Mesic Sites Research Articles

Interactions between stand thinning, site quality and host tree species on spruce budworm biological performance and host tree resistance over a 6 year period after thinning

Forest management activities such as thinning have been proposed to reduce defoliation damage incurred by spruce budworm (Choristoneura fumiferana (Clem.)). Yet, information regarding thinning effects on tree and stand vulnerability to spruce budworm is equivocal. A better understanding of thinning with respect to host tree resistance and budworm performance could be useful to reduce its impacts while respecting forest ecological integrity. Our objectives were (1) to evaluate the effects of thinning and site drainage quality on balsam fir (Abies balsamea (L.) Mill.), white spruce (Picea glauca (Moench) Voss) and black spruce (P. mariana (Mill.) BSP) resistance to spruce budworm through time (6years) and (2) to produce a resistance classification model that could be used by forest managers to determine when and where to apply this treatment. Field-rearing experiments of spruce budworm were conducted, together with foliar chemical analyses, along a gradient of stand thinning intensity (0%, control; 25%, light; 40%, heavy) and drainage class (rapidly drained, class 2; mesic with seepage, class 3; subhygric, class 4; hydric, class 5). Budworm performance was followed throughout the insect’s life cycle, including its winter biology. Balsam fir resistance, unlike that of white and black spruce, was significantly reduced 1year after thinning. Fir response was related to increased defoliation linked to reduction of certain monoterpenes on mesic sites and to decreased foliage production on subhygric sites. On hydric sites, thinning increased fir resistance by increasing foliage production. We observed an opposite response 3years after treatment. Heavy thinning (40%) positively affected balsam fir and white spruce tolerance and, therefore, tree resistance, by increasing foliage production and the amount remaining after budworm feeding throughout the study, except 6years after thinning in white spruce growing on mesic sites. From the fourth year onward, black spruce resistance was positively affected by thinning. This response was observed despite thinning also having favoured insect performance (high female pupal mass), which resulted in greater foliage destruction in thinned stands. These findings suggest that thinning may be used to increase tree and stand resistance to spruce budworm. Our resistance classification system could be useful to forest managers in planning thinning that would reduce budworm impacts at the stand level.

Persistence of pine species in late‐successional forests: evidence from habitat‐related variation in stand age structure

AbstractQuestionsBy what mechanisms do early‐successional tree species such as pines persist in late‐successional forests in regions where recurrence intervals for stand‐replacing disturbance are much longer than maximum tree life spans? How are recruitment patterns affected by habitat variation?LocationOld‐growth, mixed species stands of Pinus strobus, Pinus resinosa, Tsuga canadensis and Acer saccharum at 36 locations in three subregions of northeast Wisconsin and west upper Michigan, USA.MethodsFour to nine stands were located in each of six habitats and sampled in 200‐m2 plots. Increment cores were collected from 1456 trees, and charcoal collected and dated to characterize stand age structure and disturbance history. Stands on upland mesic sites, with strong successional trends toward late‐successional species, were compared to those on ‘marginal’ habitats (rock outcrops, sandy inclusions and lake, river and wetland edges), where competition is comparatively low. We developed a new methodology that utilizes uncertainty in establishment dates to more rigorously distinguish even‐aged stands (the null hypothesis) from those with multiple age classes.ResultsAnalysis of tree germination dates suggests that some stands with an apparently broad range of pine ages primarily reflect uncertainty in age estimates rather than actual periods of extended pine recruitment. Nevertheless, multi‐aged populations of pine (rejection of the even‐aged null hypothesis) were found in 86% of the stands and in all habitats. Marginal habitats had more age classes than mesic sites (mean 2.8 vs 1.7; P = 0.007) and greater age diversity. Disturbances that initiated pine cohorts were more often severe and more often caused by fire on mesic sites than on marginal habitats. Pine establishment was somewhat episodic in all three subregions, but age‐class peaks across subregions had limited synchrony and were not strongly correlated with severe drought.ConclusionsThese findings are consistent with growing evidence that age structures of early‐successional tree species in primary forests are often more complex than commonly believed, and that population structure and persistence are strongly influenced by habitat variation. Pine age structures provide indirect support for ‘refuge‐based’ landscape persistence, with dispersal limitation overcome by persistent seed sources in nearby marginal habitats.

Vulnerability to cavitation, hydraulic efficiency, growth and survival in an insular pine (Pinus canariensis)

It is widely accepted that hydraulic failure due to xylem embolism is a key factor contributing to drought-induced mortality in trees. In the present study, an attempt is made to disentangle phenotypic plasticity from genetic variation in hydraulic traits across the entire distribution area of a tree species to detect adaptation to local environments. A series of traits related to hydraulics (vulnerability to cavitation and hydraulic conductivity in branches), growth performance and leaf mass per area were assessed in eight Pinus canariensis populations growing in two common gardens under contrasting environments. In addition, the neutral genetic variability (FST) and the genetic differentiation of phenotypic variation (QST) were compared in order to identify the evolutionary forces acting on these traits. The variability for hydraulic traits was largely due to phenotypic plasticity. Nevertheless, the vulnerability to cavitation displayed a significant genetic variability (approx. 5 % of the explained variation), and a significant genetic × environment interaction (between 5 and 19 % of the explained variation). The strong correlation between vulnerability to cavitation and survival in the xeric common garden (r = -0·81; P < 0·05) suggests a role for the former in the adaptation to xeric environments. Populations from drier sites and higher temperature seasonality were less vulnerable to cavitation than those growing at mesic sites. No trade-off between xylem safety and efficiency was detected. QST of parameters of the vulnerability curve (0·365 for P50 and the slope of the vulnerability curve and 0·452 for P88) differed substantially from FST (0·091), indicating divergent selection. In contrast, genetic drift alone was found to be sufficient to explain patterns of differentiation for xylem efficiency and growth. The ability of P. canariensis to inhabit a wide range of ecosystems seemed to be associated with high phenotypic plasticity and some degree of local adaptations of xylem and leaf traits. Resistance to cavitation conferred adaptive potential for this species to adapt successfully to xeric conditions.

Winning and Losing Tree Species of Reassembly in Minnesota’s Mixed and Broadleaf Forests

We examined reassembly of winning and losing tree species, species traits including shade and fire tolerance, and associated disturbance filters and forest ecosystem types due to rapid forest change in the Great Lakes region since 1850. We identified winning and losing species by changes in composition, distribution, and site factors between historical and current surveys in Minnesota’s mixed and broadleaf forests. In the Laurentian Mixed Forest, shade-intolerant aspen replaced shade-intolerant tamarack as the most dominant tree species. Fire-tolerant white pine and jack pine decreased, whereas shade-tolerant ashes, maples, and white cedar increased. In the Eastern Broadleaf Forest, fire-tolerant white oaks and red oaks decreased, while shade-tolerant ashes, American basswood, and maples increased. Tamarack, pines, and oaks have become restricted to sites with either wetter or sandier and drier soils due to increases in aspen and shade-tolerant, fire-sensitive species on mesic sites. The proportion of shade-tolerant species increased in both regions, but selective harvest reduced the applicability of functional groups alone to specify winners and losers. Harvest and existing forestry practices supported aspen dominance in mixed forests, although without aspen forestry and with fire suppression, mixed forests will transition to a greater composition of shade-tolerant species, converging to forests similar to broadleaf forests. A functional group framework provided a perspective of winning and losing species and traits, selective filters, and forest ecosystems that can be generalized to other regions, regardless of species identity.

Restoring a midtolerant pine species as a component of late-successional forests: Results of gap-based planting trials

Coniferous species with emergent growth forms can have positive influences on biological diversity and ecosystem function in temperate deciduous forests, but conifer populations have declined in some regions of mixed forest because of human activities. Restoration of midtolerant conifers can be especially difficult in mesic, late-successional forests without intensive treatments. The goal of this study was to quantify the effect of stand and site characteristics on the growth and competitive success of eastern white pine (Pinus strobus) regeneration after gap-based restoration treatments in mesic hemlock-hardwood forests. We measured growth rates and competitive status of white pines averaging 7–15years of age that had been underplanted or naturally established in small canopy openings (mean expanded gap size ranging from 50 to 2200m2) in 14 hemlock-hardwood stands in northern Wisconsin, USA. Mortality of planted seedlings was very high, and few sites had more than 20% of the surviving pine saplings meeting established success criteria (relative height >80%, or relative height >50% and relative growth rate >100%) compared to understory-gap competitors. However, six sites had some pines up to 3–5m tall, with height growth rates exceeding 30cm/year. Height growth was linearly correlated with light intensity but not strongly correlated with gap size. Fast-growing pines were generally found in locations with >35% transmitted radiation and on sites lacking a well-developed understory of shade-tolerant species. Browse and disease damage did not appear to have a strong impact on current white pine growth and competitive status but may have contributed to high mortality rates in the first few years. Restoration of white pine in canopy openings appears feasible even on high productivity mesic sites, but such treatments are likely to have low success rates if openings are small and understory vegetation is not controlled. However, they may be adequate for establishing a minor pine component, especially in northern hardwood stands designated for uneven-aged management. Such low-intensity treatments would also be appropriate in natural areas or parks where pine abundance has declined relative to presettlement conditions and intensive treatments such as shelterwood harvests would not be acceptable.

Life form and water source interact to determine active time and environment in cryptogams: an example from the maritime Antarctic

Antarctica, with its almost pristine conditions and relatively simple vegetation, offers excellent opportunities to investigate the influence of environmental factors on species performance, such information being crucial if the effects of possible climate change are to be understood. Antarctic vegetation is mainly cryptogamic. Cryptogams are poikilohydric and are only metabolically and photosynthetically active when hydrated. Activity patterns of the main life forms present, bryophytes (10 species, ecto- and endohydric), lichens (5 species) and phanerogams (2species), were monitored for 21days using chlorophyll a fluorescence as an indicator of metabolic activity and, therefore, of water regime at a mesic (hydration by meltwater) and a xeric (hydration by precipitation) site on Léonie Island/West Antarctic Peninsula (67°36'S). Length of activity depended mainly on site and form of hydration. Plants at the mesic site that were hydrated by meltwater were active for long periods, up to 100% of the measurement period, whilst activity was much shorter at the xeric site where hydration was entirely by precipitation. There were also differences due to life form, with phanerogams and mesic bryophytes being most active and lichens generally much less so. The length of the active period for lichens was longer than in continental Antarctica but shorter than in the more northern Antarctic Peninsula. Light intensity when hydrated was positively related to the length of the active period. High activity species were strongly coupled to the incident light whilst low activity species were active under lower light levels and essentially uncoupled from incident light. Temperatures were little different between sites and also almost identical to temperatures, when active, for lichens in continental and peninsular Antarctica. Gradients in vegetation cover and growth rates across Antarctica are, therefore, not likely to be due to differences in temperature but more likely to the length of the hydrated (active) period. The strong effect on activity of the mode of hydration and the life form, plus the uncoupling from incident light for less active species, all make modelling of vegetation change with climate a more difficult exercise.

Is an Epic Pluvial Masking the Water Insecurity of the Greater New York City Region?*,+

Abstract Six water emergencies have occurred since 1981 for the New York City (NYC) region despite the following: 1) its perhumid climate, 2) substantial conservation of water since 1979, and 3) meteorological data showing little severe or extreme drought since 1970. This study reconstructs 472 years of moisture availability for the NYC watershed to place these emergencies in long-term hydroclimatic context. Using nested reconstruction techniques, 32 tree-ring chronologies comprised of 12 species account for up to 66.2% of the average May–August Palmer drought severity index. Verification statistics indicate good statistical skill from 1531 to 2003. The use of multiple tree species, including rarely used species that can sometimes occur on mesic sites like Liriodendron tulipifera, Betula lenta, and Carya spp., seems to aid reconstruction skill. Importantly, the reconstruction captures pluvial events in the instrumental record nearly as well as drought events and is significantly correlated to precipitation over much of the northeastern United States. While the mid-1960s drought is a severe drought in the context of the new reconstruction, the region experienced repeated droughts of similar intensity, but greater duration during the sixteenth and seventeenth centuries. The full record reveals a trend toward more pluvial conditions since ca. 1800 that is accentuated by an unprecedented 43-yr pluvial event that continues through 2011. In the context of the current pluvial, decreasing water usage, but increasing extra-urban pressures, it appears that the water supply system for the greater NYC region could be severely stressed if the current water boom shifts toward hydroclimatic regimes like the sixteenth and seventeenth centuries.

Evaluation and improvement of the Community Land Model (CLM4) in Oregon forests

Abstract. Ecosystem process models are important tools for determining the interactive effects of global change and disturbance on forest carbon dynamics. Here we evaluated and improved terrestrial carbon cycling simulated by the Community Land Model (CLM4), the land model portion of the Community Earth System Model (CESM1.0.4). Our analysis was conducted primarily in Oregon forests using FLUXNET and forest inventory data for the period 2001–2006. We go beyond prior modeling studies in the region by incorporating regional variation in physiological parameters from &gt;100 independent field sites in the region. We also compare spatial patterns of simulated forest carbon stocks and net primary production (NPP) at 15 km resolution using data collected from federal forest inventory plots (FIA) from &gt;3000 plots in the study region. Finally, we evaluate simulated gross primary production (GPP) with FLUXNET eddy covariance tower data at wet and dry sites in the region. We improved model estimates by making modifications to CLM4 to allow physiological parameters (e.g., foliage carbon to nitrogen ratios and specific leaf area), mortality rate, biological nitrogen fixation, and wood allocation to vary spatially by plant functional type (PFT) within an ecoregion based on field plot data in the region. Prior to modifications, default parameters resulted in underestimation of stem biomass in all forested ecoregions except the Blue Mountains and annual NPP was both over- and underestimated. After modifications, model estimates of mean NPP fell within the observed range of uncertainty in all ecoregions (two-sided P value = 0.8), and the underestimation of stem biomass was reduced. This was an improvement from the default configuration by 50% for stem biomass and 30% for NPP. At the tower sites, modeled monthly GPP fell within the observed range of uncertainty at both sites for the majority of the year, however summer GPP was underestimated at the Metolius semi-arid pine site and spring GPP was overestimated at the Campbell River mesic Douglas-fir site, indicating GPP may be an area for further improvement. The low bias in summer maximum GPP at the semi-arid site could be due to seasonal response of Vcmax to temperature and precipitation while overestimated spring values at the mesic site could be due to response of Vcmax to temperature and day length.

Carbon turnover in mixed stands: Modelling possible shifts under climate change

Adaptation of forest management to changing climate may lead to remarkable changes in tree species composition. Deciduous species are predicted to be favoured in a warmer climate, but the competitive capacity of different tree species requires detailed analysis. The main objectives of this study were to assess how varied initial proportions of tree species affected productivity in mixed stands, and to evaluate the biomass production and carbon sequestration potential of trees and soil in mixed stands in a changing climate. We used the individual-based simulation model EFIMOD combined with the model of soil organic matter dynamics ROMUL to simulate spatially explicit competition between trees for light and nutrients. We focused our simulations on the most common type of site (mesic site, Myrtillus type) in southern Finland. The average carbon and nitrogen pools in the forest floor and mineral soil were used as initial soil data. The ambient climate scenario was based on measured data (1961–2007) from the Finnish Meteorological Institute. Climate change scenarios were based on runs of three general circulation models. We simulated the forest stand dynamics of two- and three-species mixtures (Silver birch, Scots pine and Norway spruce) with different initial proportions. In mixed spruce–birch stands, spruce replaced birch whatever the initial density, regardless of climatic scenario. In the beginning of the development of mixed spruce–pine stands, pine grew faster and increased its proportion of biomass by 10–20%. In pine–birch stands, the dominant species maintained its dominating position, but the proportion of pine generally declined by the end of simulation in birch-dominated stands. In mixed stands of three tree species, spruce tended to dominate by the end of a 100-year simulation period, independently of the initial proportions. The highest average carbon stock in standing biomass was observed in mixed stands with three species and in spruce-dominated stands. Climate change increased stand productivity and the increase in coniferous stands was more remarkable than in birch-dominated stands. In addition, soil was affected by species composition: increased proportions of pine resulted in increasing carbon stocks in the forest floor. Climate change negatively affected accumulation of organic matter in soil, especially in the spruce-dominated stands.

Cyclic Occurrence of Fire and Its Role in Carbon Dynamics along an Edaphic Moisture Gradient in Longleaf Pine Ecosystems

Fire regulates the structure and function of savanna ecosystems, yet we lack understanding of how cyclic fire affects savanna carbon dynamics. Furthermore, it is largely unknown how predicted changes in climate may impact the interaction between fire and carbon cycling in these ecosystems. This study utilizes a novel combination of prescribed fire, eddy covariance (EC) and statistical techniques to investigate carbon dynamics in frequently burned longleaf pine savannas along a gradient of soil moisture availability (mesic, intermediate and xeric). This research approach allowed us to investigate the complex interactions between carbon exchange and cyclic fire along the ecological amplitude of longleaf pine. Over three years of EC measurement of net ecosystem exchange (NEE) show that the mesic site was a net carbon sink (NEE = −2.48 tonnes C ha−1), while intermediate and xeric sites were net carbon sources (NEE = 1.57 and 1.46 tonnes C ha−1, respectively), but when carbon losses due to fuel consumption were taken into account, all three sites were carbon sources (10.78, 7.95 and 9.69 tonnes C ha−1 at the mesic, intermediate and xeric sites, respectively). Nonetheless, rates of NEE returned to pre-fire levels 1–2 months following fire. Consumption of leaf area by prescribed fire was associated with reduction in NEE post-fire, and the system quickly recovered its carbon uptake capacity 30–60 days post fire. While losses due to fire affected carbon balances on short time scales (instantaneous to a few months), drought conditions over the final two years of the study were a more important driver of net carbon loss on yearly to multi-year time scales. However, longer-term observations over greater environmental variability and additional fire cycles would help to more precisely examine interactions between fire and climate and make future predictions about carbon dynamics in these systems.

Dendrochronological analysis of white oak growth patterns across a topographic moisture gradient in southern Ohio

Moisture availability is a key factor that influences white oak (Quercus alba L.) growth and wood production. In unglaciated eastern North America, available soil moisture varies greatly along topographic and edaphic gradients. This study was aimed at determining the effects of soil moisture variability and macroclimate on white oak growth in mixed-oak forests of southern Ohio. Using accurately dated and measured tree rings, we analyzed 119 white oaks growing across an integrated moisture index (IMI), a computer-generated GIS model that simultaneously combines topographic and edaphic features into a moisture index scale. Growth trends varied considerably across the IMI, with trees in mesic sites exhibiting patterns much different from those in either xeric or intermediate sites. BAI growth and biomass increments were higher for trees growing in the intermediate and mesic sites than those from the xeric sites. Correlation and response function analyses, and redundancy analysis revealed significant relations between ring-width indices and climate, with current year May–July PDSI, precipitation and temperature as the most important correlates of white oak growth. Additionally, climatic influences on growth rate were variable across the IMI; trees in xeric sites showed much greater coefficients relative to those from the intermediate and mesic sites. Despite these differences, xeric and intermediate trees exhibited similar growth patterns. The present results provide further evidence of the usefulness of the IMI for identifying and comparing white oak growth patterns across the complex, dissected landscape of southern Ohio.

Stand age structural dynamics of conifer, mixedwood, and hardwood stands in the boreal forest of central Canada

To study the effects of stand development and overstory composition on stand age structure, we sampled 32 stands representing conifer, mixedwood, and hardwood stand types, ranging in ages from 72 to 201 years on upland mesic sites in northwestern Ontario. We defined the stages of stand development as: stem exclusion/canopy transition, canopy transition, canopy transition/gap dynamics, and gap dynamics. Stand age structure of conifer stands changed from bimodal, bimodal, reverse-J, and bimodal, respectively, through the stages of stand development. Mixedwood and hardwood stands revealed similar trends, with the exception of missing the canopy transition/gap dynamic stage in mixedwoods. Canopy transition/gap dynamic stage in hardwoods showed a weaker reverse-J distribution than their conifer counterparts. The results suggest that forest management activities such as partial and selection harvesting and seed-tree systems may diversify standard landscape-level age structures and benefit wildlife, hasten the onset of old-growth, and create desired stand age structures. We also recommend that the determination of old-growth using the following criteria in the boreal forest: 1) canopy breakdown of pioneering cohort is complete and stand is dominated by later successional tree species, and 2) stand age structure is bimodal, with dominating canopy trees that fall within a relatively narrow range of age and height classes and a significant amount of understory regeneration.

Sudden changes in environmental conditions do not increase invasion risk in grassland

After direct habitat transformation, biological invasions are considered to be the second most important threat to biodiversity. A better understanding of the factors affecting invasion success in new areas is crucial, and may provide insight into potential control actions. We hypothesized that invasion risk increases in habitats undergoing a sudden change in the disturbance regime or environmental conditions. For testing this assumption we initiated a seed sowing experiment while introducing two novel treatments, mowing twice and fertilizer application, in two grassland sites (one dryer and one mesic) in Romania. The seeds of two invasive species, Solidago canadensis and Rudbeckia laciniata, and two resident natives of similar seed sizes, life-forms and strategies were sowed in treated and control plots, and seed germination, seedling establishment and growth were followed during four months. Contrary to our expectations, there was no difference in the treatment effects on seed germination and seedling establishment between species, while there was on seedling vigour of the larger seeded species in the dryer grassland site, where the native had a higher performance especially in increased nutrient conditions. Indifferently from applied treatments, invasive species had greater cumulative germination in the mesic site, while natives were far more successful in seedling establishment in the drier site. At the same time, seed size was found to be a very important factor explaining germination and establishment success, with large seeded species outperforming small seeded species in any circumstances. Our results call the attention upon management interventions in mesic, productive grassland sites opening colonization windows for the recruitment of those invasive species of which ecological requirements correspond to local environmental conditions.

Effect of Slope and Aspect on the Associated Flora of Pinyon Pines in Central Mexico

To test the hypothesis that Mexican pinyons (Pinus cembroides) and Johannis pines (P. johannis) grow under different environmental conditions, the flora associated with both species was sampled along two 20-m perpendicular transects at 40 sites. Patterns were analyzed with the two-way-indicator-species program, which yielded six groups with Mexican pinyons dominating xeric sites and Johannis pines dominating mesic sites. Other groups of species corresponded to transition from mesic to xeric sites. We used canonical-correspondence analysis to explore the effect of slope and aspect on species and indirectly related them to an underlying moisture gradient. The first ordination axis indicated that aspect explained 59% of the variation, and the second axis, representing slope, explained 41%.Para probar la hipótesis de que los piñoneros mexicanos (Pinus cembroides) y los piñoneros Johannis (P. johannis) crecen bajo diferentes condiciones ambientales, se muestreó la flora asociada a ambas especies a lo largo de dos transectos perpendiculares de 20 m en 40 sitios. Los patrones se analizaron con el programa TWINSPAN, el cual produjo seis grupos, con dominancia del piñonero mexicano en sitios xéricos, y del piñonero Johannis en sitios mésicos. Otros grupos de especies correspondieron a los sitios de transición mésicos a xéricos. Usamos Análisis Canónico de Correspondencias para explorar el efecto de la pendiente y la orientación sobre las especies y relacionarlas indirectamente con un gradiente subyacente de humedad. El primer eje de ordenación indicó que la exposición explicaba el 59% de la variación, y el segundo eje, el cual representa la pendiente, explicaba el 41%.

Significant mean and extreme climate sensitivity of Norway spruce and silver fir at mid-elevation mesic sites in the Alps.

Climate forcing is the major abiotic driver for forest ecosystem functioning and thus significantly affects the role of forests within the global carbon cycle and related ecosystem services. Annual radial increments of trees are probably the most valuable source of information to link tree growth and climate at long-term time scales, and have been used in a wide variety of investigations worldwide. However, especially in mountainous areas, tree-ring studies have focused on extreme environments where the climate sensitivity is perhaps greatest but are necessarily a biased representation of the forests within a region. We used tree-ring analyses to study two of the most important tree species growing in the Alps: Norway spruce (Picea abies) and silver fir (Abies alba). We developed tree-ring chronologies from 13 mesic mid-elevation sites (203 trees) and then compared them to monthly temperature and precipitation data for the period 1846–1995. Correlation functions, principal component analysis and fuzzy C-means clustering were applied to 1) assess the climate/growth relationships and their stationarity and consistency over time, and 2) extract common modes of variability in the species responses to mean and extreme climate variability. Our results highlight a clear, time-stable, and species-specific response to mean climate conditions. However, during the previous-year's growing season, which shows the strongest correlations, the primary difference between species is in their response to extreme events, not mean conditions. Mesic sites at mid-altitude are commonly underrepresented in tree-ring research; we showed that strong climatic controls of growth may exist even in those areas. Extreme climatic events may play a key role in defining the species-specific responses on climatic sensitivity and, with a global change perspective, specific divergent responses are likely to occur even where current conditions are less limited.

Sapling growth and crown expansion in canopy gaps of Nothofagus pumilio (lenga) forests in Chubut, Patagonia, Argentina.

In the province of Chubut in Patagonia, Argentina, Nothofagus pumilio forests (locally known as lenga), are managed through selective cuts, which imply the opening of canopy gaps. This management scheme is carried out without taking into consideration the sapling requirements changes through neither a cutting cycle nor the precipitation gradient in which these forests thrive. To analyze these changes, we inferred the facilitation-competition balance between the canopy and regeneration studying the effects of precipitation levels, gap size and gap age on saplings growth in height on 45 canopy gaps artificially created between 1960 and 1993. Results shown that during the first 20 years since the gap opening, the regeneration growth is determined by light availability in mesic sites and by water availability in dry sites. However, the difference due to the precipitation levels gradually decreases over time. Moreover, in the period between 20 and 35 years after gap opening, in both mesic and xeric sites, light is the limiting factor to growth. This means that in xeric sites, saplings shift from a water-dependent to a light-dependent growth. The average closing rate of gaps due to lateral growth of trees bordering the gap is high enough so that within the proposed gap size range, the gap healing can occur before regeneration reaches the upper stratum. Consequently, in mesic sites the gap opening can be done by a single operation that generates gaps with diameters of approximately twice the average height of the canopy (D/H). While in xeric environments, lenga seedling establishment and initial growth require the cover of small gaps, but advanced regeneration requires bigger gaps to reach the canopy. For this reason, gaps should be opened in two stages: the first gaps should be opened with a D/H between 0.8 and 1, and after a cutting cycle of 35 years, these openings should be enlarged to a D/H between 1.5 and 2. The close relationship is maintained between the new cohort and the upper strata require special considerations regarding the evolution of the balance between positive and negative interactions during development. Our work highlights the need to adjust management practices to these spatial and temporal variations so as to achieve an optimal growth along the entire production cycle.

The Younger Dryas in the Neotropics: paleoecological evidence from Venezuela

A continuous lacustrine sequence from the western part of Lama Lake (69°32′N, 90°12′E), complemented by a peat sequence from the lake catchment provides the first detailed environmental reconstruction for the Late Glacial and Holocene on the Taymyr Peninsula. Scarce steppe-like communities with Artemisia, Poaceae, and Cyperaceae dominated during the Late Glacial. Tundra-like communities with Betula nana, Dryas, and Salix grew on more mesic sites. There are distinct climatic signals, which may be correlated with the Bølling and Allerød warming and Middle and Younger Dryas cooling. The Late Glacial/Preboreal transition, at about 10,000 14C yr BP, was characterized by changes from predominantly open herb communities to shrub tundra ones. Larch forest might have been established as early as 9700–9600 14C yr BP, whilst shrub alder came to the area ca 9500–9400 14C yr BP, and spruce did not reach area before ca 9200 14C yr BP. Spruce-larch forests with shrub alder and tree birch dominated the vegetation around the Lama Lake from ca 9000 14C yr BP. Dwarf birch communities were also broadly distributed. The role of spruce in the forest gradually decreased after 4500 14C yr BP. The vegetation cover in the Lama Lake area became similar to the modern larch-spruce forest ca 2500 14C yr BP. A pollen-based biome reconstruction supports a quantitative interpretation of the pollen spectra. Climate reconstructions obtained with information-statistical and plan-functional-type methods show very similar trends in reconstructed July temperature since ca 12,300 14C yr BP, while precipitation anomalies are less coherent, especially during the Late Glacial–Holocene transition.

Drought response and changing mean sensitivity of European beech close to the dry distribution limit

European beech (Fagus sylvatica L.) covers a large area mainly in the colline and montane ranges in Europe, and a drier and warmer climate, as expected for the coming decades, is likely to alter its distribution. So far, an altitudinal shift has been projected using a variety of modelling approaches. However, we lack knowledge about the climatic and edaphic factors that control the growth and competitive behaviour of beech at its dry distribution limit. We applied and further developed dendroecological methods to study the drought response and sensitivity pattern of beech at sites with different moisture regimes. We compared three pairs of sites from different geographical regions near the dry distribution limit of beech in Switzerland, consisting of a dry and mesic site each. Radial growth differed between mesic and dry sites, in that average ring-width at mesic sites was around double the width at dry sites. For the whole study period (1930–2006), the sites with the lowest available soil water capacity (AWC) were found to respond most sensitively to drought. However, in recent years, sites with higher AWC have shown increasing drought sensitivity, i.e. they have responded even more strongly to drought than the dry sites. This change in sensitivity corresponds to a seasonal shift in drought response at mesic sites, with a change in the months showing significant drought response in all three studied regions compared with the past. Even though dry sites generally displayed a larger number of negative pointer years than mesic sites, it appears that the frequency of pointer years has increased at mesic sites, i.e. they have become more sensitive particularly in the last quarter of the twentieth century. Yet, the frequency of pointer years at the dry sites has remained fairly constant. These results indicate that beech trees near their dry distribution limit are adapted to extreme conditions already, while changes in the growth patterns of beech under mesic conditions have to be expected.

Long-term (13-year) effects of repeated prescribed fires on stand structure and tree regeneration in mixed-oak forests

The survival and growth of oak advance regeneration is often limited by shade-tolerant species that are abundant in the understory of oak stands. Evidence of historic burning has prompted the use of prescribed fire as a tool to improve the competitive status of oak regeneration in mature stands. A primary shortfall of fire effects research in oak forests has been a lack of long-term studies on the effects of multiple fires. Here we describe the effects of repeated fires on stand structure and tree regeneration over a 13-year period in mature mixed-oak forests located in southern Ohio, USA. Three stands were burned 3–5 times from 1996 to 2005 with low-intensity dormant-season fires, and two stands remained unburned. Woody vegetation was sampled periodically on nine 0.125ha plots per stand. Plots were located across the upland landscape and were characterized by an Integrated Moisture Index. Fire altered stand structure by reducing the density of large saplings (3.0–9.9cm DBH) and midstory trees (10–25cm DBH) by 76% and 34%, respectively. Fire had little impact on trees >25cm DBH. Small saplings (1.4m tall to 2.9cm DBH) were dynamic over time on dry plots that were burned. After being repeatedly topkilled from year 1–8, the small sapling layer had redeveloped on dry burned plots by year 13 and species composition had shifted from dominance by shade-tolerant species to a more equal distribution of shade-tolerants, oaks+hickories, and sassafras. The density of oak+hickory and sassafras advance regeneration (stems 30cm tall to 2.9cm DBH) was significantly greater on burned plots than on unburned plots in year 13, though variability among plots was high. Advance regeneration of shade-tolerant species was equally abundant on burned and unburned plots. Density of oak+hickory advance regeneration in year 13 was positively related to its weighted frequency (a surrogate for size and abundance) in year 0 (r2=0.67, p<0.0001) and inversely related to stand density (r2=0.33, p<0.0001) and canopy cover (r2=0.31, p<0.0001), both of which were reduced by fire. Although oak+hickory advance regeneration was more abundant on burned plots, we conclude that other methods (e.g., herbicide, partial cutting) are necessary to further reduce stand density and promote the development of larger oak+hickory regeneration, particularly on mesic sites.

Interactions between overstorey and understorey vegetation along an overstorey compositional gradient

AbstractQuestionsWhat is the nature of the interactions between higher vegetation strata (overstorey) and lower strata (understorey vegetation) and among understorey vegetation layers in closed canopy forests? How does the abundance or richness of one vegetation layer affect the abundance or richness of the other?LocationBoreal mixed‐wood forests ofOntario andQuebec,Canada.MethodsWe sampled fire‐origin stands of varying overstorey composition from broad‐leaf‐dominated to mixed to conifer‐dominated stands on mesic sites inOntario and subhydric sites inQuebec. Overstorey tree species composition and understorey shrub, herb, bryophyte and lichen species cover and richness were estimated within 400‐m2circular plots. In addition, soil nutrients, coarse woody debris and light conditions were measured. Overstorey composition was expressed as the percentage basal area of broad‐leaf tree species. Path analysis was used to examine interactions among the forest layers.ResultsOverstorey broad‐leaf composition had positive effects on shrub and herb layer cover and herb layer richness, and negative effects on bryophyte and lichen species cover and richness. Shrub layer cover had no effect on herb layer cover, but shrub layer richness had a positive effect on herb layer richness. Herb layer cover and richness had negative effects on bryophyte species cover and richness. Bryophyte cover had no effect on lichen cover, but its richness was positively related to lichen richness. In both regions, soil pH, total nitrogen, total phosphorus and cation exchange capacity of the forest floor were positively correlated with overstorey broad‐leaf composition and with shrub layer cover and herb layer cover.ConclusionsIncreasing overstorey broad‐leaf composition, through its influence on soil nutrients, promotes shrub and herb layer species, but limits bryophyte and lichen species. In the overstorey‐controlled understorey resource environment, cover and richness of shrub and herb layers increase with resource availability, whereas bryophytes and lichens show positive associations with abundance of coarse woody debris.