Pedro F. Quintana-Ascencio

Diversity of ageing across the tree of life

Evolution drives, and is driven by, demography. A genotype moulds its phenotype’s age patterns of mortality and fertility in an environment; these two patterns in turn determine the genotype’s fitness in that environment. Hence, to understand the evolution of ageing, age patterns of mortality and reproduction need to be compared for species across the tree of life. However, few studies have done so and only for a limited range of taxa. Here we contrast standardized patterns over age for 11 mammals, 12 other vertebrates, 10 invertebrates, 12 vascular plants and a green alga. Although it has been predicted that evolution should inevitably lead to increasing mortality and declining fertility with age after maturity, there is great variation among these species, including increasing, constant, decreasing, humped and bowed trajectories for both long- and short-lived species. This diversity challenges theoreticians to develop broader perspectives on the evolution of ageing and empiricists to study the demography of more species.

How do plant ecologists use matrix population models

Matrix projection models are among the most widely used tools in plant ecology. However, the way in which plant ecologists use and interpret these models differs from the way in which they are presented in the broader academic literature. In contrast to calls from earlier reviews, most studies of plant populations are based on < 5 matrices and present simple metrics such as deterministic population growth rates. However, plant ecologists also cautioned against literal interpretation of model predictions. Although academic studies have emphasized testing quantitative model predictions, such forecasts are not the way in which plant ecologists find matrix models to be most useful. Improving forecasting ability would necessitate increased model complexity and longer studies. Therefore, in addition to longer term studies with better links to environmental drivers, priorities for research include critically evaluating relative/comparative uses of matrix models and asking how we can use many short-term studies to understand long-term population dynamics.

Secondary succession in disturbed Pinus-Quercus forests in the highlands of Chiapas, Mexico

We present floristic and structural data on serai plant communities (Old-Field, Grassland, Shrubland, and Early Successional, Mid-Successional, and Mature Forest) resulting from the current land use pattern in the Pinus-Quercus forests in the highlands of Chiapas, Mexico. The number of species ranged from 24 in the Old-Field stage up to 100 in the Mature Forest, and the shrub layer was richest in all the forested stands. An almost complete floristic replacement exists between the open and the forested communities. Quercus and Pinus dominate the canopy of old-growth stands, but their recruitment does not occur or is very low in the shade. Vigorous seedlings and saplings of these species are found only in the open serai stages or in forest gaps. Demographic and phytosociological data are used to indicate the successional role of dominant species in the forested stages. Some implications of the regional land use patterns for conservation and succession are discussed.

Soil Seed Banks and Regeneration of Tropical Rain Forest from Milpa Fields at the Selva Lacandona, Chiapas, Mexico

The traditional milpa agriculture system (slash-burn) of the Lacandon Maya people in eastern Chiapas, Mexico has created and uses a variety of habitat patches including the whole range of seral stages during forest development. This study examines seed bank attributes in habitats associated with slash-burn agriculture and with the more recent expansion of grasslands. We estimated the abundance and floristic composition of seeds in the soil from induced grasslands, milpa fields neighboring early and mature forests, and from secondary forest and mature tropical rain forest (15 samples by site, 2-3 replicated sites per successional stage). A total of 130 species germinated from the soil samples. The seeds of late successional tree species were scarce in all the seed soil samples (<2% of the species). Twenty-five early successional species accounted for ca. 70 percent of all individuals. Soil seed density (but not species richness) increased as milpas and fallows dominated the landscape. Affinity analysis based on presence/absence and abundance data showed a different soil seed bank composition for the grasslands when compared to that of all other successional communities. Our results suggest that current expansion of pastures and agricultural fields modifies seed deposition and storage patterns in the soil that may alter the process of secondary succession. This compromises both the regenerating potential of tropical rain forest in the region, and the sustainability of traditional practices and economic relationships of the Lacandon Maya.

Competitive abilities of three narrowly endemic plant species in experimental neighborhoods along a fire gradient

We conducted field experiments manipulating lichens, shrubs, and herbs along a time-since-fire gradient and assessing effects on three endemic herbaceous species of Florida scrub: Eryngium cuneifolium, Hypericum cumulicola, and Polygonella basiramia. Responses included seed germination, survival, biomass, and fecundity. Transplants into recently burned patches generally had higher survival, larger biomass, and greater reproductive output than transplants into long-unburned patches. Open areas and sites near oaks frequently were more favorable than sites near Florida rosemary. Ground lichens did not affect germination but increased mortality rate of seedlings. Neighboring small shrubby and herbaceous species did not affect the performance of these species. Of the three species, naturally occurring E. cuneifolium were farthest from large shrubs, and their microhabitats had the least ground lichens and shrubs. Eryngium cuneifolium and H. cumulicola are capable of forming persistent seed banks and their recruitment after fire depends mostly on these dormant seeds. Polygonella basiramia relies on seed dispersal and immediate seed germination to colonize recently burned patches. Management for these species should involve variable fire regimes to allow all three species to persist along with many other scrub endemics.

Tree composition and structure in disturbed stands with varying dominance by Pinus spp. in the highlands of Chiapas, México

We studied soil and forest floor conditions, regeneration patterns oftrees, and forest structure and floristic composition along a gradient ofdominance by Pinus spp. in disturbed stands in thehighlands of Chiapas, southern Mexico. Seedlings, saplings, and adults of treespecies were counted and measured in 2-3 circular plots (1000m2 each) in 36 forest stands (a total of 38 treespecies). Dominance of broadleaved trees other thanQuercusspp. was negatively correlated with basal area of Pinusspp. (P < 0.001). Soils of pine-dominated stands weremore compacted, less acidic, and less fertile (lower C.E.C., nitrogen, andorganic matter content). Numerous broadleaved trees may depend on anoak-dominated canopy to regenerate, and changes associated to pinelandsexpansion may compromise their long-term persistence. However, seedlings andsaplings of Quercus spp. were abundant in stands with bothan oak- or pine-dominated canopy. The results suggest that a plan forsustainable forest utilization in the study region, and in other similarpopulated tropical highlands, could take advantage of the abundant oakregeneration and coexistence with pines in the canopy.

Fire-mediated effects of shrubs, lichens and herbs on the demography of Hypericum cumulicola in patchy Florida scrub

Abstract Understanding the effect of disturbance and interspecific interactions on population dynamics and availability of suitable habitats for colonization and growth is critical for conservation and management of endangered species. Hypericum cumulicola is a narrowly endemic, small perennial herb virtually restricted to open areas of well-drained white sand in Florida rosemary scrub, a naturally patchy community that burns about every 20–80 years. Over 1 year (September 1994 to September 1995) we evaluated variation in survival, growth and fecundity among 1214 individuals in 14 rosemary scrub patches of different sizes (0.09–1.85 ha) and fire histories (2, 8–10, and >20 years since the last fire). Fire kills aboveground individuals of H. cumulicola, but new individuals were present a year after fire. Recruitment decreased in patches more than a decade post-fire. Survival, annual height growth rate, and fecundity (number of flowers and fruits) were higher in recently burned patches. Scrub patch size did not affect these demographic variables. Survival was positively associated with the presence of conspecifics and negatively related to proximity to the dominant shrub Florida rosemary (Ceratiola ericoides), prior reproductive output, and ground lichen cover. Since H. cumulicola and other herbaceous species in the rosemary scrub depend on sporadic fires to decrease interference of shrubs and ground lichens, its persistence may be threatened by fire suppression.

Seed predation and dispersal in a dominant desert plant: Opuntia, ants, birds, and mammals

Several Opuntia spp. (Cactaceae) occur in large and dense stands in semiarid central Mexico. Their fruits and seeds are a major food source for rodents, and to a lesser extent, for harvester ants, birds, and other mammals. O. robusta and O. streptacantha differ in their timing of fruit production and presentation and in some fruit traits which may have different consequences for seed predation/dispersal interactions with the same group of animals. Multiple interactions suggesting a changing seed predation/seed dispersal facilitation among these plants and animals are reported. The fate of Opuntia seeds (either eaten or dispersed) after several possible seed predation/moving/dispersal interactions apparently depends on such characteristics of the habitat as the abundance of rodent populations and the availability of optional resources for these specialized granivores.

Association of ecotones with relative elevation and fire in an upland Florida landscape

Abstract Question: What are the importance of elevation and fire in maintaining ecotones of Florida scrub assemblages along a gradual topographic gradient? Location: Archbold Biological Station (ABS), 12 km south of Lake Placid, Florida, USA. Methods: Vegetation cover of upland Florida shrublands was quantified using the line-intercept method along 20 transects traversing similar elevation gradients, stratified by time since fire (TSF). We objectively identified shrubland ecotones using a split moving windows boundary analysis (SMW) with three different window widths. Non-metric multidimensional scaling ordination was used to determine relationships among plant assemblages defined by SMW. Results: We located up to four ecotones per transect, the majority of which were wide, highly heterogeneous zones. Relative elevation controlled the distribution of plant assemblages in upland Florida shrublands. Ecotones in shrublands > 30 years TSF had relatively low dissimilarity values in SMW, indicating that previously discrete plant assemblages with longer TSF were becoming more similar with time. Conclusions: Split Moving Windows (SMW) analysis identified ecotones relatively well although patches generated by oak clonal growth were sometimes identified as ecotones. Fire suppression caused ecotones to become more diffuse, suggesting that without fire at least every 30 years, discrete plant assemblages within upland Florida shrublands will be more continuous. Abbreviations: ABS = Archbold Biological Station; SMW = Split moving window; TSF =Time since fire; NMDS = Non-metric multi-dimensional scaling; OSF = Open Scrubby Flatwoods; RS = Rosemary Scrub; SF = Scrubby Flatwoods; F = Flatwoods; SCB = Squared city block dissimilarities. Nomenclature: Wunderlin (1998); Abrahamson et al. (1984).

Variation in soil moisture in relation to rainfall, vegetation, gaps, and time-since-fire in Florida scrub

ABSTRACT Florida scrub is a pyrogenic shrubland ecosystem occurring on well-drained sands derived from contemporary and relictual beach dunes. Despite average annual precipitation > 1300 mm, Florida scrub is dominated by xeromorphic plants. We monitored spatio-temporal variation in soil moisture to determine if the distribution of Florida scrub communities reflects patterns in soil moisture variation. Using frequency domain reflectometry, we measured soil moisture at 24 sampling stations (3 depths per station) in 3 Florida scrub communities (rosemary scrub, scrubby flatwoods, and oak–hickory scrub) at Archbold Biological Station for 3y (October 1998–September 2001). Stations were arrayed to sample 2 microhabitats (gaps, shrubs) and 2 burn histories. Soil moisture closely tracked cumulative rainfall across widely varying precipitation in the 3 y studied. Soil moisture changed through time and differed significantly among habitats; it was generally highest in scrubby flatwoods, particularly during the wettest periods, and lowest in oak–hickory scrub. Soil moisture was generally greater at deeper depths, in more recently burned sites, and in gaps. Burn effects were particularly pronounced in rosemary scrub, where lack of resprouting dominants after fire maintains more distinct, larger gaps. Burn and gap effects were small in absolute terms, but burned sites and gaps consistently had greater soil moisture than unburned and matrix sites. These small differences may be critical to the germination, establishment, and growth of narrowly endemic plants, particularly in Florida rosemary scrub. However, factors such as competition for nutrients, cryptobiotic soil crusts, litter accumulation, gap size, and allelopathy may be more important in influencing distributions of endemic scrub plants.