Tree seedling establishment in Andean and Alpine treeline ecotones
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Philipps-Universität Marburg
Abstract
Seedling establishment is one of the major bottlenecks in plant community dynamics and is critical for elevational advance of tree species above the treeline ecotone. Understanding seedling responses to different combinations of abiotic and biotic constraints is essential for predicting future treeline shifts. Although treeline ecology has significantly progressed, the number of studies addressing seedling establishment in treeline ecotone conditions is very modest, and those dealing with the ecology of tree seedlings from Andean treelines are scarce. In this doctoral research, I investigate tree seedling establishment at treeline ecotones, focusing on the Venezuelan Andes and the French Alps through a combination of field studies and experimental approaches. I explore how abiotic factors, microsite conditions, and plant-plant interactions influence treeline seedling dynamics in both environments.
In the Venezuelan Andes, precipitation seasonality at treeline elevations may critically influence tree seedling survival, yet scarce climate data availability limits our understanding of its effects. In the first study, I analyzed climate, microclimate, seedling demography, and ecophysiological responses of seedlings of the tree species Cybianthus marginatus and Clusia multiflora. I selected three sites in a dry treeline ecotone at 3100 m asl, using adjacent 10x10 m plots within the forest, forest border, and páramo environments. Seedlings did not occur in the páramo, while their density did not differ between the forest and the border. Survival was high in both forest and border plots, and growth rates were generally low but higher for Cybianthus at the forest border. Both species exhibited low assimilation and transpiration rates regardless of the environment. Seedlings showed spatial associations with moisture-regulating elements like mosses, ferns, and basal rosettes, suggesting they benefit from increased water availability in their neighborhood. These findings suggest a potentially slow forest expansion, with tree cover advancing as forest-border vegetation facilitates seedling establishment beyond current forest limits.
In the second study, conducted in the French Alps, I examined microsite preferences of the conifers Larix decidua, Pinus uncinata, and P. cembra in alpine treeline ecotones with different bedrock chemistry. I selected four sites, two with calcareous and two with siliceous bedrock, and compared 50 occupied microsites to 50 randomly selected controls. I assessed topography, substrate, ground cover, and shelter proximity, relating these characteristics to observations of individual health. The three species established in similar microsites, usually with some shelter, and only certain extreme microsite types remained unoccupied. Most individuals exhibited krummholz forms or were bent, showing minimal signs of recent mechanical damage, desiccation, or herbivory. These findings indicate that microsite availability does not limit seedling establishment in this study area, suggesting that seed availability likely plays a larger role. Despite harsh conditions limiting growth to tree stature, recent growth trends indicate potential for upward treeline shifts under favorable climatic conditions.
In the third study, I explored how seedlings of the treeline-forming species Larix decidua, Picea abies, Pinus uncinata, P. cembra, and Sorbus aucuparia respond to microclimatic modification in a field experiment in the French Alps. Seedlings from two consecutive cohorts were planted in 40 plots arranged in five blocks and exposed them to the following treatments: Day warming, Day warming + watering, Night warming, Night warming + shade, Shade, Watering, Vegetation cover and Control. Survival and biomass production of the two seedling cohorts, as well as chlorophyll fluorescence in the first cohort, was evaluated. Vegetation cover and warming negatively affected survival and biomass, while shading and increased water availability provided benefits in some cases. However, species and cohorts responded differently, likely due to interannual climatic variability. Chlorophyll fluorescence measurements indicated no severe photoinhibition. The findings show that, while current climatic conditions do not strongly limit seedling survival, plant-plant interactions play a key role in the expansion of the treeline ecotone. The study highlights the importance of interannual climatic variability for seedling survival and treeline dynamics.
This thesis illustrates the multidimensional controls on seedling establishment in treeline ecotones. While key abiotic factors -like temperature fluctuations and water availability - influence survival and growth, plant-plant interactions, both positive through environmental amelioration and negative through competition between grasses and seedlings, plays a key role in seedling survival of both Andean and Alpine treeline tree species. While microsite availability is not a limiting factor in the Alps, site-specific differences in seedling density in the Andes suggest that microsites availability and characteristics, particularly substrate cover, can enhance establishment. Long-term monitoring of seed availability and new recruitment, together with the inclusion of inter- annual climatic variability and strong seasonality in predictive models, could give a better understanding of the treeline dynamics under changing climatic conditions