Interaction between the cactus Neobuxbaumia tetetzo and the nurse shrub Mimosa luisana

Abstract. Pattern analysis and association analysis showed that recruitment of the giant columnar cactus Neobuxbaumia tetetzo in the semi-arid valley of Zapotitlán is largely limited to areas beneath the canopies of perennial shrubs, acting as nurse plants. Chi-square and Haberman tests revealed that young cacti were more frequently found beneath canopies of leguminous shrubs, especially Mimosa luisana, than were older cacti. Segregation analysis indicated a positive association (= negative segregation) of young cacti and M. luisana canopies, but older cacti were randomly distributed with respect to M. luisana. This, and the height class distributions of cacti associated withM luisana or not, suggested the replacement of M. luisana by N. tetetzo. Mimosa luisana plants in association withiV. tetetzo had greater amounts of dead basal area thanM luisana lacking associated N tetetzo. This suggested competition from JV. tetetzo promotes the replacement process. The roots of N tetetzo are ≤ 30 cm deep. Interception of soil water before penetration to deeper roots of M luisana may be the mode of competition between the two species.     

Spatial Associations, Size–Distance Relationships and Population Structure of Two Dominant Life Forms in a Semiarid Enclave of the Venezuelan Andes

The role of three thorny legume species as nurse plants and competitive relationships with columnar cacti were evaluated in a semiarid enclave of the Venezuelan Andes. Abundance and size of three columnar cacti species (Stenocereus griseus, Cereus repandus and Pilosocereus tillianus) under isolated shrubs of three thorny legumes species (Prosopis juliflora, Acacia farnesiana and A. macracantha) were recorded and compared with open areas. Using size–distance data we inferred the intensity of intra- and interspecific competition between both life forms in a “cardonal” (xeric zone) and “espinar” (mesic zone) of the enclave. Sixty-one columnar cacti were found beneath the three thorny legume species (92%), while only 5 cactus individuals were found in open areas (8%). Comparison of observed and expected number of cactus individuals shows that S. griseus and C. repandus are significantly associated with isolated shrubs of A. farnesiana and A. macracantha. Although P. juliflora contributed more than 40% of the total legume plant cover, the number of columnar cacti under its isolated shrubs was significantly lower than would be expected by chance. In all, 19 of 21 possible plant–plant combinations between and within columnar cacti and legume shrubs were recorded (espinar: 18, cardonal: 13, common combinations: 12). In both zones, intra- and interspecific combinations among columnar cacti species were relatively high (positive correlation between the sum of neighbour plant sizes and the distances separating them). Our results strongly suggest, at least for case of S. griseus and C. repandus, that these columnar cacti species require nurse plants for their establishment. The results on interference need still support from further research. We discuss the effects of plant–plant positive interactions on natural regeneration of these columnar cacti.     

The importance of shrub cover for four cactus species differing in growth form in an Andean semi‐desert

Question: Factors influencing seedling establishment are known to vary between open sites and those protected by plant cover. In many desert regions, protected microhabitats below shrubs are essential for establishment of many cactus species. Very little is known about these factors for Andean cacti and how the importance of vegetation cover varies with cactus species. Are Andean cacti associated more frequently to vegetation cover than to open ground? Are they associated to certain shrub species? Is the distributional pattern in relation to cover similar for different cactus species? In what microhabitat (below or away from shrubs) are cactus seeds more abundant? These questions are addressed for the case of an Andean semi‐desert. Location: Semi‐arid tropical Andes, La Paz department, Bolivia. Methods: We examined 132 isolated shrubs = 50 cm along a line across two microhabitats: areas below and away from shrubs/trees. Shrub crown size was measured. The among‐shrub samples were taken from open spaces contiguous to each of the sampled shrubs. In both microhabitats, all cactus species were recorded. The cardinal direction of the cacti was also registered. Correlation between canopy diameter and number of beneficiaries was evaluated for Prosopis flexuosa. The cactus seed bank in each microhabitat was also studied. Results and Conclusions: The four cactus species found behaved differently in relation to shrub canopies. These distributional differences could be due to differences in growth form. Columnar cacti apparently need the shade of shrubs. Only the columnar species is able to grow near the base of the tallest nurse species. The opuntioid cacti studied seem more facultative: although apparently preferring shrub un‐der‐canopies, they are able to establish in open ground. The globose cactus is the most indifferent to the presence of plant cover. These patterns parallel others found in North America. The capacity of different cacti to appear in open spaces could be related to vegetative propagation, and not necessarily to seedling tolerance of heat.     

Distribution of plant life forms along an altitudinal gradient in the semi‐arid valley of Zapotitlán,Mexico

Abstract. Plant life‐form abundance along a 600 m altitudinal gradient (1600–2200 m a.s.l.) in the semi‐arid valley of Zapotitlán, México was correlated with soil characteristics and climatic variables. One mixed soil sample was taken and analysed for each of six elevations, temperature was estimated using a terrestrial thermal gradient, and precipitation using a linear regression between total annual precipitation and the elevation of the weather stations in the valley. Rosettes, microphanerophytes, therophytes and nanophanerophytes were well represented throughout the gradient. Columnar cacti were restricted to the 1600–1800 m range, and geophytes to the 1700–1800 m range. In general, abundance of life forms was inversely associated with altitude. Multiple regression analysis did not show parameters to significantly explain the abundance of rosettes, nanophanerophytes, epiphytes, geophytes and hemiparasites; altitude and nitrogen proved significant for columnar cacti, succulents and chamaephytes; altitude, pH, electrical conductivity and nitrogen were significant for globose cacti; pH was significant for therophytes; and altitude was significant for microphanerophytes.     

The importance of Olneya tesota as a nurse plant in the Sonoran Desert

Abstract. The function of Olneya tesota (ironwood) as a nurse plant and habitat modifier species in the Sonoran Desert was evaluated at five study sites (using 75 250-m² sample plots) from Bahia Kino, Sonora to Organ Pipe Cactus National Monument, Arizona. Beneath the canopy of O. tesota trees 75 perennial plant species were found. A principal component ordination of the plots created three groups: southern, perturbed and protected sites. A strong triple association of columnar cacti, big shrubs with berry type fruits and O. tesota was detected. The relationships of Lophocereus schottii and Peniocereus striatus with O. tesota were studied in more detail. Significant differences in soil surface temperatures and stem temperatures of P. striatus were detected beneath the shade of O. tesota trees compared with soils and plants in open spaces. Olneya tesota must be considered as a habitat modifier species with ecological and conservational importance to the plant communities in the Sonoran Desert.     

Columnar cacti-shrub relationships in an Andean semiarid valley in western Venezuela

Spatial associations between columnar cacti and mimosoid shrubs were evaluated in an Andean semidesert enclave. An analysis on the capacity of other deciduous shrub species and medium-sized ephemeral plants to modify the spatial distribution of cacti was also included. The number of Stenocereus griseus (Haw.) F. Buxb., Cereus repandus (L.) Backeb. and Pilosocereus tillianus Gruber & Schaftzl that grow below the canopies of perennial plants were assessed and compared with open areas. Comparison of observed and expected number of cactus individuals shows a positive spatial association between S. griseus and Prosopis juliflora DC. Moreover, abundance of C. repandus and P. tillianus under the canopies of mimosoid shrubs were not statistically different from what was expected by chance. Positive spatial association between S. griseus and Capparis odoratissima Jacq., Cassia emarginata L., Cordia curassavica (Jacq.) Roem. & Schult., Croton rhamnifolius H.B.K. and Jatropha gossypifolia L. was also found. Evidence suggests that positive columnar cacti-shrub spatial association may occur either in the cactus thickets or in the thornscrubs. Due to the clumped distributions of S. griseus beneath P. juliflora and other perennial plants, we here postulate that S. griseus is the primary nursed columnar cactus of the enclave. The presence of cacti in open areas suggests that facilitation may be less important for establishment of columnar cacti species in this Andean semidesert landscape than in other temperate and tropical semiarid zones. We here discuss the importance of the nurse syndrome phenomenon for recruitment of S. griseus and for the conservation of this Andean semiarid environment.     

Factors affecting cactus recruitment in semiarid Chile: A role for nurse effects?

The nurse-protégé hypothesis states that adult plants of one species provide micro-environmental conditions that favor the establishment of seedlings of a second species with no effect for the first species. Several studies suggest this effect should be prevalent in arid and semiarid zones as adult plants often provide shelter from low moisture and high temperature. Echinopsis chiloensis and Eulychnia acida are endemic columnar cacti that inhabit the arid and semiarid zones of Chile. In this study, we examined the pattern of recruitment of both cactus species at Reserva Nacional Las Chinchillas, located ～60 km east from the Pacific coast. We determined number, growth and survivorship of young cacti (<30 cm height) through biannual monitoring between 2009 and 2012 in microhabitats that strongly differ in their abiotic variables (minimum and maximum temperature and mean relative humidity, moisture content, and physical and chemical soil characteristics), under five different shrub species and in open spaces, and examined the association of these cacti with potential nurse plants. Most young cacti occurred under shrubs, the microhabitat having the lowest mean and maximum temperatures and the highest relative humidity. In particular, E. chiloensis and E. acida were found under the shrubs Flourensia thurifera and Bahia ambrosioides, respectively, in a higher frequency than expected by chance, suggesting that these shrub species behave as nurse plants through species-specific effects than are not accounted for by differences in soil nutrients.     

Ecological longevity of Polaskia chende (Cactaceae) seeds in the soil seed bank,seedling emergence and survival

• Soil seed banks are essential elements of plant population dynamics, enabling species to maintain genetic variability, withstand periods of adversity and persist over time, including for cactus species. However knowledge of the soil seed bank in cacti is scanty. In this study, over a 5‐year period we studied the seed bank dynamics, seedling emergence and nurse plant facilitation of Polaskia chende, an endemic columnar cactus of central Mexico.
• P. chende seeds were collected for a wild population in Puebla, Mexico. Freshly collected seeds were sown at 25 °C and 12‐h photoperiod under white light, far‐red light and darkness. The collected seeds were divided in two lots, the first was stored in the laboratory and the second was use to bury seeds in open areas and beneath a shrub canopy. Seeds were exhumed periodically over 5 years. At the same time seeds were sown in open areas and beneath shrub canopies; seedling emergence and survival were recorded over different periods of time for 5 years.
• The species forms long‐term persistent soil seed banks. The timing of seedling emergence via germination in the field was regulated by interaction between light, temperature and soil moisture. Seeds entered secondary dormancy at specific times according to the expression of environmental factors, demonstrating irregular dormancy cycling.
• Seedling survival of P. chende was improved under Acacia constricta nurse plants. Finally, plant facilitation affected the soil seed bank dynamics as it promoted the formation of a soil seed bank, but not its persistence.

     

Differentiation diversity of Argentine cacti and its relationship to environmental factors

Abstract. We studied the differentiation diversity (β-diversity or species turnover) patterns of the three main cactus growth forms (columnar, opuntioid and globose) in 318 (1° × 1°) squares covering Argentina. We analysed the degree of association between species turnover of each growth form with the spatial variation of a set of 15 environmental variables. Species turnover was estimated in two ways: (1) by calculating species turnover along latitudinal and longitudinal gradients and (2) by evaluating the species turnover between each square and its eight surrounding neighbouring grid cells. For the three growth forms, species turnover in latitudinal transects was mostly related to the mean within-transect values of certain environmental variables, while in longitudinal transects it was related to the variation of some environmental variables within the transect rather than to their mean values. For columnar species, transect species turnover was mainly associated with variation in temperature, confirming the temperature-sensivity of this growth form. For opuntioid species, turnover along transects was mainly related to topographic variables. In the case of globose cacti, transect turnover was associated with variation in temperature and rainfall. For the three growth forms, areas of high turnover coincided with marked transitions between different biogeographic provinces, while the areas with lowest species turnover coincide with topographically and climatically uniform plains. Species turnover between individual squares was positively associated with the proportion of summer rainfall in globose cacti, the variation of mean annual temperature in columnar cacti and was negatively related to mean annual temperature in opuntioid cacti. Compared to the other growth forms, globose cacti presented a much larger proportion of squares with a high species turnover. In general, differentiation diversity was lower for the opuntioid and the columnar species, two growth forms with higher dispersal ability and was highest for the globose cacti, which have the lowest dispersal capacity. Environmentally heterogeneous areas, where large-scale transitions between biomes occur, have exceptionally high species turnover, and are important target areas for the conservation of biodiversity.     

Nurse plants vs. nurse objects: effects of woody plants and rocky cavities on the recruitment of the Pilosocereus leucocephalus columnar cactus

BACKGROUND AND AIMS: Most studies on cactus recruitment have focused on the role of woody plants as seedling facilitators. Although the spatial association of cacti with objects had been described, the mechanisms underlying this association remain unknown. The aims of this study were to identify which mechanisms facilitate the establishment of a columnar cactus under the shade and protection of objects and to compare these mechanisms with those involved in plant-plant facilitation. METHODS: Three split-split-plot field experiments were conducted to compare the effects of two microhabitats (inside rocky cavities and beneath plant canopies) on seed removal, germination, seedling survivorship and dry weight. Flat, open spaces were used as the control. For each microhabitat, the effect of seed or seedling protection and substrate limitation were explored; aboveground microclimate and some soil properties were also characterized. KEY RESULTS: The permanence of superficial seeds was greater inside rocky cavities than beneath woody plant canopies or on flat, open areas. Germination was similar in cavities and beneath plant canopies, but significantly higher than on flat, open areas. Seedling survivorship was greater beneath plant canopies than inside cavities or on flat, open spaces. CONCLUSIONS: The mechanisms of plant facilitation are different from those of object facilitation. There are seed-seedling conflicts involved in the recruitment of P. leucocephalus: nurse plants favour mainly seedling survivorship by providing a suitable microenvironment, while nurse objects mainly favour seed permanence, by protecting them from predators.     

Reproductive biology of Ferocactus recurvus (Mill.) Borg subsp. recurvus (Cactaceae) in the Tehuacán‐Cuicatlán Valley,Mexico

• Mexico has one of the highest diversities of barrel cacti species worldwide; however, all are threatened and require conservation policies. Information on their reproductive biology is crucial, but few studies are available. Ferocactus recurvus subsp. recurvus is a barrel cactus endemic to the Tehuacán‐Cuicatlán Valley. Our research aimed to characterise its floral and pollination biology. We hypothesised bee pollination, as suggested by its floral morphology and behaviour, and self‐incompatibility, like most barrel cacti studied.
• Three study sites were selected in the semiarid Zapotitlán Valley, Mexico. We examined 190 flowers from 180 plants to determine: morphometry and behaviour of flowers, flower visitors and probable pollinators, and breeding system.
• Flowers showed diurnal anthesis, lasting 2–5 days, the stigma being receptive on day 2 or 3 after the start of anthesis. Flowers produced scarce/no nectar and main visitors were bees (Apidae), followed by flies (Muscidae), ants (Formicidae), thrips (Thripidae) and hummingbirds (Throchilidae); however, only native bees and occasionally wasps contacted the stigma and anthers. Pollination experiments revealed that this species is self‐incompatible and xenogamous. In natural conditions, fruit set was 60% and cross‐pollination fruit set was 100%. Percentage seed germination resulting from cross‐pollination was higher than in the control treatment.
• Our results provide ecological information for conservation programmes to ensure a high probability of breeding and seed production in natural populations of F. recurvus.

     

The Diet and Reproductive Schedules of Leptonycteris curasoae curasoae and Glossophaga longirostris elongata (Chiroptera: Glossophaginae) on Curaçao

Two bat species, Leptonyrteris curasoae and Glossophaga longirostris, are the principal pollinators of at least two of the three species of columnar cacti that grow on the semiarid island of Curaçao, Netherlands Antilles. I examined the importance of the cacti in the diets of the bats and found that 85–91 percent of their diet samples contained cactus pollen and seeds. At least 43 percent of the samples from each species contained cactus pollen andlor seeds exclusively. Leptonycteris curasoae consumes nectar and pollen of Ceiba pentandra and Agave spp. at the beginning of the dry season and G. longirostris also consumes a few other plant products in the wet season, but both bat species depend nutritionally on cacti. Female bats give birth to one pup per year, and the periods of parturition and lactation in each species correspond to peaks in the reproductive phenology of the two most abundant columnar cactus species. From personal observations and a review of the literature, I determined that bats were unlikely to fly to the mainland to feed, although L. curasoae may do so. I conclude that the interdependence of bats and cacti is suggestive of coevolution, and that columnar cacti are critical for the survival and persistence of nectar-feeding bats on Curaçao.     

Wide-band tracheids are present in almost all species of Cactaceae

Wide-band tracheids (WBTs) have been found in seedlings of most species of cacti that have fibrous wood in their adult bodies. Consequently, this cell type is now known to be present in almost all cacti. Earlier studies of adult plants revealed WBTs to be present only in cacti with globose or short, broad bodies, whereas all species with large columnar or long slender bodies had fibrous wood without WBTs. However, even these species produce WBTs during the first several months after germination. In species with fibrous wood in their adult bodies (species with large or slender bodies), seedlings undergo a phase transition in wood morphogenesis after a few months and stop producing the juvenile (WBT) wood and begin producing adult (fibrous) wood. If adult plants have an intermediate size, the phase transition is delayed and the plant produces WBT wood for several years. Species with globose bodies repress the phase transition completely and never switch to producing adult (fibrous) wood. Because WBTs are so widespread, they probably originated only once in Cactaceae, not multiple times as suggested earlier, or there may have been just a single origin in the Cactaceae/Portulacaceae clade.     

Complex foraging ecology of the red harvester ant and its effect on the soil seed bank

Granivory is an important interaction in the arid and semi-arid zones of the world, since seeds form an abundant and nutritious resource in these areas. While species of the genus Pogonomyrmex have been studied in detail as seed predators, their impact on seed abundance in the soil has not yet been explored in sufficient depth. We studied the impact of the harvesting activities of the ant Pogonomyrmex barbatus on seed abundance in the soil of the Zapotitlán valley, Mexico. We found that P. barbatus activity significantly impacts the abundance of seeds in the soil, which is lower in the sites where P. barbatus forages than it is in sites with no recorded foraging. We also found that P. barbatus distributes intact seeds of three tree species, two of which are nurse plants, and could consequently be promoting the establishment of these species. Using tools derived from graph theory, we observed that the ant-seed interactions exhibit a nested pattern; where more depredated seed species seem to be the more spatially abundant in the environment. This study illustrates the complex foraging ecology of the harvester ant P. barbatus and elucidates its effect on the soil seed bank in a semi-arid environment.     

Seedling survival of three endemic and threatened Mexican cacti under induced climate change

Human‐induced warming may increase the risk of local extinction for plant species with low tolerance of elevated temperatures. The Chihuahuan desert harbors the highest diversity of globose cacti in the world and most of them are at risk of extinction. Predictive models of climate change indicate an increase in summer temperature of 1–2°C by 2030 for this desert. Nevertheless, studies on the vulnerability of cacti species in early development phases to future climate change are scarce. We assessed the survival of three threatened cacti species from the Chihuahuan desert under induced warming. Open‐top chambers (OTCs) were used to simulate the effect of global warming on 2‐year seedlings of Echinocactus platyacanthus f. visnaga, Ferocactus histrix and Stenocactus coptonogonus. OTCs had higher temperature and lower humidity than control plots, and these elevated temperatures reduced seedling survival. Within the OTCs, no living individuals of any species were found after 105 days. Conversely, in the control plots, the three cacti species showed variable numbers of survivors after this period. Therefore the predicted global warming scenarios will greatly limit plant recruitment and the long‐term persistence of natural populations of Mexican endemic cacti species.     

Distribution and morphology of columnar cacti in tropical deciduous woodland,Jalisco, Mexico

The distribution and branching morphologies of four species of columnar cactus species in tropical deciduous woodland in Jalisco, western Mexico, are examined. Two species, Stenocereus marginatus and especially Pachycereus pecten-aboriginum, are taller and extend through the woodland canopy, and only these two species occupy habitats where the woodland vegetation is taller, on N- and NE-facing aspects where incident radiation levels are lowest. Two species, Stenocereus standleyi and Cephalocereus purpusii, are shorter, subcanopy species, and are more restricted to low-angle slopes and slopes with S- to W-aspects, where the woodland vegetation is shorter and more open, with a greater penetration of radiation into the vegetation. Branching morphology is affected by the vegetation cover in all species, but in different ways between the two taller and the two shorter species. Ecological segregation in these four species is achieved apparently by differences among the light regimes of subhabitats correlated to the morphology of the cacti. This contrasts to segregation in Sonoran Desert columnar cactus species, which segregate by different strategies of water uptake and utilization, and in columnar cacti in woodlands in Oaxaca further south in Mexico, where adult individuals are very similar in branching morphology, but the younger individuals differ in this respect and presumably become established in different sorts of light gaps.Nomenclature follows Gibson & Horak (1978).     

Facilitation by nurse shrubs of resprouting behavior in a post-fire shrubland in northern Patagonia,Argentina

Abstract. The nurse-plant syndrome is a widely recognized example of positive (facilitative) influences of plant species on the establishment and growth of other species. Most studies of the nurse-plant syndrome have been on species that reproduce mainly from seed rather than vegetatively. In this study, we experimentally compared the influences of two species of nurse shrubs, Schinus patagonicus and Berberis buxifolia, on the survival and growth of vegetatively reproducing herbaceous and woody plants in a post-fire shrubland in northern Patagonia, Argentina. The vegetation beneath shrubs was removed by clipping and, in a paired-sample design, one half of the canopy of each shrub was removed. We determined species richness, counted number of resprouts, and measured photon flux density and soil moisture beneath cut and uncut halves of each shrubs. Abundances of resprouts were several times greater beneath the uncut vs. the cut shrubs, as was the mean number of species. Thus, shrubs have a strong facilitating influence as measured by resprout densities and the number of species. Numbers of resprouts and of species were twice as high beneath Schinus as beneath Berberis, implying important differences in the facilitative effects of the two shrubs species. Microsites beneath Schinus were characterized by lower and more heterogeneous light levels but by greater soil moisture. Even though the reproductive mode in this post-fire shrubland is overwhelmingly vegetative, facilitation by nurse shrubs is important and differentially effective for different species of nurse shrubs.     

Role of nurse plants in Araucaria Forest expansion over grassland in south Brazil

Abstract Araucaria Forest expansion over grassland takes place under wet climate conditions and low disturbance and it is hypothesized that isolated trees established on grassland facilitate the establishment of forest woody species beneath their canopies. Forest with Araucaria angustifolia is a particular type of Brazilian Atlantic Forest and the main forest type on the highland plateau in south Brazil, often forming mosaics with natural Campos grassland. The objectives of this paper were to evaluate the role of isolated shrubs and trees as colonization sites for seedlings of Araucaria Forest woody species on grassland, to determine which species function as preferential nurse plants in the process and the importance of vertebrate diaspore dispersal on the structure of seedling communities beneath nurse plants. The study was conducted in São Francisco de Paula, Rio Grande do Sul State, where we sampled isolated shrubs and trees in natural grassland near Araucaria Forest edges. Seedlings were counted and identified, and seedling diaspore dispersal syndromes, size and colour were registered. We detected 11 woody species with a potential role in nucleating grassland colonization by forest species. Beneath the canopies of nurse plants more forest species seedlings were found compared with open field grassland and the seedlings had diaspores mostly dispersed by vertebrates. Also, more seedlings were found under the canopy of A. angustifolia than beneath other nurse plant species. We conclude that A. angustifolia trees established on grassland act as nurse plants, by attracting disperser birds that promote colonization of the site by other forest species seedlings, and that under low level of grassland disturbance, conservation of frugivorous vertebrate assemblages may increase forest expansion over natural grassland and also facilitate the regeneration of degraded forest areas.     

Positive effects of native shrubs on three specially protected cacti species in Durango,México

The spatial association between the specially protected cacti Coryphanta durangensis, Echinocereus longisetus and Peniocereus greggii and potential nurse plants was evaluated, as was their relative position to the sun under the crown of the latter in the southern Chihuahuan Desert. The soil temperature under potential nurse plants was lower than under direct sunlight. There was more nitrogen and organic matter in the soil under Prosopis laevigata trees than in soil under direct sunlight. There were 68 plants of C. durangensis, 59 plants of E. longisetus and 157 of P. greggii. Only one individual of C. durangensis and one of E. longisetus grew outside the shade of the crown of other plants. Echinocereus longisetus was not associated with any particular species, but grew more often than expected by chance in the northern segment of the crown (i.e. the area most shaded in the afternoon). Coryphanta durangensis and P. greggii grew more often under P. laevigata than expected by chance. The effect of other plants, and P. laevigata in particular, on the facilitation of growth of protected cacti species should be considered in management plans of the Chihuahuan Desert, where mesquite (P. laevigata) is often harvested for charcoal production.