On the spatial distribution of woody plant species in a tropical montane cloud forest

Abstract The cloud forest is a special type of forest ecosystem that depends on suitable conditions of humidity and temperature to exist; hence, it is a very fragile ecosystem. The cloud forest is also one of the richest ecosystems in terms of species diversity and rate of endemism. However, today, it is one of the most threatened ecosystems in the world. Little is known about tree species distribution and coexistence among cloud forest trees. Trees are essential to understanding ecosystem functioning and maintenance because they support the ecosystem in important ways. For this dissertation, an analysis of woody plant species distribution at a small scale in a north-Peruvian Andean cloud forest was performed, and some of the factors implicated in the observed patterns were identified. Towards that end, different natural factors acting on species distribution within the forest were investigated: (i) intra-specific arrangements, (ii) heterospecific spatial relationships and (iii) relationships with external environmental factors. These analyses were conducted first on standing woody plants and then on seedlings. The woody plants were found to be clumped in the forest, either considering all the species together or each species separately. However, each species presented a specific pattern and specific spatial relationship among different-age individuals. Dispersal mode, growth form and shade tolerance played roles in the final distribution of the species. Furthermore, spatial associations among species, either positive or negative, were observed. These associations were more numerous when considering individuals of the interacting species at different developmental stages, i.e., younger individuals from one species and older individuals from another. Accordingly, competition and facilitation are asymmetric processes and vary throughout the life of an individual. Moreover, some species appear to prefer certain habitat conditions and avoid other habitats. The habitat definition that best explains species distribution is that which includes both environmental and stand characteristics; thus, a combination of these factors is necessary to understanding species' niche preferences. Seedling distribution was also associated with habitat conditions, but these conditions explained less than the 30% of the spatial variation. The position of conspecific adult individuals also affected seedling distribution; although the seedlings of many tree species avoid the vicinity of conspecifics, a few species appeared to prefer the formation of cohorts around their parent trees. The importance of habitat conditions and distance dependence with conspecifics varied among regions within the forest as well as on the developmental stage of the stand. The results from this thesis suggest that different species can coexist within a given space, forming a ���puzzle��� of species as a result of the intra- and interspecific spatial relationships along with niche preferences and adaptations that operate at different scales. These factors not only affect each species in a different way, but specific preferences also vary throughout species' lifespans. Resumen Resumen El bosque de niebla es uno de los ecosistemas m��s amenazados del mundo adem��s de ser uno de los m��s fr��giles. Son formaciones azonales que dependen de la existencia de unas condiciones de humedad y temperatura que permitan la formaci��n de nubes que cubran el bosque; lo que dificulta en gran medida su conservaci��n. Tambi��n es uno de los ecosistemas con mayor riqueza de especies adem��s de tener uno de los mayores porcentajes de endemismos. Uno de los aspectos m��s importantes para entender el ecosistema, es identificar y entender los elementos que lo componen y los mecanismos que regulan las relaciones entre ellos. Los ��rboles son el soporte del ecosistema. Sin embargo, apenas hay informaci��n sobre la distribuci��n y coexistencia de los ��rboles en los bosques de niebla. Esta tesis presenta un an��lisis de la distribuci��n a peque��a escala de las plantas le��osas en un bosque de niebla situado en la cordillera andina del norte de Per��; as�� como el an��lisis de algunos de los factores que pueden estar implicados en que se origine la distribuci��n observada. Para este prop��sito se estudia c��mo influyen factores de diferente naturaleza en la distribuci��n de las especies (i) organizaci��n intra-espec��fica (ii) relaciones espaciales heterospec��ficas y (iii) relaci��n con factores ambientales externos. En estos an��lisis se estudiaron primero las plantas j��venes y las adultas, y despu��s las pl��ntulas. Los ��rboles aparecieron agregados en el bosque, tanto considerando todos a la vez como cuando se estudi�� cada especie por separado. Sin embargo, cada especie mostr�� un patr��n distinto as�� como una particular relaci��n espacial entre individuos j��venes y adultos. El modo de dispersi��n, la forma de vida y la tolerancia de la especies estuvieron relacionados con el patr��n general observado. Se vio tambi��n que ciertas especies aparec��an relacionadas con otras, tanto de forma positiva (compartiendo zonas) como negativa (apareciendo en ��reas distintas). Las asociaciones fueron mucho m��s numerosas cuando se consideraron los pares de especies en diferente estado de desarrollo, es decir, individuos j��venes de una especie e individuos mayores de la otra. Eso indicar��a que los procesos de competencia y facilitaci��n son asim��tricos y adem��s var��an durante la vida de la planta. Por otro lado, algunas especies aparecen preferentemente bajo ciertas condiciones de h��bitat y evitan otras. La definici��n de h��bitat a la que mejor responden las especies es cuando se incluyen tanto variables ambientales como de masa; as�� que ambos tipos de variables son necesarias para entender la preferencia de las especies por ciertos nichos. La distribuci��n de las pl��ntulas tambi��n estuvo relacionada con condiciones de h��bitat, pero eso s��lo llegaba a explicar hasta un 30% de la variabilidad espacial. La posici��n de los adultos de la misma especie tambi��n afect�� a la distribuci��n de las pl��ntulas. En bastantes especies las pl��ntulas evitan la cercan��a de adultos de su misma especie, padres potenciales, aunque algunas especies aisladas mostraron el patr��n contrario y aparecieron preferentemente en las mismas ��reas que sus padres. La importancia de las condiciones de h��bitat y posici��n de los adultos en la disposici��n de las pl��ntulas var��a de una zona a otra del bosque y adem��s tambi��n var��a seg��n el estado de desarrollo de la masa.

Maintaining distances with the engineer: patterns of coexistence in plant communities beyond the patch-bare dichotomy

Two-phase plant communities with an engineer conforming conspicuous patches and affecting the performance and patterns of coexisting species are the norm under stressful conditions. To unveil the mechanisms governing coexistence in these communities at multiple spatial scales, we have developed a new point-raster approach of spatial pattern analysis, which was applied to a Mediterranean high mountain grassland to show how Festuca curvifolia patches affect the local distribution of coexisting species. We recorded 22 111 individuals of 17 plant perennial species. Most coexisting species were negatively associated with F. curvifolia clumps. Nevertheless, bivariate nearest-neighbor analyses revealed that the majority of coexisting species were confined at relatively short distances from F. curvifolia borders (between 0-2 cm and up to 8 cm in some cases). Our study suggests the existence of a fine-scale effect of F. curvifolia for most species promoting coexistence through a mechanism we call 'facilitation in the halo'. Most coexisting species are displaced to an interphase area between patches, where two opposite forces reach equilibrium: attenuated severe conditions by proximity to the F. curvifolia canopy (nutrient-rich islands) and competitive exclusion mitigated by avoiding direct contact with F. curvifolia.