Within-site Heterogeneity Research Articles

Reassembly of ground-dwelling ant communities in reforestation plots in SW Costa Rica

Due to agriculture and logging, Costa Rica has lost many primary forests, making reforestation an important task. To judge the progress of reforestation, it is important to follow the reassembly of organismal communities within restored habitats. The COBIGA project near La Gamba, in the Golfo Dulce region of Costa Rica, aims at reforestation of lowland sites with native tree species. Ants, as ubiquitous and highly abundant terrestrial organisms, have a substantial influence on tropical ecosystems. The multiple roles include scavenging, predation, herbivory, and mutualistic interactions. We examined ant community responses to reveal the status of community regeneration and functional integrity. We compared the composition and diversity of the ant assemblages at three different age reforestation sites (2, 8, and 10 years old) with those at an old-growth forest as a reference site. By offering canned tuna fish at ground level along replicated transects, we observed 43 ant species representing six functional groups during the 2 months of sampling. Most of the observed ant species were omnivorous, but old-growth forests harbored a substantial number of other functional groups, such as generalized predators, arboreal predators, and arboreal omnivores. In contrast, the youngest reforestation site harbored a severely impoverished ant assemblage comprising mostly generalized polygynous and polydomous ant species from lower trophic levels. The within-site heterogeneity of the ant assemblages increased from the youngest to the oldest forest. In addition, our results show the importance of monitoring the progress of forest recovery to avoid the spread of invasive species into primary habitats.

Plasticity in growth is genetically variable and highly conserved across spatial scales in a Mediterranean pine.

Phenotypic plasticity is a main mechanism for sessile organisms to cope with changing environments. Plasticity is genetically based and can evolve under natural selection so that populations within a species show distinct phenotypic responses to environment. An important question that remains elusive is whether the intraspecific variation in plasticity at different spatial scales is independent from each other. To test whether variation in plasticity to macro- and micro-environmental variation is related among each other, we used growth data of 25 Pinus pinaster populations established in seven field common gardens in NW Spain. Phenotypic plasticity to macro-environmental variation was estimated across test sites while plasticity to micro-environmental variation was estimated by using semivariography and kriging for modeling within-site heterogeneity. We provide empirical evidence of among-population variation in the magnitude of plastic responses to both micro- and macro-environmental variation. Importantly, we found that such responses were positively correlated across spatial scales. Selection for plasticity at one scale of environmental variation may impact the expression of plasticity at other scales, having important consequences on the ability of populations to buffer climate change. These results improve our understanding of the ecological drivers underlying the expression of phenotypic plasticity.

Local and landscape environmental heterogeneity drive ant community structure in temperate seminatural upland grasslands.

Environmental heterogeneity is an important driver of ecological communities. Here, we assessed the effects of local and landscape spatial environmental heterogeneity on ant community structure in temperate seminatural upland grasslands of Central Germany. We surveyed 33 grassland sites representing a gradient in elevation and landscape composition. Local environmental heterogeneity was measured in terms of variability of temperature and moisture within and between grasslands sites. Grassland management type (pasture vs. meadows) was additionally included as a local environmental heterogeneity measure. The complexity of habitat types in the surroundings of grassland sites was used as a measure of landscape environmental heterogeneity. As descriptors of ant community structure, we considered species composition in terms of nest density, community evenness, and functional response traits. We found that extensively grazed pastures and within-site heterogeneity in soil moisture at local scale, and a high diversity of land cover types at the landscape scale affected ant species composition by promoting higher nest densities of some species. Ant community evenness was high in wetter grasslands with low within-site variability in soil moisture and surrounded by a less diverse landscape. Fourth-corner models revealed that ant community structure response to environmental heterogeneity was mediated mainly by worker size, colony size, and life history traits related with colony reproduction and foundation. We discuss how within-site local variability in soil moisture and low-intensity grazing promote ant species densities and highlight the role of habitat temperature and humidity affecting community evenness. We hypothesize that a higher diversity of land cover types in a forest-dominated landscape buffers less favorable environmental conditions for ant species establishment and dispersal between grasslands. We conclude that spatial environmental heterogeneity at local and landscape scale plays an important role as deterministic force in filtering ant species and, along with neutral processes (e.g., stochastic colonization), in shaping ant community structure in temperate seminatural upland grasslands.

Road salt impact on soil electrical conductivity across an urban landscape

Abstract Road salt application is a necessary component of winter road maintenance but comes with an environmental cost. Salts are transported via stormwater drainage or overland and soil throughflow to surface waterbodies, where excess ions create unfavorable or even uninhabitable conditions for freshwater organisms. Soils may retain salts during the process of overland and subsurface flow, thus acting as reservoirs that slow the transport of salt into freshwaters. Understanding the capacity and consistency of anthropogenic salt storage in urban soils may allow us to discover when and where deicing salt applications are most harmful. This article investigates the degree to which soils across a heterogeneous urban landscape retain salts. We measured the electrical conductivity (EC) of soils in an urban setting. Land covers included forests, grasslands, open spaces, low- and medium-density developments and along roadsides. We found that across land-cover types, soil carbon and porosity were correlated to EC in late summer, which suggests that pore space is an important and long-lasting reservoir for salt. In addition, more developed areas, had higher mean soil EC and greater EC variability within and between sites, with 75% of overall variance occurring within individual sites. We hypothesize that this within-site heterogeneity is driven by anthropogenic modifications to salt inputs and soil characteristics. The high EC variance in highly developed urban soils is a previously undiscussed phenomenon and highlights the fine-scale complexity of heterogeneous urban landscapes and the need for high-resolution sampling to accurately characterize urban ecosystems.

Prediction of lithium response using clinical data

Promptly establishing maintenance therapy could reduce morbidity and mortality in patients with bipolar disorder. Using a machine learning approach, we sought to evaluate whether lithium responsiveness (LR) is predictable using clinical markers. Our data are the largest existing sample of direct interview-based clinical data from lithium-treated patients (n=1266, 34.7% responders), collected across seven sites, internationally. We trained a random forest model to classify LR-as defined by the previously validated Alda scale-against 180 clinical predictors. Under appropriate cross-validation procedures, LR was predictable in the pooled sample with an area under the receiver operating characteristic curve of 0.80 (95% CI 0.78-0.82) and a Cohen kappa of 0.46 (0.4-0.51). The model demonstrated a particularly low false-positive rate (specificity 0.91 [0.88-0.92]). Features related to clinical course and the absence of rapid cycling appeared consistently informative. Clinical data can inform out-of-sample LR prediction to a potentially clinically relevant degree. Despite the relevance of clinical course and the absence of rapid cycling, there was substantial between-site heterogeneity with respect to feature importance. Future work must focus on improving classification of true positives, better characterizing between- and within-site heterogeneity, and further testing such models on new external datasets.

LiDAR-derived topography and forest structure predict fine-scale variation in daily surface temperatures in oak savanna and conifer forest landscapes

In mountain landscapes, surface temperatures vary over short distances due to interacting influences of topography and overstory vegetation on local energy and water balances. At two study landscapes in the Sierra Nevada of California, characterized by foothill oak savanna at 276–481 m elevation and montane conifer forest at 1977–2135 m, we deployed 288 near-surface (5 cm above the surface) temperature sensors to sample site-scale (30 m) temperature variation related to hillslope orientation and vegetation structure and microsite-scale (2–10 m) variation related to microtopography and tree overstory. Daily near-surface maximum and minimum temperatures for the 2013 calendar year were related to topographic factors and vegetation overstory characterized using small footprint LiDAR imagery acquired by the National Ecological Observatory Network (NEON) Airborne Observation Platform (AOP). At both landscapes we recorded large site and microsite spatial variation in daily maximum temperatures, and less absolute variation in daily minimum temperatures. Generalized boosted regression trees were estimated to measure the influence of tree canopy density, understory solar radiation, cold-air drainage and pooling, ground cover and microtopography on daily maximum and minimum temperatures at site and microsite scales. Site-scale models based on indices of understory solar radiation and landscape position explained an average of 61–65% of daily variation in maximum temperature; site-scale models based on tree canopy density and landscape position explained 65–83% of variation in minimum temperatures. Models explained <15% of variation in microsite-scale maximum temperatures but within-site heterogeneity was significantly correlated with within-site heterogeneity in modeled understory radiation at both landscapes. Tree canopy density and slope explained 33% of microsite-scale variation in minimum temperatures at savanna sites. Our results demonstrate that it is feasible to model site-scale variation in daily surface temperature extremes and within-site heterogeneity in surface temperatures using LiDAR-derived variables, supporting efforts to understand cross-scale relationships between surface microclimates and regional climate change. Improved understanding of topographic and vegetative buffering of thermal microclimates across mountain landscapes is key to projecting microclimate heterogeneity and potential species’ range dynamics under future climate change.

Heterogeneity and optimal sampe size of soil physicochemical properties in subtropical forest.

Selecting the optimal sample size accurately is important for improving the measurement precision of soil physicochemical properties. In this study, we examined the within-site heterogeneity of soil physical and chemical properties in an evergreen broad-leaved forest and a Chinese fir (Cunninghamia lanceolata) plantation in subtropical China. We applied both traditional and bootstrapping technique to determine the optimal sample size, the number of observation (n), for predicting soil nutrient content with an allowable error of 10% at a 95% confidence level for both forest types. The results showed that soil pH and C/N had low variation, while the soil nutrients had a moderate variation in both forests. The coefficients of variation (CV) of total element concentration in the evergreen broad-leaved forest, such as total carbon and total nitrogen, were significantly larger than those in the Chinese fir plantation, while the CVs of available nutrients were similar between two forests. The evaluation error of all soil properties in two forests showed a stable trend after a sharp decline with the increase of sample size. The mean values also changed from violent fluctuation to stable, and extravagant sample could not effectively improve the accuracy of mean value measurement. The sample sizes of soil water content, total carbon, total nitrogen, nitrate (NO3--N), total potassium, total calcium, and total magnesium in the evergreen broad-leaved forest were signifi-cantly larger than those in the Chinese fir plantation, while the sample size for ammonium (NH4+-N) showed opposite pattern, and the others had no significant difference. In addition, the sample sizes of different soil physicochemical properties varied significantly in the same forest. The available elements, such as NH4+-N and NO3--N, needed a larger sample size than total elements in both forests. Accordingly, we proposed that the heterogeneity and sample size of soil physicochemical properties should be considered before soil survey. An adequate sample size may be varied according to the purpose of sample and the site-specific variation of soil characters.

Canopy Leaf Area Index in Non-Forested Marshes of the California Delta

Leaf area index (LAI, one-sided leaf area per unit ground area) is an important parameter in models of canopy-atmosphere carbon exchange and greenhouse gas fluxes that has been disproportionately under-studied in wetland ecosystems. This study assessed variation in LAI and its sensitivity to canopy height, structure and remotely sensed normalized difference vegetation index (NDVI) in the California Delta, USA in July–August 2014. Plant area index (PAI), an optically measured LAI proxy, significantly varied among wetland sites and vegetation types (p < 0.001). PAI significantly correlated with leaf inclination angle and above-water heights of canopy and standing litter, being on average higher in taller freshwater reeds with steeper leaf angles. Presence of litter contributed to within-site heterogeneity and overestimation of PAI at direct green LAI < 3. Although satellite-based NDVI strongly correlated with PAI (R2 = 0.63, p < 0.001), its application was constrained by presence of water and other non-vegetated backgrounds in wetland pixels. Overall, high within-site variation in LAI resulting from structural heterogeneity and/or vegetation diversity limits representativeness of mean LAI for ecosystem models. This constraint calls for more in-depth future analyses of the effects of vertical and horizontal wetland complexity on LAI measurements and performance as an indicator of light interception and canopy-based ecosystem function.

Temporal and small-scale spatial variation in grassland productivity, biomass quality, and nutrient limitation

Characterization of spatial and temporal variation in grassland productivity and nutrition is crucial for a comprehensive understanding of ecosystem function. Although within-site heterogeneity in soil and plant properties has been shown to be relevant for plant community stability, spatiotemporal variability in these factors is still understudied in temperate grasslands. Our study aimed to detect if soil characteristics and plant diversity could explain observed small-scale spatial and temporal variability in grassland productivity, biomass nutrient concentrations, and nutrient limitation. Therefore, we sampled 360 plots of 20 cm × 20 cm each at six consecutive dates in an unfertilized grassland in Southern Germany. Nutrient limitation was estimated using nutrient ratios in plant biomass. Absolute values of, and spatial variability in, productivity, biomass nutrient concentrations, and nutrient limitation were strongly associated with sampling date. In April, spatial heterogeneity was high and most plots showed phosphorous deficiency, while later in the season nitrogen was the major limiting nutrient. Additionally, a small significant positive association between plant diversity and biomass phosphorus concentrations was observed, but should be tested in more detail. We discuss how low biological activity e.g., of soil microbial organisms might have influenced observed heterogeneity of plant nutrition in early spring in combination with reduced active acquisition of soil resources by plants. These early-season conditions are particularly relevant for future studies as they differ substantially from more thoroughly studied later season conditions. Our study underlines the importance of considering small spatial scales and temporal variability to better elucidate mechanisms of ecosystem functioning and plant community assembly.

Heterogeneity of mollusc communities between and within forest fragments in a much modified floodplain landscape

The Rhine floodplain is much modified by human land use. What used to be the hardwood zone of the large floodplain nowadays holds only a few isolated, recent (mostly < 60 years) forest patches. The aim of the study was to check if the circumstances favoured homogeneity of the snail communities, usually observed within floodplains, or if heterogeneity prevailed. We inventoried molluscs (presenceabsence) in 21 sites within and 18 sites adjacent to a stretch of the Lower Rhine floodplain (total 39 sites). Communities from eight of the sites were quantitatively sampled for within-site analyses. Species turnover occurred with distance from the Rhine, partially along with lateral habitat turnover. Introduced species mainly occurred in young forests, in similar numbers within and beyond the floodplain. Old forests beyond the floodplain had a small number of introduced species and a small between-site heterogeneity, suggesting strong habitat filters (selection of species by environmental conditions) which might be associated with the prevalence of detritus-derived food. The occasionally flooded sites had the longest scatter along the first NMDS axis, indicating a local differentiation without a longitudinal gradient. The young forests that were occasionally inundated and those that were never flooded shared many species, resulting in the lack of significant differences based on the presence-absence data. The damp, regularly flooded forests differed from the occasionally inundated ones and the ones beyond the floodplain. Flooding introduces drift material and species to the sites. The drift is associated with an initially increased within-site heterogeneity. During the years after the flooding event, the within-site homogeneity of communities increases (suggesting local selection of species) in tandem with an increase in the between-site differences (local selection plus stochastic extinctions). key worDs: afforestation, drift dispersal, habitat fragmentation, introduced species, land use, snail fauna

Impact of introduced herbivores on understory vegetation along a regional moisture gradient in Patagonian beech forests

Introduced ungulates can alter understory structure and composition posing a serious threat to forest biodiversity. Yet how large-herbivore impacts in forested regions vary along major environmental gradients remains little explored. If ungulate effects shift with habitat conditions, then management could be tailored to protect most vulnerable forests. We tested the hypothesis that the extent of livestock impact on understory vegetation increases with habitat moisture across Nothofagus dombeyi forests in Nahuel Huapi National Park, NW Patagonia, Argentina. Understory composition and species diversity were compared for paired sites (N=5), which were historically used by cattle or remained free of livestock for more than 50yr, and were located along a regional precipitation gradient (1500–2800mm/yr). Long-term cattle presence reduced the cover of sub-canopy trees, shrubs and bamboo by 57–83%, and increased the relative cover of ground-layer herbs, regardless of habitat moisture. Livestock effects on species composition increased towards the wettest forests, which contained more species exclusive to either browsed or unbrowsed sites. Livestock presence increased species richness (α diversity) and within-site heterogeneity (β diversity) in some locations, but mostly reduced species evenness (30%) throughout the moisture gradient. Species turnover at the gradient scale was lower across browsed sites than for livestock-free sites. Our results indicate that the historical presence of domestic cattle induced region-wide changes in understory communities, highlighting the vulnerable nature of the local flora to ungulate disturbance. The greater impact of livestock browsing on the species composition of wetter forests was consistent with the role of plant growth–defence trade-offs along resource gradients. We suggest that the erosion of understory vegetation attributed to domestic herbivores in Patagonian beech forests can be mitigated by adjusting current animal stocks, while moister forests should be given the highest conservation priority.

Functional traits are more variable at the intra- than inter-population level: a study of four calcareous dry-grassland plant species

The majority of studies investigating plant functional traits have used species average trait values, and assumed that average values were sufficiently representative of each species considered. Although this approach has proven valuable in community ecology studies, plant functional traits can significantly vary at different scales, i.e. between but also within populations. The study of species functional trait variability can facilitate increasingly accurate studies in community ecology. Nevertheless, the current extent of within-site plant trait variability has been poorly addressed in the literature. Calcareous grasslands are ecosystems well-suited to study plant trait variation at small spatial scales. Many species are present on heterogeneous calcareous sites, with significant differences in hydric status due to variations in soil depth, soil moisture, aspect, and slope. This study assesses the extent of intra-population functional trait variability and tests the hypothesis that this variability can be explained by within-site environmental heterogeneity. Three functional traits (SLA-specific leaf area, LDMC-leaf dry matter content, and plant vegetative height) were assessed in three populations of four calcareous grassland species totalling 950 individuals. The heterogeneity in soil depth and potential direct incident radiation was also quantified and related to plant functional trait variability. The intra-population functional trait variability was compared to the inter-population variability of collected data and global inter-population variability data obtained from the worldwide TRY functional traits database. The results showed that SLA, LDMC, and plant height are characterized by considerable intra-population variation (SLA: 72–95%, LDMC: 78–100% and vegetative height: 70–94% of trait variability). The results also indicate higher plant height and larger SLA for individuals located in plots with deeper soils or lower potential direct incident radiation, on gentle slopes or north-facing slopes. Our findings additionally support the concept that higher plant height, higher SLA, and lower LDMC are related to higher availability of soil water. Individuals on shallow soils or in more exposed areas are better equipped to cope with environmental stress. Our results indicate plasticity or local adaptation in individuals to environmental heterogeneity. This study suggests that detailed analyses involving plant functional traits require measurements in situ from a large number of individuals, as the degree of individual response strongly depends on an individual’s location and its micro-environmental conditions. Neglecting intra-population trait variability may be critical, as intraspecific variation can be very high at the population scale, and is likely to be driven by local environmental heterogeneity. Manuscript received 26 November 2015, accepted 29 April 2016 Co-ordinating Editor: Thomas Becker 322

Variation in monsoon precipitation drives spatial and temporal patterns of Larrea tridentata growth in the Sonoran Desert

Summary 1. Broad-scale patterns of above-ground net primary production (ANPP) are closely coupled to climate features, particularly the distribution and magnitude of rainfall. In arid and semi-arid regions, however, the precipitation regime, together with local geomorphology and plant life history traits, combine to regulate soil water availability and patterns of growth, complicating simple correlations with climatic variables. 2. To better understand the drivers of plant growth in desert ecosystems, we characterized the rates and spatial heterogeneity of stem elongation by the dominant perennial shrub, creosote bush (Larrea tridentata) in the northern Sonoran Desert of Arizona (USA). Estimates of stem growth were made over a 5-year period (2006–2010) from 60 plots at 15 sites spanning c. 120 km across the Central Arizona–Phoenix (CAP) Long-term Ecological Research (LTER) area. 3. We observed both the highest and lowest rates of stem growth during summer, and these patterns were related to the amount of monsoon rainfall and local rates of water infiltration. The relationship between growth and precipitation in the summer was nonlinear, with rates increasing more than eightfold at plots receiving more than 100 mm of seasonal rainfall. Conversely, growth during the winter/spring was intermediate in magnitude, similar among years and poorly correlated with seasonal precipitation. 4. The spatial heterogeneity of stem growth also differed markedly between seasons and was greater both within and among sites during summer compared to winter/spring growing periods. At broad scales, spatial heterogeneity of shrub growth was correlated with seasonal changes in the spatial variability of rainfall across the study area. At small spatial scales, greater heterogeneity during the summer appears linked to local soil properties that influence infiltration and water availability following monsoon storms. 5. Overall, the strong, nonlinear growth response by L. tridentata to monsoon rainfall suggests that the recurrence interval of wet summer seasons is an important driver of ANPP for this long-lived shrub. More generally, our results illustrate how regional climate variability and local soil properties can interact to influence the rates and heterogeneity of desert plant growth at multiple scales.

Assessing parasite community structure in cockles Cerastoderma edule at various spatio-temporal scales

Cockles (Cerastoderma edule) are among the most exploited bivalves in Europe. They live in lagoons and estuaries where they undergo many stressors including parasites. Trematodes are the most prevalent macroparasites of cockles and can exert a significant impact on their host populations depending on parasite species and infection intensity. Monitoring these parasite–host systems in order to predict potential host mortalities require a correct knowledge of the spatio-temporal variation of infection.A yearly monitoring of cockles from six stations around Ile aux Oiseaux, Arcachon Bay (France) was conducted between 1998 and 2005. Distance between two stations was ca. 1km. Nine trematode species were identified. Despite a relative homogeneity of the parasite community structure in cockles, between three and six clusters were identified by Hierarchical Ascendant Classification showing that among-sites heterogeneity of trematode communities in cockles was higher than within-site heterogeneity. At the scale of 8 years, and for 2-year old cockles, these patterns remained stable in four out of six stations. Spatial aggregation disappeared with cockle age, since parasite communities in 3-year cockles did not reflect any particular station(s): with age, cockles eventually accumulated most trematode species and lost the site signature. On the other hand, we demonstrated that the commonly accepted theory stating that older/larger cockles accumulate more trematode larvae was not verified and that there could exist a vulnerable age/size that doesn’t correspond to largest values. This study provided a new insight in the parasite community heterogeneity in their host, and in the significance of samples in relation with space and time.

The spatial pattern of grasses in relation to tree effects in an arid savanna community: Inferring the relative importance of canopy and root effect

Both aboveground and belowground processes play important roles in tree-grass interactions in savannas. Little consideration has been given to within-site heterogeneity in the strengths of co-occurring canopy and root effects of trees on grasses in savanna communities. Here, we attempted to correlate the spatial pattern of grass morphological traits with the strengths of canopy and root effects. The results from a spatial analysis suggested that the grass traits had lower variability within the operating domain of the root effect than within that of the canopy effect in sub-canopy areas; in contrast, the operating domain of the root effect presented higher variability of grass traits than that of the canopy effect in inter-canopy areas. Combined with root investigations on vertical distribution patterns, these results suggested that the root effect appeared to outweigh the canopy effect in the sub-canopy areas, where apparent vertical root separation between trees and grasses was shown; while the canopy effect could outweigh the root effect in the inter-canopy areas, where root separation was not observed. This study could provide correlative information on the relative importance of canopy and root effects, and has some useful implications on within-site heterogeneity in terms of aboveground and belowground components in savannas.

Contrasting diversity patterns of soil mites and nematodes in secondary succession

Soil biodiversity has been recognized as a key feature of ecosystem functioning and stability. However, soil biodiversity is strongly impaired by agriculture and relatively little is known on how and at what spatial and temporal scales soil biodiversity is restored after the human disturbances have come to an end. Here, a multi-scale approach was used to compare diversity patterns of soil mites and nematodes at four stages (early, mid, late, reference site) along a secondary succession chronosequence from abandoned arable land to heath land. In each field four soil samples were taken during four successive seasons. We determined soil diversity within samples (α-diversity), between samples (β-diversity) and within field sites (γ-diversity). The patterns of α- and γ-diversity developed similarly along the chronosequence for oribatid mites, but not for nematodes. Nematode α-diversity was highest in mid- and late-successional sites, while γ-diversity was constant along the chronosequence. Oribatid mite β-diversity was initially high, but decreased thereafter, whereas nematode β-diversity increased when succession proceeded; indicating that patterns of within-site heterogeneity diverged for oribatid mites and nematodes. The spatio-temporal diversity patterns after land abandonment suggest that oribatid mite community development depends predominantly on colonization of new taxa, whereas nematode community development depends on shifts in dominance patterns. This would imply that at old fields diversity patterns of oribatid mites are mainly controlled by dispersal, whereas diversity patterns of nematodes are mainly controlled by changing abiotic or biotic soil conditions. Our study shows that the restoration of soil biodiversity along secondary successional gradients can be both scale- and phylum-dependent.

MONOTERPENOID DIVERSITY IN RELATION TO MORPHOLOGY OF PINUS BRUTIA AND PINUS HALEPENSIS IN AN EAST MEDITERRANEAN AREA(ATTIKI, GREECE): IMPLICATIONS FOR PINE EVOLUTION

Pinus halepensis and P. brutia hybridize extensively. The analysis of monoterpenoid and morphological variability of 122 pine trees at five sites showed that the within-site heterogeneity is high while the entire provenance diversity is comparable with that of a circum-Mediterranean scale. Four morphotypes and four chemotypes were recognized. The congruence between morphological and monoterpenoid classification of pines was high (coefficient of contingency 89%) while thirteen family groups of genetically closely related trees were found by means of a specifically designed sampling scheme of continuous classification – identification and re-sampling. Putative hybrids contributed substantially to the observed diversity and there is considerable heterogeneity between sites. Low-altitude sites, presumably susceptible to sea-level fluctuations, alluvial deposition and human influence, are in general more diverse at both levels of genetic complexity than inland sites. The hybrid phenotypes were not merely intermediate forms of parental taxa, and in a multivariate sense they are located on a parabola in the space of morphological properties. The arrangement of hybrids in the monoterpenoid space did not reveal any clear-cut pattern except that three major composite axes can summarize 98.9% of the entire monoterpenoid variation. The observed pattern of variation was interpreted as a result of waves of introgressive influence of eastern P. brutia population on P. halepensis, which was caused by marine transgressions in Pliocene and eustatic sea-level fluctuations in Pleistocene. The human influence on this introgression accelerated the process, and amplified the resulted pattern, although it blurred in many instances the biogeographic routes of germplasm mixing. Its bearing on the evolution of pines is discussed, and it was found that the ‘Eocene refugia hypothesis’ of C.I. Millar holds by analogy also in central Aegean pines.

Indices for characterising spatial variability of soil nitrogen semi-arid grasslands of Northwestern Australia

Within-site heterogeneity of soil N in arid and semi-arid grasslands is often less pronounced than in shrublands and, depending on the index used, can be difficult to detect. We compared biological and chemical indices of soil N from two perennial tussock grasslands of similar soil type in sub-tropical, northwestern Australia. We aimed to add to the minimal information on spatial heterogeneity of soil N in Australian arid environments and to identify indices suited to routine analysis in monitoring and rehabilitation programs in Australia's rangelands. Soils were collected from two sites per grassland type at the end of spring (dry season) from immediately adjacent to tussocks and open areas between tussocks and from 0–2 and 2–5 cm depth. Amounts of N mineralized in leached aerobic incubations (81 d, 40°C, 0–2 cm only) differed significantly between grasslands and between sampling positions (greater adjacent to tussocks) and ranged from 88 to 158 μg g −1. Only NO 3 −-N was produced during aerobic incubations and was about 20% of total N, irrespective of grassland and sampling position. In contrast, N produced during anaerobic incubations (7 d, 40°C) and by extraction in hot KCl was less than 1% of total N. Both of these indices were poorly correlated with others and were either highly variable or did not detect a pronounced depth effect. Oxidisable C and absorbance 260 of 10 mM NaHCO 3 extracts were better correlated with our aerobic index of N availability but relationships differed between grasslands. We recommend either absorbance 260 of mild-salt extracts (soils leached and then aerobically incubated) or total N for relative estimates of N mineralization potential in hot, arid environments where soils are routinely `field-dry'. Strong correlations between variables in this study indicate that heterogeneity in relatively undisturbed, semi-arid soils of similar mineralogy and history is of sufficient magnitude for identifying appropriate indices of N availability.

An application of terrain and environmental modelling in a large-scale forestry experiment

Within-site heterogeneity, particularly of soil and climatic variables is a difficulty facing forestry researchers in the design of field trials. Forest soils are notoriously variable and small changes in soil properties can have a large impact on tree productivity. Likewise, within-site variation in microclimate is known to influence growth. Within a particular climatic envelope, this variation is controlled to a large extent by topography. In this study, a range of terrain-derived attributes and indices was calculated for each of 40 growth plots from a digital elevation model (DEM) in an attempt to quantify the effects of spatial heterogeneity and establish whether they improved the interpretation of data from a field experiment. In addition, selected soil profile measurements were analyzed for their impact on tree growth and correlated with calculated terrain attributes. The presence of gleying (evidence of waterlogging) in the profile accounted for a significant proportion of the residual variation in growth response after treatment effects were removed. A combination of predicted erosion index (ei), relative available soil water (raswc), dynamic wetness index (dynwet) and tangential curvature (tancurve) contributed to significant improvements in model predictions of growth. A physiologically based model of tree growth (PROMOD) was applied to the site to model the effect of these variables on tree growth. We found that by using a terrain-derived waterlogging or gleying variable as input, PROMOD could successfully account for variation in tree growth due to waterlogging (gleying) and some additional between-plot variation in growth due to soil depth.

Early survival, growth and foliar nutrients in native Ecuadorian trees planted on degraded volcanic soil

There is a growing interest for the use of native tree species for tropical plantations. However, their silvicultural requirements are generally unknown. Fifteen native tree species, seven early- and eight late-successional species, were planted on an abandoned pasture located on a degraded Hydrandept volcanic soil in Amazonian Ecuador. The objective was to study their survival, growth and foliar nutrient levels in relation to their successional status. The experiment consisted of a random block design with three replicates, each one subdivided in 15 plots with one species per plot. Half of the plots (split-plot) were fertilized with a slow-release fertilizer (16–10–10). Height and basal diameter were measured every 6 months, for 2.5 yr. Foliar nutrients were estimated from samples taken once, at 2 yr. Soil analyses indicated that the soil was poorer and denser in the plantation compared to the adjacent forest, with a large within-site heterogeneity. Early-successional species had a significantly higher survival, growth and foliar nutrient concentrations than the late-successional species. Fertilizer addition did not affect survival or growth. Among late-successional species, Caryodendron orinocense had by far the highest survival. Four early-successional species had a high survival and good growth, with low coefficients of variation in spite of soil heterogeneity: Erythrina poeppigiana, Pollalesta discolor, Heliocarpus americanus and Inga densiflora. The better growth performances of early-successional species could be related to the relatively early stage of plantation. Our results suggest that these species could be used advantageously to rapidly create a tree cover on a degraded soil. This study highlights the importance of investigating the appropriate land use of degraded volcanic soils in the humid tropics. It also underscores the high potential of some native tree species, and recommends that more studies be done on their silviculture, as well as on the selection of those best adapted to degraded volcanic soils.