Browsing by Autor "Sergio Valdivia"

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Co-occurrence Patterns along a Regional Aridity Gradient of the Subtropical Andes Do Not Support Stress Gradient Hypotheses
(Public Library of Science, 2013) Ramiro Pablo López; Sergio Valdivia; Mónica L. Rivera; Rodrigo S. Ríos
The stress gradient hypothesis posits that facilitation and stress are positively correlated. The hump-shaped hypothesis, on the contrary, proposes that facilitation is greater at intermediate stress levels. The relationship between facilitation and environmental stress is commonly studied at small spatial scales and/or considering few species; thus, the implications of facilitation at a community level remain poorly understood. Here, we analyzed local co-occurrence patterns of all plant species at 25 sites within the subtropical Andes to evaluate the role of facilitation and competition as drivers of community structure. We considered a wide latitudinal gradient (19-26°S) that incorporates great variation in aridity. No previous studies have attempted to study these patterns across such a broad scale in warm deserts. Each locality was sampled at two scales (quadrat and patch), and co-occurrence was analyzed via null models. Furthermore, we tested for a relationship between plant co-occurrences and environmental aridity. Resulting patterns depended on life form. When all species were considered, negative associations were found, indicating competition. Woody/cactus life forms tended to be associated across communities, suggesting that there is facilitation between these life forms. Additionally, and unlike previous studies, we found positive associations among shrubs. The strength of the association between woody species changed non-monotonically with aridity. Herbs showed an inverted hump-shaped relationship, albeit ranging mostly among neutral values. Independent of the association type exhibited by different life forms, our community level results do not support current stress gradient hypotheses.
Consistent spatial patterns across several plant communities within a region indicate that the same processes may be acting on Andean deserts and semideserts
(Wiley, 2019) Ramiro Pablo López; Freddy S. Zenteno-Ruíz; Natalio Roque‐Marca; Laura Moya; David Tábara Villalba; Sergio Valdivia; Daniel M. Larrea‐Alcázar
Abstract Questions Spatial pattern analysis may be used to derive hypotheses on the processes that shape plant communities. Do different mature semidesert communities exhibit the same major spatial patterns, suggesting that similar processes shape them? Location Nine localities in the Bolivian Andes. Methods We mapped all woody individuals at each locality. We performed uni‐, bi‐, and multivariate spatial analyses of nine Andean, mature semiarid plant communities to test for similarity in spatial patterns. Results We found that the nine communities exhibited, in most of the main spatial pattern attributes, close similarities (community aggregation, importance of plant interactions, life form differences, etc.). Some peculiarities were likely due to differences in life form or site history. Conclusions At least for Andean semiarid communities, the complete analysis of a representative plot conveys reliable information on spatial relationships and, hence, on potential underlying mechanisms. This shows that the use of 1–2 representative plots can be used to extrapolate to similar communities within a given region. Our results can be used as a plausible hypothesis to be tested in other zones of the planet.
The importance of shrub cover for four cactus species differing in growth form in an Andean semi‐desert
(Wiley, 2007) Ramiro Pablo López; Sergio Valdivia
Abstract 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.
The role of nurse plants in the establishment of shrub seedlings in the semi-arid subtropical Andes
(Springer Science+Business Media, 2007) Ramiro Pablo López; Sergio Valdivia; Ninel Sanjinés; Diego de la Quintana