Do large forest trees tend towards high species mingling?

Effects caused by human management and natural disturbances as well as by ecological mechanisms such as those described, for example, by the Janzen-Connell and the herd immunity hypotheses play an important role in maintaining species diversity. These processes are often accompanied by local size-hierarchy effects and as a likely consequence of them we hypothesised that large plants generally have a tendency to show high species mingling, where plants and their nearest neighbours are heterospecific. To carry out a first benchmark study in forests from different parts of Europe, Africa and North America, we selected spatial data from twelve forest ecosystems and analysed the mingling situation based on an index of spatial species mingling. Using stem diameter at breast height and stem diameter differentiation among the nearest neighbours as explanatory variables we then applied logistic regression to explain mingling probability. Overall we found significant support for the expectation that large trees and trees growing at low local densities often (75% of all analysed forest stands) have indeed a tendency towards high species mingling. This supports our expectation that the tendency of larger trees towards high species mingling generally is either a consequence of disturbances/forest management or of the aforementioned ecological processes, although a few forest stands also deviated from the expected pattern. We also found that size differentiation and species mingling are strongly related in local neighbourhoods. Thus the results of our study strongly support the view that local species richness promotes local size hierarchy irrespective of climate zone. This leads to situations where in local neighbourhoods with large size diversity there is also high mingling and vice versa. This allows conservation to focus on maintaining or improving one diversity aspect, e.g. size diversity, whilst obtaining the other, e.g. species mingling, as a by-product.     

Do trees in urban or ornamental plantings receive more damage by insects than trees in natural forests?

Abstract.

• 1 To determine if trees in urban or ornamental plantings are more susceptible to attack and receive more damage to foliage by herbivores than trees in natural forests, we compared the amount of leaf damage caused by several guilds of insects feeding on seven species of native, broadleaf trees in two geographic locations.
• 2 Total leaf damage did not differ significantly between urban or ornamental and natural forests, although trees in natural forests tended to have slightly higher levels of leaf damage.
• 3 Damage caused by chewing insects was consistently higher on trees in natural forests than in urban or ornamental plantings. All other feeding guilds showed no consistent pattern in levels of damage between the two habitats.
• 4 Total damage levels were highest on canopy trees and lowest on understorey trees.
• 5 These results are inconsistent with the view that trees in urban or ornamental settings are more susceptible to insect attack than trees in natural forests.
• 6 The lower level of foliar damage caused by chewing insects on trees in urban or ornamental plantings may arise because of low rates of dispersal by insects into urban environments, higher levels of plant resistance to insect attack in urban or ornamental plantings, or lower survival rates of herbivorous insects in urban environments.

     

Temporary forest pools: can we see the water for the trees?

Temporary waters, in general, are fascinating habitats in which to study the properties of species adapted to living in highly variable environments. Species display a remarkable array of strategies for dealing with the periodic loss of their primary medium that sets them apart from the inhabitants of permanent water bodies. Survival of individuals typically depends on exceptional physiological tolerance or effective migrational abilities, and communities have their own, distinctive hallmarks. This paper will broadly overview the biology of temporary ponds, but will emphasize those in temperate forests. In particular, links will be sought between aquatic community properties, the nature of the riparian vegetation, and forestry practices. Quite apart from their inherent biological interest, temporary waters are now in the limelight both from a conservation perspective, as these habitats come more into conflict with human activities, and a health-control perspective, as breeding habitats for vectors of arboviruses. Traditionally, many temporary waters, be they pools, streams or wetlands, have been considered to be ȁ8wastedȁ9 areas of land, potentially convertible to agriculture/silviculture once drained. In reality, they are natural features of the global landscape representing distinct and unique habitats for many species – some that are found nowhere else, others that reach their maximum abundance there. To be effective, conservation measures must preserve the full, hydroseral range of wetland types.     

Nitrogen cycling in a northern hardwood forest: Do species matter?

To investigate the influence of individual tree species on nitrogen (N) cycling in forests, we measured key characteristics of the N cycle in small single-species plots of five dominant tree species in the Catskill Mountains of New York State. The species studied were sugar maple (Acer saccharum), American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), eastern hemlock (Tsuga canadensis), and red oak (Quercus rubra). The five species varied markedly in N cycling characteristics. For example, hemlock plots consistently showed characteristics associated with \"slow\" N cycling, including low foliar and litter N, high soil C:N, low extractable N pools, low rates of potential net N mineralization and nitrification and low NO₃^– amounts trapped in ion-exchange resin bags buried in the mineral soil. Sugar maple plots had the lowest soil C:N, and the highest levels of soil characteristics associated with NO₃^– production and loss (nitrification, extractable NO₃^–, and resin bag NO₃^–). In contrast, red oak plots had near-average net mineralization rates and soil C:N ratios, but very low values of the variables associated with NO₃^– production and loss. Correlations between soil N transformations and litter concentrations of N, lignin, lignin:N ratio, or phenolic constituents were generally weak. The inverse correlation between net nitrification rate and soil C:N that has been reported in the literature was present in this data set only if red oak plots were excluded from the analysis. This study indicates that tree species can exert a strong control on N cycling in forest ecosystems that appears to be mediated through the quality of soil organic matter, but that standard measures of litter quality cannot explain the mechanism of control.     

Do tadpoles affect leaf decomposition in neotropical streams?

1. Of the relatively few studies that have examined consequences of amphibian declines on stream ecosystems, virtually all have focused on changes in algae (or algal‐based food webs) and little is known about the potential effects of tadpoles on leaf decomposition. We compared leaf litter decomposition dynamics in two neotropical streams: one with an intact community of tadpoles (with frogs) and one where tadpoles were absent (frogless) as a result of a fungal pathogen that had driven amphibians locally extinct. The stream with tadpoles contained a diverse assemblage (23 species) of larval anurans, and we identified five species of glass frog (Centrolenidae) tadpoles that were patchily distributed but commonly associated with leaf detritus and organic sediments in pools. The latter reached total densities of 0–318 tadpoles m^?2. 2. We experimentally excluded tadpoles from single‐species leaf packs incubated over a 40‐day period in streams with and without frogs. We predicted that decomposition rates would be higher in control (allowing access of tadpoles) treatments in the study stream with frogs than in the frogless stream and, in the stream with frogs, in the control than in the tadpole exclusion treatment. 3. In the stream with frogs, Centrolene prosoblepon and Cochranella albomaculata tadpoles were patchily distributed in leaf packs (0.0–33.3 m^?2). In contrast to our predictions, leaf mass loss and temperature‐corrected leaf decomposition rates in control treatments were almost identical in our stream with frogs (41.01% AFDM lost, k_{degree day} = ?0.028 day^?1) and in the frogless stream (41.81% AFDM lost, k_{degree day} = ?0.027 day^?1) and between control and tadpole exclusion treatments within each stream. Similarly, there were no significant differences in leaf pack bacterial biomass, microbial respiration rates or macroinvertebrate abundance between treatments or streams. Invertebrate assemblages on leaf packs were similar between treatments (SIMI = 0.97) and streams (SIMI = 0.95) and were dominated by larval Chironomidae, Simuliidae (Diptera) and larval Anchytarsus spp. (Coleoptera). 4. In contrast to dramatic effects of grazing tadpoles on algal communities observed previously, tadpoles had no major effects on decomposition. While centrolenid tadpoles were common in the stream with frogs, their patchy distribution in both experimental and natural leaf packs suggests that their effects on detrital dynamics and microbes are probably more localised than those of grazing tadpoles on algae.     

Cutting the forest to see a single tree?

The development of single-molecule tools has significantly impacted the way we think about biochemical processes. Watching a single protein in action allows us to observe kinetic details and rare subpopulations that are hidden in ensemble-averaging techniques. I will discuss here the pros and cons of the single-molecule approach in studying ligand binding in macromolecular systems and how these techniques can be applied to characterize the behavior of large multicomponent biochemical systems.     

Endophytic fungi in forest trees: are they mutualists?

Forest trees form symbiotic associations with endophytic fungi which live inside healthy tissues as quiescent microthalli. All forest trees in temperate zones host endophytic fungi. The species diversity of endophyte communities can be high. Some tree species host more than 100 species in one tissue type, but communities are usually dominated by a few host-specific species. The endophyte communities in angiosperms are frequently dominated by species of Diaporthales and those in gymnosperms by species of Helotiales. Divergence of angiosperms and gymnosperms coincides with the divergence of the Diaporthales and the Helotiales in the late Carboniferous about 300 million years (Ma) ago, indicating that the Diaporthalean and Helotialean ancestors of tree endophytes had been associated, respectively, with angiosperms and gymnosperms since ≥300 Ma. Consequently, dominant tree endophytes have been evolving with their hosts for millions of years. High virulence of such endophytes can be excluded. Some are, however, opportunists and can cause disease after the host has been weakened by some other factor. Mutualism of tree endophytes is often assumed, but evidence is mostly circumstantial. The sheer impossibility of producing endophyte-free control trees impedes proof of mutualism. Some tree endophytes exhibit either a pathogenic or a putatively mutualistic behaviour depending on the situation. The lifestyle (mutualism, commensalism, parasitism) of most tree endophytes is, however, not known. They are just there in the tissue and resume growth at the onset of natural senescence of the host tissue on which they eventually sporulate. Density of colonization of conifer needles by endophytes increases with needle age. It is postulated that the needles die as soon as colonization density reaches a threshold value. Normally, the threshold is not reached before the onset of natural senescence. The threshold value may, however, be reached earlier under some adverse conditions, e.g. lack of light in dense stands. As a consequence, endophytes kill the needles prematurely. Needle endophytes could, thus, be useful to eliminate “parasitic” needle mass, i.e. needles which consume more than they produce.     

Are mangroves in the tropical Atlantic ripe for invasion? Exotic mangrove trees in the forests of South Florida

Two species of mangrove trees of Indo-Pacific origin have naturalized in tropical Atlantic mangrove forests in South Florida after they were planted and nurtured in botanic gardens. Two Bruguiera gymnorrhiza trees that were planted in the intertidal zone in 1940 have given rise to a population of at least 86 trees growing interspersed with native mangrove species Rhizophora mangle, Avicennia germinans and Laguncularia racemosa along 100 m of shoreline; the population is expanding at a rate of 5.6% year⁻¹. Molecular genetic analyses confirm very low genetic diversity, as expected from a population founded by two individuals. The maximum number of alleles at any locus was three, and we measured reduced heterozygosity compared to native-range populations. Lumnitzera racemosa was introduced multiple times during the 1960s and 1970s, it has spread rapidly into a forest composed of native R. mangle, A. germinans, Laguncularia racemosa and Conocarpus erectus and now occupies 60,500 m² of mangrove forest with stem densities of 24,735 ha⁻¹. We estimate the population growth rate of Lumnitzera racemosa to be between 17 and 23% year⁻¹. Populations of both species of naturalized mangroves are dominated by young individuals. Given the long life and water-dispersed nature of propagules of the two exotic species, it is likely that they have spread beyond our survey area. We argue that the species-depauperate nature of tropical Atlantic mangrove forests and close taxonomic relatives in the more species-rich Indo-Pacific region result in the susceptibility of tropical Atlantic mangrove forests to invasion by Indo-Pacific mangrove species.     

Do tree species characteristics influence soil respiration in tropical forests? A test based on 16 tree species planted in monospecific plots

The high spatial variability of soil respiration in tropical rainforests is well evaluated, but influences of biotic factors are not clearly understood. This study underlines the influence of tree species characteristics on soil respiration across a 16-monospecific plot design in a tropical plantation of French Guiana. A large variability of soil CO₂ fluxes was observed among plots (i.e. 2.8 to 6.8 μmol m^?2 s^?1) with the ranking being constant across seasons. There were no significant relationships between soil respiration and soil moisture or soil temperature, neither spatially, nor seasonally. The variability of soil respiration was mainly explained by quantitative factors such as leaf litterfall and basal area. Surprisingly, no significant relationship was observed between soil respiration and root biomass. However, the influence of substrate quality was revealed by a strong relationship between soil respiration and litterfall P (and litterfall N, to a lesser extent).     

Do riparian forest strips in modified forest landscapes aid in conserving bat diversity?

Agricultural practices lead to losses of natural resources and biodiversity. Maintaining forests alongside streams (riparian forest strips) has been used as a mechanism to minimize the impact of clearing for agriculture on biodiversity. To test the contribution of riparian forest strips to conserve biodiversity in production landscapes, we selected bats as a biodiversity model system and examined two dimensions of diversity: taxonomic and functional. We compared bat diversity and composition in forest, with and without stream habitat, and in narrow forest riparian strips surrounded by areas cleared for agriculture. We tested the hypothesis that riparian forest strips provide potential conservation value by providing habitat and serving as movement corridors for forest bat species. Riparian forest strips maintained 75% of the bat species registered in forested habitats. We found assemblage in sites with riparian forest strips were dominated by a few species with high abundance and included several species with low abundance. Bat species assemblage was more similar between sites with streams than between those sites to forests without stream habitat. These results highlight the importance of stream habitat in predicting presence of bat species. We registered similar number of guilds between forest sites and riparian forest strips sites. Relative to matrix habitats, stream and edge habitats in riparian forest strips sites were functionally more diverse, supporting our hypothesis about the potential conservation value of riparian forest strips. Results from this study suggest that maintaining riparian forest strips within cleared areas for agricultural areas helps conserve the taxonomic and functional diversity of bats. Also, it provides basic data to evaluate the efficacy of maintaining these landscape features for mitigating impacts of agricultural development on biodiversity. However, we caution that riparian forest strips alone are not sufficient for biodiversity maintenance; their value depends on maintenance of larger forest areas in their vicinity.     

Do seasonal temperatures,species traits and nearby timber harvest predict variation in moth species richness and abundance in unlogged deciduous forests?

1. The present study aimed to determine whether the activity of lepidopteran communities in stands adjacent to timber harvest was more variable than that for communities in unlogged landscapes.
2. Moths were sampled using light traps in 11 unlogged forest stands in Morgan‐Monroe State Forest (Indiana, U.S.A.) in 2007 (before harvest) and then again from 2009 to 2012 (post‐harvest within nearby stands). Mean winter low temperatures were correlated with higher levels of variation in species richness across all forest stands. Greater variation in species richness was also correlated with the occurrence of nearby harvests.
3. Colder winters depressed species richness of forest moths across all forest stands. Unlogged stands within 100 m of patch cuts or shelterwood cuts gained and lost species at a greater rate than stands adjacent to unlogged forest. Species specialized to a single host were subject to the most significant variability and many specialists experienced ±50% change in abundance over 5 years.
4. Timber harvest created a disturbance footprint that extended beyond the managed forest concession. Moth species with a specialized larval diet appeared to be most vulnerable to the indirect effects of timber harvest. Small logging concessions may change diversity at larger scales than presumed.

     

Do cloud forest tree species differ in their suitability as a substrate for epiphytic bromeliads?

There is evidence for the existence of varying degrees of host preference in vascular epiphytes; certain tree species can be positively, neutrally, or negatively associated with epiphytes. The objective of this study was to evaluate whether tree species of the cloud forest differ in their suitability as a substrate for epiphytic bromeliads. To evaluate the association between epiphytic bromeliad cover and host tree species, we sampled 62 plots (each of 200 m²) in four cloud forest fragments in Veracruz, Mexico. For all trees ≥10 cm in diameter at breast height (DBH), we recorded species name, DBH, and percentage cover of bromeliads in categories of tree coverage. In total, 587 trees belonging to 52 species were recorded. All of the 10 tree species used to assess differences in epiphyte cover (each with a minimum of nine individuals) supported bromeliads, but mean bromeliad cover differed significantly among the tree species. The tree species that concentrated the highest bromeliad cover were Quercus sartorii (29.86%) and Liquidambar styraciflua (21.72%). Our results indicate that, while none of the tree species analyzed was a limiting host for epiphytic bromeliads in general, varying levels of bromeliad cover occur depending on the host species in tropical montane cloud forest fragments suggesting that certain tree species are better hosts than others. The implications for conservation efforts of differential tree species suitability as epiphyte hosts are discussed.     

Conservation implications of the refugee species concept and the European bison: king of the forest or refugee in a marginal habitat?

The concept of refugee species provides a theoretical framework towards increasing the predictive power of the ‘declining population paradigm’ through identifying species which are expected to suffer from a declining population syndrome. Using a simple habitat model as a framework, refugee species are defined as those that can no longer access optimal habitat, but are confined to suboptimal habitats, with consequences of decreased fitness and density, and attendant conservation risks. Refugee species may be difficult to detect in the absence of information on prior habitat use and fitness and their observed ecology will be constrained by the habitat limits forced on them. Identification of refugee species, characterisation of pre‐refugee ecology and the restoration of such species to optimal habitat is critical to their successful conservation. The concept is showcased by addressing the conundrum of a large grazing bovid, the European bison Bison bonasus, being managed as a forest specialist, despite its evolutionary background, dental morphology, neonatal behaviour, diet and microhabitat selection being characteristic of a grazing species inhabiting open, grass‐rich habitats. It is hypothesized that a combination of increasing replacement of open steppe by forest cover after the last postglacial period and increasing human pressure forced bison into forests as a refuge habitat. This process was then reinforced through active management of bison in forests as managers committed themselves to the ‘bison as forest species’ paradigm. A research agenda to test this hypothesis using an experimental approach in the conservation management of European bison by introducing populations into diverse habitat types is suggested.