Observations of tree growth, woodland structure and elephant damage on Colophospermum mopane in Luangwa Valley, Zambia

Six samples of tagged Colophospermum mopane were monitored for five years in locations with varying soil characteristics but with similar elephant densities. Physiognomic variation among the samples was related to soil differences, which also correlated with different browsing habits by elephants. The impact of elephant browsing further influenced both the physiognomy and demography of C. mopane. Results from this study suggest that the influence of soils and elephants on C. mopane alter successional transitions from grassland to woodland. Soils that promote coppicing of C. mopane yield less stable woodlands when associated with elephants than soils promoting woodlands with large bolus, non-coppicing trees. The dynamics of the latter are determined more by tree recruitment as influenced by such agents as other browsers or frequency and seasonality of bush fires. Implications for forest/elephant management are discussed.     

Seasonal variations in the quality of mopane leaves as a source of browse for mammalian herbivores

This paper deals with seasonal variations in the chemical composition of mopane leaves in terms of their palatability to herbivores. Green mopane leaves were found to be most palatable over summer, a period when, due to very poor rainfall, the veld was at its most desolate. Senescing leaves were at their most palatable in late winter/early spring, this resource maintaining many herbivores for the short period before the spring leaf flush. Four leaf classes were identified during the spring leaf flush: young green leaves were the most palatable followed by very young leaves, mature green leaves and senescent leaves. The flush is independent of rain and coincides with the end of the dry season, a period when most tree species occurring in association with the mopane are leafless and the grass sward is very poor. It begins earlier on heavily browsed trees. Most ungulates present in the area were found to utilize the nutritious young leaves. The importance of this leaf flush is discussed with respect to the management of the area, and the importance of the heavy pressure large mammals are imposing on this veld type is elicited.     

Influence of Colophospermum mopane canopy cover on litter decomposition and nutrient dynamics in a semi‐arid African savannah

The objective of this study was to investigate the influence of mopane canopy cover on litter decomposition in a semi‐arid African savannah. We used a randomized block design with five blocks of 100 × 100 m demarcated in a 10‐ha pocket of open mopane woodland. Litterbags were placed beneath large (8.3 m crown diameter) and small mopane trees (2.7 m crown diameter) and in the intercanopy area. Decomposition was fastest in the intercanopy area exposed to solar radiation (k = 0.35 year^?1), intermediate beneath small trees (k = 0.28 year^?1) and slowest beneath large trees (k = 0.23 year^?1). Soil temperatures beneath small and large trees were 3–5 and 6–9°C lower than in the intercanopy area, respectively. Bacterial and fungal counts were significantly higher (P < 0.05) beneath large than small trees and in the intercanopy area. The amount of N and P released did not vary significantly among sampling sites. Soil moisture in the dry season was similar among sampling sites but rainy‐season soil moisture was significantly greater (P < 0.05) beneath large than small trees and in the intecanopy area. Mopane canopy cover retarded litter decomposition suggesting that photodegradation could be an important factor controlling carbon turnover in semi‐arid African savannahs.     

Effects of simulated browsing on growth and leaf chemical properties in Colophospermum mopane saplings

Browsing intensity influences a plant's response to herbivory. Plants face a trade-off between investment in the production of secondary compounds and investment in growth. To elucidate this trade-off, we simulated four browsing intensities (0%, 50%, 75% and 100%) on mopane saplings, Colophospermum mopane (J.Kirk ex Benth.) J.Léonard, in a greenhouse experiment. This showed that, with increasing defoliation intensity, plants change their investment strategy. At intermediate levels of defoliation (50%), mopane saplings increased the synthesis of condensed tannins, so that tannin concentrations followed a hump-shaped relation with defoliation intensity, with significantly higher tannin concentration at intermediate defoliation levels. When defoliated heavily (75% and 100%), tannin concentrations dropped, and plants were carbon stressed as indicated by a reduced growth rate of the stem diameter, and leaf production and mean individual leaf mass were reduced. This suggests that, at intermediate defoliation intensity, the strategy of the plants is towards induced chemical defences. With increasing defoliation, the relative costs of the secondary metabolite synthesis become too high, and therefore, the plants change their growing strategy. Hence, browsers should be able to benefit from earlier browsing by either adopting a low or a relatively high browsing pressure.     

Effects of large herbivores and fire on the regeneration of Acacia erioloba woodlands in Chobe National Park, Botswana

Acacia erioloba woodlands provide important forage and shade for wildlife in northern Botswana. Mortality of mature trees caused by browsing elephants has been well documented but the lack of regeneration of new trees has received little attention. Annual growth of new shoots and changes in height were measured to determine the influence of elephants and small ungulate browsers, rainfall and fire on the growth and survival of established A. erioloba seedlings from 1995 to 1997 in the Savuti area of Chobe National Park. All above‐ground vegetation was removed from 40% of established seedlings in 1995 and 28% in 1997 by browsing elephants, and the mean height of remaining seedlings decreased from >550 mm to <300 mm. When seedlings browsed by kudu, impala and steenbok but not elephants are considered, mean seedling height increased <50 mm per year, even though mean new shoot growth remaining at the end of the dry season was 100–200 mm. Fires burned portions of the study area in 1993 and 1997, killing above‐ground vegetation, but most established A. erioloba seedlings survived, producing coppice growth from roots. While elephants and fire caused the greatest reduction in established seedling height and number, small browsers suppressed growth, keeping seedlings vulnerable to fire and delaying growth to reproductive maturity.     

The role of water abundance, thermoregulation, perceived predation risk and interference competition in water access by African herbivores

In African savannas, surface water can become limiting and an understanding of how animals address the trade-offs between different constraints to access this resource is needed. Here, we describe water access by ten African herbivore species in Hwange National Park, Zimbabwe, and we explore four possible determinants of the observed behaviours: water abundance, thermoregulation, perceived predation risk and interference competition. On average, herbivores were observed to drink in 80% of visits to a waterhole. The probability of drinking was higher in 2003 (474 mm) than in 2004 (770 mm), and at the end of the dry season than at its beginning. For larger species, this probability may also be related to risks of interference competition with elephants or other herbivores. For smaller species, this probability may also be related to the perceived risk of predation. We also investigate the time spent accessing water to drink. The influence of herd size and the presence of young on the time spent accessing water for most species suggests that perceived predation risk plays a role. Thermoregulation also affects this time: during the hottest periods, herbivores spend less time in open areas, unless when wind is strong, probably owing to evapotranspired heat loss.     

The chemical composition of montane plants and its influence on the diet of the large mammalian herbivores in the Pare National des Volcans, Rwanda

The diets of five large mammalian herbivores in the Pare National des Volcans, a tropical montane forest, were determined using microhistological analysis of faecal material. Correction factors were used to calculate the biomass intake from the relative areas of leaf cuticle found in the faeces. Dietary nitrogen was calculated from the biomass intake obtained in the faecal analysis. This compared well with faecal nitrogen values corrected for dietary intake using equations from the literature. Both measures indicated that each of the five herbivores was obtaining a nitrogen-rich diet. Preferences for food plants were calculated using Manly's alpha as a measure. Nutrient content and other measures of dietary quality of the plants were determined and a multiple regression analysis used to determine which were important for the dietary intake of each herbivore. Mountain gorillas showed a selection of digestible food plants whilst buffalo selected food plants that were not easily digested. Other than in vitro digestibility, none of the measures of dietary quality correlated with the intake of the herbivores. Dietary overlap between the species was significantly lower than expected overlap values calculated using Monte Carlo analyses of neutral models. It is suggested that nutrients were not limiting the populations of these five herbivores.     

Potential influence of midgut pH and redox potential on protein utilization in insect herbivores

Early insect physiologists recognized the importance of gut physicochemistry, primarily pH, redox potential, and ionic strength, on digestive processes, but studies to date have been primarily restricted to keratin- and wood-feeding insects. Recent investigations show that herbivorous insects have a broad range of gut redox conditions, with pHs ranging from 6.0 to 11.8 and measured redox potentials from −200 to +240 mV. The redox state of the gut is largely dependent on pH, which is well regulated, and the redox activity of ingested material, including plant chemicals, at that pH. Inter- and intraspecific variation in midgut redox conditions appears to be substantial enough to affect digestion via effects on the structure and function of dietary proteins and proteolytic enzymes. The impact of reducing conditions on proteins probably depends on characteristics such as tertiary structure and the number and arrangement of disulfide linkages. In addition to the effects of reducing conditions in dietary proteins, there can be effects on the activities of digestive enzymes, depending on their structure and the nature of their catalytic site. We speculate that phylogenetic and environmental determinants of gut physicochemistry may place constraints on the efficacy of different digestive processes, and may thus influence the evolution of digestive strategies in insects. © 1996 Wiley-Liss, Inc.     

No effects of fire,large herbivores and their interaction on regrowth of harvested trees in two West African savannahs

Theory and empirical evidence for the impacts of fire and herbivory in savannahs is well established – they are top‐down disturbances that maintain savannahs in disequilibrium states away from potential tree cover. In African savannahs, the demand for fuelwood is extremely high, so tree harvest likely also has an impact, both directly and indirectly, on tree cover, density and biomass. Many savannah trees resprout vigorously from the base after harvest. However, harvested trees regenerate as saplings susceptible to fire and browsing, so harvest may have important demographic consequences. Here, we report the effects of tree harvest, and its interaction with fire and herbivory, on savannah dynamics by analysing woody regrowth following a harvest in arid Sahelian and mesic Guinean savannahs in Mali, West Africa. Tree harvest resulted in an overall reduction in wood production per tree compared to growth in nonharvested trees. Regrowth, either biomass or height, did not differ among fire and herbivory treatments. Our results suggest that the resprouting abilities that savannah trees have evolved to cope with frequent fire are essential for surviving tree harvest and subsequent disturbance. In these savannahs, regrowth is rapid enough in the first growing season to escape the impact of dry season fires.     

Rapid, landscape scale responses in riparian tundra vegetation to exclusion of small and large mammalian herbivores

Productive tundra plant communities composed of a variety of fast growing herbaceous and woody plants are likely to attract mammalian herbivores. Such vegetation is likely to respond to different-sized herbivores more rapidly than currently acknowledged from the tundra. Accentuated by currently changing populations of arctic mammals there is a need to understand impacts of different-sized herbivores on the dynamics of productive tundra plant communities. Here we assess the differential effects of ungulate (reindeer) and small rodent herbivores (voles and lemmings) on high productive tundra vegetation. A spatially extensive exclosure experiment was run for three years on river sediment plains along two river catchments in low-arctic Norway. The river catchments were similar in species pools but differed in species abundance composition of both plants and vertebrate herbivores. Biomass of forbs, deciduous shrubs and silica-poor grasses increased by 40–50% in response to release from herbivory, whereas biomass of silica-rich grasses decreased by 50–75%. Hence both additive and compensatory effects of small rodents and reindeer exclusion caused these significant changes in abundance composition of the plant communities. Changes were also rapid, evident after only one growing season, and are among the fastest and strongest ever documented in Arctic vegetation. The rate of changes indicates a tight link between the dynamics of productive tundra vegetation and both small and large herbivores. Responses were however not spatially consistent, being highly different between the catchments. We conclude that despite similar species pools, variation in plant species abundance and herbivore species dynamics give different prerequisites for change.     

阔叶红松林大径级个体对树木空间分布的影响

对森林空间结构的研究一直是生态学重要研究内容之一,林分中大径级个体直接影响周围林分的空间格局,当大径级个体成为倒木或枯立木时会造成较大林隙,这也将导致周围空间格局发生变化。利用小兴安岭凉水自然保护区内10.4 hm2样地的调查数据,统计了样地内乔木树种组成,分析计算样地内树木个体间的分布规律,探究大径级个体对周围邻木空间分布的影响。研究结果表明:样地中共有乔木24种,共计7412株(去除分株),隶属于11科18属,林分中大径级个体周围存在由小径级树木聚集生长形成的环形结构,该环形结构随着邻木径级的变化而变化,2级邻木(5.0≤DBH10.0 cm)主要在距离大径级个体3—7m范围内形成环形结构,3级邻木(10.0≤DBH30.0 cm)主要在距离大径级个体3—6 m范围内形成环形结构,而1级邻木(1.0≤DBH5.0 cm)与4级邻木(30.0≤DBH50.0 cm)并未形成环形结构,经分析环形结构的产生与林分内个体间的竞争存在密切关系,并且该结构与林隙斑块动态理论间存在内在联系,该结构是森林动态变化的一种表现形式,会伴随林木的生长产生或消失。另外研究表明大径级个体胸径的变化对邻木的环形结构分布范围也有一定影响,当大径级个体胸径增加时邻木的聚集范围有收缩的趋势。本次实验采用单次获取的林木数据,对于环形结构随时间的变化情况还有待进一步探究。     

The influence of ants on the guild structure of Acacia insect communities in Mkomazi Game Reserve, north-east Tanzania

A total of 41,099 insect specimens of 133 families and 492 morphospecies were collected from 31 trees of six species of Acacia in north-east Tanzania, representing one of the largest insect samples ever analysed from a tropical savannah habitat. Herbivores (sapsuckers and chewers) and parasitoids had the highest diversity shares, whereas the highest biomass shares were obtained by phytophagous chewers, ants and predators. The percentage biomass of ants was correlated positively with the diversity share of sapsuckers and negatively with the diversity share of tourists. A positive correlation was found with the residual biomass share of phytophagous sapsuckers, indicating a protective function of ants for this guild. Diversity and abundance share was much higher in egg and coccoid parasitoids compared to larval parasitoids, probably due to predation by ants on larval parasitoids. Their low diversity supports the hypothesis of a decline towards the equator in ichneumonid diversity.     

小兴安岭阔叶红松林大径级个体对同种邻木分布的影响

利用小兴安岭凉水自然保护区内10.4 hm²样地的调查数据,统计并分析样地内乔木树种的组成与分布,借助混交度计算方法探究大径级个体周围同种邻木分布情况,并分析其产生的原因及大径级个体在其中发挥的作用,研究结果表明：（1）大径级个体（DBH ≥ 50.0 cm）与其周围5-6 m范围内的邻木的混交程度最低,易形成围绕大径级个体（以下简称中央木）的同种邻木环形结构,该环形结构在一定程度上推动着未来的森林演替与更新;（2）距离中央木15 m范围内的中小径级个体（1级木（1.0 cm ≤ DBH < 5.0 cm）、2级木（5.0 cm ≤ DBH < 10.0 cm）、3级木（10.0 cm ≤ DBH < 30.0 cm））与中央木均保持较高的混交程度,而4级木（30.0 cm ≤ DBH < 50.0 cm）、5级木（DBH ≥ 50.0 cm）与中央木的混交程度明显下降,以5级木最低,这种现象的产生与密度制约及物种共存机制存在密切关系。通过研究不同物种及不同生长阶段（不同径级）的林木分布,揭示了凉水地区阔叶红松林中大径级个体周围邻木分布格局特征及其成因,论证了大径级个体在控制周围林分结构、维持物种多样性及推动森林演替中的重要作用,为阔叶红松林的经营及管理提供了数据支撑。     

Annual growth rings, rainfall-dependent growth and long-term growth patterns of tropical trees from the Caparo Forest Reserve in Venezuela

1 Tree-ring analyses and dendrometer measurements were carried out on 37 tree species in a semi-deciduous forest of the Reserva Forestal de Caparo, Venezuela, where the mean annual rainfall is about 1700 mm and there is a dry season from December to March. The main purposes of the investigation were to show the seasonality of cambial growth, and the connection between precipitation patterns and tree-ring curves. Long-term rates of wood increment were also estimated.
2 Cambial markings in consecutive years showed that annual rings were formed by many species.
3 The distinctiveness of growth zones was usually greater in deciduous species than in evergreen species, although not all deciduous species had distinct rings.
4 Dendrometer measurements showed that the annual growth rhythm was related to precipitation patterns. Evergreen species tended to show only a short interruption of wood growth (during the later part of the dry season), whereas deciduous species stopped growth completely at the end of the rainy season.
5 For deciduous species, regression analyses showed close relations between tree-ring width and the sum of precipitation outside the rainy seasons (i.e. November to April). Evergreen species reacted to the total annual amount of precipitation.
6 Variation in longest available ring chronology (for Terminalia guianensis ) showed little correlation with the El Niño–Southern Oscillation effect.
7 On average trees from natural forests showed relatively constant growth over the entire life span. Plantation trees grew fast up to an age of 15–20 years, but annual increments then decreased to values seen in natural forest trees.     

The influence of stem guying on radial growth,stem form and internal resin features in radiata pine

Key message

Stem guying to prevent wind-induced swaying of radiata pine trees resulted in significant changes in radial growth, but did not affect the frequency of compression wood or resin features.

Abstract

Mechanical stress resulting from wind forces acting on trees can cause a number of direct and indirect effects ranging from microscopic changes in cambial activity through to stem breakage and uprooting. To better understand these effects on radial stem growth and wood properties, an experiment was established in a 13-year-old radiata pine (Pinus radiata D Don) stand in which 20 trees were guyed to prevent them from swaying. Radial growth was monitored in these trees and 20 matched controls at monthly intervals for 5 years. The trees were then felled and radial growth, resin features and compression wood were assessed on cross-sectional discs taken at fixed locations up the stem. There was a significant reduction in radial growth at breast height (1.4 m above the ground) in the guyed trees, but an increase in growth immediately above the guying point. A total of 277 resin features were observed in the growth rings formed following guying. The overall frequency of such features was related to height within the stem and annual ring number. No effect of stem guying was found on the incidence of compression wood. Interestingly, the distribution of resin features also did not differ between guyed and un-guyed trees. There was no evidence of a link between stem restraint as a result of guying and the incidence of resin features, suggesting that other factors, such as soil moisture may be more influential.     

The impact of large herbivores on floral composition and vegetation structure in the Naukluft Mountains, Namibia

Floral composition and vegetation structure were investigated on the Naukluft Mountain Plateau to determine (i) whether or not grazing has had an impact on the flora of the Naukluft Mountains during the last two decades and (ii) if so, whether vegetation recovery is influenced by the nature of the rainy season. Vegetation subjected to grazing to large herbivores and vegetation excluded from grazing for 18 and 19 years was compared. Based on classification (TWINSPAN) and ordination (detrended and canonical correspondence analysis) of 20, 10 × 10 m stands within and outside exclosures in two seasons, a clear difference in floristic composition between grazed and not grazed areas was indicated. Although neither the height of the vegetation, not the growth form spectrum differed significantly between grazed and not grazed sites, leaf-succulents were much more abundant when grazing was excluded. This suggests that changes in the vegetation due to grazing pressure have occurred in the last two decades, but the degree of grazing impact is not severe and may be part of the natural savanna ecosystem of the Naukluft Mountains. Whether this trend also occurs in other vegetation types in the Naukluft Mountains remains to the investigated. This revised version was published online in November 2006 with corrections to the Cover Date.     

The influence of cellulose filler on feeding, growth and utilization of protein and energy in rainbow trout, Salmo gairdnerii Richardson

A nutrient source was diluted with indigestible filler in the form of α-cellulose. The trout compensated for up to 30% cellulose by increasing the total weight of food eaten, thereby stabilizing both nutrient intake and growth rate. The fish failed to compensate for levels of 40–50% cellulose and, despite evidence for an increase in the size of the stomach, nutrient intake was approximately halved. There was a slight reduction in food conversion efficiency and the growth rate declined accordingly. By contrast, faecal output rose considerably.