The cocoa yield gap in Ghana: a quantification and an analysis of factors that could narrow the gap

Open Access Article; Published online: 28 Jul 2022CONTEXT Global cocoa production is largely concentrated in West Africa where over 70% of cocoa is produced. Here, cocoa farming is largely a rain-fed, low-input system with low average yields, which are expected to decline with climate change. With increasing demand, there is a need to evaluate opportunities to increase production whilst avoiding deforestation and expansion to croplands. Thus, it is important to know how much additional cocoa can be produced on existing farmland, and what factors determine this potential for increased yield. OBJECTIVE The objective was to quantify the cocoa yield gap in Ghana and identify the factors that can contribute to narrowing the gap. METHODS We calculated the cocoa yield gap as the difference between potential yield (i. water-limited potential(Yw) quantified using a crop model, ii. attainable yield in high-input systems(YE), iii. attainable yield in low-input systems(YF)) and actual farmer yield. Both absolute and relative yield gaps were calculated. We then related each yield gap (absolute & relative) as a function of environment and management variables using mixed-effects models. RESULTS AND CONCLUSIONS There were considerable yield gaps on all cocoa farms. Maximum water-limited yield gaps (YGW) were very large with a mean absolute gap of 4577 kg/ha representing 86% of Yw. Attainable yield gap in high-input (YGE) was lower with mean absolute gap of 1930 kg/ha representing 73% of YE. The yield gap in low-input (YGF) was even lower with mean absolute gap of 469 kg/ha representing 42% of YF. Mixed-effects models showed that, absolute YGW were larger at sites with higher precipitation in the minor wet and minimum temperature in the minor dry season explaining 22% of the variability in YGW. These same factors and cocoa planting density explained 28% of variability in absolute YGE. Regardless of climate, absolute YGF and relative YGW, YGE and YGF were reduced by increasing cocoa planting density and application of fungicide against black pod. The models explained 25% of the variability in absolute YGF, and 33%, 33% and 25% in relative YGW, YGE and YGF respectively. In conclusion, climate determined absolute YGW in Ghana whilst absolute YGE were determined by both climate and management. In contrast, absolute YGF and relative YGW, YGE and YGF can be reduced by agronomic management practices. SIGNIFICANCE Our study is one of the first to quantify cocoa yield gaps in West Africa and shows that these can be closed by improved agronomic practices

Spatio-temporal assessment of beech growth in relation to climate extremes in Slovenia – An integrated approach using remote sensing and tree-ring data

Climate change is predicted to affect tree growth due to increased frequency and intensity of extreme events such as ice storms, droughts and heatwaves. Yet, there is still a lot of uncertainty on how trees respond to an increase in frequency of extreme events. Use of both ground-based wood increment (i.e. ring width) and remotely sensed data (i.e. vegetation indices) can be used to scale-up ground measurements, where there is a link between the two, but this has only been demonstrated in a few studies. We used tree-ring data together with crown features derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) to assess the effect of extreme climate events on the growth of beech (Fagus sylvatica L.) in Slovenia. We found evidence that years with climate extremes during the growing season (drought, high temperatures) had a lower ring width index (RWI) but we could not find such evidence for the remotely sensed EVI (Enhanced Vegetation Index). However, when assessing specific events where leaf burning or wilting has been reported (e.g. August 2011) we did see large EVI anomalies. This implies that the impact of drought or heatwave events cannot be captured by EVI anomalies until physical damage on the canopy is caused. This also means that upscaling the effect of climate extremes on RWI by using EVI anomalies is not straightforward. An exception is the 2014 ice storm that caused a large decline in both RWI and EVI. Extreme climatic parameters explained just a small part of the variation in both RWI and EVI by, which could indicate an effect of other climate variables (e.g. late frost) or biotic stressors such as insect outbreaks. Furthermore, we found that RWI was lower in the year after a climate extreme occurred in the late summer. Most likely due to the gradual increase in temperature and more frequent drought we found negative trends in RWI and EVI. EVI maps could indicate where beech is sensitive to climate changes and could be used for planning mitigation interventions. Logical next steps should focus on a tree-based understanding of the short -and long-term effects of climate extremes on tree growth and survival, taking into account differential carbon allocation to the crown (EVI) and to wood-based variables. This research highlights the value of an integrated approach for upscaling tree-based knowledge to the forest level

The global forest above-ground biomass pool for 2010 estimated from high-resolution satellite observations

The terrestrial forest carbon pool is poorly quantified, in particular in regions with low forest inventory capacity. By combining multiple satellite observations of synthetic aperture radar (SAR) backscatter around the year 2010, we generated a global, spatially explicit dataset of above-ground live biomass (AGB; dry mass) stored in forests with a spatial resolution of 1 ha. Using an extensive database of 110 897 AGB measurements from field inventory plots, we show that the spatial patterns and magnitude of AGB are well captured in our map with the exception of regional uncertainties in high-carbon-stock forests with AGB >250 Mg ha−1, where the retrieval was effectively based on a single radar observation. With a total global AGB of 522 Pg, our estimate of the terrestrial biomass pool in forests is lower than most estimates published in the literature (426–571 Pg). Nonetheless, our dataset increases knowledge on the spatial distribution of AGB compared to the Global Forest Resources Assessment (FRA) by the Food and Agriculture Organization (FAO) and highlights the impact of a country's national inventory capacity on the accuracy of the biomass statistics reported to the FRA. We also reassessed previous remote sensing AGB maps and identified major biases compared to inventory data, up to 120 % of the inventory value in dry tropical forests, in the subtropics and temperate zone. Because of the high level of detail and the overall reliability of the AGB spatial patterns, our global dataset of AGB is likely to have significant impacts on climate, carbon, and socio-economic modelling schemes and provides a crucial baseline in future carbon stock change estimates. The dataset is available at https://doi.org/10.1594/PANGAEA.894711 (Santoro, 2018)

Densidade, tamanho e distribuição estomática em 35 espécies de árvores na Amazônia Central

Stomata are turgor-operated valves that control water loss and CO2 uptake during photosynthesis, and thereby water relation and plant biomass accumulation is closely related to stomatal functioning. The aims of this work were to document how stomata are distributed on the leaf surface and to determine if there is any significant variation in stomatal characteristics among Amazonian tree species, and finally to study the relationship between stomatal density (S D) and tree height. Thirty five trees (>17 m tall) of different species were selected. Stomatal type, density (S D), size (S S) and stomatal distribution on the leaf surface were determined using nail polish imprints taken from both leaf surfaces. Irrespective of tree species, stomata were located only on the abaxial surface (hypostomaty), with large variation in both S D and S S among species. S D ranged from 110 mm-2 in Neea altissima to 846 mm-2 in Qualea acuminata. However, in most species S D ranges between 271 and 543 mm-2, with a negative relationship between S D and S S. We also found a positive relationship between S D and tree height (r² = 0.14, p 17 m de altura) de diferentes espécies foram selecionadas. Tipo de complexo estomático, S D, tamanho (S S) e distribuição na superfície foliar foram determinados utilizando impressões de ambas as superfícies foliares com esmalte incolor. Independente da espécie, os estômatos foram encontrados apenas na superfície abaxial (hipoestomatia) com ampla variação na S D e no S S entre espécies. A densidade estomática variou de 110 mm-2 em Neea altissima a 846 mm-2 em Qualea acuminata. Entretanto, a maioria das espécies apresentou S D entre 271 e 543 mm-2, com uma relação negativa entre S D e S S. Observou-se uma relação positiva entre S D e altura arbórea (r² = 0.14, p < 0.01), não havendo relação entre S D e espessura foliar. Os tipos estomáticos mais comuns foram: anomocíticos (37%), seguidos de paracíticos (26%) e anisocíticos (11%). Concluiu-se que em espécies da Amazônia, a distribuição de estômatos na superfície foliar está mais relacionada a fatores genéticos de cada espécie do que a variações ambientais. Entretanto, S D é fortemente influenciada por fatores ambientais concernentes à altura da árvore

Strong floristic distinctiveness across Neotropical successional forests

Forests that regrow naturally on abandoned fields are important for restoring biodiversity and ecosystem services, but can they also preserve the distinct regional tree floras? Using the floristic composition of 1215 early successional forests (≤20 years) in 75 human-modified landscapes across the Neotropic realm, we identified 14 distinct floristic groups, with a between-group dissimilarity of 0.97. Floristic groups were associated with location, bioregions, soil pH, temperature seasonality, and water availability. Hence, there is large continental-scale variation in the species composition of early successional forests, which is mainly associated with biogeographic and environmental factors but not with human disturbance indicators. This floristic distinctiveness is partially driven by regionally restricted species belonging to widespread genera. Early secondary forests contribute therefore to restoring and conserving the distinctiveness of bioregions across the Neotropical realm, and forest restoration initiatives should use local species to assure that these distinct floras are maintained

Looking backwards : using tree rings to evaluate long-term growth patterns

To improve our understanding of the ecology of tropical forest trees, it is essential to obtain information on tree growth over periods of decades to centuries. Using tree-ring analysis such data can be derived as this technique allows reconstructing the growth history over the entire lifespan of a tree. This PhD thesis reports on the use of tree-ring analysis for reconstructing long-term growth patterns of Bolivian forest trees. A large number of discs was collected from five tree species, and tree rings were identified and measured. First, I evaluated whether those trees that are currently large have had faster rates of diameter growth when they were small compared to present-day small trees. This ‘juvenile selection effect’ would imply that fast-growers have a higher probability to reach the forest canopy and become reproductive. For three out of five species, I could indeed detect a juvenile selection effect. Thus, for these three species fast growth of small trees may be essential to reach the canopy. The above finding has consequences for growth models that are often used to simulate tropical timber yield. If fast-growing trees have a higher chance to reach the size at which they can be harvested, it is important to include these higher growth rates in such models. I found that this was indeed the case: simulated timber yield was higher when this juvenile selection effect was taken into account. Nevertheless, even with the higher growth rates, the recuperation of timber volume during one logging cycle remained low. Only 20-33% of the timber volume harvested at the first harvest could be obtained at second harvest after 20 years. Reaching the forest canopy for tropical trees implies large investments in height. Such investments may imply that less carbohydrates are invested in diameter growth and stem volume. I collected stem discs at various heights of juvenile trees to reconstruct height, diameter and volume growth. I found that individual trees differed very strongly in growth rates (height, stem volume, diameter). These growth rates were related: the faster height growers grew relatively little in stem volume, and vice versa. This suggests that juvenile trees in favourable (light) conditions invest relatively more in stability and crown development than in height growth Tree-ring analysis allows evaluating whether diameter growth rates have increased or decreased over time. For four out of five species I found that growth rates of small trees increased over the last two centuries. This pattern is consistent with what would be expected due to CO2-fertilization, but other causes cannot be ruled out. I then checked for indications of changes in forest dynamics over time. To this end, I calculated the frequency of releases (periods of fast growth) and autocorrelation strength. No indications for a change in forest dynamics were found. In this thesis I showed that tree-ring analysis can strongly contribute to improving our understanding of long-term ecological processes in tropical forests. This knowledge is much needed in assessing the response of tropical forests to the predicted climate changes in the future

Looking backwards : using tree rings to evaluate long-term growth patterns

To improve our understanding of the ecology of tropical forest trees, it is essential to obtain information on tree growth over periods of decades to centuries. Using tree-ring analysis such data can be derived as this technique allows reconstructing the growth history over the entire lifespan of a tree. This PhD thesis reports on the use of tree-ring analysis for reconstructing long-term growth patterns of Bolivian forest trees. A large number of discs was collected from five tree species, and tree rings were identified and measured. First, I evaluated whether those trees that are currently large have had faster rates of diameter growth when they were small compared to present-day small trees. This ‘juvenile selection effect’ would imply that fast-growers have a higher probability to reach the forest canopy and become reproductive. For three out of five species, I could indeed detect a juvenile selection effect. Thus, for these three species fast growth of small trees may be essential to reach the canopy. The above finding has consequences for growth models that are often used to simulate tropical timber yield. If fast-growing trees have a higher chance to reach the size at which they can be harvested, it is important to include these higher growth rates in such models. I found that this was indeed the case: simulated timber yield was higher when this juvenile selection effect was taken into account. Nevertheless, even with the higher growth rates, the recuperation of timber volume during one logging cycle remained low. Only 20-33% of the timber volume harvested at the first harvest could be obtained at second harvest after 20 years. Reaching the forest canopy for tropical trees implies large investments in height. Such investments may imply that less carbohydrates are invested in diameter growth and stem volume. I collected stem discs at various heights of juvenile trees to reconstruct height, diameter and volume growth. I found that individual trees differed very strongly in growth rates (height, stem volume, diameter). These growth rates were related: the faster height growers grew relatively little in stem volume, and vice versa. This suggests that juvenile trees in favourable (light) conditions invest relatively more in stability and crown development than in height growth Tree-ring analysis allows evaluating whether diameter growth rates have increased or decreased over time. For four out of five species I found that growth rates of small trees increased over the last two centuries. This pattern is consistent with what would be expected due to CO2-fertilization, but other causes cannot be ruled out. I then checked for indications of changes in forest dynamics over time. To this end, I calculated the frequency of releases (periods of fast growth) and autocorrelation strength. No indications for a change in forest dynamics were found. In this thesis I showed that tree-ring analysis can strongly contribute to improving our understanding of long-term ecological processes in tropical forests. This knowledge is much needed in assessing the response of tropical forests to the predicted climate changes in the future

Leaf size and leaf display of thirty-eight tropical tree species

Trees forage for light through optimal leaf display. Effective leaf display is determined by metamer traits (i.e., the internode, petiole, and corresponding leaf), and thus these traits strongly co-determine carbon gain and as a result competitive advantage in a light-limited environment. We examined 11 metamer traits of sun and shade trees of 38 coexisting moist forest tree species and determined the relative strengths of intra- and interspecific variation. Species-specific metamer traits were related to two variables that represent important life history variation; the regeneration light requirements and average leaf size of the species. Metamer traits varied strongly across species and, in contrast to our expectation, showed only modest changes in response to light. Intra- and interspecific responses to light were only congruent for a third of the traits evaluated. Four traits, amongst which leaf size, specific leaf area (SLA), and leaf area ratio at the metamer level (LAR) showed even opposite intra- and interspecific responses to light. Strikingly, these are classic traits that are thought to be of paramount importance for plant performance but that have completely different consequences within and across species. Sun trees of a given species had small leaves to reduce the heat load, but light-demanding species had large leaves compared to shade-tolerants, probably to outcompete their neighbors. Shade trees of a given species had a high SLA and LAR to capture more light in a light-limited environment, whereas shade-tolerant species have well-protected leaves with a low SLA compared to light-demanding species, probably to deter herbivores and enhance leaf lifespan. There was a leaf-size-mediated trade-off between biomechanical and hydraulic safety, and the efficiency with which species can space their leaves and forage for light. Unexpectedly, metamer traits were more closely linked to leaf size than to regeneration light requirements, probably because leaf-size-related biomechanical and vascular constraints limit the trait combinations that are physically possible. This suggests that the leaf size spectrum overrules more subtle variation caused by the leaf economics spectrum, and that leaf size represents a more important strategy axis than previously though

Plasticity in leaf traits of 38 tropical tree species in response to light; relationships with light demand and adult stature.

1. The sun-shade acclimation and plasticity of 16 functional leaf traits of 38 tropical tree species were studied in relation to their light demand, maximum adult stature and ontogenetic changes in crown exposure. 2. Species differed significantly in all leaf traits, which explained a large part of the observed variation (average R2 = 0·72). Light had a significant effect on 12 traits and species showed a similar proportional response to light, indicating that the species ranking in trait performance is largely maintained in different light environments. 3. Specific leaf area, leaf nutrient content and chlorophyll : nitrogen ratio showed the largest plasticity to irradiance. These traits are important for maximizing growth in different light conditions because they are closely linked to the photosynthetic capacity and carbon balance of the plant. 4. Plasticity is generally thought to be greatest for pioneer species that occupy early successional habitats with a large variation in irradiance. This hypothesis was rejected because short-lived pioneers showed the lowest plasticity to irradiance. 5. An alternative hypothesis states that plasticity is largest for tall species that experience large ontogenetic changes in irradiance during their life cycle. Yet plasticity was barely related to adult stature or ontogenetic changes in crown exposure. Short-lived pioneers that experience consistently high light levels did have low plasticity, but shade-tolerant species that experience consistently low light levels had high plasticity. 6. Tropical rainforest species show a large variation in plasticity. Plasticity is a compromise between many factors and constraints, and all of these may explain the observed patterns to some extent