Summary 0We examined the abundance and distribution patterns of pioneer seeds in the soil seed bank\ and of pioneer seedlings in 42 recently formed gaps\ in a 49!ha forest dynamics plot on Barro Colorado Island "BCI#\ Panama[ The aim was to assess the importance of dispersal limitation "failure of seeds to arrive at all sites suitable for their germination# and establishment limitation "failure of seeds having reached a site to germinate successfully and establish as seedlings# in determining patterns of gap occupancy[ 1 The abundance of seeds in the soil seed bank was strongly negatively correlated with seed size\ but was not correlated with the abundance of reproductive!sized adult trees in the plot[ In contrast\ the abundance of pioneer seedlings × 09 cm height in natural gaps was strongly correlated with adult abundance\ but was not correlated with seed size[ 2 Seedlings were non!randomly distributed among gaps\ but seedling abundance was not directly related to gap size\ and there was no evidence of partitioning of the light environment of gaps by small seedlings[ Large di}erences in growth and mortality rates among species were observed after 0 year\ and this may result in the gap size partitioning previously found in saplings of the same species[ 3 Seedlings of most species\ particularly those with large seeds\ were relatively more abundant than expected in gaps close to their conspeci_c adults[ Proximity to repro! ductives\ and by inference dispersal limitation\ therefore exerts some e}ect on seedling distribution[ None the less\ large di}erences between seed and seedling abundances for some species\ and low seedling occupancy rates in some gaps close to adult conspeci_cs\ suggest that seedling emergence probabilities and species!speci_c estab! lishment requirements may also be important determinants of local abundance[

Summary Crassulacean acid metabolism (CAM) is a specialized mode of photosynthesis that features nocturnal CO2 uptake, facilitates increased water‐use efficiency (WUE), and enables CAM plants to inhabit water‐limited environments such as semi‐arid deserts or seasonally dry forests. Human population growth and global climate change now present challenges for agricultural production systems to increase food, feed, forage, fiber, and fuel production. One approach to meet these challenges is to increase reliance on CAM crops, such as Agave and Opuntia, for biomass production on semi‐arid, abandoned, marginal, or degraded agricultural lands. Major research efforts are now underway to assess the productivity of CAM crop species and to harness the WUE of CAM by engineering this pathway into existing food, feed, and bioenergy crops. An improved understanding of CAM has potential for high returns on research investment. To exploit the potential of CAM crops and CAM bioengineering, it will be necessary to elucidate the evolution, genomic features, and regulatory mechanisms of CAM. Field trials and predictive models will be required to assess the productivity of CAM crops, while new synthetic biology approaches need to be developed for CAM engineering. Infrastructure will be needed for CAM model systems, field trials, mutant collections, and data management.

Abstract. Crassulacean acid metabolism (CAM) is a specialised mode of photosynthesis that improves atmospheric CO 2 assimilation in water-limited terrestrial and epiphytic habitats and in CO 2 -limited aquatic environments. In contrast with C 3 and C 4 plants, CAM plants take up CO 2 from the atmosphere partially or predominantly at night. CAM is taxonomically widespread among vascular plants and is present in many succulent species that occupy semiarid regions, as well as in tropical epiphytes and in some aquatic macrophytes. This water-conserving photosynthetic pathway has evolved multiple times and is found in close to 6% of vascular plant species from at least 35 families. Although many aspects of CAM molecular biology, biochemistry and ecophysiology are well understood, relatively little is known about the evolutionary origins of CAM. This review focuses on five main topics: (1) the permutations and plasticity of CAM, (2) the requirements for CAM evolution, (3) the drivers of CAM evolution, (4) the prevalence and taxonomic distribution of CAM among vascular plants with emphasis on the Orchidaceae and (5) the molecular underpinnings of CAM evolution including circadian clock regulation of gene expression.

Species of the large family Orchidaceae display a spectacular array of adaptations and rapid speciations that are linked to several innovative features, including specialized pollination syndromes, colonization of epiphytic habitats, and the presence of Crassulacean acid metabolism (CAM), a water-conserving photosynthetic pathway. To better understand the role of CAM and epiphytism in the evolutionary expansion of tropical orchids, we sampled leaf carbon isotopic composition of 1,103 species native to Panama and Costa Rica, performed character state reconstruction and phylogenetic trait analysis of CAM and epiphytism, and related strong CAM, present in 10% of species surveyed, to climatic variables and the evolution of epiphytism in tropical regions. Altitude was the most important predictor of photosynthetic pathway when all environmental variables were taken into account, with CAM being most prevalent at low altitudes. By creating integrated orchid trees to reconstruct ancestral character states, we found that C 3 photosynthesis is the ancestral state and that CAM has evolved at least 10 independent times with several reversals. A large CAM radiation event within the Epidendroideae, the most species-rich epiphytic clade of any known plant group, is linked to a Tertiary species radiation that originated 65 million years ago. Our study shows that parallel evolution of CAM is present among subfamilies of orchids, and correlated divergence between photosynthetic pathways and epiphytism can be explained by the prevalence of CAM in low-elevation epiphytes and rapid speciation of high-elevation epiphytes in the Neotropics, contributing to the astounding diversity in the Orchidaceae.Crassulacean acid metabolism (CAM) is a taxonomically widespread photosynthetic pathway that has evolved in plants of CO 2 -and water-limited environments, including tropical forest canopies with intermittent or seasonal water availability, hot semiarid regions, and some aquatic environments. The CAM pathway is characterized by the temporal separation of carbon fixation between nocturnal CO 2 fixation by phosphoenolpyruvate carboxylase in the cytosol and daytime decarboxylation of organic acids to release CO 2 that is then refixed by Rubisco in the chloroplast (Ting, 1985). CAM photosynthesis is found in approx-