RESUMO
Background and Aims: Dioecious plants are of particular concern in view of global environmental changes because reproductive females are more sensitive to abiotic stresses, thus compromising population viability. Positive interactions with other plants may counteract the direct effects of any abiotic environmental stress, allowing them to thrive and maintain a viable population in suboptimal habitats, although this process has not been tested for dioecious species. Furthermore, almost no data are available on the outcome of such species interactions and their link with local spatial patterns and sex ratios. Methods: We set up a field experiment with Poa ligularis, a dioecious native grass from the arid grasslands of South America. We studied the interaction of male and female plants with cushion shrubs of contrasting ecological strategies. We experimentally limited direct shrub-grass competition for soil moisture and transplanted plants to evaluate the amelioration of abiotic stress by shrub canopies (i.e. sun and wind) on grasses. We also studied the distribution of naturally established female and male plants to infer process-pattern relationships. Key Results: Positive canopy effects as well as negative below-ground effects were more intense for females than for males. Deep-rooted Mulinum spinosum shrubs strongly facilitated survival, growth and reproduction of P. ligularis females. Naturally established female plants tended to distribute more closely to Mulinum than co-occurring males. Female growth suffered intensive negative root competition from the shallow-rooted Senecio filaginoides shrub. Conclusions: Interactions with other plants may reduce or enhance the effect of abiotic stresses on the seemingly maladapted sex to arid environments. We found that these biased interactions are evident in the current organization of sexes in the field, confirming our experimental findings. Therefore, indirect effects of climate change on population sex ratios may be expected if benefactor species abundances are differentially affected.
Assuntos
Apiaceae/fisiologia , Poa/fisiologia , Estresse Fisiológico , Apiaceae/crescimento & desenvolvimento , Mudança Climática , Ecologia , Ecossistema , Poa/crescimento & desenvolvimento , Reprodução , SoloRESUMO
The environmental relationships and ecophysiology of Azorella compacta, a giant cushion plant, were investigated in Parque Nacional Lauca, Chile (18 degrees 10'-18 degrees 25' S and 69 degrees 16' W, 4400 m asl). The diurnal temperature range can reach 42 degrees C on some days of the year. The surface temperature of A. compacta was 13 degrees C below that of the air temperature of -7 degrees C at dawn, but from midmorning to late afternoon, the plant surface temperature remained within a few degrees of the air temperature. Soil surface temperatures did not differ between north- and south-facing slopes, but a model showed an increase in radiation reception by north-facing slopes throughout most of the year. Gas exchange measurements of A. compacta measured at the onset of the wet season ranged from -0.6662 to 11.4 micromol.m(-2).s(-1), and maximum stomatal conductance (Gs) was 410 mmol.m(-2).s(-1). The estimated light compensation point was 89 micromol.m(-2).s(-1) and estimated light saturation occurred at about 1280 mumol.m(-2).s(-1). Diurnal water potential measurements for A. compacta ranged from -1.67 to -2.65 MPa. This is one of the first ecophysiological studies of a tropical alpine cushion plant.
Assuntos
Aclimatação/fisiologia , Apiaceae/fisiologia , Ecologia , Fotossíntese/fisiologia , Temperatura , Ar , Dióxido de Carbono/metabolismo , Chile , Ecossistema , Luz , Oxigênio/metabolismo , Estômatos de Plantas , Estações do Ano , Solo , Luz Solar , Água/fisiologiaRESUMO
Ecosystem engineers are organisms that change the distribution of materials and energy in the abiotic environment, usually creating and maintaining new habitat patches in the landscape. Such changes in habitat conditions have been widely documented to affect the distributions and performances of other species but up to now no studies have addressed how such effects can impact the biotically driven physicochemical processes associated with these landscapes, or ecosystem functions. Based on the widely accepted positive relationship between species diversity and ecosystem functions, we propose that the effects of ecosystem engineers on other species could have an impact on ecosystem functions via two mutually inclusive mechanisms: (1) by adding new species into landscapes, hence increasing species diversity; and (2) by improving the performances of species already present in the landscape. To test these hypotheses, we focused on the effects of a high-Andean ecosystem engineer, the cushion plant Azorella monantha, by comparing the accumulation of plant biomass and nitrogen fixed in plant tissues as species richness increases in landscapes with and without the engineer species. Our results show that both ecosystem functions increased with species richness in both landscape types, but landscapes including A. monantha cushions reached higher outcomes of plant biomass and nitrogen fixed in plant tissues than landscapes without cushions. Moreover, our results indicate that such positive effects on ecosystem functions could be mediated by the two mechanisms proposed above. Then, given the conspicuousness of ecosystem engineering in nature and its strong influence on species diversity, and given the well-known relationship between species diversity and ecosystem function, we suggest that the application of the conceptual framework proposed herein to other ecosystems would help to advance our understanding of the forces driving ecosystem functioning.
Assuntos
Apiaceae/fisiologia , Ecossistema , Altitude , Biodiversidade , Biomassa , ChileRESUMO
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.