Litterfall of epiphytic macrolichens in Nothofagus forests of northern Patagonia,Argentina: Relation to stand age and precipitation

Abstract: The objective of this study was to analyse how stand age and precipitation influence abundance and diversity of epiphytic macrolichens in southern beech Nothofagus forests, estimated by lichen litter sampling. Five sites of Nothofagus dombeyi (Mirbel) Oersted were selected in Nahuel Huapi National Park, Argentina. At each site, lichen fragments from the forest floor were collected at 12.5 m² plots in pairs of young and mature N. dombeyi forest. Additionally, two sites with multi‐aged subalpine Nothofagus pumilio (Poepp. et Endl.) Krasser forest were investigated in a similar manner. Average litterfall biomass per stand varied from less than 1 kg ha^?1 in a young low‐precipitation stand to a maximum of 20 kg ha^?1 in a mature high‐precipitation stand. In places with higher precipitation, litterfall biomass in N. dombeyi forest was considerably higher in old stands as compared with young ones. In places with less than 2000 mm of precipitation, differences in biomass were less pronounced. Old humid stands contained about twice as many taxa in the litter as old low‐precipitation stands and young stands in general. Mature stands in low‐precipitation sites only contained 17% of the litter biomass as compared with mature stands in high‐precipitation sites. Epiphytic lichen composition changed from predominating fruticose lichens (Usnea spp. and Protousnea spp.) in low‐precipitation stands to Pseudocyphellaria spp., Nephroma spp. and other foliose lichens, in the high‐precipitation stands. There were no clear differences in the proportion of fruticose and foliose lichens between young and old stands. Fruticose lichens dominated litter biomass in both N. pumilio sites.     

Pathways of stand development in ageing Pinus sylvestris forests

Question: What are the main pathways of long‐term stand development in forest ecosystems on oligotrophic and acidic sandy soils? Location: Nine forest reserves at different locations in The Netherlands; all ageing Pinus sylvestris forests that are no longer managed and where massive regeneration of broad‐leaved species is often reported. Methods: Agglomerative cluster analysis was used to define structural classes from forest reserve data. Sequences of structural classes, representing different trajectories of stand development, were constructed with the aid of a process based gap model. Results: Four main pathways of stand development could be distinguished. Three pathways are linked to gap dynamics, and lead towards dominance of Betula, Quercus or Fagus. They differ in light availability for regeneration and/or seed tree availability. The fourth pathway comprises of development patterns after major disturbances. Conclusions: The new methodological approach, combining the empirical strength of forest reserve data with the predictive ability of a process based model, made it possible to detail and quantify insights into structure and dynamics of forests on poor sandy soils. Some factors not included in the study can substantially influence pathways, especially those where Quercus and Fagus potentially play an important role.     

Early post-hurricane stand development in Fringe mangrove forests of contrasting productivity

We examined the immediate effects of a hurricane (Hurricane Andrew, August 1992) in a coastal landscape in sub-tropical Florida, and then monitored stand recovery in Fringe mangrove sites of different productive capacity for 9 years after the disturbance. Structural impacts of the hurricane were confined almost entirely to forests within 200–300 m of the coast. Mortality and damage were concentrated on canopy individuals. Following the hurricane, rapid canopy recovery and the early onset of competition among Fringe forest stems, as evidenced by relatively high mortality of smaller individuals, magnified the initial dominance of hurricane survivors and early-established seedlings over later cohorts, and limited recruitment to the brief period prior to canopy closure. Changes in the relative abundance of the two dominant mangrove species following disturbance varied strongly along the productivity gradient. The shade-tolerant Rhizophora mangle L. generally became the overwhelming canopy dominant in the competitive environment of the recovering Coastal Fringe forest following hurricane, but the shade-intolerant Laguncularia racemosa (L.) C.F. Gaertn was better represented in less productive Interior Fringe sites, where canopy closure was delayed. Site productivity is an important determinant of the success of mangrove species during post-hurricane stand development, and consequently of the zonation of communities in the coastal landscape.     

Post-fire recovery of ant communities in Submediterranean Pinus nigra forests

This study analyzes the variations in the structure and composition of ant communities in burned Pinus nigra forests in central Catalonia (NE Spain). Pinus nigra forests do not recover after fire, changing to shrublands and oak coppices. For this reason, we suggest that ant communities of burned P. nigra forests will change after fire, because the post‐fire scenario, in particular with the increase of open areas, is different to the unburned one, and more favourable for some species than for others. In four locations previously occupied by P. nigra forests where different fires occurred 1, 5, 13 and 19 yr before the sampling, we sampled the structure and composition of ant communities with pitfall traps, tree traps and net sweeping in unburned plots and in plots affected by canopy and understory fire. The results obtained suggest that canopy and understory fire had little effect on the structure of ant communities. Thus, many variables concerning ant communities were not modified either by fire type (understory or canopy fire) or by time since fire. However, a number of particular species were affected, either positively or negatively, by canopy fire: three species characteristic of forest habitats decreased after fire, while eight species characteristic of open habitats increased in areas affected by canopy fire, especially in the first few years after fire. These differences in ant community composition between burned and unburned plots imply that the maximum richness is achieved when there is a mixture of unburned forests and areas burned with canopy fire. Moreover, as canopy cover in P. nigra forests burned with canopy fire is not completed in the period of time studied, the presence of the species that are characteristic of burned areas remains along the chronosequence studied, while the species that disappear after fire do not recover in the period of time considered. Overall, the results obtained indicate that there is a persistent replacement of ant species in burned P. nigra forests, as is also the case with vegetation.     

Post-fire succession in the northern jarrah forest of Western Australia

Post-fire species changes were documented over a portion of the northern section of the jarrah forest of Western Australia from a series of sites of known age since last fire. Species richness and species diversity were observed to increase following fire for several years, followed by a gradual decline. Dominance-diversity curves indicated that maximum diversity occurred from 3 to 5 years following fire. The species composition of sites of different regions was more similar than sites of similar time since burn. The significance of the results to the programme of controlled burns is discussed.     

Structure and stand development of secondary forests in Eastern Prealps (Italy)

The composition and temporal variations in species recruitment were examined by means of annual dendrochronological data, to determine the historical development and successional history of ash and sycamore mixed stands in five secondary forests on the Eastern Prealps. The first step of the investigation was to describe species composition and age structure. In accordance with data derived from land register and time series of orthophoto images, differences in time of colonization among the five stands were detected. The second step was to describe spatial colonization patterns: only in one case a frontal pattern was found while in others the colonization started from some old alder or ash trees preserved by farmers in the meadows.     

Effects of introduced ungulates on forest understory communities in northern Patagonia are modified by timing and severity of stand mortality

Natural disturbances such as fires, windstorms, floods, and herbivory often act on plant communities, affecting their structure and the abundance and composition of their species. Most research has focused on the effects of single disturbances on plant communities whereas the synergistic effects of several disturbances have received less attention. In this study, we evaluated how timing and severity of tree mortality modified plant use by introduced deer and early post-mortality successional trajectories in northern Patagonian conifer forests. We sampled understory composition and deer use in Austrocedrus chilensis (ciprés de la cordillera) forest stands undergoing varying timing and severity of forest mortality as reconstructed using dendroecological techniques. In addition, we evaluated the effect of fallen logs on plant composition and deer use of plants by monitoring areas of massive dieback where fallen logs had been removed for fire hazard reduction, and nearby control areas not subjected to such removal. Stepwise regression analyses showed that history and severity of tree mortality strongly influence plant composition and deer use of plants. For deer use (with pellet counts and browsing index as response variables), results showed a positive relationship with degree of stand mortality and a negative relationship with cover of fallen logs. Similarly, cover of unpalatable shrub species was explained by canopy mortality history, whereas cover of palatable shrub species was positively associated with severity of canopy mortality. In areas where fallen logs had been removed, pellet counts were six times higher than those in control areas. Though total shrub species cover was similar between log removal and control areas, proportion of unpalatable shrubs increased in areas where fallen logs had been removed. In conclusion, deer use of plants was strongly limited by tall fallen logs, allowing palatable species to establish and grow. Fallen log removal accelerated deer entrance and changed understory composition toward more browse-resistant and unpalatable species. These results underscore the importance of considering the dynamics (timing, severity, and extent) of fallen woody debris influencing understory herbivory and post-disturbance succession. In addition, experimental results underpin the importance of maintaining snags and large woody debris in disturbed landscapes where salvage logging is a routine procedure.     

Natural and anthropogenic influences on coarse woody debris stocks in Nothofagus–Araucaria forests of northern Patagonia,Argentina

Coarse woody debris (CWD) is an important element driving ecological processes, strengthening ecosystem resilience and for biodiversity within forest ecosystems. However, the abundance and distribution of CWD and their relation to natural and human factors are poorly known in southern South America. In this work we studied the density and volume of CWD types in Nothofagus–Araucaria stands in northern Patagonia (Neuquén – Argentina) and relationships with forest composition and structure. We also studied their relationships with fire history, topography and human‐related variables. Twenty‐three stands with Nothofagus pumilio, Nothofagus antarctica and/or Araucaria araucana were sampled to estimate quantities of logs, snags and dead branches using the planar‐intersect method. CWD density and volume in these forests were moderate and varied across the landscape with a spatial pattern determined by biotic, abiotic and human use–related variables. Mean CWD volume was 52.9 m³ ha^?1 (range: 1.6–143.7) and significantly varied among forest types and watersheds. CWD was positively related to dbh, tree height and slope, but negatively related to tree density. CWD was clearly influenced by composition and structural characteristics of stands, where the tree species traits had an important role. As well, the observed amount and type of CWD, whereby most of the stands showed low levels of old (pre‐disturbance) logs/snags and poor new inputs of deadwood, may be explained by fire frequency. Firewood gathering and livestock grazing negatively affected deadwood stocks and topography counteracts this effect by limiting human access. Fire disturbance history, windthrow and dieback pulses produced by insect outbreaks and human access seemed to be the main causes that best explained CWD spatial distribution and abundance patterns in north‐western Patagonian forests.     

Post-fire restoration of Mediterranean forests: Testing assembly rules mediated by facilitation

In view of the importance of facilitative interactions between plants, nurse-based planting has been proposed as a restoration technique for Mediterranean vegetation. However, facilitation efficiency is known to depend on the environmental context and the particular pair of interacting species. Understanding these context- and pair-specific dependences is fundamental to understanding Mediterranean vegetation dynamics and to improving the use of nurse-based plantation for restoration. We assessed the effectiveness of nurse-based plantation and the significance for post-fire restoration of some assembly rules mediated by facilitation. In two nearby areas of different burning ages, we compared seedling establishment of 13 tall shrubs and trees planted in open ground and under nurses. Nurses and planted seedlings were selected from different life-forms. Tests of the assembly rules were provided by the partitioning of the statistical interaction effect between nurse and planted seedling life-forms in a two-factor design. Nurse-based plantation increased seedling survival 2–9 times compared to plantation in open ground, depending on the year. The higher efficiency of nurse-based plantation was consistent for the two burned areas and occurred in many species even in years with contrasting rainfall. We detected pair-specific differences in the efficiency of facilitation. This pair-specificity was partly explained by the dependence between life-forms of nurse and guest species, suggesting the existence of assembly rules. Our results confirm that nurse-based plantation would be an appropriate post-fire restoration technique in Mediterranean mountains under dry-subhumid climate, but suggest that attention to the life-form assemblage rules is needed for an efficient implementation of such technique.     

Nitrogen oligotrophication in northern hardwood forests

While much research over the past 30 years has focused on the deleterious effects of excess N on forests and associated aquatic ecosystems, recent declines in atmospheric N deposition and unexplained declines in N export from these ecosystems have raised new concerns about N oligotrophication, limitations of forest productivity, and the capacity for forests to respond dynamically to disturbance and environmental change. Here we show multiple data streams from long-term ecological research at the Hubbard Brook Experimental Forest in New Hampshire, USA suggesting that N oligotrophication in forest soils is driven by increased carbon flow from the atmosphere through soils that stimulates microbial immobilization of N and decreases available N for plants. Decreased available N in soils can result in increased N resorption by trees, which reduces litterfall N input to soils, further limiting available N supply and leading to further declines in soil N availability. Moreover, N oligotrophication has been likely exacerbated by changes in climate that increase the length of the growing season and decrease production of available N by mineralization during both winter and spring. These results suggest a need to re-evaluate the nature and extent of N cycling in temperate forests and assess how changing conditions will influence forest ecosystem response to multiple, dynamic stresses of global environmental change.     

Nitrogen mineralization in high-elevation forests of the appalachians. II. Patterns with stand development in fir waves

Wave-like patterns of mortality and regeneration of balsam fir (Abies balsamea {L.} Mill.) and Fraser fir (A. fraseri {Pursh.} Poir.) forests at high elevations in the Appalachian Mountains offer a unique opportunity to study the effects of stand development on nitrogen cycling. We sampled two fir waves, one with Fraser fir on Mt. LeConte in the Great Smoky Mountains National Park, and one with balsam fir on Whiteface Mountain in New York. Net nitrogen mineralization for 3 summer months at Mt. LeConte was high in the dead fir zone (47 kg-N/ha), lower in the regeneration and juvenile zones (24 and 21 kg-N/ha), and highest in the mature zone (61 kg-N/ha). This sampling period probably accounted for about 60% of the annual total. The pattern was similar in the balsam fir wave on Whiteface Mountain, with N mineralization rates of 39 and 33 kg-N/ha over 2 months for the regenerating and juvenile zones, and 43 and 54 kg-N/ha for the mature and dead zones. Throughfall nitrogen followed a fairly similar pattern, with rates ranging from 4.5 to 10 kg-N/ha for 2 or 3 months across all zones at both sites. Tension-free lysimeters indicated very little leaching of nitrogen below 30 cm depth (the maximum was 2.6 kg-N/ha), but these estimates may be low. We conclude that nitrogen mineralization is high at all stages of stand development, perhaps exceeding the uptake capacity of the trees. Rates of nitrogen leaching may be high in these ecosystems and should receive further attention. Request for offprints     

Shell shape variation in the Nassariid Buccinanops globulosus in northern Patagonia

Morphological variation among natural populations is a phenomenon commonly observed in marine invertebrates and well studied, particularly, in shelled gastropods. The nassariid Buccinanops globulosus is interesting to study shell shape variation because it exhibits strong interpopulation differences in life history features, including maximum size, fecundity and growth rate. In this study, we examined the pattern of variation in size and shell shape among populations and between sexes of B. globulosus (Bahía San Antonio 40°29′S 63°01′W, Playa Villarino 40°45′S 64°40′W and Bahía Nueva 42°46′S 65°02′W). In particular, we used geometric morphometric techniques to test: (1) whether the two components of shell morphology (size and shape) are independent and (2) whether shape differences between sexes are consistently found among populations, regardless of their body sizes. Our results show shell shape variation between the populations of B. globulosus of northern Patagonia. Intra-specific shell shape variation is affected by body size, indicating allometry. Regardless of the size differences, individuals from Playa Villarino have high-spired shells, and shorter apertures and wider columellar area than individuals from the other populations. Also, sex-related shape differences were consistently found at each population, thus suggesting a common sexual dimorphism in shell morphology for this species. The functional significance of the variability found is discussed in terms of the flexibility of developmental programmes for morphology as well as the evolution of phenotypic plasticity.     

The direct regeneration hypothesis in northern forests

Question: Can the direct regeneration hypothesis (DRH) be used to predict post‐disturbance regeneration after fire, wind disturbance, and clearcutting in northern forests? Do life‐history traits such as regeneration strategy and shade tolerance influence post‐disturbance regeneration success of tree species? Location: Northern forests in North America. Methods: A meta‐analysis was conducted by collecting published data on pre‐ and post‐disturbance stand compositional characteristics in the northern forests. For each tree species, compositional difference (CD) was calculated as the difference between basal area proportions of the post‐ and pre‐disturbance stands, but for post‐disturbance stands <25 years of age, post‐disturbance proportions were calculated based on relative stem density. Results: Species response to disturbances was best explained by regeneration strategy, while disturbance type had no effect on CD. The proportion of broadleaf trees with either strong or weak vegetative reproduction ability increased after all disturbances. Serotinous species had CD values not significantly different from zero after fire, while CD for semi‐serotinous species was negative. The post‐disturbance proportions of non‐serotinous conifers decreased after all forms of disturbance. Conclusions: All disturbances promote broadleaf trees, regardless of regeneration strategy (suckering, sprouting, or seeding). The DRH is supported for conifers with serotinous cones after fire. Fire causes local extinction of non‐serotinous conifers, while wind and clearcutting only decrease the proportion of non‐serotinous conifers because of partial survival of seed sources and advanced regeneration. This study suggests that increasing stand‐replacing disturbances associated with global climate change will promote broadleaf trees in northern forests.     

Effects of pasture development on the ecological functions of riparian forests in Hokkaido in northern Japan

In the summer, the forest canopy lowers the water temperature, which is very important for anadromous fish, and its population density is significantly lower in grassland streams. Leaf litter and terrestrial invertebrates are the critical food resources for stream organisms. In a basin where the riparian forest is preserved, but other areas have been cut, the amount of leaf litter is almost equivalent to that in an intact natural basin. The annual input of terrestrial invertebrates falling into the forested reaches was 1.7 times greater than that in the grassland reaches, and fish biomass was significantly less in the grassland reaches. In-stream large woody debris creates storage sites for organic and inorganic matter and enhances habitat diversity for aquatic biota. However, the volume and number of large wood pieces decreased significantly with pasture development, because it clears the riparian forests and covers the riverbanks with grass. Fine sediment is a prominent by-product of agricultural development and adversely impacts periphyton productivity, the density and diversity of aquatic invertebrates, fish feeding, fish spawning and egg survival. We also examine the adequate width of a riparian buffer if it is to be able to satisfy its ecological functions.     

Disentangling stand and environmental correlates of aboveground biomass in Amazonian forests

Tropical forests contain an important proportion of the carbon stored in terrestrial vegetation, but estimated aboveground biomass (AGB) in tropical forests varies two‐fold, with little consensus on the relative importance of climate, soil and forest structure in explaining spatial patterns. Here, we present analyses from a plot network designed to examine differences among contrasting forest habitats (terra firme, seasonally flooded, and white‐sand forests) that span the gradient of climate and soil conditions of the Amazon basin. We installed 0.5‐ha plots in 74 sites representing the three lowland forest habitats in both Loreto, Peru and French Guiana, and we integrated data describing climate, soil physical and chemical characteristics and stand variables, including local measures of wood specific gravity (WSG). We use a hierarchical model to separate the contributions of stand variables from climate and soil variables in explaining spatial variation in AGB. AGB differed among both habitats and regions, varying from 78 Mg ha^?1 in white‐sand forest in Peru to 605 Mg ha^?1 in terra firme clay forest of French Guiana. Stand variables including tree size and basal area, and to a lesser extent WSG, were strong predictors of spatial variation in AGB. In contrast, soil and climate variables explained little overall variation in AGB, though they did co‐vary to a limited extent with stand parameters that explained AGB. Our results suggest that positive feedbacks in forest structure and turnover control AGB in Amazonian forests, with richer soils (Peruvian terra firme and all seasonally flooded habitats) supporting smaller trees with lower wood density and moderate soils (French Guianan terra firme) supporting many larger trees with high wood density. The weak direct relationships we observed between soil and climate variables and AGB suggest that the most appropriate approaches to landscape scale modeling of AGB in the Amazon would be based on remote sensing methods to map stand structure.     

Leaf area index development in temperate oak and beech forests is driven by stand characteristics and weather conditions

Using data from 20 even-aged and homogeneous mature beech and oak study plots in Flanders (Northern Belgium), an analysis of the empirical relationships between the rates of leaf area index (LAI) change throughout the leaf development of 2008 and stand, site and meteorological variables was performed. Species-specific multiple linear regressions were fitted between the rates of LAI change and the predictors for two distinct periods from April until August. After a sharp increase in LAI following budburst, the seasonal LAI development for both species showed a marked period of stationary LAI development over all study plots. The cause for the cessation of LAI growth was assumed to be the decline of air temperature and radiation during this period. Later on, the rate of LAI development restarted similarly in every plot. The influence of weather on LAI development was high and its effects were different between species, with beech mostly affected by radiation and oak negatively related to minimal and maximal values of air temperature. Furthermore, our analysis suggested that stand structural (tree density and stand basal area for both species) and tree growth characteristics (average tree-ring width ratio for oak) variables were major drivers of the LAI development during early spring. Later during the growth period, stand variables became less predominant in affecting LAI development. Site quality variables affected LAI development to a lesser extent. The seasonal LAI development was found very similar among stands. This study adds a more accurate and comprehensive approach to the modelling of LAI development during leaf growth of two important European temperate deciduous forest species.