the cost of the mechanism. Specifically, short-lived annual species should be freezing avoidant, whereas perennial long-lived species should be freezing tolerant. We used thermal analysis to determine the mechanism of freezing resistance of leaves and flowers for 10 annual and 14 perennial herb species from an alpine ecosystem in the Central Chilean Andes. We found that 70% of the annual species, their flowers and leaves were freezing avoidant, indicating that avoidance was their predominant freezing-resistance mechanism. In the case of perennial species, both mechanisms were almost equally represented in flowers and leaves. Overall, our results showed that a species freezing-resistance mechanism depends on its life-history strategy, and that leaves and flowers of single species exhibit the same freezing resistance mechanism, suggesting a common whole plant strategy. Further, freezing resistance strategies were not found to be mutually exclusive. In some cases, a specific combination of phenological, structural, and functional strategies may determine how freezing resistant vegetative and reproductive organs are to freezing during the growing season.

In high-mountain habitats, summer frost events can have negative consequences for plant fitness. Despite this, most studies have evaluated the consequences of frosts for vegetative structures of perennial plants, and neither for leaves nor for flowers of annual plants. We hypothesize that the degree of freezing resistance of flowers and leaves of a species depends on its life-history strategy (LHS), and is probably the consequence of a trade-off between growth/reproduction and the cost of the freezing resistance. Specifically, flowers and leaves of short-lived annual species should be less freezing resistant than those of perennial plant species. We compared the freezing resistance of flowers and leaves of 10 annual and 12 perennial plant species from the Andes of central Chile using the electrolyte leakage method. Temperature damage for 50% tissue (LT50) of annual species was − 9.6 °C in flowers and − 11.9 °C in leaves. In perennial species, LT50 was similar in flowers (− 12.3 °C) and leaves (− 12.5 °C). Despite that, these differences were not significant (except the flowers of annual species), we found remarkable differences between LHS when freezing resistance was analyzed species by species. Like this, 58% and 83% of perennial species resist temperatures ≤ − 10 °C in their flowers and leaves, respectively, compared with only 30% and 40% of annual species. Additionally, in most of the species, the freezing resistance of leaves was greater than that of flowers, with this proportion being greater in annual (58%) than in perennial species (43%). Thus, we concluded that the degree of freezing resistance depends on the LHS, such that annual species, which are less freezing resistant than perennial species, have an infrequent occurrence and a distribution restricted to low elevation in high-mountain habitats.

Snow cover duration, which depends on winter precipitation and temperature, is decreasing along several alpine areas around the world as a consequence of the global warming and further increases are expected in the future. Thus, alpine species that requires cold stratification to initiate seed germination may be threatened in the future. Here we report the results of an exploratory experimental survey to assess the importance of cold stratification for the seed germination in alpine plant species from the High-Andes of central Chile. In addition, we assessed how this requirement varies with elevation at the community level. Seeds of a total of fifty plant species were collected along an altitudinal range from 2200 to 3600 m a.s.l. Eighteen species corresponded to the Low-Andean vegetation belt, while thirteen and nineteen species corresponded to the Mid- and High-Andean vegetation belts, respectively. For each species, seeds that experienced cold-stratification at 4 °C for 3 months and control seeds (stored at room temperature and dry conditions for 3 months) were placed on moist paper in 4 Petri dishes containing 50 seeds each, and arranged in a growth chamber set at 20 °C/10 °C and a photoperiod of 14/10 h. While 36% of the studied species were able to germinate without cold-stratification, after this pre-treatment 74% of these species germinated, suggesting that in general cold-stratification promotes seed germination of these alpine plant species. This positive effect was particularly evident on the species from the lower elevations. At highest elevation both the number of species that showed seed germination and the percentage and velocity of seed germination were low. Nonetheless, non-germinated seeds remained viable, suggesting a deeper dormancy. Our results suggest that cold stratification could be an important requirement for successful seed germination in species from lower elevations, while species from higher elevations could require other factors than cold-stratification to break seed dormancy. Nonetheless, as we did not use temperatures as those experienced in the field, further studies are needed to gain insights into the importance of the duration of cold stratification and the underlying mechanisms involved in the seed germination in the field of high-Andean plant species.

Background and aims In Mediterranean annual plants, germination mainly occurs during the autumn and only those seedlings that survive winter freezing can flower and produce seedlings in spring. Surprisingly, the effect of freezing events as an abiotic determinant of these communities remains unexplored. The present study aimed to investigate how freezing events affect annual Mediterranean communities and whether their functional structure as related to freezing resistance is linked to the main biotic and abiotic determinants of these communities. Methods In 120 plots located on a semi-arid Mediterranean steppe (Spain), the community functional structure related to the lethal temperature causing 50 % frost damage (LT50 trait) in seedlings was estimated and summarized as the community-weighted mean (CWM-LT50) and its functional diversity (FD-LT50). Plots were stratified according to distance to rabbit shelters and latrines as a proxy for rabbit density, proximity to Stipa tenacissima and spring water availability, where annual species abundance was recorded in all plots over three consecutive years. Key results Annual species were able to resist a threshold temperature of -4 °C and most had LT50 values around the absolute minimum temperature (-9.5 °C) in the three years. Higher rabbit densities led to lower CWM-LT50 and higher FD-LT50 values. Plots close to Stipa tussocks had higher CWM-LT50 values whereas water availability had no effects. Conclusions High freezing resistance was extended among winter annual species, suggesting the presence of an association between historical environmental filtering and low winter temperatures. However, the community functional structure related to freezing resistance remained variable among scenarios with differences in herbivory pressure and distance to perennial vegetation. The trends observed indicate that traits that allow plants to deal with herbivory may also promote freezing resistance, and that tussocks can act as nurses via microclimatic amelioration of harsher winter conditions.

Only two vascular plants have been able to colonize some of the ice and snow-free lands of the Antarctic Peninsula: the hair grass Deschampsia antarctica (Poaceae) and the pearlwort Colobanthus quitensis (Caryophyllaceae). This low species diversity may be due to the permanent low temperature even during summer time. Beside low temperature, Antarctic plants must be able to cope with other severe physiological stressors such as desiccation, low soil water availability, and high irradiance. However, these factors are found in other cold areas of the globe. Thus, what is so special about these two species that has enabled them to be the only successful flowering plants in the Antarctica? Although this question has been addressed in other articles, we still lack of an integrative ecophysiological framework that helps to disentangle what it is unique of these species in terms of adaptations to the Antarctic environments, and how these adaptations will help or preclude their responses to future climate change. Several adaptations seem to help to withstand the Antarctic climate: xerophytic anatomical characteristics, sufficient freezing tolerance, ability to maintain positive net photosynthesis at near 0 °C, adequate management of excess photosynthetic active radiation, resistance to photoinhibitory conditions, tolerance to water stress, and ability to form associations with endophytes that help in their mineral nutrition. Besides the very effective stress tolerance strategies, several ecophysiological traits show considerable response flexibility. The evidence reviewed here indicates that small increases in air temperature may be beneficial in terms of photosynthetic performance. However, increased frequency of leaf temperatures over 20 °C could be harmful affecting photosynthesis and reducing the ability of these plants to tolerate freezing temperatures and photoprotection at low temperatures, attributes by which they are able to colonize these very harsh environment.

Summary Positive interactions have started to gain a place as important drivers of community structure and biological diversity. Defined as non-trophic interspecific interactions that increase the average individual fitness of one species, by definition, the presence of one plant species enhances the chances that another species co-occur in the same place, indicating that positive interactions may determine biological diversity. However, this has been poorly explored. The majority of the studies addressing community-level consequences of facilitation have compared the diversity of the plant assemblages growing within nurses vs. those growing outside them, reporting contrasting results among them. Nonetheless, nurses and their alternative microhabitats (open areas among nurses) are part of the same community. Thus, if nurses allow for the persistence of species that otherwise would be excluded from the community, a net increase in the species diversity at the entire community level will be generated even though nurse plants contained fewer species than open areas. Here, we conducted a bibliographic search using the ISI Web of Knowledge data base and reviewed the literature conducted on alpine plant communities where assessments of the diversity of plants growing within and outside a nurse species were available. In most cases nurse species substantially increased species richness at the community level, despite the fact that in some cases, they contained lower species numbers than open areas. Nurse species enhanced species richness more in systems with impoverished local diversity, suggesting that facilitative interactions in alpine habitats act as an insurance policy that sustains diversity under very harsh conditions.

In high-elevation habitats, an apparent trade-off between high growth-rates and freezing resistance has led some authors to assume that seedlings are less freezing resistant than adults. However, because seedlings are exposed to lower and longer freezing events near the ground, they could be more freezing-resistant than adults. As freezing resistance is related to plant height, ontogenetic changes in the intensity of freezing resistance and the mechanism involved should differ between seedlings and adults of tall vs. short plants, with those changes being greater for tall plants. We evaluated differences between seedlings and adult plants in the ability to resist freezing temperatures in 13 high-Andean species from central Chile. In a field laboratory, we determined the low temperature damage, ice nucleation temperature and freezing resistance mechanisms in their leaves. Seedlings were on average 3.4 K less freezing resistant than adults with greater differences found for tall plant species. Six species changed their freezing resistance mechanism with ontogeny, including short and tall plants, although changes were greater for tall plant species. Ontogenetic changes in the freezing resistance of high-Andean species seem to be more related to a trade-off between plant-growth and stress resistance than to differences in thermal microhabitats between seedlings and adults.

Questions: How frost resistant are the early development stages (seeds, seedlings, plantlets and juveniles) of alpine plant species? Do summer frosts impair establishment of plant species typical of different successional stages on a central alpine glacier foreland? Location: Rotmoos glacier foreland, Austrian Central Alps (Obergurgl, Tyrol, Austria). Methods: Seeds of 12 species typical of different successional stages were collected in the glacier foreland and either sown directly in the field or in a growth chamber (25/10 °C, 16/8 h) and grown to the investigated development stages. Frost resistance of the early development and adult stages was determined by exposing them to a set of freezing temperatures and assessing viability with the tetrazolium test (LT50, i.e. 50% of samples being lethally frost damaged). Results: Dry seeds had the highest frost resistance (LT50: 19 °C), followed by wet seeds after imbibition (LT50: 8 °C). With the onset of germination, frost resistance decreased rapidly. While germinated seeds tolerated a mean of 3.2 ° C, seedlings and juveniles were less frost resistant (LT50: 2.5 °C). Along the primary succession, seedlings of pioneer species were significantly less frost resistant than early- and late-successional species. However, field grown seedlings, mainly of pioneer species, showed higher frost resistance (mean: 5 °C) than the growth chamber seedlings (mean: 3 °C), indicating that frost hardening (transition from a lower to a higher level of frost resistance) is already possible during these early stages of development.

Studies performed in the fire-prone Mediterranean-type climate shrublands of Australia, California, and South Africa have shown that plant-derived smoke enhances seed germination in many species. Unlike other areas with similar climate, central Chile stands out for the absence of natural fires, suggesting that smoke may not be expected to promote germination. However, anthropogenic fires have been frequent since several millennia, and the role of fire on shaping fire functional traits is not clear at this point. The aim of this study was to evaluate the effects of plant-derived smoke on seed germination of some predominant native woody species from the Mediterranean matorral of central Chile. We exposed seeds of 18 woody species to plant-derived smoke for 30 min and assessed their germination. Five species failed to germinate under either the experimental and control conditions. Smoke significantly stimulated germination in three species, while decreasing it in eight. Species showing smoke-inhibited germination tend to be major dominants in the Chilean matorral vegetation. The three smoke-stimulated species are known colonizers. This suggests that current human-caused fires could drastically change the structure of Chilean matorral. Although our results suggest fire have not played a major role in shaping adaptations for seed germination of woody species in the Chilean matorral, more investigation about ephemeral species is needed. This study provides the first results about smoke-related germination in the Mediterranean-type zone of central Chile, generating the opportunity to investigate the evolutionary context and distribution of smoke-stimulated germination in all the Mediterranean-type ecosystems.

Freezing temperatures and summer droughts shape plant life in Mediterranean high-elevation habitats. Thus, the impacts of climate change on plant survival for these species could be quite different to those from mesic mountains. We exposed 12 alpine species to experimental irrigation and warming in the Central Chilean Andes to assess whether irrigation decreases freezing resistance, irrigation influences freezing resistance when plants are exposed to warming, and to assess the relative importance of irrigation and temperature in controlling plant freezing resistance. Freezing resistance was determined as the freezing temperature that produced 50 % photoinactivation [lethal temperature (LT50)] and the freezing point (FP). In seven out of 12 high-Andean species, LT50 of drought-exposed plants was on average 3.5 K lower than that of irrigated plants. In contrast, most species did not show differences in FP. Warming changed the effect of irrigation on LT50. Depending on species, warming was found to have (1) no effect, (2) to increase, or (3) to decrease the irrigation effect on LT50. However, the effect size of irrigation on LT50 was greater than that of warming for almost all species. The effect of irrigation on FP was slightly changed by warming and was sometimes in disagreement with LT50 responses. Our data show that drought increases the freezing resistance of high-Andean plant species as a general plant response. Although freezing resistance increases depended on species-specific traits, our results show that warmer and moister growing seasons due to climate change will seriously threaten plant survival and persistence of these and other alpine species in dry mountains.

Se ha propuesto que los ambientes de alta montana son especialmente vulnerables al calentamiento global. La interaccion de variables como deficit hidrico y las condiciones de temperatura ambiental puede restringir el intercambio gaseoso en zonas de alta-montana. Debido a las distintas limitaciones abioticas que se encuentran en un gradiente altitudinal, se propone que el aumento de la temperatura afectara positivamente la fotosintesis de la planta alto-andina Phacelia secunda en elevadas altitudes y negativamente a las plantas de bajas altitudes. En el presente estudio se evaluo el efecto del aumento de la temperatura ambiental sobre el intercambio gaseoso de P. secunda en dos altitudes: 2900 m y 3600 m. En cada altitud se expusieron seis individuos de P. secunda a un sistema de calentamiento pasivo denominado "Open Top Chamber" (OTC) que aumenta la temperatura del aire en ca. 3°C. Adicionalmente, en cada altitud se seleccionaron otros seis individuos en espacios abiertos para utilizarlos como control y que se encontraban a 2 m de la OTC mas cercana. En cada individuo se midio el intercambio gaseoso y el potencial hidrico xilematico. La temperatura ambiental fue en promedio 3,5°C mayor dentro de las OTC en ambas altitudes. En contraste, el potencial hidrico del suelo y de las plantas se redujo un 28% al interior de las OTC, pero solo a 2900 m. En esta altitud, las tasas de fotosintesis al interior de las OTC se redujeron un 40,7%, mientras que a 3600 m las tasas aumentaron un 24,4%. Los efectos diferenciales en las tasas de fotosintesis debido a la temperatura fueron acompanados con cambios en la conductancia estomatica. Esto sugiere que los efectos del calentamiento global sobre la fotosintesis de P. secunda son contrastantes entre ambas altitudes debido a los efectos secundarios que este tiene sobre la disponibilidad hidrica en cada altitud.

Positive interactions between species are known to play an important role in the structure and dynamics of alpine plant communities. The balance between negative and positive interactions is known to shift along spatial and temporal gradients, with positive effects prevailing over negative ones as the environmental stress increases. Thus, this balance is likely to be affected by climate change. We hypothesized that increases in temperature (a global warming scenario) should decrease the importance of positive interactions for the survival and growth of alpine plant species. To test this hypothesis, we selected individuals of the native grass species Hordeum comosum growing within the nurse cushion species Azorella madreporica at 3,600 m.a.s.l. in Los Andes (Chile), and performed nurse removal and seedling survival experiments under natural and warmer conditions. For warmer conditions, we used open-top chambers, which increased the temperature by 4 °C. After two growing seasons, we compared the effect of nurse removal on the survival, biomass, and photochemical efficiency of H. comosum individuals under warmer and natural conditions. Nurse removal significantly decreased the survival, biomass, and photochemical efficiency of H. comosum, demonstrating the facilitative effects of nurse cushions. Seedling survival was also enhanced by cushions, even under warmer conditions. However, warmer conditions only partially mitigated the negative effects of nurse removal, suggesting that facilitative effects of cushions do not wane under warmer conditions. Thus, facilitative interactions are vital to the performance and survival of alpine species, and these positive interactions will continue to be important in the warmer conditions of the future in high-alpine habitats.

The level and mechanism of resistance to damage from freezing temperatures is related to the ambient temperatures experienced by plants. High-elevation habitats exhibit a thermal gradient in the air–soil profile, exposing ground-level species to more rigorous thermal microhabitats than well aboveground species. Such gradients could explain an inverse relationship between freezing temperature damage (LT50) and plant height, such that leaves of smaller plants tolerate freezing conditions, while leaves of taller plants avoid them. However, microclimatic data have not been included in previous studies evaluating those patterns. On the other hand, a proposed trade-off between plant functional traits and freezing resistance suggest that leaves of growth-forms with higher intensity of growth are less freezing resistant than those with lower growth intensity. In this work, we determined whether the ability to resist freezing temperatures is related to plant height or growth-form in 37 high-elevation species from the Central Chilean Andes. We proposed that (1) plants with contrasting heights are exposed to different thermal conditions during freezing events, (2) the level and the mechanism of freezing resistance are inversely related to plant height, and that (3) the level of freezing resistance varied between plants with low- and high-growth intensities. We found that high-Andean species of different heights are exposed to different thermal conditions depending on the distance from the ground. However, neither level nor mechanisms of freezing resistance were related to plant height. Leaves of both short and tall plants showed similar LT50 and their main freezing resistance mechanism was tolerance. Moreover, leaves of growth-forms with high- and low-growth intensity resisted similar freezing temperatures. Our results are discussed in relation to environmental conditions that characterize the Central Chilean Andes.

Alpine habitats have been proposed as particularly sensitive to climate change. Shorter snow cover could expose high-elevation plants to very low temperatures, increasing their risk of suffering damage by freezing, hence decreasing their population viability. In addition, a longer and warmer growing season could affect the hardening process on these species. Thus, understanding the ability of these species to withstand freezing events under warmer conditions is essential for predicting how alpine species may respond to future climate changes. Here we assessed the freezing resistance of 11 species from the central Chilean Andes by determining their low temperature damage (LT(50)) and freezing point (FP) after experimental warming in the field. Plants were exposed during two growing seasons to a passive increase in the air temperature using open top chambers (OTCs). OTCs increased by ca. 3 K the mean air and soil daytime temperatures, but had smaller effects on freezing temperatures. Leaf temperature of the different species was on average 5.5 K warmer inside OTCs at midday. While LT(50) of control plants ranged from -9.9 to -22.4, that of warmed plants ranged from -7.4 to -17.3 degrees C. Overall, high-Andean species growing inside OTCs increased their LT(50) ca. 4 K, indicating that warming decreased their ability to survive severe freezing events. Moreover, plants inside OTCs increased the FP ca. 2 K in some studied species, indicating that warming altered processes of ice crystal formation. Resistance of very low temperatures is a key feature of high-elevation species; our results suggest that current climate warming trends will seriously threaten the survival of high-elevation plants by decreasing their ability to withstand severe freezing events.

Summary • Predicted increases in the length of the growing season as a result of climate change may more frequently expose high-elevation plants to severe frosts. Understanding the ability of these species to resist frosts during the growing season is essential for predicting how species may respond to changes in temperature regimes. • Here, we assessed the freezing resistance of 24 species from the central Chilean Andes by determining their low temperature damage (LT50), ice nucleation temperature (NT), freezing point (FP) and freezing resistance mechanism (i.e. avoidance or tolerance). • The Andean species were found to resist frosts from −8.2 to −19.5°C during the growing season, and freezing tolerance was the most common resistance mechanism. Freezing resistance (LT50) varied within the growing season, decreasing towards the end of this period in most of the studied species. However, the FP showed the opposite trend. LT50 increased with elevation, whilst FP was lower in plants from lower elevations, especially late in the growing season. • Andean species have the potential to withstand severe freezing conditions during the growing season, and the aridity of this high-elevation environment seems to play an important role in determining this high freezing resistance.

Colobanthus quitensis es una especie ampliamente distribuida, que crece desde los Andes tropicales hasta la Peninsula Antartica. A pesar de que ambos habitats se caracterizan por ser frios y con temperaturas congelantes, existen importantes diferencias microclimaticas entre estos habitats durante el periodo de crecimiento. Por ello, seria esperable encontrar importantes diferencias en la respuesta del aparato fotosintetico a factores abioticos entre poblaciones antarticas y andinas de C. quitensis. Estudiamos la relacion de la fotosintesis neta con la temperatura foliar y la intensidad luminica en dos poblaciones de C. quitensis, una proveniente de La Parva en Los Andes de Chile central y la otra de la Isla Rey Jorge, en la Antartica maritima. Plantas de ambas poblaciones fueron crecidas en laboratorio a 15 °C y 250 μmol fotones m-2 s-1, con un fotoperiodo de 16/8 h luz/oscuridad. Veinte plantas de cada poblacion fueron transferidas a una camara a 4 °C, con las mismas condiciones de luz y fotoperiodo antes mencionadas, de modo de evaluar la capacidad de aclimatacion de la fotosintesis. A 15 °C, las poblaciones presentaron diferencias en su temperatura foliar optima para la fotosintesis. Estos optimos fueron 24,0 °C en individuos provenientes de los Andes y 18,6 °C en individuos de la Antartida. En contraste, los individuos antarticos mostraron la mas alta tasa de fotosintesis neta bajo ambos tratamientos de temperatura, con puntos de compensacion y saturacion mas bajos. Los individuos antarticos tuvieron mayor capacidad fotosintetica a menores temperaturas que los individuos andinos. Nuestros resultados sugieren que C. quitensis ha adaptado el desempeno fotosintetico de individuos creciendo en diferentes localidades a las condiciones climaticas predominantes en dichas localidades. Las diferencias en las respuestas fotosinteticas a la temperatura y a la luz son discutidas en relacion a la diferenciacion ecotipica entre las dos poblaciones estudiadas

Cushion plants are one of the most common growth forms in alpine habitats. Their low stature, dense canopy, and compact form allow them to decouple their microclimate from the surrounding environment, mitigating the effect of low temperatures and drought, enhancing the survival of other species. In this study, we evaluated the modifications on soil temperature and moisture over an entire growing season by two cushion species (Laretia acaulis and Azorella monantha) in alpine communities located at two different elevations in the central Chilean Andes. Additionally, w ep erformed seedling survival experiments with two native herbaceous species (Hordeum comosum and Erigeron andicola) and the non-native forb Cerastium arvense to assess if seedling survival is higher within cushions than outside them. Our results indicated that cushions ameliorated extreme low and high substrate temperatures, improved soil moisture, and enhanced seedling survival of the three herbaceous plant species evaluated. Our results suggest that microclimatic modifications associated with cushion plant sc ould be important for the establishment and survival of other plant species, both native and non-native, in the high alpine communities of central Chile.

Some epiphytic Hymenophyllaceae are restricted to lower parts of the host (< 60 cm; 10-100 μmol photons m(-2) s(-1)) in a secondary forest of Southern Chile; other species occupy the whole host height (≥ 10 m; max PPFD > 1000 μmol photons m(-2) s(-1)). Our aim was to study the photosynthetic light responses of two Hymenophyllaceae species in relation to their contrasting distribution. We determined light tolerance of Hymenoglossum cruentum and Hymenophyllum dentatum by measuring gas exchange, PSI and PSII light energy partitioning, NPQ components, and pigment contents. H. dentatum showed lower maximum photosynthesis rates (A max) than H. cruentum, but the former species kept its net rates (An) near Amax across a wide light range. In contrast, in the latter one, An declined at PPFDs > 60 μmol photons m(-2) s(-1). H. cruentum, the shadiest plant, showed higher chlorophyll contents than H. dentatum. Differences in energy partitioning at PSI and PSII were consistent with gas exchange results. H. dentatum exhibited a higher light compensation point of the partitioning of absorbed energy between photochemical Y(PSII) and non-photochemical Y(NPQ) processes. Hence, both species allocated energy mainly toward photochemistry instead of heat dissipation at their light saturation points. Above saturation, H. cruentum had higher heat dissipation than H. dentatum. PSI yield (YPSI) remained higher in H. dentatum than H. cruentum in a wider light range. In both species, the main cause of heat dissipation at PSI was a donor side limitation. An early dynamic photo-inhibition of PSII may have caused an over reduction of the Qa+ pool decreasing the efficiency of electron donation to PSI. In H. dentatum, a slight increase in heat dissipation due to acceptor side limitation of PSI was observed above 300 μmol photons m(-2)s(-1). Differences in photosynthetic responses to light suggest that light tolerance and species plasticity could explain their contrasting vertical distribution.

Summary • In alpine habitats, positive interactions among plants tend to increase with elevation as a result of altitudinal increase in environmental harshness. However, in mountains located in arid zones, lower elevations are also stressful because of scarce availability of water, suggesting that positive interactions may not necessarily increase with elevation. • Here we analysed the spatial association of plant species with the nurse cushion plant Laretia acaulis at two contrasting elevations, and monitored the survival of seedlings of two species experimentally planted within and outside cushions in the semiarid Andes of central Chile. • Positive spatial associations with cushions were more frequent at lower elevations. Species growing at the two elevations changed the nature of their association with cushions from neutral or negative at higher elevations to positive at lower elevations. Survival of seedlings was higher within cushions, particularly at lower elevations. • The increased facilitation by cushions at lower elevations seems to be related to provision of moisture. This result suggests that cushion plants play a critical role in structuring alpine plant communities at lower elevations, and that climatic changes in rainfall could be very relevant for persistence of plant communities.