Questions: Do local plant-plant interactions and facilitation play an important role in vegetation dynamics after disturbance in the high tropical Andes? In particular, does the dominant shrub Baccharis prunifolia have an effect on microclimate and local vegetation structure within old-fields at the upper forest line? Does distance from the border of the surrounding forests also influence community structure within fallow plots? Is there an interaction between facilitation and edge effects on vegetation regeneration?Location: Gavidia Valley, Sierra Nevada de M erida, Venezuela.Methods: Plant cover, species richness and community structure were compared in circular micro-plots placed under the canopy of B. prunifolia shrubs and in open areas (12 micro-plots in each case) both near and far from the forest border within two 10-12 yr successional plots at 3400 m a.s.l. (a total of 96 micro-plots). Additionally, topsoil temperature was monitored for 1 mo in the four situations under study.Results: Average and maximum temperature and temperature amplitudes were lower under the canopy of B. prunifolia shrubs than in open areas; this effect being more marked far from the forest edge. Species richness, vegetation cover and the density of dominant forest trees were higher under the shrubs' canopy than in the inter-shrub spaces. The proximity to the edge of the surrounding forest also had a positive effect on species richness, with no significant interaction with the local sampling situation (under shrub vs outside). Conclusions:Our results suggest that both facilitation and edge effects influence the effectiveness of vegetation regeneration within old-fields in the high tropical Andes. Consequently, these processes should be considered for interpreting vegetation dynamics in environmental change scenarios at the tropical mountain tree line and for the design of ecological restoration strategies in these high-diversity ecotones, which have been subjected to large-scale anthropogenic transformations in recent decades.

Assemblages associated with intertidal rocky shores were examined for large scale distribution patterns with specific emphasis on identifying latitudinal trends of species richness and taxonomic distinctiveness. Seventy-two sites distributed around the globe were evaluated following the standardized sampling protocol of the Census of Marine Life NaGISA project (www.nagisa.coml.org). There were no clear patterns of standardized estimators of species richness along latitudinal gradients or among Large Marine Ecosystems (LMEs); however, a strong latitudinal gradient in taxonomic composition (i.e., proportion of different taxonomic groups in a given sample) was observed. Environmental variables related to natural influences were strongly related to the distribution patterns of the assemblages on the LME scale, particularly photoperiod, sea surface temperature (SST) and rainfall. In contrast, no environmental variables directly associated with human influences (with the exception of the inorganic pollution index) were related to assemblage patterns among LMEs. Correlations of the natural assemblages with either latitudinal gradients or environmental variables were equally strong suggesting that neither neutral models nor models based solely on environmental variables sufficiently explain spatial variation of these assemblages at a global scale. Despite the data shortcomings in this study (e.g., unbalanced sample distribution), we show the importance of generating biological global databases for the use in large-scale diversity comparisons of rocky intertidal assemblages to stimulate continued sampling and analyses.

Abstract:The soil seed bank is the basis for community establishment and permanence and plays a primary role in natural restoration of degraded or altered ecosystems. As part of a restoration project, this study aimed to quantify the soil seed bank and to evaluate the effect of the needle litter layer on seedling emergence. Soil samples from a pine plantation were collected at random in the field and set to germinate in a greenhouse. Half of them were covered by a 6cm layer of dead pine needles simulating field conditions. in the field, 20x20cm plots were established, half were left intact and half were cleaned from the litter needles. All four treatments had 15 replicates and seedling emergence was recorded during six months. Soil seed bank density was 1 222/m 2 from 17 morphotypes. in the field, the number of morphotypes and seedlings was only 9% and 6% respectively, of those emerged in the greenhouse, possibly due to watering and lack of predation in the latter. in both cases, herbs and graminoids were the dominant emerging seedlings, making up to 70-90% of the total. The needle layer didn't prevent seeds from reaching the soil but strongly reduced (>50%) seedling emergence, although high variability within treatments resulted in no statistically significant differences. These results show that the needle layer hinders germination and/or emergence of seedlings from the seed bank. its removal may be a recommended technique to accelerate natural restoration in pine plantations. Rev. Biol. Trop. 59 (3): 1071-1079. Epub 2011 September 01.

The outcome of competition for nitrogen (N) between native and invasive tree species is a major concern when considering increasing anthropogenic N deposition. Our study investigated whether three native (i.e., Fagus sylvatica, Quercus robur , and Pinus sylvestris ) and two invasive woody species (i.e., Prunus serotina and Robinia pseudoacacia ) showed different responses regarding morphological and physiological parameters (i.e., biomass and growth indices, inorganic vs. organic N acquisition strategies, and N allocation to N pools) depending on the identity of the competing species, and whether these responses were mediated by soil N availability. In a greenhouse experiment, tree seedlings were planted either single or in native-invasive competition at low and high soil N availability. We measured inorganic and organic N acquisition using 15 N labeling, total biomass, growth indices, as well as total soluble amino acid-N and protein-N levels in the leaves and fine roots of the seedlings. Our results indicate that invasive species have a competitive advantage via high growth rates, whereas native species could avoid competition with invasives via their higher organic N acquisition suggesting a better access to organic soil N sources. Moreover, native species responded to competition with distinct species- and parameter-specific strategies that were partly mediated by soil N availability. Native tree seedlings in general showed a stronger response to invasive P. serotina than R. pseudoacacia , and their strategies to cope with competition reflect the different species’ life history strategies and physiological traits. Considering the responses of native and invasive species, our results suggest that specifically Q. robur seedlings have a competitive advantage over those of R. pseudoacacia but not P. serotina . Furthermore, native and invasive species show stronger responses to higher soil N availability under competition compared to when growing single. In conclusion, our study provides insights into the potential for niche differentiation between native and invasive species by using different N forms available in the soil, the combined effects of increased soil N availability and competition on tree seedling N nutrition, as well as the species-specific nature of competition between native and invasive tree seedlings which could be relevant for forest management strategies.

Competitive interactions between native tree seedlings and exotic grasses frequently hinder forest restoration. We investigated the consequences of competition with exotic grasses on the growth and net nitrogen (N) uptake capacity of native rainforest seedlings used for reforestation depending on soil N availability and N source. Tree seedlings and grasses were grown in the greenhouse in different competition regimes (one tree species vs one grass species) and controls (grass monocultures or single tree seedlings) at low and high soil N. After 8 weeks, we quantified net N uptake capacity using 15N-labelled organic (i.e., glutamine and arginine) and inorganic (i.e., ammonium and nitrate) N sources and biomass indices. Depending on soil N availability, we observed different species-specific responses to growth and N acquisition. Tree seedlings generally increased their net N uptake capacity in response to competition with grasses, although overall seedling growth was unaffected. In contrast, the responses to competition by the grasses were species-specific and varied with soil N availability. The different N acquisition strategies suggest the avoidance of competition for N between trees and grasses. Overall, the results highlight that quantifying underlying mechanisms of N acquisition complements the information on biomass allocation as a measure of responses to competition, particularly with varying environmental conditions.