In alpine ecosystems, greater overlap in flowering phenology among species at higher elevations could be due to evolutionary convergence among lineages or environmental filtering for taxa preadapted to colder conditions. We hypothesize that the flowering phenology of high alpine communities, subjected to colder and shorter reproductive seasons, is the result of convergence due to strong selective pressure imposed by the environment rather than environmental filtering for conservated traits. To test this hypothesis, we analyzed phylogenetic signal for first flowering date, peak flowering date, flowering duration and thermal sums to first and peak flowering and community phylogenetic structure considering Mean Nearest Taxon Distance (MNTD) and Mean Pairwise Distance (MPD) on four sites encompassing a total of 86 species derived from the subalpine and high alpine in the central Chilean Andes. After discarding possible richness effects on phylogenetic signal, the high alpine sites continued to show significant phylogenetic signal for a smaller number of floral traits than the subalpine sites. This was particularly evident for thermal sums. The two high elevation communities show significant values of SES(MNTD) but not for SES(MPD), indicating clustering related to the tips of the phylogeny. Overall, results suggest that environmental filtering for preadapted lineages is not the main driver of the phylogenetic structure and composition in high alpine communities. Rather, species at higher elevation have been subjected to greater environmental pressures leading to trait convergence. We conclude that phylogenetic conservatism in floral phenology has been overridden by the harsh environmental conditions in the high Andes. The high alpine environment can be seen as an evolutionary promoter rather than a gatekeeper of lineages preadapted to cold conditions.

Aim High alpine regions are threatened but understudied ecosystems that harbor diverse endemic species, making them an important biome for testing the role of environmental factors in driving functional trait-mediated community assembly processes. We tested the hypothesis that plant-soil feedbacks along a climatic and elevation gradient influence plant community assembly through shifts in habitat suitability, which drive plant functional, phylogenetic, and spectral diversity. Location In a high mountain system (2400-3500 m) of Región Metropolitana in the Chilean Andes (33°S, 70°W). Methods We surveyed vegetation and spectroscopic reflectance (400-2400 nm) to quantify taxonomic, phylogenetic, functional, and spectral diversity at five sites from 2400 m to 3500 m elevation. We characterized soil attributes and processes by measuring water content, carbon and nitrogen, and net nitrogen mineralization rates. Results At high elevation, colder temperatures reduced available soil nitrogen, while at warmer, lower elevations, soil moisture was lower. Metrics of taxonomic, functional, and spectral alpha diversity peaked at mid-elevations, while phylogenetic species richness was highest at low elevation. Leaf nitrogen increased with elevation at the community level and within individual species, consistent with global patterns of increasing leaf nitrogen with colder temperatures. Main conclusions The increase in leaf nitrogen, coupled with shifts in taxonomic and functional diversity associated with turnover in lineages, indicate that the ability to acquire and retain nitrogen in colder temperatures may be important in plant community assembly in this range. Such environmental filters have important implications for forecasting shifts in alpine plant communities under a warming climate.

In closed-canopy forests, the availability of photosynthetically active light has been a focal point of research, emphasizing the role of light as a resource in limiting carbon assimilation and individual tree growth. However, light shapes the functioning of forest ecosystems through multiple mechanisms. Here, using a series of studies from a network of tree diversity experiments, we explore the multifaceted ways in which light---in terms of both quantity and quality---shapes productivity in mixed-species forests. Spectral reflectance from remote sensing of forest canopies is being increasingly used to detect how tree diversity influences productivity. We demonstrate that airborne imaging spectroscopy captures functionally important differences among canopies related to their structure, chemistry, and underlying biological interactions. Ground-based analyses can show in detail how photosynthetically active light is partitioned among species in mixed-species communities. We show that greater interception of light and greater efficiency of light use, generated by inter- and intra-specific differences, combine to enhance productivity in mixed-species forests. Light may shape forest function not only as a resource but also as a stressor and cue. Plants can perceive light at various wavelengths, use this information to assess their neighborhoods, and subsequently adjust their physiology and allocation. We characterize how light quality---from the ultraviolet to shortwave infrared---varies among and within canopies of differing diversity. We explore how these diversity-light quality relationships arise and connect across levels of biological organization from leaf-level trait expression to forest function. Together these studies lend insight into light-mediated mechanisms that drive relationships between biodiversity and productivity in forest ecosystems---insights that are crucial to predict how biodiversity change will affect future forest function.