Tag: grevillea robusta
Native coffee agroforestry in the Western Ghats of India maintains higher carbon storage and tree diversity compared to exotic agroforestry
The ongoing introduction of the exotic Grevillea robusta tree species into agroforestry systems (AFS) of the Indian Western Ghats could become a threat to both climate change mitigation and tree diversity conservation.
Here, we quantified carbon (C) storage and shade tree diversity in native forests and coffee AFS under contrasted management (native versus exotic shade trees, Robusta versus Arabica systems) at 67 plots along a 3500 mm precipitation gradient in the Cauvery watershed, India.
Despite a substantial reduction of shade tree cover in native AFS compared to forest (from 90% to 32% in the high precipitation area), native AFS and forests displayed high and comparable C stocks (max. 228 MgC ha−1 and 234 MgC ha-1, respectively) and tree diversity (max. 44 and 45 species, respectively). Both variables were negatively impacted by the introduction of G. robusta, especially in Robusta coffee systems (max. 158 MgC ha−1, 12 species).
The current trend toward the introduction of G. robusta in coffee AFS of the study area (exotic agroforestry) negatively affects C storage and tree diversity, especially in Robusta coffee systems. Policy makers should take advantage of the carbon-tree diversity positive correlation found in the agroforestry landscape of the Wester
Decreasing crop diversity leads to food insecurity and off-farm food reliance and varies with land degradation status
Pollination and fruit-set of Grevillea robusta in western Kenya
The floral visitors of silky oak, Grevillea robusta A. Cunn. ex R.Br., their foraging behaviour and their effects on fruit-set were studied at Malava, western Kenya. Grevillea robusta is a popular tree for farm plantings in the eastern and central African highlands. Yield of seed has been disappointingly low in some areas and a lack of appropriate pollinators has been suggested as a possible cause. Investigations involved the monitoring of visitors on active inflorescences, assessment of the rewards available to potential pollinators, and exclusion experiments to establish the effects of various visitors on fruit-set. The flowers are visited mainly by birds and insects. The likely pollinators of G. robusta are sunbirds (Nectarinia amethystina, N. cyanolaema, N. olivacea, N. superba and N. venusta) and white-eyes (Zosterops kikuyuensis and Z. senegalensis). Very little aggressive behaviour between birds was recorded. No nocturnal pollinators were observed. Nectar was the major floral reward for pollinators, but is likely depleted by ants and honey bees, the foraging behaviour of which confirmed them to be nectar-robbers. These insects hardly ever touched stigmas during their visits. Eighty-nine per cent of bird visits were in the morning (07.00–10.00 hours) when nectar volume was highest. Inflorescences bagged to exclude birds set no fruits, and unmanipulated flowers and flowers bagged with self-pollen set no fruits, indicating a self-incompatibility mechanism. Control cross-pollinated flowers displayed greatly increased fruit-set (25.1%) compared with natural open-pollination (0.9%). All these findings confirm the importance of cross-pollen transfer to flowers and the necessity of pollinators for fruit-set. Effective seed production requires activity of pollinators for self-pollen removal and cross-pollen deposition. Seed production stands for G. robusta should be established where flowering is prolific and bird pollinators are abundant.
Productivity, microclimate and water use in Grevillea robusta-based agroforestry systems on hillslopes in semi-arid Kenya
This paper describes a multi-disciplinary project to examine the changing interactions between trees and crops as the trees in semi-arid agroforestry systems establish and mature; the project is one of the most detailed and highly instrumented long-term studies of tree and crop growth, system performance, resource capture, hydrology and microclimate ever carried out within an agroforestry context. Its primary objective was to compile a comprehensive experimental database to improve the mechanistic understanding of tree/crop interactions and support the development and validation of process-based simulation models describing resource capture and tree and crop growth in semi-arid agroforestry systems.Grevillea robusta A. Cunn. (grevillea) trees were grown as mono-cultures or in mixtures with cowpea (Vigna unguiculata L.) or maize (Zea mays L.) over a 68-month period. Allometric approaches were used to determine seasonal and annual growth increments for leaf area and leaf, branch and trunk biomass in grevillea. Crop performance was examined during each growing season, while the spatial distribution of tree and crop roots was established during the latter stages of the experiment using coring and mini-rhizotron approaches. Detailed hydrological studies examined effects on the soil water balance and its components (precipitation, interception, runoff and soil moisture status); equivalent measurements of spatial and temporal variation in microclimatic conditions allowed the mechanistic basis for beneficial and detrimental effects on understorey crops and the influence of proximity to trees on crop performance to be examined. Transpiration by grevillea and water movement through lateral and tap roots were measured using sap flow methodology, and light interception by the tree and crop canopies was routinely determined.This multi-disciplinary study has provided a detailed understanding of the changing patterns of resource capture by trees and crops as agroforestry systems mature. This paper provides an overview of the underlying rationale, experimental design and core measurements, outlines key results and conclusions, and draws the attention of readers to further papers providing more detailed consideration of specific aspects of the study.
Floral structure, stigma receptivity and pollen viability in relation to protandry and self-incompatibility in silky oak (Grevillea robusta A. Cunn.)
The reproductive biology of Grevillea robusta growing under exotic conditions in Kenya and Australia is reported.The species showed both protandry and a self-incompatibility mechanism. The stigma was wet and papillate with adistinct groove in the middle. The anthers dehisced prior to anthesis, when the perianth opened. Stigmatic receptivitybegan 1 d after anthesis, with the greatest pollen germination rates and longest pollen tubes obtained 2 d afteranthesis. Nectar secretion commenced with pollen dehiscence and was abundant at anthesis. Most stigmatic groovesopened widely 1±2 d after anthesis and stigmas showed taller papillae and abundant secretion. Controlled pollina-tions gave a greater fruit set from cross-pollination (5.9% in April and 17.5% in July) than open-pollination (0.1%in April and 3.3% in July). No fruit set from self-pollination was obtained in April, and very few fruit set forgeitonogamous (two out of 1622; 0.1%) or for autogamous (one out of 2707 ¯owers; 0.04 %) pollination treatmentsin July. Following self-pollination, growth of pollen tubes was poorer than in other treatments, and was generallyarrested in the upper style. Cross-pollinated ¯owers produced normal and straight pollen tubes, while self-pollentubes had growth abnormalities. Most of the open-pollinated ¯owers were found without pollen or with only self-pollen on their stigmas indicating that the amount of cross-pollen reaching the stigma under open-pollination may bea factor limiting seed production. Flowers shed soon after the fertilization phase were those with ungerminated pollenor no pollen. Although a very low rate of sel®ng may occur, G. robusta presents a self-incompatibility system andallogamy is its primary breeding behaviour.
Allometric estimation of above-ground biomass and leaf area in managed Grevillea robusta agroforestry systems
Non-destructive methods for determining the biomass and leaf area of individual trees throughout their growing cycle are an essential tool in agroforestry research, but must be capable of providing reliable estimates despite the influence that management strategies such as pruning may have on tree form. In the present study, allometric methods involving measurements of the diameter of all branches provided reliable estimates of canopy leaf area and biomass for grevillea trees (Grevillea robusta A. Cunn.; Proteaceae) grown as poles, but proved unsuitable for routine measurements because of their time-consuming nature. An alternative, less laborious method based on measurements of trunk cross-sectional area immediately below the first branch of the canopy provided satisfactory allometric estimates of leaf area and canopy biomass. Trunk biomass was determined from measurements of tree height and diameter at breast height using established methodology based on the assumption that trunk volume may be calculated using a quadratic paraboloid model; biomass was determined as the product of trunk volume and the specific gravity of the wood. The theoretical basis, development and validation of allometric methods for estimating tree growth are discussed and their wider applicability to other agroforestry systems is assessed.
Scattered shade trees improve low-input smallholder Arabica coffee productivity in the Northern Lake Kivu region of Rwanda
In suboptimal conditions for coffee growth, the use of shade tree is usually considered beneficial to production. This study aimed to evaluate this benefit in the poorly documented East African smallholder Arabica coffee systems in optimal climatic but suboptimal management conditions. In a 4 year observational trial in 50 coffee farms, the association of shade trees and coffee generated an average 55 % cherry production increase. Neither delay in berry maturation nor buffer in alternate bearing patterns were observed, probably due to the low productivity of unfertilized coffee plants. Quality wise, the presence of shade trees did not result in an increase of larger green beans, but it reduced the proportion of altered and lighter cherries in 2009, a low production year. A shade species effect was detected through the positive influence of two non-leguminous shade trees, Persea americana and Ficus thonningii on production. The effect was correlated to greater canopy openness and increase in K soil content. In general, soil mineral content was not influenced by the presence of trees, but the legume species Inga oerstediana appeared responsible for a slight increase of total C and N soil content. It is concluded that in the small holder context of the Northern Lake Kivu region, the association of mature trees is beneficial to coffee production and can contribute to the improvement of producer’s income.
Occurrence and abundance of arbuscular mycorrhizal fungi (AMF) in agroforestry systems of Rubavu and Bugesera Districts in Rwanda
Arbuscular mycorrhizal fungi (AMF) help to facilitate mobilization of nutrients from soil to plant. The study was carried out in humid Rubavu and semi-arid Bugesera districts in Rwanda. We hypothesized that the presence of tree species in farming systems enhances mycorrhizal fungal density. The occurrence and abundance of AMF in the soil around main agroforestry tree species in these regions was studied. Tree species in Rubavu included Alnus acuminata, Markhamia lutea, Grevillea robusta and Eucalyptus sp. and in Bugesera Acacia polyacantha, Senna spectabilis, Grevillea robusta and Eucalyptus sp. AMF spores were isolated from soil samples collected under and outside the trees canopies. Results show significant differences in spore density between species. The density of AMF spores was highest under A. acuminata and A. polyacantha and lowest in Eucalyptus sp. and G. robusta in Rubavu and Bugesera, respectively. Generally, the mean spore abundance (spores/g of soil) was significantly higher in Bugesera (3.1-6.6) than Rubavu (1.6-4.4). Spores abundance was also affected by distance from the tree trunk and tree size. The present work is the first attempt to study the AMF communities associated with tree species in agroforestry systems in Rwanda. We propose further studies relating mycorrhizal diversity in the agroforestry systems to performance and yields of crops
Modelling soil evaporation in an agroforestry system in Kenya
Soil evaporation measurements from bare soil and shaded soil under an agroforestry tree canopy were used to construct a model to predict soil evaporation with and without tree shade. It was found that a simple daily time step model based on the Ritchie (1972)approach was unable to predict daily soil evaporation accurately, but was capable of providing good estimates of cumulative soil evaporation over hydrologically significant periods (weeks–months). This model was used to show how trees could reduce annual soil evaporation directly beneath their canopy by an average of 35% (compared to completely bare soil), equivalent to 21% of rainfall. In sparse agroforestry tree canopies the area average saving is smaller, depending on tree leaf area index (LAI). The model also demonstrated how annual saving in soil evaporation due to a tree canopy might vary with rainfall, with a maximum of around 180 mm being achieved once rainfall exceeded 1000 mm year1. This saving in soil water is very significant and will help offset the enhanced evaporative losses associated with tree canopies due to interception and re-evaporation of rainfall or as tree transpiration