Nico de Ridder

Effects of fire and herbivory on the stability of savanna ecosystems

Savanna ecosystems are characterized by the co-occurrence of trees and grass- es. In this paper, we argue that the balance between trees and grasses is, to a large extent, determined by the indirect interactive effects of herbivory and fire. These effects are based on the positive feedback between fuel load (grass biomass) and fire intensity. An increase in the level of grazing leads to reduced fuel load, which makes fire less intense and, thus, less damaging to trees and, consequently, results in an increase in woody vegetation. The system then switches from a state with trees and grasses to a state with solely trees. Similarly, browsers may enhance the effect of fire on trees because they reduce woody biomass, thus indirectly stimulating grass growth. This consequent increase in fuel load results in more intense fire and increased decline of biomass. The system then switches from a state with solely trees to a state with trees and grasses. We maintain that the interaction between fire and herbivory provides a mechanistic explanation for observed discontinuous changes in woody and grass biomass. This is an alternative for the soil degradation mechanism, in which there is a positive feedback between the amount of grass biomass and the amount of water that infiltrates into the soil. The soil degradation mechanism predicts no discontinuous chang- es, such as bush encroachment, on sandy soils. Such changes, however, are frequently ob- served. Therefore, the interactive effects of fire and herbivory provide a more plausible explanation for the occurrence of discontinuous changes in savanna ecosystems.

Spatial Heterogeneity and Irreversible Vegetation Change in Semiarid Grazing Systems

Recent theoretical studies have shown that spatial redistribution of surface water may explain the occurrence of patterns of alternating vegetated and degraded patches in semiarid grasslands. These results implied, however, that spatial redistribution processes cannot explain the collapse of production on coarser scales observed in these systems. We present a spatially explicit vegetation model to investigate possible mechanisms explaining irreversible vegetation collapse on coarse spatial scales. The model results indicate that the dynamics of vegetation on coarse scales are determined by the interaction of two spatial feedback processes. Loss of plant cover in a certain area results in increased availability of water in remaining vegetated patches through run‐on of surface water, promoting within‐patch plant production. Hence, spatial redistribution of surface water creates negative feedback between reduced plant cover and increased plant growth in remaining vegetation. Reduced plant cover, however, results in focusing of herbivore grazing in the remaining vegetation. Hence, redistribution of herbivores creates positive feedback between reduced plant cover and increased losses due to grazing in remaining vegetated patches, leading to collapse of the entire vegetation. This may explain irreversible vegetation shifts in semiarid grasslands on coarse spatial scales.

Drivers of land use change and household determinants of sustainability in smallholder farming systems of Eastern Uganda.

Smallholder farming systems in sub-Saharan Africa have undergone changes in land use, productivity and sustainability. Understanding of the drivers that have led to changes in land use in these systems and factors that influence the systems’ sustainability is useful to guide appropriate targeting of intervention strategies for improvement. We studied low input Teso farming systems in eastern Uganda from 1960 to 2001 in a place-based analysis combined with a comparative analysis of similar low input systems in southern Mali. This study showed that policy-institutional factors next to population growth have driven land use changes in the Teso systems, and that nutrient balances of farm households are useful indicators to identify their sustainability. During the period of analysis, the fraction of land under cultivation increased from 46 to 78%, and communal grazing lands nearly completely disappeared. Cropping diversified over time; cassava overtook cotton and millet in importance, and rice emerged as an alternative cash crop. Impacts of political instability, such as the collapse of cotton marketing and land management institutions, of communal labour arrangements and aggravation of cattle rustling were linked to the changes. Crop productivity in the farming systems is poor and nutrient balances differed between farm types. Balances of N, P and K were all positive for larger farms (LF) that had more cattle and derived a larger proportion of their income from off-farm activities, whereas on the medium farms (MF), small farms with cattle (SF1) and without cattle (SF2) balances were mostly negative. Sustainability of the farming system is driven by livestock, crop production, labour and access to off-farm income. Building private public partnerships around market-oriented crops can be an entry point for encouraging investment in use of external nutrient inputs to boost productivity in such African farming systems. However, intervention strategies should recognise the diversity and heterogeneity between farms to ensure efficient use of these external inputs.

Computed evapotranspiration of annual and perennial crops at different temporal and spatial scales using published parameter values

Land use changes from perennial to annual crops and vice versa may affect water balances, in particular through changing evapotranspiration rates with great implications for modelling. Direct comparison of evapotranspiration rates published in the literature is hardly possible, since differences in soils and climate make data incomparable. This paper presents literature data on crop parameters and environmental conditions that determine transpiration for four crops (oil palm, cocoa, maize and rice) and evaporation. Transpiration at leaf scale and soil evaporation as well as evaporation of intercepted rainfall have been computed using these data and are scaled-up to canopy scale. The objective is to quantify evapotranspiration of the crops under identical environmental conditions at two temporal scales: daily and annual evapotranspiration. Variation among crops in transpiration at leaf scale owing to crop characteristics is levelled out in scaling up to canopy scale. Differences in annual evapotranspiration between perennial and annual crops mainly are due to the fact that perennial crops transpire during the dry season, although at low rates, but still considerably higher than evaporation rates of bare and dry soils. Apparently, the degree of soil cover with vegetation in space and in time is of major importance to evaluate differences in annual evapotranspiration caused by changes in land use.

Designing a “Target-Tree” for Maximizing Gross Value of Product in Patagonian Sweet Cherry Orchards

Abstract A “target-tree” approach to maximize gross value of product (GVP; US

Resource use dynamics and interactions in the tropics: Scaling up in space and time

/ha) at the farm gate was developed and applied to sweet cherry orchards, integrating eco-physiological information, model estimates, and expert knowledge. During the 2003/04 growing season, the effect of the ratio of Fruit Number to Leaf Area (FNLAR; fruit/m2 LA) on Mean Fruit Weight (MFW; g/fruit) was analyzed at both the spur and whole-tree level, for different combinations “training system-cultivar” (Experiment 1). During the 2004/05 season, the effect of FNLAR on MFW, soluble solids content (SSC), titratable acidity (TA), and firmness (F), was evaluated at whole-tree level in a ‘Bing’/‘Mahaleb’ orchard (Experiment 2). In Experiment 1, there were no significant interactions between training system and cultivar for the effect of FNLAR on MFW at spur and whole-tree level. No significant differences were observed between vase and Tatura-trellis training systems. The R2-values for the relationships per cultivar were higher at whole-tree level than at spur level. At both the spur and whole-tree level, ‘Lapins’ had the highest Y-intercept and ‘Van’ the lowest. At spur level, no differences among cultivars were detected in their sensitivity to increments in FNLAR, but at whole-tree level ‘Van’ showed less sensitivity than ‘Lapins’ and ‘Bing’. In Experiment 2, MFW, TA and SSC decreased linearly with increasing FNLAR (P < 0.05). Firmness showed the same tendency, but the relationship was not significant (P = 0.082). Minimum fruit quality thresholds define the suitable market for the fruit (export, domestic, or industry), with their associated price ranges. In addition, in both the domestic and export markets, price depends mainly on fruit size. FNLAR determines fruit quality (and indirectly fruit price), and yield, in combination with MFW and leaf area index (LAI). GVP is calculated as the product of yield and fruit price. The combination ‘Bing’/‘Mahaleb’ on vase was used to illustrate parameter estimation (LAI and FNLAR) for a “target-tree” in Patagonian orchards, using the results of Experiment 2. Under these conditions, a LAI of 3.07 is required to intercept 75% of PAR at harvest. With these parameters, and considering “price-fruit quality” relationships based on expert knowledge, maximum GVP was obtained with 80 fruit/m2 LA and a yield of 18.25 t-ha−1. Although this example is limited to a single combination of cultivar and training system in a specific location, the methodology can be applied to other situations, provided reliable relevant eco-physiological information is available.