Differences between understorey and canopy in herbivore community composition and leaf quality for two oak species in Missouri

1. From July 1994 to September 1995, at six censuses, the herbivore community associated with understorey (< 2.5 m height) and canopy (15–20 m) leaves of Quercus alba and Q. velutina was sampled in south-eastern Missouri, U.S.A. 2. Across all censuses, herbivore densities were not significantly different between canopy and understorey for Q. alba and Q. velutina, except in August 1994 when herbivore densities were 60% higher in the canopy on Q. alba. Little significant spatial variation in herbivore densities or community composition was found during the study years. 3. The herbivore community was diverse, consisting of 138 species of leaf-chewing insects. Species richness was significantly greater (by 5–20%) in the understorey than in the canopy for both tree species, and the relative abundance of the main families, different feeding guilds, and most common species differed significantly between understorey and canopy. 4. To determine the extent to which leaf quality explained the observed patterns, percentage nitrogen and protein binding capacity were measured in canopy and understorey leaves of Q. alba and Q. velutina. Per cent nitrogen was higher in canopy leaves for Q. velutina while protein binding capacity was higher in canopy leaves for Q. alba. 5. These results suggest that the herbivore community associated with these two species of Quercus comprises species that appear to respond individually to environmental and biological conditions encountered in the understorey and the canopy.     

Abundance of white lace lerp psyllids on understorey and canopy river red gums and relationships with foliar sugars and tannins

1. Trees present herbivorous insects with the greatest diversity of resources of any plant growth form. Both ontogeny and shading can alter suitability for arboreal insect herbivores. 2. We conducted a longitudinal study of tagged ‘mature’ (>12 months old) Eucalyptus camaldulensis leaves to compare the suitability of understorey and canopy trees for the leaf senescence-inducing psyllid, Cardiaspina albitextura. We quantified sugars and tannins as possible predictors of nymphal abundance. 3. Canopy leaves hosted double the number of nymphs as understorey leaves. Variation among individual trees (understorey and canopy) was the most important source of heterogeneity explaining psyllid abundance, although relative leaf age significantly influenced oviposition on canopy leaves. The diversity of foliar sugars was higher among canopy leaves than among understorey leaves. There was significant between-tree diversity in total hydrolysable tannins (HTs) and total condensed tannins (CTs) among understorey trees but not among canopy trees. Heterogeneity among understorey and canopy trees was explained by greater diversity of ellagitannins (HTs) than of CTs. 4. Shading is detrimental to the survival of nymphs on both host types, but sugars are unlikely to explain variation in suitability. Vescalagin (an ellagitannin) was negatively correlated with the abundance of nymphs on both host types.     

Plastic leaf morphology in three species of Quercus: The more exposed leaves are smaller,more lobated and denser

Phenotypic plasticity and developmental instability in leaf traits are common in oak species but the role of environmental factors is not well understood. To decipher possible correlations between different leaf traits and effects of the position of leaves within the tree canopy, we quantified the plasticity of three leaf traits of 30 trees of Quercus alba L., Quercus palustris Muench and Quercus velutina Lam. We hypothesized that trees could modify the shape of their leaves for better adaptation to the variable microclimate within the canopy. Our results demonstrated that the south and north outer leaves were significantly smaller, more lobed and denser than those situated in the inner canopy. The order of leaves on the branch accounted for the plasticity of leaf traits in Q. alba only. Plasticity of lobing in Q. alba and Q. velutina depended on the height of the trees. We detected fluctuating asymmetry (FA) in all three species, but the source of variation depended on branch position in Q. velutina only. FA was more pronounced in north-facing leaves. Plasticity of the leaf traits ranged from small to medium. Plasticity of leaf area and leaf mass per area (LMA) depended on the branch position. However, the plasticity of lobation was not affected by the location of a branch within the tree canopy. Quercus alba and Q. palustris had similar plastic responses but the plasticity of Q. velutina was significantly smaller. We concluded that individual plants detect and cope with environmental stress through vegetative organ modification.     

Differential effects of host plant hybridization on herbivore community structure and grazing pressure on forest canopies

Interspecific hybridization in plants is known to have ecological effects on associated organisms. We examined the differences in insect herbivore community structure and grazing pressure on tree canopy leaves among natural hybrids and their parental oak species. We measured leaf traits, herbivore community structure, and grazing pressure on leaves of two oak species, Quercus crispula and Q. dentata, and their hybrids. The concentration of nitrogen in canopy leaves was greater in hybrids and in Q. dentata than in Q. crispula. The concentration of total phenolics was lower in hybrids than in Q. crispula. The concentration of condensed tannin was greater in hybrids than in Q. crispula. Relative herbivore abundance and species richness were greater on oak hybrids than on either parental species; herbivore species diversity and composition on hybrids were close to those on Q. crispula. Herbivore grazing pressure was lower on hybrids and Q. dentata than on Q. crispula. There was a negative correlation between herbivore grazing pressure and leaf nitrogen, suggesting that interspecific variation among oak taxa in herbivore pressure may be explained by leaf nitrogen; variation in herbivore community structure among oak taxa is likely to be controlled by polygenic leaf traits. Differing responses of (1) herbivore community structure and (2) herbivore grazing pressure to host plant hybridization may play important roles in regulating herbivore biodiversity in cool‐temperate forest canopies.     

Tannin assays in ecological studies: Lack of correlation between phenolics,proanthocyanidins and protein-precipitating constituents in mature foliage of six oak species

Summary There is no correlation between protein-precipitating capacity and either total phenolic or proanthocyanidin content of extracts of mature foliage from six species of oaks: Quercus alba (white oak), Q. bicolor (swamp white oak), Q. macrocarpa (bur oak), Q. palustris (pin oak), Q. rubra (red oak), and Q. velutina (black oak). It is argued that studies which probe the role of tannins in the selection and utilization of food by herbivores should include a protein-precipitation assay, since such an assay provides a measure of the property of tannins which is presumed to contribute to their utility as defensive compounds. A convenient modification of the bovine serum albumin (BSA) precipitation assay, which measures the amount of protein precipitated when a plant extract is added to a BSA solution, is described. Advantages of this procedure recommend its routine adoption in studies of the role of tannins in plant-herbivore interactions.     

Environmentally-based maternal effects: a hidden force in insect population dynamics?

Summary The nutritional environment of the parental generation of the polyphagous gypsy moth, Lymantria dispar, can significantly influence the growth and reproductive potential of the next generation through environmentally-based maternal effects. In the first experiment, members of the parental generation were reared on red oak trees (Quercus rubra L.) with known defoliation and phenolic levels. Diet in the offspring generation was homogeneous (synthetic diet). With genetic effects accounted for 1) offspring attained greater pupal weights when their mothers fed on trees with higher leaf damage levels, 2) daughters had a shorter prefeeding stage, a trait associated with dispersal tendency, when their mothers experienced higher condensed tannin levels, and 3) sons had lower pupal weights when their mothers experienced greater condensed tannin levels. In the second experiment, members of the parental generation were reared on either red or black oak (Q. velutina) trees. Offspring of each mother were divided among four diets: red oak, chestnut oak (Q. prinus L.), a standard synthetic diet, and a low-protein synthetic diet. The parental host species accounted for 24% of the variation in daughters' development time; offspring diet accounted for 52%. When mothers were reared on black oak rather than red oak, their offspring developed significantly faster when the F1 diet was chestnut oak. Environmentally-based maternal effects can significantly influence the expression of offspring dispersal potential, growth rate, and offspring fecundity. These traits contribute to natality and survival in natural populations and, hence, to population growth potential. Theoretical models of insect population dynamics demonstrate that the presence of a time delay in a density dependent response can induce destabilization. Maternal effects act on a time delay and may participate in the destabilization characteristic of outbreak species.     

Tree resistance to Lymantria dispar caterpillars: importance and limitations of foliar tannin composition

The ability of foliar tannins to increase plant resistance to herbivores is potentially determined by the composition of the tannins; hydrolyzable tannins are much more active as prooxidants in the guts of caterpillars than are condensed tannins. By manipulating the tannin compositions of two contrasting tree species, this work examined: (1) whether increased levels of hydrolyzable tannins increase the resistance of red oak (Quercus rubra L.), a tree with low resistance that produces mainly condensed tannins, and (2) whether increased levels of condensed tannins decrease the resistance of sugar maple (Acer saccharum Marsh.), a tree with relatively high resistance that produces high levels of hydrolyzable tannins. As expected, when Lymantria dispar L. caterpillars ingested oak leaves coated with hydrolyzable tannins, levels of hydrolyzable tannin oxidation increased in their midgut contents. However, increased tannin oxidation had no significant impact on oxidative stress in the surrounding midgut tissues. Although growth efficiencies were decreased by hydrolyzable tannins, growth rates remained unchanged, suggesting that additional hydrolyzable tannins are not sufficient to increase the resistance of oak. In larvae on condensed tannin-coated maple, no antioxidant effects were observed in the midgut, and levels of tannin oxidation remained high. Consequently, neither oxidative stress in midgut tissues nor larval performance were significantly affected by high levels of condensed tannins. Post hoc comparisons of physiological mechanisms related to tree resistance revealed that maple produced not only higher levels of oxidative stress in the midgut lumen and midgut tissues of L. dispar, but also decreased protein utilization efficiency compared with oak. Our results suggest that high levels of hydrolyzable tannins are important for producing oxidative stress, but increased tree resistance to caterpillars may require additional factors, such as those that produce nutritional stress.     

Specialization pays off: contrasting effects of two types of tannins on oak specialist and generalist moth species

Phytochemical coevolution theory posits that specialist herbivores will be less sensitive than generalists to the defensive compounds of their host. On the other hand, both types of herbivores should allegedly be similarly sensitive to ‘quantitative’ defences, such as tannin compounds. In this paper, we critically examine the biological effects of two types of tannins: vescalagin (a quantitatively dominant hydrolysable tannin of Quercus robur), and a mix of condensed tannins. In a phylogenetically controlled design, we compare the response of two specialist moth species (Dichonia aprilina and Catocala sponsa) and two generalist species (Acronicta psi and Amphipyra pyramidea) to four artificial diets: a control diet, a diet with 50 mg g^?1 vescalagin, a diet with 15 mg/g condensed tannins, and a diet with both 50 mg g^?1 vescalagin and 15 mg g^?1 condensed tannins. Overall, we find drastic effects of vescalagin and pronounced differences in the responses of generalist and specialist herbivores, but no detectable effects of condensed tannins, and no interaction between the two types of compounds. More specifically, vescalagin reduced the growth of generalist species to one‐half of control levels over 72 h. The compound served as a strong feeding deterrent to generalists, reducing ingestion rates by two‐thirds. Vescalagin also reduced the metabolic and growth efficiency of generalist species to between 16% and 56% of control levels – effects which were lacking or even reversed in specialist species. These patterns suggest that vescalagin forms an important part of the oak's defence against herbivores, and that specialist species have adapted to deal with such substances. In terms of biological effects, condensed tannins seem much less important. Given a quantitative dominance of hydrolysable tannins over condensed tannins in oak leaves, and a seasonal decline in overall tannin levels, these findings contradict the previous notion that widespread spring feeding among oak herbivores could be attributed to tannins.     

Temporal and spatial variations in leaf herbivory within a canopy of<Emphasis Type="Italic"> Fagus crenata</Emphasis>

This study investigated spatio-temporal variation in the leaf area consumed by insect herbivores within a canopy of Fagus crenata, with reference to the light conditions of leaf clusters. There was no clear relationship between photosynthetic photon flux density (PPFD) and consumed leaf area (CLA) in May, immediately after leaf flush, but CLA decreased with an increase in PPFD after June. Leaf mass per area, carbon concentration, C/N ratio, concentration of total phenolics, and condensed tannin concentration were higher in leaves under high light intensity than those of leaves under low light. On the other hand, the nitrogen concentration of leaves decreased as light availability increased. Consequently, within-tree variation in light availability affects the consumption of leaves by insect herbivores through temporal changes in leaf characteristics.     

Interactions between oak tannins and parasite community structure: Unexpected benefits of tannins to cynipid gall-wasps

Plant species vary tremendously in the number of phytophagous species they support. May (1979) and Price (1980) proposed that some of this variation may be due to variation in biochemical defenses. We find that variation between oak species in leaf tannin levels is positively correlated with 1) variation in the numbers of species of leaf-galling cynipid wasps those trees host; and 2) the density of individual galls per oak leaf. We hypothesize that leaf and gall tannins serve a protective function for cynipids, decreasing the amount of cynipid larval mortality due to fungal infestation. This defensive function would explain the observed positive relationships between oak tannin levels and cynipid diversity as well as cynipid abundance.     

Phenolics,nutrition and insect herbivory in some garrigue and maquis plant species

Summary Garrigue plant species growing on a calcareous substrate in southern France had higher foliar N levels than the same species growing on a relatively lower nutrient siliceous substrate (maquis). However maquis species had significantly higher foliar levels of P, more water, higher phenolic concentrations and larger leaf areas. The cumulative amount of insect damage on garrigue and maquis plants was similar, presumably due to different nutritional advantages in each case. Soil fertilization signifincantly elevated N levels in Q. coccifera, increased total leaf areas, decreased condensed tannin levels, and these leaves showed significantly more insect damage. Some effects of burning on Q. coccifera are also described. In these shrublands, fertilization may render leaf material more nutritional for herbivores by increasing nitrogen content and decreasing condensed tannin concentration, although very heavy grazing pressure may increase levels of leaf phenolics.     

The influence of Prosopis canopies on understorey vegetation: Effects of landscape position

Abstract. The influence of canopy trees and shrubs on under‐storey plants is complex and context‐dependent. Canopy plants can exert positive, negative or neutral effects on production, composition and diversity of understorey plant communities, depending on local environmental conditions and position in the landscape. We studied the influence of Prosopis velutina (mesquite) on soil moisture and nitrogen availability, and understorey vegetation along a topographic gradient in the Sonoran Desert. We found significant increases in both soil moisture and N along the gradient from desert to riparian zone. In addition, P. velutina canopies had positive effects, relative to open areas, on soil moisture in the desert, and soil N in both desert and intermediate terrace. Biomass of understorey vegetation was highest and species richness was lowest in the riparian zone. Canopies had a positive effect on biomass in both desert and terrace, and a negative effect on species richness in the terrace. The effect of the canopy depended on landscape position, with desert canopies more strongly influencing soil moisture and biomass and terrace canopies more strongly influencing soil N and species richness. Individual species distributions suggested interspecific variation in response to water‐ vs. N‐availability; they strongly influence species composition at both patch and landscape position levels.     

Does the differential seedling mortality caused by slugs alter the foliar traits and subsequent susceptibility of hybrid willows to a generalist herbivore?

Abstract 1. Many Salicaceae species naturally form hybrid swarms with parental and hybrid taxa that differ in secondary chemical profile and in resistance to herbivores. Theoretically, the differential mortality in the seedling stage can lead to changes in trait expression and alter subsequent interactions between plants and herbivores. This study examines whether herbivory by the generalist slug Arion subfuscus, which causes extensive mortality in young willow seedlings, causes shifts in (a) the foliar chemistry of F2 willow hybrids (Salix sericea and Salix eriocephala), and (b) the subsequent susceptibility to Japanese Beetles, Popillia japonica. 2. In 2001, two populations of F2 seedlings were generated: those that survived slug herbivory (80–90% of seedlings placed in the field were killed by the slugs) were designated as S-plants, whereas C-plants (controls) experienced no mortality. 3. Common garden experiments with cuttings from these populations, in 2001 and 2002, revealed extensive variation in the phenolic chemistry of F2 hybrids, but revealed no significant difference between S- and C-plants, although the levels of foliar nutrients, proteins and nitrogen tended to be higher in S-plants. 4. Concentrations of salicortin and 2′-cinnamoylsalicortin explained 55 and 38% of the the variation in leaf damage caused by Japanese beetles, and secondary chemistry was highly correlated within replicate clones (salicortin R²= 0.85, 2-cinnamoylsalicortin R²= 0.77, condensed tannins R²= 0.68). 5. Interestingly, Japanese beetle damage and condensed tannins were positively correlated within the S-plants, but not in the C-plants, suggesting that slugs had selected for plants with a positive relationship between tannins and P. japonica damage. This is unlikely to be a consequence of a preference for tannins, but is suggested to be related to the elevated nutrient levels in the S-plants, perhaps in combination with the complex-binding properties of tannins. 6. The damage was highly correlated within replicate clones and a model choice analysis suggested that Japanese beetle damage may be explained by four factors: concentrations of salicortin, condensed tannins, and nitrogen, as well as the specific leaf area (thick leaves were damaged less).     

Seasonal variation in leaf chemistry of the coast live oak Quercus agrifolia and implications for the California oak moth Phryganidia californica

Summary The perennial foliage of the California coast live oak (Quercus agrifolia Nee) permits herbivores to feed on this oak species throughout the year. Patterns of herbivory for a two-year period on Q. agrifolia were observed in relation to seasonal and age-related changes in the nutritional and defensive characteristics of leaves. Nitrogen and phosphorus contents were higher in new leaves compared to mature foliage. Structural compounds (e.g., cellulose) in leaves rapidly increased with age. Concentrations of tatal phenolics (Folin-Denis) and astringency were higher in new foliage, and concentrations of condensed tannins gradually increased as the leaves matured. Peaks of herbivore damage were observed in June and in September–October, and were caused by outbreaks of the California oak moth (Phryganidia californica). P. californica, a bivoltine oak specialist, exhibited feeding preferences in June for old leaves over emerging leaves, and showed no preferences for leaf classes in September. These results suggest that P. californica is adapted to survive on nutritionally poor foliage and to circumvent quantitative defenses such as condensed tannins.     

Infidelity of leafcutting ants to host plants: resource heterogeneity or defense induction?

Summary Leafcutting ants have strong among- and within-plant preferences, and generally abandon plants long before they are completely defoliated. Two tropical deciduous forest tree species preferred by the leafcutting ant Atta colombica were studied to determine how variation in resource quality affects ant selectivity and partial defoliation of plants. Significant differences in palatability and leaf characteristics of Spondias mombin and Bursera simaruba were found among trees and among leaf types within trees, but not among branches within trees. No short-term responses to experimental defoliation of up to 50% of total canopy were found in either species. Leaf nutrient and poisture content were positively correlated, and phenolic content negatively correlated, with the palatability of Spondias mombin, a species containing hydrolyzable tannins. Leaf moisture and phenolic content were both positively correlated with the palatability of Bursera simaruba, which contains predominantly condensed tannins. The results suggest that variation in leaf quality among and within plants is at least a partial explanation for ant selectivity and partial defoliation of preferred species. There is no evidence that rapidly induced changes in plant chemistry affect ant decisions to abandon these plants. Instead, it appears likely that ants abandon plants once high-quality leaf patches are exhausted. Quantitative variation in leaf nutrients, moisture, and secondary chemicals all appear to contribute to ant preferences for individuals and tissues of highly palatable plants.     

Seeing the forest for the trees: long-term exposure to elevated CO2 increases some herbivore densities

The effects of elevated CO₂ on plant growth and insect herbivory have been frequently investigated over the past 20 years. Most studies have shown an increase in plant growth, a decrease in plant nitrogen concentration, an increase in plant secondary metabolites and a decrease in herbivory. However, such studies have generally overlooked the fact that increases in plant production could cause increases of herbivores per unit area of habitat. Our study investigated leaf production, herbivory levels and herbivore abundance per unit area of leaf litter in a scrub‐oak system at Kennedy Space Center, Florida, under conditions of ambient and elevated CO₂, over an 11‐year period, from 1996 to 2007. In every year, herbivory, that is leafminer and leaftier abundance per 200 leaves, was lower under elevated CO₂ than ambient CO₂ for each of three species of oaks, Quercus myrtifolia, Quercus chapmanii and Quercus geminata. However, leaf litter production per 0.1143 m² was greater under elevated CO₂ than ambient CO₂ for Q. myrtifolia and Q. chapmanii, and this difference increased over the 11 years of the study. Leaf production of Q. geminata under elevated CO₂ did not increase. Leafminer densities per 0.1143 m² of litterfall for Q. myrtifolia and Q. chapmanii were initially lower under elevated CO₂. However, shortly after canopy closure in 2001, leafminer densities per 0.1143 m² of litter fall became higher under elevated CO₂ and remained higher for the remainder of the experiment. Leaftier densities per 0.1143 m² were also higher under elevated CO₂ for Q. myrtifolia and Q. chapmanii over the last 6 years of the experiment. There were no differences in leafminer or leaftier densities per 0.1143 m² of litter for Q. geminata. These results show three phenomena. First, they show that elevated CO₂ decreases herbivory on all oak species in the Florida scrub‐oak system. Second, despite lower numbers of herbivores per 200 leaves in elevated CO₂, increased leaf production resulted in higher herbivore densities per unit area of leaf litter for two oak species. Third, they corroborate other studies which suggest that the effects of elevated CO₂ on herbivores are species specific, meaning they depend on the particular plant species involved. Two oak species showed increases in leaf production and herbivore densities per 0.1143 m² in elevated CO₂ over time while another oak species did not. Our results point to a future world of elevated CO₂ where, despite lower plant herbivory, some insect herbivores may become more common.     

The role of leaf defensive traits in oaks on the preference and performance of a polyphagous herbivore,Orgyia vetusta

1. Leaves possess traits that mediate the preference and performance of herbivores. Most evidence for the importance of leaf traits as defences against herbivory comes from studies of few model plant species. 2. In a phylogenetically explicit comparison, I explain the differences in preference and performance of tussock moth (Orgyia vetusta Boisduval) larvae on leaves of 27 oak (Quercus) species using nine putative leaf defences. 3. The preference for an oak species correlated positively with the survival of caterpillars. The correlation between preference and performance did not differ between oak species native to the range of tussock moth versus those from outside the herbivore's range. 4. The first principal component of leaf traits predicted survival of caterpillars on oak leaves but only marginally predicted their preference between oak species. A multiple regression model showed that evergreenness, toughness, and condensed tannin content were the best predictors of caterpillar survival, and leaf toughness was the best predictor of host preference. 5. Generalist caterpillars may accurately assess the value of novel food sources. Moreover, many leaf traits that have been found to affect herbivory within a plant species can also be used to predict the fitness of a generalist herbivore between species.     

Seasonal changes in the density of the symbionts Buchnera and Wolbachia of the aphid Tuberculatus macrotuberculatus on Quercus dentata

Seasonal changes in leaf traits and tannins, including hydrolysable tannins (HTs) and condensed tannins (CTs), affect aphid body size and demographic patterns. The aphid Tuberculatus macrotuberculatus Essig & Kuwana (Hemiptera: Aphididae) feeds on the leaves of the daimyo oak, Quercus dentata Thunberg (Fagaceae), does not alternate hosts, and is attended continuously by ants. Buchnera aphidicola Munson et al. (a γ‐proteobacterium, hereafter Buchnera) is the primary symbiont of most aphid species. It provides essential amino acids to host aphids. Wolbachia sp. (an α‐proteobacterium) is present in some aphid species. This study investigated the effects of seasonal tannin and leaf trait fluctuations in Q. dentata on aphid performance and Buchnera and Wolbachia densities in T. macrotuberculatus. As the season progressed, the water content and HT concentration in Q. dentata leaves decreased, CT concentration in Q. dentata leaves increased, and T. macrotuberculatus performance decreased. Buchnera density varied in accordance with host aphid performance, whereas Wolbachia density did not vary throughout the season, suggesting that although Buchnera depends on the host plant and host aphid performance for growth, Wolbachia may have a high tolerance for oligotrophic environments and may exist independent of the nutritional metabolism of the host aphid. Although the role of Wolbachia in T. macrotuberculatus remains unclear, it may be associated with resistance to parasitoid wasps and plant secondary metabolites.     

Stratification and diel activity of arthropods in a lowland rainforest in Gabon

The abundance, activity and species richness of arthropods, particularly of insect herbivores, were investigated in the upper canopy and understorey of a lowland rainforest at La Makandé, Gabon. In total 14 161 arthropods were collected with beating, flight interception and sticky traps, from six canopy sites, during the day and at night, from mid-January to mid-March 1999. The effects of stratum were most important, representing between 40 and 70% of the explained variance in arthropod distribution. Site effects represented between 20 and 40% of the variance and emphasized the néed for replication of sampling among canopy sites. Time effects (diel activity) explained a much lower percentage of variance (6–9%). The density and abundance of many arthropod taxa and species were significantly higher in the upper canopy than in the understorey. Arthropod activity was also higher during the day than at night. In particular, insect herbivores were 2.5 times more abundant and twice as speciose in the upper canopy than in the understorey, a probable response to the greater and more diverse food resources in the former stratum. Faunal overlap between the upper canopy and understorey was low. The most dissimilar herbivore communities foraged in the understorey at night and the upper canopy during the day. Further, a taxonomic study of a species-rich genus of herbivore collected there (Agrilus , Coleoptera Buprestidae) confirmed that the fauna of the upper canopy was different, diverse and very poorly known in comparison to that of the understorey. Herbivore turnover between day and night was rather high in the upper canopy and no strong influx of insect herbivores from lower foliage to the upper canopy was detected at night. This suggests that insect herbivores of the upper canopy may be resident and well adapted to environmental conditions there.     

African rainforest vegetation and rumen microbes: Phenolic compounds and nutrients as correlates of digestibility

Summary In order to refine hypotheses concerning food selection by generalist herbivores with ruminant-like digestive systems the chemical correlates of digestibility in such a system have been studied. Samples of seeds and leaves from tree species growing in two African rainforests (Douala-Edea Forest Reserve, Cameroon, and Kibale Forest, Uganda) were assayed for phenolic content and nutrient content, and in-vitro dry matter digestibility was analysed utilizing rumen inoculum from a fistulated steer. Both forests studied carry populations of colobine monkeys with ruminant like digestive tracts. Content of condensed tannins and, to a lesser extent of total phenolics, was found to be negatively correlated with digestibility; a result that may be attributable to the inctivation of microbial enzymes by tannins. The negative association of tannin content and digestibility was stronger in material from the Cameroon site, the vegetation of which contains considerably higher concentrations of tannins and is generally less digestible than that from the Ugandan site. Gross energy content of leaves was also found to be persistently negatively correlated with digestibility. The interpretation of this result is uncertain; however, gross energy yield may well reflect variation in content of cell wall polymers, especially lignin. For the complete set of data, tannins presented the strongest observed correlation with digestibility, but when only mature leaves were considered the relationship with gross energy appeared stronger. No strong association was noted between high nutrient content and high digestibility. This was attributed to the fact that the assay measured the extent of digestion under standardised and very favourable conditions of nutrient supply. The results obtained are discussed in relation to observations of leaf and seed selection preferences of Colobus spp. in these two forests.Publication 19-013 of the Wisconsin Regional Primate Research Center