Consistency of effects of tropical-forest disturbance on species composition and richness relative to use of indicator taxa.

Lawton et al. (1998) found, in a highly cited study, that the species richness of 8 taxa each responds differently to anthropogenic disturbance in Cameroon forests. Recent developments in conservation science suggest that net number of species is an insensitive measure of change and that understanding which species are affected by disturbance is more important. It is also recognized that all disturbance types are not equal in their effect on species and that grouping species according to function rather than taxonomy is more informative of responses of biodiversity to change. In a reanalysis of most of the original Cameroon data set (canopy and ground ants, termites, canopy beetles, nematodes, and butterflies), we focused on changes in species and functional composition rather than richness and used a more inclusive measure of forest disturbance based on 4 component drivers of change: years since disturbance, tree cover, soil compaction, and degree of tree removal. Effects of disturbance on compositional change were largely concordant between taxa. Contrary to Lawton et al.'s findings, species richness for most groups did not decline with disturbance level, providing support for the view that trends in species richness at local scales do not reflect the resilience of ecosystems to disturbance. Disturbance affected species composition more strongly than species richness for butterflies, canopy beetles, and litter ants. For these groups, disturbance caused species replacements rather than just species loss. Only termites showed effects of disturbance on species richness but not composition, indicating species loss without replacement. Although disturbance generally caused changes in composition, the strength of this relationship depended on the disturbance driver. Butterflies, litter ants, and nematodes were correlated with amount of tree cover, canopy beetles were most strongly correlated with time since disturbance, and termites were most strongly correlated with degree of soil disturbance. There were moderately divergent responses to disturbance between functional feeding groups. Disturbance was most strongly correlated with compositional differences of herbivores within beetles and nematodes and humus feeders within termites. Our results suggest that consideration of the impact of different forms of disturbance on species and functional composition, rather than on net numbers of species, is important when assessing the impacts of disturbance on biodiversity.

Influence of nutrient application rate on growth and rooting potential of the West African hardwood Triplochiton scleroxylon.

Effects of stock plant nutrition on growth and subsequent rooting of leafy stem cuttings of the commercially important west African tree Triplochiton scleroxylon K. Schum. were investigated to identify the primary morphological variables influencing adventitious root formation. Potted plants were watered as required with one of four balanced nutrient solutions (0, 15, 63 or 125 mg N l(-1)). Increasing the nutrient supply to stock plants increased growth rate and rate of adventitious root production of subsequently harvested cuttings, but an optimum nutrient regime was not identified. Rooting percentage increased from 27% in cuttings harvested from stock plants receiving a low nutrient supply to 64% for cuttings harvested from stock plants receiving eight times the amount of nutrients extractable from plants under typical field conditions in West Africa. The ability of a cutting to retain its leaf during propagation and the length of the cutting were the primary measured characteristics associated with rooting and accounted for 43% of the variance fitted in the model. The length of the new shoot, total leaf area and within-shoot position (node position) were also significantly associated with rooting. After these characteristics were fitted to the model, the genetic origin (clone) was not statistically significant. We conclude that genetic variation in rooting potential is mediated through variations in morphological and physiological traits, rather than through genetic variation in cell differentiation or related aspects of root initiation.

Influence of female cones on the vegetative growth of Pinus contorta trees.

Branches of Pinus contorta Dougl. bearing two-year-old female cones initiated fewer lateral buds than vegetative branches. However, the number of lateral shoots that differentiated and grew was not reduced on female cone-bearing branches. Neither the number nor the weight of female cones influenced the length of the terminal shoot. The total length of all lateral shoots was positively associated with the weight of two-year-old female cones. Branch units with two-year-old female cones produced significantly more total dry weight in the current year than vegetative branch units. There was, however, no significant reduction in the dry weight of terminal and lateral shoots. Branches bearing female cones allocated between 17 and 45% of the current year's dry weight to two-year-old cones and between 1 and 5% was allocated to one-year-old female cones. Female cones therefore apparently do not reduce the photosynthetic potential of trees. The influence of female cones compared with male cones on the growth of trees is discussed.

Influence of male cones on early season vegetative growth of Pinus contorta trees.

On average, branches of Pinus contorta Dougl. bearing male cones had 35 fewer needle pairs than equivalent vegetative branches, and significantly fewer differentiated primordia (i.e., male cones + needle pairs + sterile cataphylls). It was estimated that the formation of male cones results in a 27-50% reduction in the number of needles per male cone-bearing branch. In early spring, branches bearing male cones had on average 23% (0.44 g) more dry weight than vegetative branches. On average, 95% of the dry weight of male cone-bearing branches was allocated to the terminal shoot (54% of which was male cones) and 5% to the lateral shoots. By comparison, vegetative branches allocated 85% of their total dry weight to the terminal shoot and 15% to the lateral shoots. These findings suggest that male cones may reduce the photosynthetic potential of the trees which bear them.

The dynamics of rooting in Triplochiton scleroxylon cuttings: their relation to leaf area, node position, dry weight accumulation, leaf water potential and carbohydrate composition.

Single-node, leafy stem cuttings of Triplochiton scleroxylon K. Schum. were collected from successive nodes down the uppermost shoot of 2-shoot stockplants. The leaves were trimmed to 10, 50 and 100 cm(2) before the cuttings were set under intermittent mist to root. Batches of cuttings were harvested after 0, 14, 28 and 42 days to assess leaf water potential, dry weight and carbohydrate content of their leaf and stem portions. Cuttings with leaf areas of 10, 50 and 100 cm(2) increased in total dry weight by 29, 61 and 90%, respectively, during the 6-week period. The increase in dry weight was accompanied by increases in reflux-extracted soluble carbohydrates (RSC), water-soluble carbohydrates (WSC) and starch. By contrast, increase in leaf area reduced leaf water potential of cuttings before root emergence. Fewer large-leaved cuttings rooted than smaller-leaved cuttings, suggesting that rooting ability is at least partially determined by the balance between photosynthesis and transpiration. Fewer roots per cutting were produced on cuttings with 10 cm(2) leaves than on cuttings with larger leaves. Node position affected increments in dry weight, carbohydrate content and leaf water potential, with differences between nodes on day 0 generally being lost or slightly reversed by day 14. Rooting ability was not related to initial (day 0) carbohydrate content, suggesting that rooting is dependent on carbohydrates formed after severance. During the rooting period, the proportions of total non-structural carbohydrate as WSC and starch were reversed, from mostly WSC on day 0 to mostly starch by day 42. These changes in WSC and starch occurred most rapidly in large-leaved cuttings.

Physiological, environmental and genetic variation in apical dominance as determined by decapitation in Triplochiton scleroxylon.

Patterns of lateral shoot growth following decapitation in 1-meter tall, rooted Triplochiton scleroxylon K. Schum. cuttings varied with clone and in response to a range of environmental conditions and growth regulator treatments. Two phases of bud activity were identified, the Sprouting Phase, in which many buds were released from correlative inhibition, and the Dominance Phase (starting 3-4 weeks after decapitation) in which uppermost laterals began to dominate and suppress growth, and sometimes cause apical abscission of lower lateral shoots. Except in non-erect plants, the most distal lateral to elongate became the new leading shoot. During the Sprouting Phase, the proportion of active buds was increased by removing leaves from the upper part of the stem, and by reducing the photoperiod from 13-15 h to 11 h, particularly at 20 degrees C rather than 25 degrees C. Conversely, the proportion of sprouting buds was decreased by injecting plant stems with NAA (250 microg/plant). During the Dominance Phase, suppression of lateral shoot growth was hastened by stem injection with GA(3) (200 microg/plant), especially when applied to the uppermost shoot at the end of the Sprouting Phase. Reimposition of dominance was delayed, however, by (1) high rates of N:P:K fertilization, (2) low temperature (20 versus 25 degrees C) under relatively long days (13 and 15 h), (3) low photon flux density (160 micromol m(-2) s(-1)) and (4) severe defoliation. Plant orientation had no effect on bud activity of decapitated plants, but affected the relative vigor and orientation of new lateral shoots. High temperature (25 versus 20 degrees C) and injection with GA(3) increased the erectness of newly developing lateral shoots.

Parental factors in dominance of lateral buds on rhizomes of Agropyron repens (L.) beauv.

A mixture of 1-naphthylacetic acid and 6-benzylaminopurine was found to be an effective substitute for the rhizome apex of attached rhizomes, when the plants were not at anthesis. The same mixture was not effective, however, when the plants were at anthesis. The same growth-regulators applied to the apical end of detached multi-node rhizome fragments did not maintain correlative inhibition unless growth-regulators were simultaneously applied to the basal end. Various interactions between apical and basal applications are discussed with regard to their role as a parental factor in apical dominance.