Tree seedling recruitment dynamics in forest-savanna transitions : Trait responses to vegetation controls mediate differential seedling establishment success of tree functional types

In the transition between tropical forests and savannas, forest may be lost to savanna-like vegetation or may occasionally encroach on savannas affecting climate and other benefits to humans. Vegetation in the forest savanna-transition of Ghana often appears stable, except in forests with significant human-induced deforestation where forests are being replaced by savanna-like vegetation. This phenomenon is still poorly understood, but may be explained by how changes in vegetation controls (e.g. fire and precipitation) affect recruitment of forest and savanna-transition tree seedlings differently. Savanna-transition species occur both in forest and humid savanna. I showed that high grass biomass in savanna both directly (via competition) and indirectly (via dry season fire) select for species which invest higher in belowground resource capture and carbohydrate storage. Lower precipitation decreases chances of tree seedling recovery from defoliation disturbance, but allocation trait differences between forest and savanna-transition tree species may explain relative stability of transition forests and the lack of success of true forest species in the forest-savanna transition

Using Remote Sensing and Biogeographic Modeling to Understand the Oak Savannas of the Sheyenne National Grassland, North Dakota, USA

Oak savannas are valuable and complex ecosystems that provide multiple ecosystem goods and services, including grazing for livestock, watershed regulation, and recreation. These ecosystems of the woodland-prairie ecoregion of the Midwestern United States are, however, in danger of disappearing. The Sheyenne National Grassland, North Dakota, a protected Prairie grassland-savanna, is a representative of such rare habitats, where oak savanna is found at the landscape scale. In this research, I map the distribution patterns of oak savanna in the Sheyenne using a combination of remote sensing and geospatial datasets, including landscape topography, soils, and fire disturbance. Further, I interpret the performance of a suite of advanced Species Distribution Modeling approaches including Maximum Entropy, Random Forest, Generalized Boosted Model, and Classification Tree to analyze the primary environmental and management factors influencing oak distributions at landscape scales. Woody canopy cover was estimated with high classification accuracy (80-95%) for two study areas of the Sheyenne National Grassland. Among the four species distribution modeling approaches tested, the Random Forest (RF) approach provided the best predictive model. RF model parameters indicate that oak trees favor gently sloping locations, on well-drained upland and sandy soils, with north-facing aspect. While no direct data on water relationships were possible in this research, the importance of the topographic and soil variables in the SDM presumably reflect oak preference for locations and soils that are not prone to water saturation, with milder summer temperatures (i.e. northern aspects), providing conditions suitable for seedling establishment and growth. This research increases our understanding of the biogeography of Midwestern tall-grass oak savannas and provides a decision-support tool for oak savanna management

Effects of restoration on Midwestern oak savanna biodiversity, structure, and oak regeneration

Over the last century, savannas throughout the world have been encroached by woody plants, altering over- and understory plant composition and distributions of understory resources. This dissertation investigates woody encroachment removal from Midwestern oak savannas, using a large-scale restoration experiment in Iowa. Four sites received a woody encroachment removal treatment (restoration) and four sites were retained as encroached controls. Chapter two details impacts on woody plant regeneration dynamics. Encroachment removal restored savanna canopy structure and overstory dominance by Querucs (oak) species; however, advanced regeneration was dominated by encroaching species within three years. I suggest that the encroached savannas represent an alternative stable state and that further management actions, potentially involving prescribed fire, will be necessary to maintain the savanna state.;Chapter three investigates spatiotemporal effects of encroachment removal on understory biodiversity. Restoration sites had increased alpha (within sample) Simpson\u27s diversity and alpha and gamma (site-level) species richness relative to control sites, while gamma and beta (among-sample) Simpson\u27s diversity, beta richness, and alpha species evenness were not affected. These changes were driven by widespread establishment of new species at the site-level (notably graminoids) and within-site proliferation of pre-existing species (predominantly graminoids and woody species). I highlight the utility of restoration experiments, like this one, for conducting research on multi-scale processes, such as species diversity.;Chapter four assesses development of understory resource and vegetation gradients. I found that encroachment removal restored light and soil moisture gradients and that these gradients were important for structuring post-restoration plant communities. The savannas in this study appear to be remarkably resilient to degradation, as important biophysical gradients were reestablished within years of restoration, even after decades of encroachment. These results are encouraging for future restoration at these sites and for woody encroachment removal efforts elsewhere.;Chapter five determines impacts of encroachment removal on patterns of Quercus alba (overstory dominant tree species) seedling success. Seedlings had greater survival and growth parameters in treatment sites, with generally better performance at further distances from trees. Thus, the mesic savannas in this study appear inherently unstable, as seedling recruitment is promoted in inter-canopy gaps. These results further support chapter two\u27s conclusion that the encroached savannas represent an alternative stable state

The role of economic, venation and morphological leaf traits in plant and ecosystem function along forest-savanna gradients in the tropics

Predictions about the impacts of current and projected anthropogenic climate change on tropical forest ecosystems require a better understanding of sensitivity and variability of plant attributes to biotic and abiotic factors. Leaf functional traits are particularly informative because they determine carbon and water fluxes between plants and atmosphere. However, due to the lack of data in the tropics, especially, in Africa, many uncertainties surround the interpretations of trait-environment relationships as well as trait-trait trade-offs and their importance on plant and ecosystem functioning. Therefore, the principal aim of the thesis was to better understand, and test widely held assumptions surrounding the functional significance of key leaf economic, morphological and venation traits in the tropical forest ecosystems. The data was collected for 90 angiosperm adult tree species along a strong environmental gradient (rainfall, fertility, midday temperature and vegetation) in Ghana, West Africa and presented in four data chapters. The first data chapter of the thesis (Chapter 4) focuses on the links between leaf photosynthetic capacity and key nutrients along a vegetation gradient from forest to savanna. This is important because these relationships are employed in the modelling of global primary productivity, with great uncertainties evident across the tropical forest-savanna boundaries. To allow for better interpretation of the results, this chapter includes an additional data set of neotropical forest-savanna boundary in Cerrado, Brazil. Special attention is paid towards patterns of nitrogen allocation between leaf structure and photosynthetic apparatus. In contrary to the expectations, the key findings of this chapter suggest that studied species do not maximise photosynthetic capacity per available foliar nitrogen, phosphorus or potassium due to adaptation to intermittent water availability in these strongly seasonal environments. The two following data chapters explore morphological adaptations of leaves to the extremes of the studied environmental gradient. Chapter 5 examines the drivers and functions of leaf morphology, specifically, leaf shape. Based on the leaf energy budget theory, the findings of this chapter demonstrate the importance of narrow and elongated leaves in thermoregulation and maintenance of photosynthetic rates in the savanna study plot, which is characteristic of high irradiance, midday temperatures and seasonal water deficits. Additionally, this chapter suggests that narrow and elongated leaf blades are also important for optimising canopy light capture in shaded canopies of wet evergreen forests. Including leaf shape metrics in plant performance modelling can improve understanding of ecosystem response to projected increases in and extreme temperatures. The penultimate data chapter of the thesis (Chapter 6) explores the role and diversity of functional traits that describe leaf venation network. The data presented in this chapter test and support the hypothesis that minor vein traits play an important hydraulic function that underlies plants adaptation/acclimation to intermittent water availability. This is seen in denser and more reticulated networks present at the driest end of the gradient and strong links between minor veins and potassium (important nutrient for osmoregulation), leaf size, leaf deciduousness and wood density. These relationships were shown to be affected more significantly by environmental filtering than by phylogenetic relatedness among species. The final data chapter (Chapter 7) joins the recurring themes of the thesis by assessing the relationships between studied leaf economic, morphological and minor venation trait dimensions and plant and community ecological strategies. Leaf functional traits clustered along three significant ecophysiological axes of variability related to gas exchange, structure and water transport. Additionally, the findings highlight the role that leaf functional traits play in the ecosystem functioning as leaf trait dimensions relate to aboveground net primary productivity and biomass. Finally, this chapter emphasises the need for more large-scale studies linking functional traits with plant and ecosystem function at different scales of tropical forests. Such studies would enable the development of a significant mechanistic understanding of what creates and drives functional trait variability across tropical forests and in modelling their effects on water and carbon fluxes in tropical forest ecosystems

How climate, topography, soils, herbivores, and fire control forest–grassland coexistence in the Eurasian forest-steppe

Recent advances in ecology and biogeography demonstrate the importance of fire and large herbivores - and challenge the primacy of climate - to our understanding of the distribution, stability, and antiquity of forests and grasslands. Among grassland ecologists, particularly those working in savannas of the seasonally dry tropics, an emerging fire-herbivore paradigm is generally accepted to explain grass dominance in climates and on soils that would otherwise permit development of closed-canopy forests. By contrast, adherents of the climate-soil paradigm, particularly foresters working in the humid tropics or temperate latitudes, tend to view fire and herbivores as disturbances, often human-caused, which damage forests and reset succession. Towards integration of these two paradigms, we developed a series of conceptual models to explain the existence of an extensive temperate forest-grassland mosaic that occurs within a 4.7 million km(2) belt spanning from central Europe through eastern Asia. The Eurasian forest-steppe is reminiscent of many regions globally where forests and grasslands occur side-by-side with stark boundaries. Our conceptual models illustrate that if mean climate was the only factor, forests should dominate in humid continental regions and grasslands should prevail in semi-arid regions, but that extensive mosaics would not occur. By contrast, conceptual models that also integrate climate variability, soils, topography, herbivores, and fire depict how these factors collectively expand suitable conditions for forests and grasslands, such that grasslands may occur in more humid regions and forests in more arid regions than predicted by mean climate alone. Furthermore, boundaries between forests and grasslands are reinforced by vegetation-fire, vegetation-herbivore, and vegetation-microclimate feedbacks, which limit tree establishment in grasslands and promote tree survival in forests. Such feedbacks suggest that forests and grasslands of the Eurasian forest-steppe are governed by ecological dynamics that are similar to those hypothesised to maintain boundaries between tropical forests and savannas. Unfortunately, the grasslands of the Eurasian forest-steppe are sometimes misinterpreted as deforested or otherwise degraded vegetation. In fact, the grasslands of this region provide valuable ecosystem services, support a high diversity of plants and animals, and offer critical habitat for endangered large herbivores. We suggest that a better understanding of the fundamental ecological controls that permit forest-grassland coexistence could help us prioritise conservation and restoration of the Eurasian forest-steppe for biodiversity, climate adaptation, and pastoral livelihoods. Currently, these goals are being undermined by tree-planting campaigns that view the open grasslands as opportunities for afforestation. Improved understanding of the interactive roles of climate variability, soils, topography, fire, and herbivores will help scientists and policymakers recognise the antiquity of the grasslands of the Eurasian forest-steppe

Biotiske og abiotiske faktorers rolle for dynamikk i tregrenseøkotonen : studier basert på feltobservasjoner og fjernmåling

With global warming, the forest extent is expected to change, and the transition zone between the upper forest boundary and treeless alpine landscape (i.e., the treeline ecotone) is expected to migrate to higher elevations. This will influence the coupling between the land and atmosphere by altering climate conditions and feedbacks. Climate, and especially temperature, is thought to be the main driver of treeline ecotone elevation. However, the factors affecting the treeline ecotone are complex and their relative roles in treeline ecotone dynamics are not fully understood. Remote sensing by airborne laser scanning (ALS) and unmanned aerial vehicles (UAVs) can be useful for acquiring data with complete coverage of various factors such as topography and vegetation for increasing our understanding of treeline shifts. In this thesis, the main research objective was to investigate the relative role of climate, herbivory, topography, and vegetation on treeline ecotone dynamics. To do this, we used data acquired along an 1,100 km latitudinal transect covering 36 treeline ecotones in Norway (59.9–69.5 °N). In addition, we also used data from a treeline ecotone site in Norway with experimental herbivore density treatments. By using data recordings on trees acquired from the latitudinal transect and experimental site, we examined the relative role of temperature and herbivory on treeline ecotone dynamics. Using UAV survey imagery and ALS together with field observations on vegetation, we investigated the potential of using remote sensing for treeline ecotone land cover classification. Further, using the resulting land cover classification maps, we aimed to disentangle the roles of climate and herbivory in addition to site-specific factors such as topography and vegetation on tree establishment in the treeline ecotone. We found that climate, and especially temperature, explained more of the variation in tree performance than herbivory along the latitudinal gradient. However, the roles were reversed at the experimental site, as the different herbivore density treatments clearly affected tree prevalence and annual radial growth. Thus, the role of temperature and herbivory seem to depend on the life stage class of the tree. The models for predicting land cover classes had overall high prediction accuracies, and field vegetation data can thus be upscaled using UAV imagery. The resulting land cover maps together with the ALS-derived topographic variables were also important in explaining the variation in tree establishment along the latitudinal gradient. However, considering on-site variations was not enough to predict treeline ecotone dynamics, thus suggesting context dependency. The findings presented in this thesis stress the importance of taking multiple factors into account when explaining variations in treeline ecotones. This can help explain differences in treeline ecotone migrations in the last decade. The importance of both winter and summer precipitation and moisture availability together with temperature should be investigated further. More accurate herbivore data is needed to quantify the relationship between browsing and tree establishment along the latitudinal gradient. Even if predicting treeline ecotone dynamics is challenging, it is important to monitor the treeline ecotone. Detecting short-term changes across spatial scales can be done using various remote sensing platforms and sensors. It is also important to understand the consequences of upward treeline shifts, such as changes in albedo and carbon storage, which likely further will affect climate change.Med global oppvarming forventes det at skogens utbredelse vil endre seg, og overgangen mellom den øvre skoggrensen og åpne fjellområder (dvs. tregrenseøkotonen) forventes å flytte seg oppover i fjellet. Dette vil påvirke koblingen mellom jorda og atmosfæren ved å endre på klimaets tilstand og tilbakekobling. Klima, og spesielt temperatur, anses som hovedpådriveren av tregrenseøkotonens høyde over havet. Til tross for dette, er faktorene som påvirker tregrenseøkotonen komplekse, og deres relative rolle for dynamikken i tregrenseøkotonen er ikke fullstendig forstått. Fjernmåling ved bruk av flybåren laserskanning (FLS) og droner kan være nyttige for å innhente data med heldekkende informasjon om ulike faktorer som terreng og vegetasjon, for å øke vår kunnskap om tregrenseforflytninger. I denne avhandlingen var forskningsformålet å undersøke den relative rollen til klima, beiting, terreng og vegetasjon på dynamikken i tregrenseøkotonen. For å gjøre dette, brukte vi data samlet inn fra 36 lokaliteter langs en 1100-km lang breddegradient i Norge (59.9–69.5 °N). I tillegg brukte vi data fra én tregrenseøkotonlokalitet i Norge med eksperimentelle beitetetthets-behandlinger. Ved å bruke data samlet inn fra trær fra de ulike lokalitetene, undersøkte vi den relative rollen til temperatur og beiting for dynamikk i tregrenseøkotonen. Vi undersøkte også om data samlet fra droneflyvninger og FLS kunne oppskalere feltobservasjoner på trær og vegetasjon for å klassifisere naturtyper i tregrensa. Videre, ved å bruke de resulterende naturtypekartene, hadde vi som mål å identifisere betydningen av klima og beiting i tillegg til lokale faktorer som terreng og vegetasjon, for etablering av trær i tregrenseøkotonen. Vi fant ut at klima, og spesielt temperatur, forklarte mer av variasjonen i trærnes utvikling enn beiting. Til tross for dette, var rollene byttet om på den eksperimentelle lokaliteten ettersom de ulike beitedyrtetthetene hadde klare påvirkninger på trærnes forekomst og årlige stammevekst. Resultatene tyder derfor på at effekten av temperatur og beiting avhenger av trærnes livsstadier. Modellene for å lage naturtypekart i tregrensa ga generelt sett presise prediksjoner, og feltobservasjoner av vegetasjon kan derfor oppskaleres ved å bruke dronebilder. Naturtypekartene, sammen med terrengvariabler fra FLS, var i tillegg til ulike klima- og beitevariabler viktige for å forklare variasjon i etablering av trær i tregrenseøkotonen. Til tross for dette, var ikke det å inkludere lokale faktorer nok til å predikere dynamikken i tregrenseøkotonen, og dette tyder derfor på at faktorenes påvirkning avhenger av kontekst. Resultatene i denne avhandlingen understreker viktigheten av å inkludere flere faktorer for å forklare forskjeller mellom ulike tregrenseøkotoner. Dette kan hjelpe med å forklare forskjellene i høydeforflytninger de siste tiårene. Viktigheten av både vinter- og sommernedbør og tilgjengelighet av vann sammen med temperatur burde undersøkes videre. Mer presise beitedata trengs for å kunne kvantifisere forholdet mellom beiting og etablering av trær langs breddegradienten. Selv om det er utfordrende å predikere dynamikk i tregrenseøkotonen, så er det viktig å overvåke økotonen. Detektering av endringer over relativt kort tid over ulike romlige skalaer kan gjøres ved å bruke ulike fjernmålingsplattformer og -sensorer. Det er også viktig å undersøke konsekvensene av en forflytning av tregrenseøkotonen, som endringer i albedo og karbonlagring, som sannsynligvis vil videre påvirke klimaendringene

Herbivores reduce seedling recruitment in alpine plant communities

publishedVersio

Herbivores reduce seedling recruitment in alpine plant communities

publishedVersio

Carbon fluxes in a mature deciduous forest under elevated CO₂

This PhD thesis addressed several major aspects of the carbon (C) cycle in a c. 100-year-old, mixed deciduous forest under elevated CO₂ with an emphasis on below-ground processes. The aim was to assess the responses of tree fine roots and soil respiration to canopy CO₂ enrichment (? 550 ppm) in this tallest forest studied to date. Furthermore, leaf gas-exchange of the five study species was examined to ascertain the long-term response of photosynthetic carbon uptake to elevated atmospheric CO₂. Investigations at the Swiss Canopy Crane (SCC) experimental site were guided by the following key questions: (1) Does below-ground C allocation to fine root production increase in response to CO₂ enrichment in order to acquire more nutrients to match the enhanced C supply in the forest canopy? (2) Is below-ground metabolism enhanced and therefore forest soil respiration stimulated by canopy CO₂ enrichment? (3) Is leaf-level photosynthesis persistently stimulated by elevated CO₂ in this stand or had these mature broad-leaved trees reduced their carbon up- take by photosynthetic down-regulation under long-term CO₂ enrichment? Findings from earlier studies at the SCC site, including 13C isotope tracing, all point towards an in- creased flux of C through CO₂-enriched trees to the soil but neither fine root biomass nor soil respiration were stimulated by elevated CO₂. Surprisingly, fine root biomass in bulk soil and ingrowth cores showed strong reductions by ? 30% in year five and six but were unaffected in the following seventh year of CO₂ enrichment. Given the absence of a positive biomass response of fine roots, we assumed that the extra C assimilated in the CO₂-enriched forest canopy was largely respired back to the atmosphere via increases in fine root and rhizosphere respiration and the metabolization of increased root derived exudates by soil microbes. Indeed, 52% higher soil air CO₂ concentration during the growing season and 14% greater soil microbial biomass both in- dicated enhanced below-ground metabolism in soil under CO₂-enriched trees. However, this did not translate into a persistent stimulation of soil respiration. At times of high or continuous precipitation soil water savings under CO₂-exposed trees (resulting from reduced sapflow) led to excessive soil moisture (> 45 vol.-%) impeding soil gas-exchange and thus soil respiration. Depending on the interplay between soil temperature and the consistently high soil water content in this stand, instantaneous rates of soil respiration were periodically reduced or increased under elevated CO₂ but on a diel scale and integrated over the growing season soil CO₂ emissions were similar under CO₂-enriched and control trees. Soil respiration could therefore not explain the fate of the extra C. The lacking sink capacity for additional assimilates led us to assume downward adjustment of photosynthetic capacity in CO₂-enriched trees thereby reducing carbon uptake in the forest canopy. Photosynthetic acclimation cannot completely eliminate the CO₂-driven stimulation in carbon uptake, but a reduction could hamper the detection of a CO₂ effect considering the low statistical power inevitably involved with such large-scale experiments. However, after eight years of CO₂ enrichment we found sustained stimulation in leaf photosynthesis (42-49%) indicating a lack of closure in the carbon budget for this stand under elevated atmospheric CO₂