Iñigo Granzow de la Cerda

Successional dynamics in Neotropical forests are as uncertain as they are predictable

Significance Although forest succession has been approached as a predictable process, successional trajectories vary widely, even among nearby stands with similar environmental conditions and disturbance histories. We quantified predictability and uncertainty during tropical forest succession using dynamical models describing the interactions among stem density, basal area, and species density over time. We showed that the trajectories of these forest attributes were poorly predicted by stand age and varied significantly within and among sites. Our models reproduced the general successional trends observed, but high levels of noise were needed to increase model predictability. These levels of uncertainty call into question the premise that successional processes are consistent over space and time, and challenge the way ecologists view tropical forest regeneration. Although forest succession has traditionally been approached as a deterministic process, successional trajectories of vegetation change vary widely, even among nearby stands with similar environmental conditions and disturbance histories. Here, we provide the first attempt, to our knowledge, to quantify predictability and uncertainty during succession based on the most extensive long-term datasets ever assembled for Neotropical forests. We develop a novel approach that integrates deterministic and stochastic components into different candidate models describing the dynamical interactions among three widely used and interrelated forest attributes—stem density, basal area, and species density. Within each of the seven study sites, successional trajectories were highly idiosyncratic, even when controlling for prior land use, environment, and initial conditions in these attributes. Plot factors were far more important than stand age in explaining successional trajectories. For each site, the best-fit model was able to capture the complete set of time series in certain attributes only when both the deterministic and stochastic components were set to similar magnitudes. Surprisingly, predictability of stem density, basal area, and species density did not show consistent trends across attributes, study sites, or land use history, and was independent of plot size and time series length. The model developed here represents the best approach, to date, for characterizing autogenic successional dynamics and demonstrates the low predictability of successional trajectories. These high levels of uncertainty suggest that the impacts of allogenic factors on rates of change during tropical forest succession are far more pervasive than previously thought, challenging the way ecologists view and investigate forest regeneration.

A THEORY OF DISTURBANCE AND SPECIES DIVERSITY : EVIDENCE FROM NICARAGUA AFTER HURRICANE JOAN

The theory of competition is modified to include the effects of disturbance as a force that can set back the process of competitive exclusion. Disturbance may be of three forms, density dependent, density independent and quasidependent. It is shown analytically that the quasi-dependent form is incapable of setting back the competitive process to avoid competitive exclusion, while either the density dependent form or the density independent form can. The basic theory is tested with data on changes in tree population densities since the landfall of Hurricane Joan in Nicaragua. It is found that this catastrophic event indeed did have the effect of reducing competitive dominance and thus preserving species diversity. Furthermore, the effect was density independent.

Growth and development of the thinning canopy in a post-hurricane tropical rain forest in Nicaragua

Abstract The effect of catastrophic disturbance on forest structure was studied for 10 years subsequent to the landfall of Hurricane Joan in 1988. Four sites within the damage area and one control site outside of the area were established in the early 1990s and positions and sizes (DBH and crown height) were measured annually. Results of the study are reported in the context of the debate between equilibrium versus non-equilibrium models of tropical forest dynamics. Equilibrium models imply some sort of niche segregation on a species to species basis, whereas non-equilibrium models emphasise chance and history in the development of canopies. In either case, the details of the dynamics of building and thinning after a disturbance are key to resolve the debate. Physical aspects of the developing canopy began with a severely damaged forest with effectively no canopy at all. Two distinct canopies had developed 4 or 5 years after the disturbance; (1) a larger but diffuse canopy made up of the tress that had survived the hurricane standing and had begun resprouting high in their damaged crowns and (2) a ‘thinning canopy’ of very densely packed crowns of small individual trees, resulting from the growth of surviving saplings and seedlings and trees that had sprouted near to the ground. Competitive thinning has begun to occur in this dense lower canopy. Since the disturbance, the increase in number of species has been great, ranging from 134 to 208% over what would be expected in a mature forest. Mortality patterns in the thinning canopy suggest that trees are not dying at random and thus supports a niche-based model of forest regeneration. Qualitative analysis of the thinning canopy also suggests that some species have come to occupy the upper position in the canopy (and are thus likely to be competitive dominants) while other species have come to occupy the lower position (and are thus likely to be competitively suppressed). A qualitative model is proposed illustrating where, during the process of regeneration, various authors have emphasized either a niche (equilibrium) or non-niche (non-equilibrium) model. It is suggested that in the building phase of the post-disturbance forest a non-niche interpretation is most appropriate while in the thinning phase a niche interpretation may be warranted.

Confirmed presence of Gigaspermum repens Hook. Lindb. in the New World

The family Gigaspermaceae consists of six, mostly Southern Hemisphere, genera. Three are monotypic: Costesia Ther. from Chile, Chamaebryum Ther. & Dixon from southern Africa, and Oedipodiella Dixon from South Africa and eastern Spain. The remaining three genera are each represented by just two species. Lorentziella Mull.Hal. is present in southern South America (Bolivia to Uruguay and Argentina), and from central Mexico to southeastern Texas (Fife, 1980). Of the two species of Neosharpiella H.Rob. & Delgad., N. turgida (Mitt.) H.Rob. & Delgad. was described from Ecuador and is also known from a few localities in western Bolivia and Chile. The other, N. aztecorum H.Rob. & Delgad., was once considered restricted to central Mexico (Fife, 1980). It has recently been reported from southern Africa (van Rooy & Perold, 2006), and also under its synonym Quathlamba debilicostata Magill (Magill, 1987). The genus Gigaspermum Lindb. is also represented by two species, G. repens (Hook.) Lindb. and G. mouretii Corb.; the latter is mainly restricted to the

Anomodon longifolius and A. thraustus (Anomodontaceae) new to North America, north of Mexico

Abstract Anomodon longifolius, a montane species known from Europe and eastern and southern Asia, was collected for the first time in the New World, from the Adirondack Mountains of New York. Also, Anomodon thraustus, an Asian species with known records from Mexico, has its range extended to the eastern United States. It is a very rare species in North America known only from a few specimens from New Jersey, New York and North Carolina that had been mistaken for depauperate forms of A. minor.

The Bryophyte Flora of the Montseny Massif (Northeastern Iberian Peninsula): Conservation Issues and an Updated Check-List

Abstract An annotated check-list of the bryophytes of the Montseny massif (northeastern Iberian Peninsula) is presented, based on literature and herbarium records revision as well as collections made by the authors. A total of 352 taxa were recorded, out of which three were hornworts, 80 liverworts and 269 mosses. Intensive surveys of original locations of threatened, extinct and protected species in the massif were carried out. Species whose local populations became extinct were those with a strong preference for wet and oligotrophic habitats, such habitats have been severely damaged in recent decades. We discuss possible strategies for effective conservation of those bryophytes whose local population have suffered a decline, like targeted legistation and establishing specific protected areas.