Physical damage by litterfall to canopy tree seedlings in two temperate New Zealand forests

Abstract. The importance of litterfall as an agent of physical damage and mortality to canopy tree seedlings was investigated in two New Zealand forests: Huapai Scientific Reserve, a warm temperate forest and Pureora Forest Park, a cool temperate forest. 200 natural seedlings within the 10–30 cm height class and 200 artificial seedlings, made from plastic straws and wire, were monitored at 4–6 week intervals over two years in each forest. Litterfall was the cause of a significant proportion of seedling mortality in each forest (11–18%). Seedling growth rates were measured and used to estimate the mortality that a cohort of seedlings may suffer due to litterfall in the time taken to grow through the 10–30 cm height class. Up to 38% mortality due to litterfall may occur to such a cohort at Huapai and up to 29% at Pureora. Although mortality due to litterfall of natural seedlings was similar in both forests, damage at Huapai (7%/yr) was almost 3 × greater than at Pureora (2.5%/yr) (P < 0.001). Artificial seedlings were damaged due to litterfall at almost five times the rate at Huapai (16%/yr) than at Pureora (3%/yr) (P < 0.0001). The rate of damage to artificial seedlings at Pureora is similar to that reported for a Northern Hemisphere temperate forest, however, the rate of damage to artificial seedlings at Huapai is much greater, and of a similar rate to those reported from tropical forests. The evolutionary and ecological implications of the differences between the rates of damage at Huapai and at Pureora, and the mechanisms promoting litterfall damage to seedlings, are discussed.     

Seedling mortality and damage due to non‐trophic animal interactions in a northern New Zealand forest

Abstract The importance of non‐trophic animal damage (biting and uprooting without consumption) and mortality of canopy tree seedlings were investigated in a warm temperate forest, in northern New Zealand. Two hundred seedlings 10‐30 cm in height were monitored at 4‐6‐week intervals for 2 years. Non‐trophic animal damage accounted for more seedling mortality in the first year (37.5% of all mortalities) than any other cause. Of the seedlings damaged in non‐trophic animal interactions 73% were bitten off close to the ground and left uneaten and the remainder were uprooted. In the second year all non‐trophic animal damage and mortality ceased following the control of rabbits (Oryctolagus cuniculus), suggesting that rabbits were the major cause of this damage. Total annual mortality rates (6‐8%) were low. However, measured seedling growth rates indicated an average time for seedlings to grow from 10 to 30 cm of 37 years. Therefore, in the absence of rabbit control, mortality due to non‐trophic animal interactions (3% per year) can have an important cumulative effect. Non‐trophic animal damage found in the present study before rabbits were culled (5% per year) was similar to that reported for two tropical forests, but much less than that reported for some other tropical and temperate forests.     

The influence of macro‐litterfall and forest structure on litterfall damage to seedlings

Abstract Microdisturbance to seedlings is important because it can differentially affect the mortality and recruitment of seedlings of forest tree species and thereby ultimately affect community composition. Microdisturbance due to litterfall has been shown to vary greatly in its influence on seedling survival among and within forests, and yet there have been no previous studies that investigate the cause of these differences. In this study the influence of macro‐litterfall on seedling damage is investigated in five complex temperate forests in New Zealand. Litterfall damage to artificial seedlings in these forests was strongly correlated with macro‐leaf‐fall (leaves > 30 cm × 1.5 cm) dry weight and total macro‐litterfall (leaves and deadwood > 30 cm × 1.5 cm) surface area (R² = 0.99, P < 0.005 for each). Protective vegetation within 2 m of the ground (mostly lianes and woody shrubs) reduced the risk of litterfall damage by up to 84%. Hitherto unexplained differences in litterfall damage to seedlings found among, and within, forests (tropical and temperate) may therefore be due to differences in rates of macro‐leaf‐fall and forest structure. These results are important because they suggest that subtle differences in forest structure, and species composition, may influence regeneration patterns through the litterfall microdisturbance regime.     

Microsite heterogeneity in litterfall risk to seedlings

Abstract Litterfall is an important cause of damage and mortality to seedlings in many forest ecosystems. This study is the first to investigate the contribution of variable risk of litterfall damage to microsite heterogeneity. Two hundred artificial seedlings were ‘planted’ in the ground at 2‐m intervals along transects in each of two New Zealand forests, and all plant species with foliage directly above each artificial seedling were recorded. Additional artificial seedlings were planted on the ground underneath treeferns and on their trunks (epiphytically). The artificial seedlings were monitored monthly for damage over 2 years. Three overtopping species that cause litterfall damage, along with one species that impedes litterfall, produced different microsites with a hierarchy of litterfall damage risk to seedlings (2–30% per year). This risk differed significantly among microsites (P < 0.0005). Seedlings differ in resilience to litterfall and, therefore, microsites with different litterfall risks provided the potential for regeneration niche differentiation. More seedlings were damaged beneath Cyathea dealbata (Cyatheaceae), which drops whole fronds, than on its trunks. The reverse was found for Dicksonia fibrosa (Dicksoniaceae), which retains dead fronds as a ‘skirt’. We suggest that shedding whole fronds, or producing a skirt of fronds, are alternative ‘strategies’ that can reduce competition from terrestrial and epiphytic seedlings, respectively.     

Susceptibility of Tree Seedlings to Biotic and Abiotic Hazards in the Understory of a Moist Tropical Forest in Panama

We evaluated temporal patterns of seedling survival of eight Neotropical tree species generated under multiple abiotic and biotic hazards (vertebrates, disease, litterfall) in the forest understory on Barro Colorado Island, Panama. Seedlings were transplanted at first leaf expansion in low densities along a 6-km transect and damage and mortality were recorded for 1 yr. We also planted and monitored small and large artificial seedlings to estimate physical disturbance regimes. During 0–2 mo after transplant, vertebrate consumers of reserve cotyledons caused high mortality of real seedlings, but little damage to artificial seedlings. On real seedlings after 2 mo, disease became an important agent of mortality, despite a decrease in overall mortality rates. Damage by litterfall remained relatively low during the 1-yr study period. Survival ranks among species showed ontogenetic shifts over time, as species changed susceptibility to the mortality agents. Survival after 2 mo was positively correlated with stem toughness, not because species with tough stems were less likely to receive mechanical damage, but because they survived better after receiving mechanical damage. Within each transplant station, artificial seedlings were not good predictors of litterfall damage experienced by real seedlings. Forest-wide litterfall damage level, however, was similar for both real and artificial seedlings ( ca 10%/yr), a moderate level compared to other tropical forests. In conclusion, species traits including biomechanical traits interact to create complex temporal patterns of first year seedling survival, resulting in ontogenetic shifts that largely reflect changes in the relative importance of vertebrate consumers relative to other hazards.     

Response of forest tree seedlings to simulated litterfall damage.

Litterfall is an important cause of seedling mortality in many forests ranging from wet tropical to boreal. However, there is a lack of studies that investigate differences between species in seedling resilience to litterfall damage. We selected seedling pairs of seven tree species and simulated litterfall damage by pinning one of each pair to the ground. The mortality and growth rates varied significantly between species for pinned individuals, but were similar for unpinned seedlings. The mortality of pinned Nestegis cunninghamii and Prumnopitys ferruginea was significantly greater than that of unpinned individuals (P < 0.05). However, contrary to expectations, the growth rates of pinned Hedycarya arborea and Nothofagus menziesii were much greater than for those left unpinned (P < 0.05). In general, seedling resilience to the bending damage differed substantially between species. N. cunninghamii and P. ferruginea suffered high mortalities and did not increase growth rates in response to damage, whereas, H. arborea and N. menziesii suffered few mortalities and regained height quickly. Other study species demonstrated intermediate resilience. This study demonstrates that some species are more likely to survive in high-risk litterfall regimes than are others. Given that litterfall risk can vary greatly between microsites, these results suggest that litterfall can contribute to regeneration niche differentiation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Damage to artificial seedlings across a disturbed Afromontane forest landscape

Seedling mortality is an important demographic bottleneck for forest regeneration, yet the factors influencing recruitment are often poorly characterized across space and time. In African highlands, where extensive patches of previously disturbed forests remain in persistent shrub‐dominated states, patterns of damage to seedlings have not been examined in detail. We used artificial seedlings to determine how (non‐herbivory‐related) damage varies across a fragmented forest landscape in the Bwindi Impenetrable National Park, Uganda. A total of 848 artificial seedlings were established in 106 plots, each with 8 seedlings, across a gradient of environmental factors, including habitat type, topographic position, distance to water, and distance to human activity. We divided damage into that due to vertebrates, plant debris, and unknown agents. From our 16,960 monthly stem assessments, 1,289 damage events were recorded. The mean “seedling” damage per year (± 1 SE) was 59.5 ± 2.3%. Damage was significantly more frequent in sites dominated by short herbs (66.1 ± 3.0) than in sites dominated by shrubs (60.1 ± 2.6) and ferns (47.9 ± 2.7). The mean percentage of damage by each class of agents was 45.8 ± 2.1% for vertebrates, 21.4 ± 2.1% for plant debris, and 3.8 ± 0.6% for unknown agents. These rates surpass those reported in other montane forests, likely reflecting the density of large vertebrates in our site. Our results indicate that most damage to artificial seedlings arises from larger vertebrates but that most spatial variation in this damage reflects the impacts of falling plant debris. These damage processes appear sufficient to constrain the regeneration of tree species in areas where forest cover is patchy and fragmented.     

Changes of community composition at multiple trophic levels due to hunting in Nigerian tropical forests

Hunting in tropical forests decimates large mammals, and this may have direct and indirect effects on other trophic levels and lead to trophic cascades. We compared replicated sites of hunted and protected forests in southeastern Nigeria, with respect to community composition of primates, other mammals, birds, plant seedlings, and mature trees. We make predictions regarding the community composition at the different trophic levels. In forests where large primates are rare, we hypothesize that their ecological role will not be fully compensated for by small frugivores. We apply multivariate methods to assess changes in community composition of mammals, birds, and seedlings, controlling for any differences between sites in the other groups, including mature trees. Medium and large (4–180 kg) primates were much rarer in hunted sites, while porcupine and rock hyrax increased in abundance with hunting. In contrast, the community composition of birds was similar in both types of forests. Seedling communities were significantly related to the community composition of mammals, and thus strongly affected by hunting. In protected forests primate dispersed plant seedling species dominated, whereas in hunted forests the seedling community was shifted towards one dominated by abiotically dispersed species. This was probably both a consequence of reduced seed dispersal by primates, and increased seed predation by rodents and hyrax. Hence we found no evidence for buffering effects on tree regeneration through functional compensation by non‐hunted animals (such as birds). Our results highlight how seedling communities are changed by the complex plant–animal intera ctions, triggered by the loss of seed dispersers. The results predict a rarity of primate‐dispersed trees in future tropical forest canopies; a forest less diverse in timber and non‐timber resources.     

Early emergence increases survival of tree seedlings in Central European temperate forests despite severe late frost

Global warming is expected to result in earlier emergence of tree seedlings that may experience higher damages and mortality due to late frost in spring. We monitored emergence, characteristics, and survival of seedlings across ten tree species in temperate mixed deciduous forests of Central Europe over one and a half year. We tested whether the timing of emergence represents a trade‐off for seedling survival between minimizing frost risk and maximizing the length of the growing period. Almost two‐thirds of the seedlings died during the first growing period. The timing of emergence was decisive for seedling survival. Although seedlings that emerged early faced a severe late frost event, they benefited from a longer growing period resulting in increased overall survival. Larger seedling height and higher number of leaves positively influenced survival. Seedlings growing on moss had higher survival compared to mineral soil, litter, or herbaceous vegetation. Synthesis. Our findings demonstrate the importance of emergence time for survival of tree seedlings, with early‐emerging seedlings more likely surviving the first growing period.     

Effects of herbivory on growth and survival of seedlings of a rainforest tree,Alphitonia whitei (Rhamnaceae)

This study examined the effects of insect herbivory on growth and mortality of seedlings of a mid- successional rainforest tree, Alphitonia whitei Braid. Two experiments were conducted in which seedlings were exposed to 0% and 50% natural defoliation by insect herbivores and placed in light gaps in simple notophyll vine forest at Paluma, near Townsville, North Queensland. In the first experiment, insect herbivory significantly increased mortality of 2-month-old seedlings. Smaller seedlings had significantly greater mortality rates than larger seedlings, irrespective of herbivory. A significantly greater proportion of smaller seedlings died from being smothered by fallen leaves and soil as a result of digging by vertebrates than for larger seedlings. In the second experiment, the effects of seedling age were examined by comparing 2-month and 4-month-old seedlings. Mortality rates were significantly influenced by seedling age, with eight times greater survival of older seedlings than of younger seedlings. Although insect herbivory was correlated with a significant decrease in shoot mass and a significant increase in root:shoot ratio, there was no effect of insect herbivory on seedling survival in the second experiment. Thus, mortality rates were greater for seedlings if they were young or small (which indirectly results from insect herbivory), because small, young seedlings were more susceptibile to other mortality factors, such as burial by fallen debris and digging by vertebrates.     

Seedling Mortality in Hawaiian Rain Forest: The Role of Small‐Scale Physical Disturbance1

Most montane rain forests on the island of Hawaii consist of a closed canopy formed by Cibotium spp. tree ferns beneath an open canopy of emergent Metrosideros polymorpha trees. We used artificial seedlings to assess the extent to which physical disturbance caused by the senescing fronds of tree ferns and the activities of feral pigs might limit tree regeneration. Artificial seedlings were established terrestrially (N= 300) or epiphytically (N = 300) on tree fern stems. Half of the seedlings on each substrate were in an exclosure lacking feral pigs and half were in forest with pigs present. After one year, the percentage of seedlings damaged was significantly greater among terrestrial seedlings (25.7%) than epiphytic seedlings (11.3%). Significantly more terrestrial seedlings were damaged in the presence of pigs (31.3%) than in the absence of pigs (20.0%). Senescing fronds of tree ferns were responsible for 60.3 percent of the damaged seedlings. Physical disturbance is potentially a major cause of seedling mortality and may reduce the expected half‐life of a seedling cohort to less than two years.     

Establishment and growth of Quercus floribunda seedlings after a mast year

Abstract. Population dynamics, mortality factors, growth and dry weight changes in Quercus floribunda seedlings were studied to determine the implication of sun and shade microsites and the role of mast years on regeneration and maintenance of oak forests in Kumaun Himalaya. The number of seedlings was significantly greater at the sun microsites, but seedling survival was not, as ground herbage clearance at sun microsites accounted for 53.5% mortality. Insect herbivores (e.g. acorn worm) were also one of the major mortality factors. At both microsites, most mortality was encountered between June and August. The initial seedling height at sun microsites was significantly lower compared to shade microsites. Seedlings at sun microsites were superior in terms of number of leaves and peak dry weight. Dry mass allocation in seedling components and root:shoot ratio was similar at both microsites. The study shows the importance of sun microsites and of masting in the potential for regeneration and expansion of Q. floribunda forests.     

Positive density dependence in seedlings of the neotropical tree species Garcinia macrophylla and Xylopia micans

Question: The Janzen‐Connell hypothesis predicts that herbivores and pathogens prevent seedlings from establishing in dense patches near adult conspecifics. Although many studies have investigated the Janzen‐Connell hypothesis, the environmental context – local or regional – in which juveniles establish is often overlooked. The objectives of this study were: (1) to evaluate Janzen‐Connell effects in contrasting environments, and (2) to incorporate microsite variation into the study of this hypothesis. Location: Pacaya‐Samiria Reserve, Peru. Methods: I assessed seedling performance of two tree species, Garcinia macrophylla and Xylopia micans, during one growing season. In an observational study, mortality and growth rates were regressed against distance to the nearest adult conspecific, conspecific seedling density, heterospecific plant density, and several abiotic variables in upland and floodplain forests. Field and shadehouse experiments were used to isolate distance‐ and density‐dependent effects. Results: Contrary to predictions, seedling survivorship increased in the presence of conspecific seedlings (Garcinia) and heterospecific understory plants (Garcinia and Xylopia) in the observational study. Survivorship in the field experiment, however, was unaffected by conspecific seedling density or adult proximity. In the shadehouse, Garcinia growth rates were highest in floodplain soils collected near adult conspecifics, but mortality was unrelated to the soil's habitat or proximity to an adult. Conclusions: The positive density dependence found in this study could have been produced by: (1) environmental factors that increase both density and survivorship, or (2) interspecific facilitation, if heterospecifics reduce herbivore or pathogen pressure on the focal species. Such interactions could help explain species coexistence in tropical forests.     

Non‐trophic plant–animal interactions mediate positive density dependence among conspecific saplings

Trophic plant–animal interactions (e.g. browsing by ungulates, insect attack) are an important and well‐studied source of mortality in many tree populations. Non‐trophic tree–animal interactions (e.g. deer antler rubbing) also frequently lead to tree death, and thus have significant effects on forest ecosystem functioning, but they are much less well studied than trophic interactions are. As deer populations have increased in recent decades in the Northern Hemisphere, their impact on tree populations via browsing and antler rubbing will increase. The aim of the study was to illustrate the potential ability of non‐trophic plant–animal interactions to regulate the dynamics of a natural forest. Specifically, we wanted to determine whether and how density and distance‐dependent processes affect sapling mortality caused by an antler rubbing by red deer Cervus elaphus. We used a spatially explicit approach to examine density and distance‐dependent mortality effects in almost two thousand Picea abies saplings over 20 years, based on a fully mapped permanent 14.4 ha plot in a natural subalpine old‐growth spruce forest. Antler rubbing by deer was the main identified cause of sapling mortality, and it showed a strong spatial pattern: positive density dependence of survival among spruce saplings. Deer selectively killed spruce saplings that were isolated from conspecifics. In consequence, non‐trophic plant–deer interactions were a major driver of the spatial pattern of P. abies sapling survival. The other mortality causes (e.g. breaking, overturning) did not show density‐dependent patterns or their effects were much weaker. In the medium and long term, the density‐dependent pattern of sapling mortality due to antler rubbing can alter the tree stand structure. Our results highlight the ecological relevance of non‐trophic plant–animal interactions for forest ecosystem functioning.     

Abiotic constraints on the establishment of Quercus seedlings in grassland

High evaporative demand and periodic drought characterize the growing season in midwestern grasslands relative to deciduous forests of the eastern US, and predicted climatic changes suggest that these climatic extremes may be exacerbated. Despite this less than optimal environment for tree seedling establishment, deciduous trees have expanded into adjacent tallgrass prairie within the last century leading to a dramatic land cover change. In order to determine the role of light and temperature on seedling establishment, we assessed carbon and water relations and aboveground growth of first‐year Quercus macrocarpa seedlings exposed to one of three conditions: (1) intact tallgrass prairie communities (control), (2) aboveground herbaceous biomass removed (grass removal), and (3) shade plus biomass removal to reduce light (PFD) to levels typical of the grassland‐forest ecotone (shade). In the 2000 growing season, precipitation was 35% below the long‐term average, which had a significant negative effect on oak seedling carbon gain at midseason (photosynthesis declined to 10% of maximum rates). However, net photosynthesis and stomatal conductance in the shade treatment was ca. 2.5 and 1.5 times greater, respectively, than in control treatment seedlings during this drought. During this period, leaf and air temperatures in control seedlings were similar whereas leaf temperatures in the shade treatment remained below air temperature. A late‐season recovery period, coincident with decreased air temperatures, resulted in increased net photosynthesis for all seedlings. Increased photosynthetic rates and water relations in shaded seedlings compared to seedlings in full sun suggest that, at least in dry years, high light and temperature may negatively impact oak seedling performance. However, high survival rates for all seedlings indicate that Q. macrocarpa seedlings are capable of tolerating both present‐day and future climatic extremes. Unless historic fire regimes are restored, forest expansion and land cover change are likely to continue.     

Performance and fate of tree seedlings on and around nests of the leaf‐cutting ant Atta cephalotes: Ecological filters in a fragmented forest

Habitat fragmentation is currently the most pervasive anthropogenic disturbance in tropical forests and some species of leaf‐cutting ants of the genus Atta (dominant herbivores in the neotropics) have become hyper‐abundant in forest edges where their nests directly impact up to 6% of the forest area. Yet, their impacts on the regeneration dynamics of fragmented forests remain poorly investigated. Here we examine the potential of Atta cephalotes nests to function as ecological filters impacting tree recruitment. Growth, survival and biomass partitioning of experimentally planted seedlings (six tree species) were examined at eight spatially independent A. cephalotes colonies in a large Atlantic Forest fragment. Seedling performance and fate (leaf numbers and damage) were monitored up to 27 months across three habitats (nest centre, nest edge and forest understorey). Plants at illuminated nest centres showed twice the gross leaf gain as understorey individuals. Simultaneously, seedlings of all species lost many more leaves at nests than in the forest understorey, causing a negative net leaf gain. Net leaf gain in the shaded understorey ranged from zero (Licania and Thyrsodium species) to substantial growth for Copaifera and Virola, and intermediate levels little above zero for Protium and Pouteria. Also seedling survival differed across habitats and species, being typically low in the centre and at the edge of nests where seedlings were often completely defoliated by the ants. Lastly, seedling survival increased strongly with seed size at nest edges while there was no such correlation in the forest. Our results suggest that Atta nests operate as ecological filters by creating a specific disturbance regime that differs from other disturbances in tropical forests. Apparently, Atta nests favour large‐seeded tree species with resprouting abilities and the potential to profit from a moderate, nest‐mediated increase in light availability.     

Association with mature plants protects seedlings from predation in an arid grassland shrub,Gutierrezia microcephala

Summary Survivorship of Gutierrezia microcephala (Compositae) seedlings was studied in an undisturbed arid grassland and in experimental plots where various components of the natural vegetation were removed following seed germination. The major causes of seedling mortality were herbivore damage from the specialist grasshopper, Hesperotettix viridis, and drought stress associated with competition from established plants. The relative intensity of these mortality factors varied strongly with seedling size. Large seedlings had higher overall survivorship but were most likely to be killed by defoliation; most small seedlings died of drought stress.In plots where all perennial grasses were removed (leaving established G. microcephala plants), seedling survivorship was 5 times greater than in undisturbed vegetation. Surviorship in plots where both grasses and mature G. microcephala were removed was slightly poorer than in undisturbed vegetation due to a large increase in mortality from defoliation. From May–August, when the herbivore H. viridis was abundant, seedling survivorship was better in the immediate vicinity of mature conspecific plants than in plots lacking mature G. microcephala, both in the presence and absence of perennial grasses. These results provide a counter example to theories predicting that the impact of specialist herbivores on seedling recruitment is greatest in the vicinity of parent plants.     

Long-distance dispersal helps germinating mahogany seedlings escape defoliation by a specialist caterpillar

Herbivores and pathogens with acute host specificity may promote high tree diversity in tropical forests by causing distance- and density-dependent mortality of seedlings, but evidence is scarce. Although Lepidoptera larvae are the most abundant and host-specific guild of herbivores in these forests, their impact upon seedling distributions remains largely unknown. A firm test of the mechanism underpinning the Janzen–Connell hypothesis is difficult, even for a single tree species, because it requires more than just manipulating seeds and seedlings and recording their fates. Experimental tests require: (1) an insect herbivore that is identified and highly specialised, (2) linkage to an in situ measure (or prevention) of herbivory, and (3) evaluation and confirmation among many conspecific adult trees across years. Here we present experimental evidence for a spatially explicit interaction between newly germinating seedlings of a Neotropical emergent tree, big-leaf mahogany (Swietenia macrophylla, Meliaceae), and caterpillars of a noctuid moth (Steniscadia poliophaea). In the understory of a southeastern Amazon forest, the proportion of attacks, leaf area lost, and seedling mortality due to this specialised herbivore peaked near Swietenia trees, but declined significantly with increasing distance from mature fruiting trees, as predicted by the Janzen–Connell hypothesis. We conclude that long-distance dispersal events (>50 m) provided an early survival advantage for Swietenia seedlings, and propose that the role of larval Lepidoptera as Janzen–Connell vectors may be underappreciated in tropical forests.     

Ants protect conifer seedlings from feeding damage by the pine weevil Hylobius abietis

• 1 Ants that protect food resources on plants may prey on (or deter) herbivores and thereby reduce damage. Red wood ants (of the Formica rufa group) are dominant ants in boreal forests of Eurasia and affect the local abundance of several herbivorous species.
• 2 The pine weevil Hylobius abietis (L.) is a herbivore that causes severe damage by feeding on the bark of coniferous seedlings within areas of forest regeneration.
• 3 We investigated whether ants can protect conifer seedlings from pine weevil feeding. In a manipulative experiment, ants were attracted to sugar baits attached to spruce seedlings and the damage caused by pine weevils was compared with control seedlings without ant‐baits.
• 4 The feeding‐scar area was approximately one‐third lower on the seedlings with ant‐baits compared with the controls. Besides red wood ants, Myrmica ants were also attracted in high numbers to the ant baits and the relative effects of these species are discussed.
• 5 The results obtained in the present study support the trophic cascade hypothesis (i.e. damage to herbivores is suppressed in the presence of predators). The decreased pine weevil feeding on the baited seedlings was probably a result of nonconsumptive interactions [i.e. the presence of (or harassment by) ants distracting pine weevils from feeding].
• 6 Understanding the role of ants may have important implications for future strategies aiming to control pine weevil damage. For example, maintaining suitable conditions for ants after harvesting stands may be an environmentally friendly but currently unexploited method of for decreasing weevil damage.

     

Interactive effects of introduced herbivores and post‐flowering die‐off of bamboos in Patagonian Nothofagus forests

Question: In November 2000, Chusquea culeou, a bamboo species dominating Andean forest understories in southern Argentina and Chile, massively flowered and died over a north‐south distance of ca. 120 km. Because bamboo is the major forage for large herbivores in these forests, we examined the interactive influences of the bamboo die‐off and herbivory by introduced cattle on understory and tree regeneration. Location: Lanín National Park, Argentina. Methods: Permanent plots, in and outside livestock exclosures, were installed in a Nothofagus dombeyi forest in patches of flowered and non‐flowered C. culeou. Plots were monitored over four years for changes in understory composition and tree seedling densities and heights. Results: After the C. culeou die‐off, new establishment of N. dombeyi was low, both with and without herbivory. Livestock alone directly increased N. dombeyi seedling mortality through physical damage. However, tree seedling browse ratings and height growth were interactively affected by bamboo flowering and herbivory; unfenced plots in flowered bamboo patches had the shortest seedlings, highest browse ratings, and lowest tree seedling annual growth rates. Understory cover was higher where livestock were excluded, and this effect was intensified in the patches of flowered bamboo. Neither herbivory nor bamboo flowering resulted in major changes in species composition, with the exception of Alstroemeria aurea. Conclusion: Effects of livestock on N. dombeyi regeneration were contingent on flowering of C. culeou. Prior to introduction of livestock, N. dombeyi regeneration was probably successful beneath canopy gaps during windows of opportunity following bamboo die‐off, but now livestock impede tree regeneration. Herbivory during bamboo withering periods also produces more open understories, particularly affecting palatable heliophyllous herb species such as Alstroemeria aurea. The results underscore the importance of assessing herbivore impacts on tree regeneration during relatively short periods of potential tree regeneration immediately following rare bamboo flowering and die‐off.