Measuring Woody Encroachment along a Forest–Savanna Boundary in Central Africa

Mitchard, Edward T. A.; Saatchi, Sassan; Gerard, F.; Lewis, Simon L.; Meir, Patrick

doi:10.1175/2009ei278.1

Cited by 93 publications

(98 citation statements)

References 54 publications

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“…First, we showed the value of multi-temporal L-band SAR backscatter for estimation of vegetation height. It is well known that, depending on forest structure, L-band SAR backscatter saturates in dense forests at biomass levels around 100 t/ha [65][66][67]. Our results indicate that including more L-band observations in a statistical model can partly help to reduce under-and overestimation at low and high ranges of a forest parameter, respectively (Figure 8).…”

Section: Discussion and Summarymentioning

confidence: 63%

Potential of Multi-Temporal ALOS-2 PALSAR-2 ScanSAR Data for Vegetation Height Estimation in Tropical Forests of Mexico

et al. 2018

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Information on the spatial distribution of forest structure parameters (e.g., aboveground biomass, vegetation height) are crucial for assessing terrestrial carbon stocks and emissions. In this study, we sought to assess the potential and merit of multi-temporal dual-polarised L-band observations for vegetation height estimation in tropical deciduous and evergreen forests of Mexico. We estimated vegetation height using dual-polarised L-band observations and a machine learning approach. We used airborne LiDAR-based vegetation height for model training and for result validation. We split LiDAR-based vegetation height into training and test data using two different approaches, i.e., considering and ignoring spatial autocorrelation between training and test data. Our results indicate that ignoring spatial autocorrelation leads to an overoptimistic model's predictive performance. Accordingly, a spatial splitting of the reference data should be preferred in order to provide realistic retrieval accuracies. Moreover, the model's predictive performance increases with an increasing number of spatial predictors and training samples, but saturates at a specific level (i.e., at 12 dual-polarised L-band backscatter measurements and at around 20% of all training samples). In consideration of spatial autocorrelation between training and test data, we determined an optimal number of L-band observations and training samples as a trade-off between retrieval accuracy and data collection effort. In summary, our study demonstrates the merit of multi-temporal ScanSAR L-band observations for estimation of vegetation height at a larger scale and provides a workflow for robust predictions of this parameter.

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Section: Discussion and Summarymentioning

confidence: 63%

Potential of Multi-Temporal ALOS-2 PALSAR-2 ScanSAR Data for Vegetation Height Estimation in Tropical Forests of Mexico

et al. 2018

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“…I have also discussed examples where this general understanding does not apply. In particular, in fragmented forest-savanna ecotones human ignitions can potentially impact the extent and frequency of fire with important implications for the conservation and management of these systems [87].…”

Section: Conclusion and The Futurementioning

confidence: 99%

Managing the human component of fire regimes: lessons from Africa

Archibald

2016

Phil. Trans. R. Soc. B

172

145

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One contribution of 24 to a discussion meeting issue 'The interaction of fire and mankind'. Human impacts on fire regimes accumulated slowly with the evolution of modern humans able to ignite fires and manipulate landscapes. Today, myriad voices aim to influence fire in grassy ecosystems to different ends, and this is complicated by a colonial past focused on suppressing fire and preventing human ignitions. Here, I review available evidence on the impacts of people on various fire characteristics such as the number and size of fires, fire intensity, fire frequency and seasonality of fire in African grassy ecosystems, with the intention of focusing the debate and identifying areas of uncertainty. Humans alter seasonal patterns of fire in grassy systems but tend to decrease total fire emissions: livestock have replaced fire as the dominant consumer in many parts of Africa, and fragmented landscapes reduce area burned. Humans alter the season and time of day when fires occur, with important implications for fire intensity, tree-grass dynamics and greenhouse gas (GHG) emissions. Late season fires are more common when fire is banned or illegal: these later fires are far more intense but emit fewer GHGs. The types of fires which preserve human livelihoods and biodiversity are not always aligned with the goal of reducing GHG concentrations. Current fire management challenges therefore involve balancing the needs of a large rural population against national and global perspectives on the desirability of different types of fire, but this cannot happen unless the interests of all parties are equally represented. In the future, Africa is expected to urbanize and land use to intensify, which will imply different trajectories for the continent's fire regimes.This article is part of the themed issue 'The interaction of fire and mankind.

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“…Expansion is the most conspicuous in landscapes with mosaics of grasslands and forests. Gallery forests (along riparian areas) and other forest patches have expanded into flammable grassy ecosystems in South America [55], North America [56,57], Australia [58,59] and different parts of Africa [60][61][62][63]. Forest expansion seems to be the most common in wetter climates [59].…”

Section: Forest Expansion Into Grasslandsmentioning

confidence: 99%

Carbon dioxide and the uneasy interactions of trees and savannah grasses

Bond

Midgley

2012

Phil. Trans. R. Soc. B

378

320

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Savannahs are a mixture of trees and grasses often occurring as alternate states to closed forests. Savannah fires are frequent where grass productivity is high in the wet season. Fires help maintain grassy vegetation where the climate is suitable for woodlands or forests. Saplings in savannahs are particularly vulnerable to topkill of above-ground biomass. Larger trees are more fire-resistant and suffer little damage when burnt. Recruitment to large mature tree size classes depends on sapling growth rates to fire-resistant sizes and the time between fires. Carbon dioxide (CO 2 ) can influence the growth rate of juvenile plants, thereby affecting tree recruitment and the conversion of open savannahs to woodlands. Trees have increased in many savannahs throughout the world, whereas some humid savannahs are being invaded by forests. CO 2 has been implicated in this woody increase but attribution to global drivers has been controversial where changes in grazing and fire have also occurred. We report on diverse tests of the magnitude of CO 2 effects on both ancient and modern ecosystems with a particular focus on African savannahs. Large increases in trees of mesic savannahs in the region cannot easily be explained by land use change but are consistent with experimental and simulation studies of CO 2 effects. Changes in arid savannahs seem less obviously linked to CO 2 effects and may be driven more by overgrazing. Large-scale shifts in the tree -grass balance in the past and the future need to be better understood. They not only have major impacts on the ecology of grassy ecosystems but also on Earth -atmosphere linkages and the global carbon cycle in ways that are still being discovered.

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Measuring Woody Encroachment along a Forest–Savanna Boundary in Central Africa

Cited by 93 publications

References 54 publications

Potential of Multi-Temporal ALOS-2 PALSAR-2 ScanSAR Data for Vegetation Height Estimation in Tropical Forests of Mexico

Potential of Multi-Temporal ALOS-2 PALSAR-2 ScanSAR Data for Vegetation Height Estimation in Tropical Forests of Mexico

Managing the human component of fire regimes: lessons from Africa

Carbon dioxide and the uneasy interactions of trees and savannah grasses

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