MODIS Vegetation Continuous Fields tree cover needs calibrating in tropical savannas

The Moderate Resolution Imaging Spectroradiometer Vegetation Continuous Fields (MODIS VCF) Earth observation product is widely used to estimate forest cover changes and to parameterize vegetation and Earth system models and as a reference for validation or calibration where field data are limited. However, although limited independent validations of MODIS VCF have shown that MODIS VCF's accuracy decreases when estimating tree cover in sparsely vegetated areas such as tropical savannas, no study has yet assessed the impact this may have on the VCF-based tree cover data used by many in their research. Using tropical forest and savanna inventory data collected by the Tropical Biomes in Transition (TROBIT) project, we produce a series of calibration scenarios that take into account (i) the spatial disparity between the in situ plot size and the MODIS VCF pixel and (ii) the trees' spatial distribution within in situ plots. To identify if a disparity also exists in products trained using VCF, we used a similar approach to evaluate the finer-scale Landsat Tree Canopy Cover (TCC) product. For MODIS VCF, we then applied our calibrations to areas identified as forest or savanna in the International Geosphere-Biosphere Programme (IGBP) land cover mapping product. All IGBP classes identified as “savanna” show substantial increases in cover after calibration, indicating that the most recent version of MODIS VCF consistently underestimates woody cover in tropical savannas. We also found that these biases are propagated in the finer-scale Landsat TCC. Our scenarios suggest that MODIS VCF accuracy can vary substantially, with tree cover underestimation ranging from 0 % to 29 %. Models that use MODIS VCF as their benchmark could therefore be underestimating the carbon uptake in forest–savanna areas and misrepresenting forest–savanna dynamics. Because of the limited in situ plot number, our results are designed to be used as an indicator of where the product is potentially more or less reliable. Until more in situ data are available to produce more accurate calibrations, we recommend caution when using uncalibrated MODIS VCF data in tropical savannas

Environmental controls on wood density in tropical forests

Tropical forests store a larger amount of carbon (C) than boreal and temperate forests. However, the determination of above ground biomass and C stocks in tropical ecosystems usually relies on a combination of remote sensing data together with ground data to develop models that predict forest biomass. One of the major determinants of above ground C stocks that cannot be determined in the field is wood density (ρ) and tropical ecosystems usually contain a high diversity of tree species, with a wide range of wood densities in comparison with temperate forests. Summary of the data chapters This thesis aimed to understand the importance of accounting for the intra and interspecific variation of the ρ with changing environmental conditions on the calculation of the above ground C on tropical forests. Not including this variation of ρ could misestimate the C reservoirs on these ecosystems. In total, this thesis includes four data chapters. The first data chapter that is Chapter 2 comprises a novel method to study the ρ within and between tree species at different study sites in Australia, Papua New Guinea and Vanuatu across different forest types. Chapter 3 and 4 studied the variation of ρ with elevation (within and between tree species) from a low to mid montane tropical forests in Papua New Guinea and in range from lowland to montane tropical forest in Australia on respectively. Additionally, Chapter 4 also examined the variation of above ground biomass with increasing elevation. Chapter 5 investigated the importance of including ρ on the estimations of the coarse woody debris residence times. The study was conducted along an altitudinal gradient in a tropical forest in Australia to observe the variation of the decomposition of the wood with the decreasing of mean annual temperature. The aim of using altitudinal gradients as sampling design was to understand the potential effects of climate change on the forest dynamics. Description of the data chapters Traditional methods for both field core extraction and laboratory processing for determining the ρ of trees are generally time consuming and costly. Some trees are very hard to core and the field and the laboratory equipment required to obtain accurate numbers are expensive. There is also the risk of sample damaging during the process of collecting and transporting the core samples. Because of all of the listed inconveniences, a fast, cheap, nondestructive and efficient field method to determine wood density would help the researchers to facilitate biomass census on tropical forests and with special emphasis those located in remote areas. Chapter 2 develops a field-based method to determine ρ that relies on a non-destructive ultrasonic technique. We tested the technique on living trees, from different ecosystem types and under a range of climate conditions. The results suggest a positive relationship between ρ and ultrasonic velocity with a coefficient of variation of 0.66 for intraspecific variation of ρ and 0.81 for interspecific variation of ρ. Due to interferences with the ultrasonic pulse velocity, measurements should be conducted in dry weather when temperatures are less than 30 °C. Potential improvements of this technique would include reducing the size of the transducers of the instrument and the development of conversion factors for measurements carried out above 30°C. Few studies have described ρ values for trees located on remote forests of Papua New Guinea. Besides, studies of the quantification of the intraspecific and interspecific variation of ρ in relation to environmental variables are scarce for tropical forests and inexistent for Papua New Guinea. Thus, in this thesis study site elevation was chosen as an environmental gradient for the study of the climatic effects on the variation of ρ. Therefore, chapter 3 reports the ρ values from the most common sun-exposed and shaded tree species in the YUS Conservation Area, Papua New Guinea over an elevation range from 1800 m to 3050 m above sea level. Besides, one key focus of this chapter is the study of intra-specific variation of ρ with elevation (temperature). The results indicate ρ was negatively related to elevation for sun-exposed trees but with a positive ρ /elevation relationship for shaded species. Both within and across species, tree size was found to be a significant covariate in models attempting to explain individual tree variations on ρ: a positive relationship between ρ with 1/DBH provided the best model fit. The results of this study have important implication for future studies on ground-based tropical forest biomass estimates for tropical montane forests. Australian forests are located in a World Heritage Area and little is known about their role of carbon reservoirs and how they will respond to the effects of climate change. Chapter 4 presents a study along an elevational gradient from 50 m to 1500 m in North Queensland on the a) plot above ground biomass, basal area and tree diameter variation, b) the intra and interspecific variation of wood density for common species along the elevation range and c) the tree growth rates variation according to tree diameter and species. This study also included the effects of soil fertility as potential cause of ρ variability. The results indicated that tree forest strata, diameter and growth rates were related. Sun-exposed trees that had bigger average diameters presented bigger growth rates than smaller trees. In addition, results suggested a relationship between forest strata and ρ. For sun-exposed trees, wood density decreased with elevation and the inverse case was for shaded trees. A multi-level statistical analysis of the dataset explained that soil fertility in addition to site elevation were significant drivers of tree wood density variation. The coarse woody debris (CWD) pool of a forest in composed mainly by death trees, large branches and death leaves. The nutrients that are confined on the death material return to the ecosystems to be reused by living organisms. Throughout this cycle, CWD provides food and habitat to living organisms and increases forest biodiversity. The pace that CWD might degrade can be related to environmental variables (i.e. soil fertility, forest mature stage). However, due to the imminent consequences of climate change on actual and future forest dynamics, this study has focused on the effects of mean annual temperature of the CWD residence times. Thus, Chapter 5 examines coarse woody debris residence time (τ), for different tree species (and therefore with different wood traits) - ranging from low to hard woods – along an elevation gradient from 102 m above sea level (MAT = 23.7 °C) to 1500 m above sea level (MAT = 16.7 °C) in a tropical forests in Australia. The aim was to understand the effects of elevation (temperature) on the chemical and physical decay of CWD. Results suggested that wood density together with Carbon:Nitrogen ratio enable prediction of the variation in τ within decay classes and tree species along an elevation gradient. In addition, τ decreased with increasing decay status of the wood, with temperature also playing an important role, as τ increased with increasing site elevation. The study also suggested the importance of further studies of the effects of seasonal variations in climate in short term field studies, as a single wet season reduced the observed τ of the CWD faster than two wet season and a dry season

Environmental controls on wood density in tropical forests

Tropical forests store a larger amount of carbon (C) than boreal and temperate forests. However, the determination of above ground biomass and C stocks in tropical ecosystems usually relies on a combination of remote sensing data together with ground data to develop models that predict forest biomass. One of the major determinants of above ground C stocks that cannot be determined in the field is wood density (ρ) and tropical ecosystems usually contain a high diversity of tree species, with a wide range of wood densities in comparison with temperate forests. Summary of the data chapters This thesis aimed to understand the importance of accounting for the intra and interspecific variation of the ρ with changing environmental conditions on the calculation of the above ground C on tropical forests. Not including this variation of ρ could misestimate the C reservoirs on these ecosystems. In total, this thesis includes four data chapters. The first data chapter that is Chapter 2 comprises a novel method to study the ρ within and between tree species at different study sites in Australia, Papua New Guinea and Vanuatu across different forest types. Chapter 3 and 4 studied the variation of ρ with elevation (within and between tree species) from a low to mid montane tropical forests in Papua New Guinea and in range from lowland to montane tropical forest in Australia on respectively. Additionally, Chapter 4 also examined the variation of above ground biomass with increasing elevation. Chapter 5 investigated the importance of including ρ on the estimations of the coarse woody debris residence times. The study was conducted along an altitudinal gradient in a tropical forest in Australia to observe the variation of the decomposition of the wood with the decreasing of mean annual temperature. The aim of using altitudinal gradients as sampling design was to understand the potential effects of climate change on the forest dynamics. Description of the data chapters Traditional methods for both field core extraction and laboratory processing for determining the ρ of trees are generally time consuming and costly. Some trees are very hard to core and the field and the laboratory equipment required to obtain accurate numbers are expensive. There is also the risk of sample damaging during the process of collecting and transporting the core samples. Because of all of the listed inconveniences, a fast, cheap, nondestructive and efficient field method to determine wood density would help the researchers to facilitate biomass census on tropical forests and with special emphasis those located in remote areas. Chapter 2 develops a field-based method to determine ρ that relies on a non-destructive ultrasonic technique. We tested the technique on living trees, from different ecosystem types and under a range of climate conditions. The results suggest a positive relationship between ρ and ultrasonic velocity with a coefficient of variation of 0.66 for intraspecific variation of ρ and 0.81 for interspecific variation of ρ. Due to interferences with the ultrasonic pulse velocity, measurements should be conducted in dry weather when temperatures are less than 30 °C. Potential improvements of this technique would include reducing the size of the transducers of the instrument and the development of conversion factors for measurements carried out above 30°C. Few studies have described ρ values for trees located on remote forests of Papua New Guinea. Besides, studies of the quantification of the intraspecific and interspecific variation of ρ in relation to environmental variables are scarce for tropical forests and inexistent for Papua New Guinea. Thus, in this thesis study site elevation was chosen as an environmental gradient for the study of the climatic effects on the variation of ρ. Therefore, chapter 3 reports the ρ values from the most common sun-exposed and shaded tree species in the YUS Conservation Area, Papua New Guinea over an elevation range from 1800 m to 3050 m above sea level. Besides, one key focus of this chapter is the study of intra-specific variation of ρ with elevation (temperature). The results indicate ρ was negatively related to elevation for sun-exposed trees but with a positive ρ /elevation relationship for shaded species. Both within and across species, tree size was found to be a significant covariate in models attempting to explain individual tree variations on ρ: a positive relationship between ρ with 1/DBH provided the best model fit. The results of this study have important implication for future studies on ground-based tropical forest biomass estimates for tropical montane forests. Australian forests are located in a World Heritage Area and little is known about their role of carbon reservoirs and how they will respond to the effects of climate change. Chapter 4 presents a study along an elevational gradient from 50 m to 1500 m in North Queensland on the a) plot above ground biomass, basal area and tree diameter variation, b) the intra and interspecific variation of wood density for common species along the elevation range and c) the tree growth rates variation according to tree diameter and species. This study also included the effects of soil fertility as potential cause of ρ variability. The results indicated that tree forest strata, diameter and growth rates were related. Sun-exposed trees that had bigger average diameters presented bigger growth rates than smaller trees. In addition, results suggested a relationship between forest strata and ρ. For sun-exposed trees, wood density decreased with elevation and the inverse case was for shaded trees. A multi-level statistical analysis of the dataset explained that soil fertility in addition to site elevation were significant drivers of tree wood density variation. The coarse woody debris (CWD) pool of a forest in composed mainly by death trees, large branches and death leaves. The nutrients that are confined on the death material return to the ecosystems to be reused by living organisms. Throughout this cycle, CWD provides food and habitat to living organisms and increases forest biodiversity. The pace that CWD might degrade can be related to environmental variables (i.e. soil fertility, forest mature stage). However, due to the imminent consequences of climate change on actual and future forest dynamics, this study has focused on the effects of mean annual temperature of the CWD residence times. Thus, Chapter 5 examines coarse woody debris residence time (τ), for different tree species (and therefore with different wood traits) - ranging from low to hard woods – along an elevation gradient from 102 m above sea level (MAT = 23.7 °C) to 1500 m above sea level (MAT = 16.7 °C) in a tropical forests in Australia. The aim was to understand the effects of elevation (temperature) on the chemical and physical decay of CWD. Results suggested that wood density together with Carbon:Nitrogen ratio enable prediction of the variation in τ within decay classes and tree species along an elevation gradient. In addition, τ decreased with increasing decay status of the wood, with temperature also playing an important role, as τ increased with increasing site elevation. The study also suggested the importance of further studies of the effects of seasonal variations in climate in short term field studies, as a single wet season reduced the observed τ of the CWD faster than two wet season and a dry season

Fire regimes, fire experiments and alternative stable states in mesic savannas:A response to Laris &amp; Jacobs (2021) 'On the problem of <i>natural</i> savanna fires'

[Extract] In their comment on Veenendaal et al. (2018), Laris & Jacobs (2021; in this issue of New Phytologist, pp. 11–13) question the appropriateness of fire experiments to simulate effects of fire on tropical vegetation cover as well as objecting to our use of the word ‘natural’ to describe nonanthropogenic fire regimes. They also challenge some of the conclusions we drew as regards the likelihood of fire-mediated feedbacks causing alternate stable states (ASS) in forest–savanna transitions

On the delineation of tropical vegetation types with an emphasis on forest/savanna transitions

Background: There is no generally agreed classification scheme for the many different vegetation formation types occurring in the tropics. This hinders cross-continental comparisons and causes confusion as words such as 'forest' and 'savanna' have different meanings to different people. Tropical vegetation formations are therefore usually imprecisely and/or ambiguously defined in modelling, remote sensing and ecological studies.Aims: To integrate observed variations in tropical vegetation structure and floristic composition into a single classification scheme.Methods: Using structural and floristic measurements made on three continents, discrete tropical vegetation groupings were defined on the basis of overstorey and understorey structure and species compositions by using clustering techniques.Results: Twelve structural groupings were identified based on height and canopy cover of the dominant upper stratum and the extent of lower-strata woody shrub cover and grass cover. Structural classifications did not, however, always agree with those based on floristic composition, especially for plots located in the forest-savanna transition zone. This duality is incorporated into a new tropical vegetation classification scheme.Conclusions: Both floristics and stand structure are important criteria for the meaningful delineation of tropical vegetation formations, especially in the forest/savanna transition zone. A new tropical vegetation classification scheme incorporating this information has been developed. © 2013 Copyright 2013 Botanical Society of Scotland and Taylor & Francis