Search

To obtain a better understanding of the functioning of a globally important biome, the BOREAS project set out to make measurements of the interaction between a number of boreal ecosystems and the atmosphere. This study is concerned with measurements made in one such ecosystem, the old-growth black spruce (OBS) site of the BOREAS Southern study area (SSA), located in Saskatchewan, Canada, during the period 1993-1996. This thesis focuses on efforts to understand the net ecosystem exchange of CO₂ and water vapour in terms of the sub-component fluxes operating within the ecosystem. The component fluxes of primary interest are photosynthesis, respiration and evapotranspiration of foliage, and the efflux of CO₂ from the forest floor. Woody biomass respiration and changes in the storage of CO₂ in the air mass within the ecosystem are also considered. Methodologies were developed to study these fluxes on a continuous basis. An "open" system gas exchange chamber for measuring soil CO₂ efflux was designed that eliminated the major problems that have been associated with this methodology in the past. The system was used to investigate temporal and spatial variation in soil CO₂ efflux at the field site. A scheme that integrated this temporal and spatial variation was used to estimate the CO₂ efflux from the forest floor for an entire year. Spatial variability in soil CO₂ efflux was high, and was related empirically to the thickness of the dead moss layer. Hour to hour variation was well described as an exponential function of soil temperature, and was significantly related to atmospheric turbulence.

Forests provide numerous crucial ecosystem services to society that are sensitive to climate and to extreme climate events such as droughts. One of the services of interest is the ability to sequester the greenhouse gas carbon dioxide (CO2). Species mixture and structural heterogeneity has been suggested to reduce the effect of climate variability on forests. It is important to understand how increasing diversity and heterogeneity will help dampen the effect of climate events on CO2 uptake. Currently, knowledge about the ability of different forests to resist or recover from the effect of climate events is limited. To mitigate this knowledge gap, this thesis examines the implications of forest structure on CO2 fluxes. Chapter 1 provides the general background of the topic. Chapter 2 examines the implications of structural diversity for seasonal and annual CO2 fluxes in two temperate deciduous forests for a period of 11 years. The two forest sites have similar mean stand age and near-identical climate conditions but different stand structure. The main question asked was how management and related structural diversity may affect CO2 fluxes. We show that the annual net ecosystem productivity (NEP) was on average 13 % higher in the managed, even-aged, and homogenous forest, than in the unmanaged, uneven-aged, and structurally diverse forest. The homogenous forest was observed to have, however, stronger sensitivities of seasonal NEP and gross primary productivity (GPP) to environmental variables. Chapter 3 relies on data from 21 Fluxnet sites to explore the effect of nine structural parameters on the temporal stability of light-saturated photosynthetic capacity (GPP1000) and on its resistance to changes in water availability during droughts. The study addresses two questions, (a) Do structurally diverse forests have lower variation in annual GPP1000? (b) Are structurally diverse forests more resistant to drought events? The results show that unmanaged forests and forests managed as high forests, which have higher basal areas and tend to be older and more diverse in size than coppice forests, had more stable annual GPP1000. The differences between individual sites in anomalies in GPP1000 in response to droughts were mostly explained by growing season air temperature. Forest structure could have influenced the response to droughts, but in our case the structure effect could not be separated from environmental effects. Chapter 4 presents a new model of soil water and related fluxes in forests, Forest Soil Water Model - FSWM, developed in the R environment. The model is suitable for predicting soil water in a wide range of forest soils. FSWM incorporates the Gash model for interception, the Ritchie model for soil evaporation and the Richards equation for soil water movement. FSWM’s performance was evaluated against soil water measurements at 12 sites. The model performance was good for deciduous broadleaf forests, moderate for mixed forests and evergreen needle leaf forests. FSWM offers flexibility in simulating soil horizons with different depths and it is helpful when comparing modelled with observed values at different soil depths. With these characteristics, FSWM is a flexible and freely available tool for ecosystem and hydrological research. Additionally, two co-author papers are included in the appendix. The first paper assessed the net ecosystem CO2 exchange (NEE), total evapotranspiration and net primary production of two neighbouring beech (Fagus sylvatica L.) forests in central Germany differing in site management. We found the interannual variability was higher in the managed, even-aged stand, and the unmanaged forest was a weaker sink of CO2 during a dry year. The second paper investigated the factors influencing the interannual variability (IAV) of photosynthetic capacity at light saturation, a key ecosystem functional property determining gross primary productivity. The study found that the older and species rich forest had reduced IAV of GPP1000. In general, the results of this thesis support the idea that unmanaged forests, mostly older and diverse, have lower interannual variability in NEP, GPP and GPP1000 as the result of their adaptation to the habitat by selecting appropriate species, developing structure to make best use of the light, water, and nutrient resources. During droughts, the effect of the forest structure was not clear. More research covering a large range of different sites is still required to get definitive results involving more structural attributes and sites from different climates.

In Northern Nigeria, the savannah parklands are mixed dryland agroecosystems having landscape productivity and deforestation challenge, induced by the changing climate and affecting sustainable livelihoods. The productivity of parkland is attributed to different factors such as erratic precipitation, drought and plant mortality. Here, the study predicts the impact of climate change on tree regeneration on parklands of three savannah agroecological zones (AEZs), namely the Sudan savannah (SS), Northern Guinea savannah (NGS) and Southern Guinea savannah (SGS) AEZs across a north-south transect using a multidisciplinary approach. The approach involves the employment of Random Forest model framework to predict the future shift in AEZs from the current climate using bioclimatic variables and climatic scenarios. Three random predicted locations within each studied AEZ identified as drought-threatened areas were selected for tree preference and identification study by analysing local farmer ethnobotanical knowledge of their favourite species, before tree identification and estimation of diversity and abundance of the current parkland species on farms in the respective local communities. The research further predicted the future distribution of selected important species using Maximum Entropy and assess the efficacy of simple propagation methods (cuttings and marcotting) on respondents’ farms, after an extreme drought survival test in a greenhouse. The results showed that agroecological zones are getting warmer and drier southwards, replacing the current NGS and SGS with SS between 2050 and 2070, by up to 65%. Parkia biglobosa, Mangifera indica and Vitellaria paradoxa are the top-ranked species by 90% of the 92 respondents in the study and are the most abundant species on farmers’ field with relative species dominance ranging from 19.61-42.64%. The strong relationship between species preference and abundance increases northwards. On the future distribution and improvement of parkland species to survive under stress conditions. Maximum entropy model predicted a north-eastern shift for future spatial species distribution of Parkia biglobosa and Vitellaria paradoxa under most extreme climate scenario. For tree species at jvenile stage to survive under 6 months drought regime, the research indicated that 9months old Parkia biglobosa seedling from SGS resisted more than other species at different ages from different AEZ species. On local adaptation strategy to improve parklands survival, there were up to 40% and 38% success rate of vegetative propagation of Anarcadium occidentale using cuttings and marcotting, respectively.

Mangroves are an assemblage of salt-tolerant trees and plants that exist in the intertidal coastal zones of countries in the tropics and sub-tropics. Healthy mangroves can greatly assist the sustainable existence of local coastal villagers because of the wealth of ecosystem goods and services they provide. However, a significant proportion of this ecosystem has been degraded or converted to other land uses. Many government mangrove agencies are realising that because of the diffuse nature of this ecosystem and limited government resources, they need to work with local communities that are based within or near these forests, encouraging some form of community management and involvement to counter mangrove losses. The overall goal of this research was to explore some of the barriers that inhibit successful community mangrove management, across two countries in Southeast Asia in order to contribute to the discussion about how best to enable this process. The first objective was to assess whether communities were able to rehabilitate mangroves successfully and the role of government assistance within this. The second objective was to study in a more qualitative, detailed manner how villagers carried out this rehabilitation work and to identify the knowledge and understanding that underpinned their decisions, contrasting these with actual outcomes. Environmental organisations typically encourage protection of existing mangroves over rehabilitation of degraded areas because rehabilitation projects have uncertain outcomes, and the full suite of ecosystem benefits are only provided by mature stands. Therefore, a third objective was to explore how communities preserve their own mangroves and inhibit cutting of mangrove through the development of their own management rules, or the use of national law. A final objective was to help independent organisations to assist mangrove communities by suggesting a simple method that would allow comparison and therefore ranking of the status of mangroves across a group of communities, to indicate which were most in need of, and likely to benefit from, outside assistance. Outcomes from the first two objectives suggested that inappropriate targets, set centrally by mangrove agencies, together with gaps in villager and mangrove agency field office staff knowledge of mangrove ecosystems, have led to sub-optimal rehabilitation outcomes. Almost all rehabilitation projects relied on planting rather than assisting natural regeneration, and much of this planting proved either unnecessary or was conducted in inappropriate locations. There was confusion about the suitability of mudflats for planting, normally considered below an appropriate tidal elevation for mangrove establishment. Effective tidal flushing and drainage was demonstrated to have a significantly positive effect on planting results. Villagers were aware of the possibility of rehabilitating some sites simply by improving the hydrology, but this appeared not to result in activities on site to improve site topography or hydrology. Consideration of the community management rules of this sample of villages suggested that many of the principles suggested by terrestrial community forest researchers hold true in a mangrove environment, particularly the need for strong social capital and effective leadership, but as has been suggested previously, local context and parameters play such a significant role that the wider application of these conclusions should be done with caution. Finally, to help external organisations that wish to assist mangrove communities, I have suggested a series of indicators for the development of mangrove quality and sustainability criteria, that when combined with other bio-physical measures, are easier to aggregate and assess than some of the existing terrestrial forest indicators. A suggested indicator weighting system is proposed, which with further testing in mangrove ecosystems other than river deltas, might provide a simple way to prioritise the location of mangrove management interventions.

The aim of this study was to evaluate some of the physical and bioeconomic potentials of a silvopastoral agroforestry system with focus on the Henfaes Silvopastoral Systems Experimental Farm (SSEF) of Bangor University, North Wales. The study reviewed research studies written on the SSEF from 1992 to 2012; assessed changes in pasture species composition and abundance since establishment; developed allometric equations for the estimation of aboveground biomass (AGB), carbon (C) stock and carbon dioxide (CO2) emission potentials of red alder (Alnus rubra Bong); studied the effect of tree/solar radiation on pasture productivity and quality; and conducted bioeconomic analysis to compare treeless pasture/livestock, forestry, and agroforestry scenarios. Review of the research studies show that as far as can be determined 66 research studies were conducted on ecosystem services of the UK’s Silvopastoral National Network Experiment (SNNE) and temperate Europe during the period 1992 - 2012. These papers were sourced mainly from the Henfaes SSEF, the UK’s SNNE, other UK and, other European research sites. The studied ecosystem services dealt with provisioning services (40%), regulating services (13%), and supporting services (47%). The scientific domains addressed include timber or wood-fuel potential (20%), pasture/livestock management (20%), biodiversity (20%), carbon sequestration (13%), water management (15%), and soils (12%). The response of pasture species to thinning varied. The percentage composition by weight of the sown species declined, while that of the grass weeds and the forb weeds increased slightly one year after thinning (2013 – 2014) compared to the adjacent open pastures. The change was not statistically significant. The understory pasture species composition, abundance and diversity changed significantly 20 years (1992 – 2012) after the establishment of the Silvopastoral National Network Experiment at Henfaes. Generally, pasture on the three red alder blocks was found to be largely grass weeds (46-48%) followed by forbs or broadleaf weeds while the sown species declined significantly. In 2012, 20 years after field planting, the mean AGB were found to vary from 130 kg tree-1 (26 Mg ha-1) to 246 kg tree-1 (49 Mg ha-1) in poor form and good form red alder trees, respectively, based on a stocking density of 200 stems ha-1. Mean C stock was 65 kg C tree-1 (13 Mg C ha-1) in poor form trees and 123 kg C tree-1 (25 Mg C ha-1) in good form trees. Mean CO2 potential was 237 kg CO2 tree-1 (48 Mg CO2 ha-1) in poor form trees and 450 kg CO2 tree-1 (90 Mg CO2 ha-1) in good form trees. Pasture productivity increased significantly with increasing solar transmission, and with increasing distance from each grazing exclusion cage to the nearest alder tree. Concentration and availability of CP, ADF, NDF and ME were greater in the with-leaves than in the without-leaves growing seasons in response to variation of photoperiod (the duration of sunshine/day length) in the United Kingdom. The bioeconomic analysis considered three land-use plausible scenarios (‘forestry’, ‘pasture / livestock’ and ‘agroforestry’) and found that, in the absence of grants/subsidies, none were viable. However, application of grants/subsidies, at the baseline assumptions, revealed that forestry was the most viable option with the highest net present value and annual equivalent value, followed by pasture/livestock and agroforestry options.

Accurately quantifying land-atmosphere exchanges is essential at every spatial scale, from aiding a better understanding of climate change globally to informing land management decisions at the smallest scale (e.g. agricultural land management). This quantification may be dealt with relatively easily for homogeneous land surfaces, but in the real world, landscapes are spatially heterogeneous and simple approaches are often inadequate. This thesis uses mathematically advanced methods and/or models to find robust solutions to landatmosphere exchange problems that accommodate spatial heterogeneity. A two-stage sampling strategy (2SS) was developed to reduce the uncertainties in the estimation of chamber-based GHG fluxes when sample size is inadequate to fully capture spatial heterogeneity. A Monte Carlo simulation showed that 2SS improves the estimation of soil GHG fluxes in all but the most homogeneous situations, with the improvement being directly related to the amount of spatial heterogeneity present. EC-based measurements of GHG fluxes invariably contain data gaps that require filling to generate long-term cumulative fluxes, i.e. integrating over a temporally heterogeneous timeseries. Gap-filling methods introduce uncertainty. A robust method based on image inpainting is introduced to fill gaps via a two-dimensional representation of a onedimensional data, i.e. the flux fingerprint. Results show that this unsupervised method, using a more compact and simple form, compares favourably with a widely-used traditional method and can outperform it when applied to de-noised data. The most robust measurements of surface carbon fluxes will be generated when using two independent measurement methods simultaneously. To investigate CO2 and CH4 fluxes from a heterogeneous fen, EC- and chamber-based measurements of surface carbon fluxes were implemented from 2013 to 2015. To implement a direct comparison between these measurements made at differing scales, the chamber-measured data were up-scaled, both temporally by model-based interpolations and spatially by flux footprint modelling. Results show a good linear correlation in CO2 flux and a near zero correlation in CH4 flux between methods. Further analysis on CH4 flux, however, show that the two differed only by a Gaussian distribution, implying the existence of white noise in the signal. The cumulative CO2 flux for the whole season measured by chambers was -376.5 g/m2, 33% higher than the estimated measured by EC (-281.8 g/m2). Similarly, the final cumulative CH4 flux was 4.01 g/m2 by chamber-based estimates, 43% more than EC (2.81 g/m2). The final part of this study investigates the surface flux of momentum in a structured heterogeneous land surface. A logarithmic normal distribution was developed to model the wind speed reduction around a tree-based windbreak. The model showed an excellent fit to field observations made at a real-world windbreak on farm land. A graphical method that describes a 3-d space of wind-chill temperature vs. ambient temperature and wind speed was created to quantify the potential thermal benefits gained by introducing windbreaks and reducing wind speed. The wind-chill thermal tolerance (WTT) of sheep was estimated and compared for a lowland and an upland site. Distinct differences to reduced wind speed were found between the sites, with greater thermal benefits at the upland site. The methods and models generated and developed in this study contribute to an improved quantification of land-atmosphere exchanges, and have potential to be applied to surface fluxes generally, either of mass (GHGs) or energy (heat, momentum), and to landscapes other than those dominated by vegetation. For example, the statistical idea of the two-stage sampling approach provides a generic solution to sample size deficiency in heterogeneous land surfaces; The inpainting-based gap-filling method, as an image processing technique, may be applicable to any signals that can be represented as an image, i.e. a two-dimensional space in which individual locations (pixels) have numerical attributes that can be used as RGB values; The WTT plot/analysis, used here in the context of sheep in upland sites, provides an intuitive and powerful scheme for analysing the thermal tolerance of any animal in any energetically heterogeneous landscape.

Integrated pest management calls for new biocontrol solutions in management of forest diseases. Endophytic fungi that are commonly found in tree tissue may have potential in biocontrol. However, the links between endophyte status and disease tolerance are still unclear, and we know little about the mechanisms by which the endophytes can influence tree pathogens. The first goal of the thesis was to compare the endophyte status in elm (Ulmus spp.) trees with low vs. high susceptibility to Dutch elm disease (DED), caused by Ophiostoma fungi, and to find correlations between endophytes and the susceptibility pattern of the trees. The second goal was to investigate the potential mechanisms of antagonism by the endophytes towards the pathogen. Thus, endophytes were isolated from leaves, bark and xylem of elms that differed in DED susceptibility. The isolates were screened for their potential to counteract the pathogen in dual cultures. Selected strains were investigated using Phenotype MicroArrays to obtain the substrate utilization profiles that reflect the endophytes’ ability to compete with the pathogen for a nutritional niche. To test for a protective effect against the disease, promising isolates were injected into young elms. Preliminary analyses were done to identify the extracellular chemicals that some of the endophytes released into the growth medium. The results showed that the frequency and diversity of endophytes was higher in xylem of elms with high susceptibility to DED. Some endophytes deadlocked the pathogen with extracellular chemicals in vitro, while others had a faster growth rate. Several endophytes were able to utilize substrates more effectively than the pathogen. A preventive treatment with endophytes protected elms against DED, but the effect was unstable across years. Bioactive fungal extracts had a complex chemical profile, and the individual compounds in the extracts remain to be identified. Because endophytes antagonized the pathogen through different mechanisms, I suggest that an endophytebased biocontrol of DED could be best achieved through a synergistic effect of several endophyte strains.