Wildlife Recovery During Tropical Forest Succession: Assessing Ecological Drivers of Community Change

Despite the high proportion of secondary forests in the tropics, their conservation value remains poorly understood, particularly with regard to animals. Most theoretical studies of succession have focused on plants, linking life history trade‐offs to well‐known patterns of community change. However, the same trade‐offs proposed for plants should apply to animals, and indeed, animal studies show a change in community dominance from habitat generalist to forest specialist species during succession. Focusing on the diverse terrestrial small mammals of the endangered Atlantic Forest, we assessed which ecological drivers (habitat structure and food availability) affect community changes during succession. If the change in community dominance is driven by trade‐offs between productivity and efficiency, it should be mainly associated with a decrease in food availability. As expected, from younger to older forest, habitat generalists decreased in richness and total abundance, concurrent with a decrease in arthropod biomass. By contrast, the increase in richness and total abundance of forest specialists was not clearly supported by the data; however, this group was not affected by food availability. These results are congruent with a trade‐off between competitive ability and ability to use abundant resources, and indicate that the major community change during succession involves habitat generalists. Secondary forests may thus be valuable for conservation, at least where habitat loss and fragmentation are not high, and old growth forest is available.     

Litter Nutrient Dynamics During Succession in Dry Tropical Forests of the Yucatan: Regional and Seasonal Effects

Land-use change in the tropics is creating secondary forest at an unprecedented rate. In the tropical Americas, mature dry tropical forest is rapidly being converted to secondary forest during the fallow period of shifting cultivation. We investigated litter phosphorus (P) and nitrogen (N) dynamics in forests recovering from shifting cultivation of maize (corn) in three regions of the Southern Yucatan Peninsula, Mexico. Our goal was to understand how nutrient and water availability affect forest recovery following conversion of mature forest to agricultural land. To investigate such changes at a regional scale, newly fallen litter was collected monthly along a seasonal, a successional, and a precipitation gradient. Reflecting possible P limitation, litter P concentration declined with forest age, while litter N concentration did not differ between age classes. Average litter P concentration from the southern, wettest region was 0.87 mg/g, almost twice the litter P concentration in the drier central and northern regions (0.44 and 0.45 mg/g, respectively). Average N concentrations of litter from the three regions ranged from 1.1% to 1.2%, with no regional differences. However, minima in both P and N concentration from all regions were pronouncedly timed with peak litterfall, suggesting nutrient retranslocation during periods of water stress. Additionally, successional differences in litter P were clearest during wetter months. P nutrient-use efficiency was lowest in the southern region and highest in the central and northern study regions. N nutrient-use efficiency was up to 40 times lower than P nutrient-use efficiency and showed no regional differences. Overall, our results suggest that litter nutrient dynamics in secondary dry tropical forests of the Southern Yucatan are strongly influenced by water and nutrient availability, especially P, as well as land-use history.     

Nitrogen Fixation in Bryophytes, Lichens, and Decaying Wood along a Soil-age Gradient in Hawaiian Montane Rain Forest

We determined rates of acetylene reduction and estimated total nitrogen fixation associated with bryophytes, lichens, and decaying wood in Hawaiian montane rain forest sites with underlying substrate ranging in age from 300 to 4.1 million years. Potential N fixation ranged from ca 0.2 kg/ha annually in the 300‐year‐old site to ca 1 kg/ha annually in the 150,000‐year‐old site. Rates of acetylene reduction were surprisingly uniform along the soil‐age gradient, except for high rates in symbiotic/associative fixers at the 150,000‐year‐old site and in heterotrophic fixers at the 2100‐year‐old site. Low fixation at the youngest site, where plant production is known to be N‐limited, suggests that demand for N alone does not govern N fixation. Total N fixation was highest in sites with low N:P ratios in leaves and stem wood, perhaps because epiphytic bryophytes and lichens depend on canopy leachate for mineral nutrients and because heterotrophic fixation is partly controlled by nutrient supply in the decomposing substrate; however, differences in substrate cover, rather than in fixation rates, had the largest effect on the total N input from fixation at these sites.     

桂西北喀斯特区原生林与次生林凋落叶降解和养分释放

凋落叶降解及养分释放研究对喀斯特生态脆弱区森林生态系统的恢复与重建具有重要指导意义。本文选取桂西北喀斯特区3种原生林与3种次生林进行比较,研究其凋落叶降解与降解过程中的营养元素释放规律以及降解速率的影响因子。结果表明,原生林凋落叶的降解速率略大于次生林。C、N、K元素在前180天释放速率较快,随后趋于稳定。次生林凋落叶总P含量在降解初始阶段呈净积累,随后净释放,而原生林的凋落叶在降解360天后仍呈现P素净积累。相关分析表明,凋落叶降解速率与凋落叶初始总N、木质素含量及木质素:N比值呈负相关,与C:N比呈正相关。综合比较发现,次生林圆叶乌桕(Sapium rotundifolium Hemsl)凋落叶的降解速率与养分释放速率较快,是喀斯特退化土地及植被恢复过程中潜在的优势种和建群种。     

Flower and Fruit Availability along a Forest Restoration Gradient

Frugivores and pollinators are two functional groups of animals that help ensure gene flow of plants among sites in landscapes under restoration and to accelerate restoration processes. Resource availability is postulated to be a key factor to structure animal communities using restoration sites, but it remains poorly studied. We expected that diverse forests with many plant growth forms that have less‐seasonal phenological patterns will provide more resources for animals than forests with fewer plant growth forms and strongly seasonal phenological patterns. We studied forests where original plantings included high tree species diversity. We studied resource provision (richness and abundance of flowers and fruits) of all plant growth forms, in three restoration sites of different ages compared to a reference forest, investigating whether plant phenology changes with restoration process. We recorded phenological data for reproductive plant individuals (351 species) with monthly sampling over 2 years, and found that flower and fruit production have been recovered after one decade of restoration, indicating resource provision for fauna. Our data suggest that a wide range of plant growth forms provides resource complementarities to those of planted tree species. Different flower phenologies between trees and non‐trees seem to be more evident in a forest with high non‐tree species diversity. We recommend examples of ideal species for planting, both at the time of initial planting and post‐planting during enrichment. These management actions can minimize shortage and periods of resource scarcity for frugivorous and nectarivorous fauna, increasing probability of restoring ecological processes and sustainability in restoration sites.     

Contrasting Effects of Substrate and Fertilizer Nitrogen on the Early Stages of Litter Decomposition

Commonly observed positive correlations between litter nitrogen (N) concentrations and decomposition rates suggest that N frequently limits decomposition in its early stages. However, numerous studies have found little, if any, effect of N fertilization on decomposition. I directly compared internal substrate N and externally supplied inorganic N effects on decomposition in sites varying in soil N availability. I decomposed eight substrates (with initial %N from 0–2.5) in control and N-fertilized plots at eight grassland and forest sites in central Minnesota. N fertilization increased decomposition at only two of eight sites, even though decomposition was positively related to litter N at all sites and to soil N availability across sites. The effect of externally supplied N on decomposition was independent of litter N concentration, but was greater at sites with low N availability. The inconsistent effects of substrate and externally supplied N may have arisen because decomposers use organic N preferentially as an N source; because inorganic N availability across sites or with fertilization induced changes in microbial community attributes (for example, lower C:N or greater efficiency) that reduced the response of decomposition to increased inorganic N supply; or because the positive correlation between litter N or site N availability with decomposition was spurious, caused by tight correlations between litter or site N and some other factor that truly limited decomposition. These inconsistent effects of substrate N and external N supply on decomposition suggest that the oft-observed relationship between litter N and decomposition may not indicate N limitation of decomposition.     

Leaf‐cutting ants as ecosystem engineers: topsoil and litter perturbations around Atta cephalotes nests reduce nutrient availability

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Variation in Canopy Litterfall Along a Precipitation and Soil Fertility Gradient in a Panamanian Lower Montane Forest

Fertilization experiments in tropical forests have shown that litterfall increases in response to the addition of one or more soil nutrients. However, the relationship between soil nutrient availability and litterfall is poorly defined along natural soil fertility gradients, especially in tropical montane forests. Here, we measured litterfall for two years in five lower montane 1‐ha plots spanning a soil fertility and precipitation gradient in lower montane forest at Fortuna, Panama. Litterfall was also measured in a concurrent nitrogen fertilization experiment at one site. Repeated‐measures ANOVA was used to test for site (or treatment), year, and season effects on vegetative, reproductive and total litterfall. We predicted that total litterfall, and the ratio of reproductive to leaf litterfall, would increase with nutrient availability along the fertility gradient, and in response to nitrogen addition. We found that total annual litterfall varied substantially among 1‐ha plots (4.78 Mg/ha/yr to 7.96 Mg/ha/yr), and all but the most aseasonal plot showed significant seasonality in litterfall. However, litterfall accumulation did not track soil nutrient availability; instead forest growing on relatively infertile soil, but dominated by an ectomycorrhizal tree species, had the highest total litterfall accumulation. In the fertilization plots, significantly more total litter fell in nitrogen addition relative to control plots, but this increase in response to nitrogen (13%) was small compared to variation observed among 1‐ha plots. These results suggest that while litterfall at Fortuna is nutrient‐limited, compositional and functional turnover along the fertility gradient obscure any direct relationship between soil resource availability and canopy productivity.     

Determinants of Leaf Litter Nutrient Cycling in a Tropical Rain Forest: Soil Fertility Versus Topography

We investigated the influence of landscape-level variation in soil fertility and topographic position on leaf litter nutrient dynamics in a tropical rain forest in Costa Rica. We sampled across the three main edaphic conditions (ultisol slope, ultisol plateau, and inceptisol) to determine the effect of soil nutrients on leaf litter nutrient concentrations while controlling for topography, and to examine topographic effects while controlling for soil nutrients. Both leaf litter macronutrient [phosphorus (P), nitrogen (N), sulfur (S), calcium (Ca), potassium (K), magnesium (Mg)] and micronutrient concentrations were quantified throughout a 4-year period. Leaf litter [P], [N] and [K] varied significantly among soil types. The variation in [P], [N], and [K] was explained by soil fertility alone. Leaf litter [S], [Ca], and [Mg] did not vary among the three soil types. Macronutrient (P, K, Mg, S, Ca) concentrations in the leaf litter were much less variable than those of Fe and Al. Lower variability in essential plant nutrients suggests a great deal of plant control over the amount of nutrients resorbed before senescense. Leaf litter macronutrient concentrations varied significantly over the 4-year period, but the temporal variation did not differ among the three edaphic types as anticipated. Hence, although the magnitude of nutrient fluxes may be controlled by local factors such as soil fertility, temporal patterns are likely regulated by a common environmental variable such as precipitation or temperature.     

氮沉降增加对森林凋落物分解酶活性的影响

氮沉降增加对森林凋落物分解酶产生的影响在世界范围受到关注。综述了凋落物分解酶的种类、影响酶的因素、酶的生态学意义和土壤酶研究技术的研究发展趋势。根据森林凋落物底物性质的不同,将凋落物分解酶分为纤维素分解酶类、木质素分解酶类、蛋白水解酶类和磷酸酶类。目前普遍认为,氮沉降增加,磷酸酶类活性随之增加,其它三类酶活性未呈现规律性变化。此外,还对氮沉降增加与土壤酶之间关系的研究前景进行了探讨。     

Ants on the Move: Resource Limitation of a Litter‐nesting Ant Community in Costa Rica1

The leaf litter of tropical wet forests is replete with itinerant ant nests. Nest movement may help ants evade the constraints of stress and disturbance and increase access to resources. I studied how nest relocation and environmental factors may explain the density, size, and growth of leaf litter ant nests. I decoupled the relationships among litter depth, food abundance, and nest availability in a 4‐mo manipulation of food and leaf litter in a community of litter‐nesting ants in a lowland wet forest in Costa Rica. Over 4 mo, 290 1 m² treatment and control plots were sampled without replacement. Nest densities doubled in response to food supplementation, but did not decrease in response to litter removal or stress (from litter trampling). The supplementation of food increased the utilization of less favored nesting materials. In response to food supplementation and litter trampling, arboreal ants established nests in the litter, and growth rates of the most common ants (Pheidole spp.) increased. Colony growth was independent of colony size and growth rates of the most abundant ants. In general, I conclude that litter‐nesting ant density is driven primarily by food limitation, that nest relocation behavior significantly affects access to resource and the demographic structure of this community, and that nest fission may be a method to break the growth–reproduction trade‐off.     

Asymbiotic Nitrogen Fixation and Litter Decomposition on a Long Soil-Age Gradient in Hawaiian Montane Rain Forest1

We determined rates of decomposition and asymbiotic nitrogen fixation in the leaf litter of Cheirodendron spp. on the Hawaiian Islands. Leaf litter was collected from four sites on a long soil-age gradient (300 yr to 4.1 M yr) and decomposed at two sites that differed widely in substrate age and nutrient availability. Rates of decomposition were higher in litter decomposed at the older site, where nutrient availability was greater. A substantial amount of nitrogen and phosphorus immobilization occurred in litter decomposed at the older site, with more immobilization occurring in litter with lower initial nitrogen and phosphorus concentrations, suggesting both supply and demand controls on nutrient immobilization. Potential rates of nitrogen fixation were very low in the first 25 d (0–5 nmol acetylene/gdw/h), rose to much higher rates by 70 d (20–45 nmol), and then declined by 140 d. We found no significant difference in rates of potential nitrogen fixation between sites of decomposition, but there was a strong substrate effect, with higher rates in litter with low lignin, low nitrogen, and high phosphorus. Where significant immobilization of nitrogen occurred for decomposing Cheirodendron, nitrogen fixation could have comprised no more than 10 percent of immobilized nitrogen. Overall, rates of nitrogen fixation were dependent on the source of the decomposing substrate but not on the site of decomposition, while short-term decomposition and nutrient immobilization were strongly dependent on the site of decomposition but not as much on the source of the decomposing substrate.     

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.     

Eurhopalothrix depressa sp. n. (Hymenoptera: Formicidae) from Southern Brazil with a Key to the Neotropical Taxa of the Genus

Eurhopalothrix depressa sp. n. is described from Southern Brazil. The species belongs to the bolaui group characterised by well developed propodeal teeth and large specialized clavate hairs on the body. It can be distinguished from all other neotropical Eurhopalothrix species by a distinct ridge dividing the head into a lower anterior and a higher posterior part and by the reduced number of 16 specialised hairs on the head. A key to all New World species of Eurhopalothrix is added.     

Litter Decomposition and Nutrient Release in a Tropical Seasonal Rain Forest of Xishuangbanna,Southwest China1

We studied litter decomposition and nutrient release in a tropical seasonal rain forest of Xishuangbanna, Southwest China. The monthly decay rates (k) of leaf litter ranged from 0.02 to 0.21/mo, and correlated with rainfall and soil moisture. Annual k values for leaf litter (1.79/yr) averaged 4.2 times of those for coarse wood (2.5–3.5 cm in diameter). The turnover coefficients of forest floor mass (annual litterfall input/mean floor mass) were: 4.11/yr for flowers and fruits, 2.07/yr for leaves, and 1.17/yr for fine wood (≤2 cm in diameter), with resident time decreasing from fine woods (0.85 yr) to leaves (0.48 yr) and to flower and fruits (0.24 yr). Nutrient residence times in the forest floor mass were ranked as: Ca (1.0 yr) > P (0.92 yr) > Mg (0.64 yr) > N (0.36 yr) > K (0.31 yr). Our data suggest that rates of litter decomposition and nutrient release in the seasonal rain forest of Xishuangbanna are slower than those in typical lowland rain forests, but similar to those in tropical semideciduous forests.     

Effects of Forest Fragmentation on Pteridophyte Diversity in a Tropical Rain Forest in Brazil

The impacts of forest fragmentation on the pteridophyte communities of the Una region of Bahia, Brazil, were investigated by comparing species richness and ensemble diversity among areas of large forest fragments (>900 ha), small forest fragments (<100 ha), and landscape matrix. We inventoried the pteridophytes below 1 m in height in interiors of small fragments, interiors of large fragments (control areas), edges of fragments, edges of continuous forest, capoeiras (initial stages of forest regeneration) and cabrucas (cocoa plantations). All ferns were collected following the plot method (plots of 120×10 m, each). Sampling units were established in the six main ecotypes of the Una region. These units were allocated within three sampling blocks of 5 per 5 km, which were chosen in order to include the largest forest patches that still remain. Results suggest that fragmentation has a negative impact on species richness at the matrix and the edges of forest remnants. A similar negative matrix end edge effect is reported for diversity of those sites measured by the α Log-series Index. However, small forest fragments have pteridophyte species richness and diversity rates similar to large ones so they should be considered of utmost importance to the conservation of forest-related species in the region.     

Forest Succession Endangers the Special Ant Fauna of Abandoned Quarries along the River Meuse (Wallonia,Belgium)

In Western Europe, old abandoned mining sites and quarries are often of high biodiversity and conservation value due to the presence of a number of endangered species. In the southern part of Belgium (Wallonia), many ancient quarries near the river Meuse are rather small and were abandoned from 50 to more than 100 years ago. In 2003, we collected 26 ant species by pitfall trapping in four of these quarries. In addition to common ones, several rare species, usually associated with mesomorphic to xeromorphic grasslands, were found in high numbers. Quarries undergoing forest succession were dominated by eurytopic species and by species typical of wet shadowy places, a fauna far less valuable in terms of nature conservation. Therefore, we suggest a management that halts further forest succession of open mesomorphic and xeromorphic habitat patches in these quarries. To assess and monitor the nature value of the ant fauna of these sites, we propose a so‐called habitat preference approach, wherein each species is assigned to one of the following three habitat preference categories: (1) eurytopic, (2) bound to wet shadow‐rich habitats, or (3) bound to dry open habitats. The stenotopic species of the last category are all endangered in Belgium and of high conservation value. The proportion of the total number of captured specimens included in the latter habitat preference category group is strongly reduced as scrub and tree encroachment advances. This proportion can therefore be used as a proxy to monitor the effects of management measures that prevent further forest succession.     

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.