CAMBIOS EN LA ESTRUCTURA Y COMPOSICIÓN DE LA VEGETACIÓN RELACIONADOS CON LOS AÑOS DE ABANDONO DE TIERRAS AGROPECUARIAS EN LA SELVA BAJA CADUCIFOLIA ESPINOSA DE LA RESERVA DE DZILAM, YUCATÁN

The dry forests in the northern YucatanPeninsula are highly important not only forscience but also for the conservation of biodiversity because of their unique biogeographical and ecological characteristics. Thecombination of arid climate, stony soil andrelative isolation from other biogeographical regions, has resulted in their shelteringan important proportion of the peninsula’sendemic taxa. Nevertheless, this regionsuffers from such human pressures as cattleranches and the exploitation of limestonebanks. This scenario makes it necessary todescribe the ecological changes on the landonce it has been abandoned after being usedfor agriculture, because there is no pristineforest in Yucatan. We compared the structure and composition of trees and shrubson abandoned agricultural land of differentages. All sites were derived from tropicaldry forest, a vegetation type which containsseveral endangered species. The changesin composition and relative abundance ofthe subfamilies belonging to the Leguminosae (Papilonoideae, Mimosoideae andCaesalpinoideae) were also reported, sincethis family was the most abundant at thestudy site. Average tree height and densityand shrub richness showed larger changeswith increasing number of years withoutmanagement (ANOVA, P< 0.05). Youngerplots (1-5 years and currently in use) were more similar among themselves than to older sites (more than five and more than tenyears). The Leguminosae were the richestfamily in species in all the plots and oneof the most abundant, possibly indicatinga history of intense use. Agriculture tendsto deplete nitrogen from the soil, whichgives a competitive advantage to legumes.Papilonoideae were the most abundant inolder sites (more than five and more thanten years), whereas Mimosoideae dominated recently abandoned sites (five years orless) and those currently in use. Metopiumbrownei, Bursera simaruba and Plumeriarubra were the most abundant species in theoldest site. The first two are found in almostall types of terrestrial vegetation on the Yucatan Peninsula, and all three are reportedas being very resistant to disturbance, suchas fire or damage associated with woodharvest, and to regenerate readily. Theirpresence may indicate a history of intenseuse of this region.Las selvas bajas del norte de Yucatán sonde gran importancia científica y conservacionista; sus condiciones de aridez, pedregosidad del suelo y su relativo aislamientode otras regiones biogeográficas le permitenalbergar una importante proporción de losendemismos de la península. No obstantelo anterior, esta área sufre de presionespor actividades humanas, principalmentela ganadería, agricultura y la extracción depiedra caliza. Por esta razón se hace necesario estudiar los cambios en la vegetaciónen las tierras agropecuarias después de suabandono. Se comparó la estructura y composición de árboles y arbustos de potrerosderivados de selva baja caducifolia espinosaen Dzilam, Yucatán con diferentes tiemposde abandono. Se reportan los cambios enla composición y abundancia de las tressubfamilias pertenecientes a la familia Leguminosae (Papilonoideae, Mimosoideae yCaesalpinoideae) ya que es una familia muyimportante en los trópicos secos y fue la másabundante y rica en nuestra área de estudio.Se encontró que la altura y la abundancia deindividuos del estrato arbóreo y la riquezadel estrato arbustivo fueron las variables que cambiaron de manera más intensa conla edad de abandono (ANOVA, P < 0.05).Los sitios de pasto en uso y sitios con menos de cinco años de abandono mostraronuna mayor semejanza en la composición deespecies entre ellos que con los más antiguos. Las parcelas con más de 10 y 5 añosde abandono formaron otro grupo, aunquecon un bajo valor de similitud (Sorensen <0.2). La familia Leguminosae fue la másrica en especies en los estratos arbustivoy arbóreo. Las leguminosas también seencontraron entre las más abundantes. Estose explica por un suelo pobre en nutrientesy la historia de manejo intenso, ya quesu capacidad de establecer simbiosis conmicroorganismos fijadores de nitrógenoles confiere una ventaja competitiva. Lasabundancias relativas de las mimosoideasfueron las más altas en los sitios de recienteabandono, en tanto que las de papilonoideasdominaron en los sitios más antiguos. Lasespecies más abundantes en el sitio de másde 10 años resultaron ser Bursera simaruba,Metopium brownei y Plumeria rubra. Lasprimeras dos se encuentran en casi todos lostipos de vegetación terrestre de la Penínsulade Yucatán. Las tres especies son resistentesa disturbios, como incendios o destrucción asociada a la explotación maderera y poseenuna alta capacidad de regeneración, lo quepuede indicar una historia de uso intenso deeste tipo de vegetación

Noncorrelated effects of seed predation and pollination on the perennial herb Ruellia nudiflora remain spatially consistent

By simultaneously manipulating both seed predator and pollinator effects on the perennial herb Ruellia nudiflora at two sites in Yucatan (Mexico), the present study evaluated (1) whether a correlation (interaction) existed between seed predator and pollinator effects on R. nudiflora seed production and (2) whether such an interaction varied geographically. We used three populations per site, and a total of 20 plants per population (N = 120). Groups of five plants were randomly chosen at each population to simultaneously receive one of two seed predator and pollinator exclosure levels (present or excluded in each case). These two factors were fully crossed, resulting in each group being subjected to one of four possible combinations: pollinators excluded/herbivores present; herbivores excluded/pollinators present; herbivores excluded/pollinators excluded; or control (neither excluded). Response variables were the number of seeds produced per plant and the proportion of attacked fruits by seed predators per plant. Seed predators had a large impact on R. nudiflora seed production but did not show any preference for fruits from plants not excluded from pollinators. In addition, the pollination treatment was not significant, indicating no effect of pollinators on reproductive success. These findings resulted in a nonsignificant herbivory ¥ pollination interaction, which was consistent across sites, indicating lack of correlated selection of these two guilds on R. nudiflora seed productionPeer reviewe

Noncorrelated effects of seed predation and pollination on the perennial herb Ruellia nudiflora remain spatially consistent

By simultaneously manipulating both seed predator and pollinator effects on the perennial herb Ruellia nudiflora at two sites in Yucatan (Mexico), the present study evaluated (1) whether a correlation (interaction) existed between seed predator and pollinator effects on R. nudiflora seed production and (2) whether such an interaction varied geographically. We used three populations per site, and a total of 20 plants per population (N = 120). Groups of five plants were randomly chosen at each population to simultaneously receive one of two seed predator and pollinator exclosure levels (present or excluded in each case). These two factors were fully crossed, resulting in each group being subjected to one of four possible combinations: pollinators excluded/herbivores present; herbivores excluded/pollinators present; herbivores excluded/pollinators excluded; or control (neither excluded). Response variables were the number of seeds produced per plant and the proportion of attacked fruits by seed predators per plant. Seed predators had a large impact on R. nudiflora seed production but did not show any preference for fruits from plants not excluded from pollinators. In addition, the pollination treatment was not significant, indicating no effect of pollinators on reproductive success. These findings resulted in a nonsignificant herbivory ¥ pollination interaction, which was consistent across sites, indicating lack of correlated selection of these two guilds on R. nudiflora seed productionPeer reviewe

Cambios en la estructura y composición de la vegetación relacionados con los años de abandono de tierras agropecuarias en la selva baja caducifolia espinosa de la reserva de Dzilam, Yucatán

The dry forests in the northern Yucatan Peninsula are highly important not only for science but also for the conservation of biodiversity because of their unique biogeographical and ecological characteristics. The combination of arid climate, stony soil and relative isolation from other biogeographical regions, has resulted in their sheltering an important proportion of the peninsula's endemic taxa. Nevertheless, this region suffers from such human pressures as cattle ranches and the exploitation of limestone banks. This scenario makes it necessary to describe the ecological changes on the land once it has been abandoned after being used for agriculture, because there is no pristine forest in Yucatan. We compared the structure and composition of trees and shrubs on abandoned agricultural land of different ages. All sites were derived from tropical dry forest, a vegetation type which contains several endangered species. The changes in composition and relative abundance of the subfamilies belonging to the Leguminosae (Papilonoideae, Mimosoideae and Caesalpinoideae) were also reported, since this family was the most abundant at the study site. Average tree height and density and shrub richness showed larger changes with increasing number of years without management (ANOVA, P< 0.05). Younger plots (1-5 years and currently in use) were more similar among themselves than to older sites (more than five and more than ten years). The Leguminosae were the richest family in species in all the plots and one of the most abundant, possibly indicating a history of intense use. Agriculture tends to deplete nitrogen from the soil, which gives a competitive advantage to legumes. Papilonoideae were the most abundant in older sites (more than five and more than ten years), whereas Mimosoideae dominated recently abandoned sites (five years or less) and those currently in use. Metopium brownei, Bursera simaruba and Plumeria rubra were the most abundant species in the oldest site. The first two are found in almost all types of terrestrial vegetation on the Yucatan Peninsula, and all three are reported as being very resistant to disturbance, such as fire or damage associated with wood harvest, and to regenerate readily. Their presence may indicate a history of intense use of this region.Las selvas bajas del norte de Yucatán son de gran importancia científica y conservacionista; sus condiciones de aridez, pedregosidad del suelo y su relativo aislamiento de otras regiones biogeográficas le permiten albergar una importante proporción de los endemismos de la península. No obstante lo anterior, esta área sufre de presiones por actividades humanas, principalmente la ganadería, agricultura y la extracción de piedra caliza. Por esta razón se hace necesario estudiar los cambios en la vegetación en las tierras agropecuarias después de su abandono. Se comparó la estructura y composición de árboles y arbustos de potreros derivados de selva baja caducifolia espinosa en Dzilam, Yucatán con diferentes tiempos de abandono. Se reportan los cambios en la composición y abundancia de las tres subfamilias pertenecientes a la familia Leguminosae (Papilonoideae, Mimosoideae y Caesalpinoideae) ya que es una familia muy importante en los trópicos secos y fue la más abundante y rica en nuestra área de estudio. Se encontró que la altura y la abundancia de individuos del estrato arbóreo y la riqueza del estrato arbustivo fueron las variables que cambiaron de manera más intensa con la edad de abandono (ANOVA, P < 0.05). Los sitios de pasto en uso y sitios con menos de cinco años de abandono mostraron una mayor semejanza en la composición de especies entre ellos que con los más antiguos. Las parcelas con más de 10 y 5 años de abandono formaron otro grupo, aunque con un bajo valor de similitud (Sorensen < 0.2). La familia Leguminosae fue la más rica en especies en los estratos arbustivo y arbóreo. Las leguminosas también se encontraron entre las más abundantes. Esto se explica por un suelo pobre en nutrientes y la historia de manejo intenso, ya que su capacidad de establecer simbiosis con microorganismos fijadores de nitrógeno les confiere una ventaja competitiva. Las abundancias relativas de las mimosoideas fueron las más altas en los sitios de reciente abandono, en tanto que las de papilonoideas dominaron en los sitios más antiguos. Las especies más abundantes en el sitio de más de 10 años resultaron ser Bursera simaruba, Metopium brownei y Plumeria rubra. Las primeras dos se encuentran en casi todos los tipos de vegetación terrestre de la Península de Yucatán. Las tres especies son resistentes a disturbios, como incendios o destrucción asociada a la explotación maderera y poseen una alta capacidad de regeneración, lo que puede indicar una historia de uso intenso de este tipo de vegetación

RUDERAL PLANTS: TEMPORARY HOSTS OF ARBUSCULAR MYCORRHIZAL FUNGI IN TRADITIONAL AGRICULTURAL SYSTEMS?

Ruderal plants may serve as temporary hosts of arbuscular mycorrhizal fungi (AMF), by maintaining the availability of active propagules in the soil, which in turn favors rapid colonization of roots of cultivated species during the agricultural cycle. The goals of this study were to: 1) estimate the richness of ruderal plant species in an agricultural plot and determine their mycorrhizal status, 2) quantify the number of live AMF spores in soil samples, and 3) estimate the infection potential and number of active propagules in soil samples from the agricultural site. The agricultural site used was located in Yucatan, Mexico, and consisted of a monoculture of corn subjected to low-impact agricultural practices during the last five years. A total of 20 species of ruderal plants were found at the experimental site, belonging to 11 families. All the sampled species exhibited associations with AMF, and colonization percentages ranged from 11.7±0.07 to 79.6±0.01 among species. The rhizosphere presented an average of 565±324 spores in 50 g of dry soil, of which 58.76% of the spores were alive. The inoculum potential of the soil was 50.4±0.05%, while the number of infective propagules was 193.37 (both in 50 mL of soil). Results from this study show that the presence of ruderal species in agricultural sites may promote the maintenance of AMF communities by acting as temporary hosts of these fungal species. In doing so, ruderal species can favor a higher production of infective AMF propagules and thus stronger mycorrhizal interactions with cultivated species