Plant functional types as predictors of transient responses of arctic vegetation to global change

Abstract. The plant functional types (growth forms) traditionally recognized by arctic ecologists provide a useful framework for predicting vegetation responses to, and effects on, ecosystem processes. These functional types are similar to those objectively defined by cluster analysis based on traits expected to influence ecosystem processes. Principal components analysis showed that two major suites of traits (related to growth rate and woodiness) explain the grouping of species into functional types. These plant functional types are useful because they (1) influence many ecological processes (e.g. productivity, transpiration, and nutrient cycling) in similar ways, (2) predict both responses to and effects on environment, including disturbance regime, and (3) show no strong relationship with traits determining migratory ability (so that no functional type will be eliminated by climatic change simply because it cannot migrate). Circumstantial evidence for the ecological importance of these functional types comes from the distribution of types along environmental gradients and the known ecological effects of traits (e.g., effects of litter quality on decomposition and of plant height on winter albedo) that characterize each functional type. The paleorecord provides independent evidence that some of these functional types have responded predictably to past climatic changes. Field experiments also show that plant functional types respond predictably to changes in soil resources (water and nutrients) but less predictably to temperature. We suggest that evidence for the validity of arctic plant functional types is strong enough to warrant their use in regional models seeking to predict the transient response of arctic ecosystems to global change.     

Effects of overstory and understory vegetation on the understory light environment in mixed boreal forests

Abstract. The percentage of above-canopy Photosynthetic Photon Flux Density (%PPFD) was measured at 0, 50 and 100 cm above the forest floor and above the main understory vegetation in stands of (1) pure Betula papyrifera (White birch), (2) pure Populus tremuloides (Trembling aspen), (3) mixed broad-leaf-conifer, (4) shade-tolerant conifer and (5) pure Pinus banksiana (Jack pine) occurring on both clay and till soil types. %PPFD was measured instantaneously under overcast sky conditions (nine locations within each of 29 stands) and continuously for a full day under clear sky conditions (five locations within each of eight stands). The percentage cover of the understory layer was estimated at the same locations as light measurements. Mean %PPFD varied from 2% at the forest floor under Populus forests to 15% above the understory vegetation cover under Betula forests. Percent PPFD above the understory vegetation cover was significantly higher under shade intolerant tree species such as Populus, Betula and Pinus than under shade tolerant conifers. No significant differences were found in %PPFD above the understory vegetation cover under similar tree species between clay and till soil types. The coefficient of variation in %PPFD measured in the nine locations within each stand was significantly lower under deciduous dominated forests (mean of 19%) than under coniferous dominated forests (mean of 40%). %PPFD measured at the forest floor was positively correlated with %PPFD measured above the understory vegetation and negatively correlated with cumulative total percent cover of the understory vegetation (R² = 0.852). The proportion of sunflecks above 250 and 500 mmol m^–2 s^–1 was much lower and %PPFD in shade much higher under Populus and Betula forests than under the other forests. Differences in the mean, variability and nature of the light environment found among forest and soil types are discussed in relation to their possible influences on tree succession.     

Reproducibility of vegetation cover estimates in south‐central Alaska forests

Abstract. Reproducibility of vegetation measurements is critical for large‐scale or long‐term studies, where numerous observers collect data, but past studies have questioned repro‐ducibility of some techniques. Five methods of evaluating understory composition were appraised for reproducibility among six observers in two forest types in south‐central Alaska: ocular estimates in quadrats, overall community species rank and cover estimates, nested rooted frequency, horizontal‐vertical profiles, and pin drop (systematic points). One forest type was selected to represent structure of coastal communities, another to represent structure of interior Alaska communities. Three general methods of evaluating reproducibility were considered: standard deviations (precision among observers), components of variance (percentage of total variance attributable to observers), and analysis of variance (significance of observer variance). Observer variances were generally similar among techniques and significant in most cases. No technique stood out as being more reproducible than others. Features of techniques other than reproducibility may be more important when selecting a technique. Management decisions based on vegetation cover data should consider the observer errors involved as well as biological significance.     

Effects of stand composition on fire hazard in mixed‐wood Canadian boreal forest

Abstract. Surface fuels were examined in 48 stands of the Canadian mixed‐wood boreal forest. Tree canopy was characterized with the point‐centred quadrant method and stands were characterized as deciduous, mixed‐deciduous, mixed‐coniferous or coniferous according to the percentage of conifer basal area. Woody debris loadings were measured with the line intersect method and the litter, duff, shrub loads and depths or heights were sampled with various quadrats. No significant difference was found among stand types for total woody debris load, large basal diameter shrub loads and load or depth of litter and duff. However, conifer stands had significantly heavier loads of small diameter elements (twigs and shrubs) and conifer pieces were more numerous within these stands than in deciduous stands. The BEHAVE prediction system was used to evaluate the impact of these differences on the potential of fire ignition in situations where topography and weather were constant. The qualitative and quantitative changes in fuels, resulting from species replacement and fast decay rates, influence fire hazard. Simulations of fire behaviour showed that in the mixed‐wood boreal forest fires were less intense and spread more slowly in deciduous stands than in mixed or coniferous stands. Moreover, spring fires were more intense than summer fires, and differences between seasons increased with the increase of deciduous basal area.     

Radial growth response of four dominant boreal tree species to climate along a latitudinal gradient in the eastern Canadian boreal forest

To address the central question of how climate change influences tree growth within the context of global warming, we used dendroclimatological analysis to understand the reactions of four major boreal tree species –Populus tremuloides, Betula papyrifera, Picea mariana, and Pinus banksiana– to climatic variations along a broad latitudinal gradient from 46 to 54°N in the eastern Canadian boreal forest. Tree‐ring chronologies from 34 forested stands distributed at a 1° interval were built, transformed into principal components (PCs), and analyzed through bootstrapped correlation analysis over the period 1950–2003 to identify climate factors limiting the radial growth and the detailed radial growth–climate association along the gradient. All species taken together, previous summer temperature (negative influences), and current January and March–April temperatures (positive influences) showed the most consistent relationships with radial growth across the gradient. Combined with the identified species/site‐specific climate factors, our study suggested that moisture conditions during the year before radial growth played a dominant role in positively regulating P. tremuloides growth, whereas January temperature and growing season moisture conditions positively impacted growth of B. papyrifera. Both P. mariana and P. banksiana were positively affected by the current‐year winter and spring or whole growing season temperatures over the entire range of our corridor. Owing to the impacts of different climate factors on growth, these boreal species showed inconsistent responsiveness to recent warming at the transition zone, where B. papyrifera, P. mariana, and P. banksiana would be the most responsive species, whereas P. tremuloides might be the least. Under continued warming, B. papyrifera stands located north of 49°N, P. tremuloides at northern latitudes, and P. mariana and P. banksiana stands located north of 47°N might benefit from warming winter and spring temperatures to enhance their radial growth in the coming decades, whereas other southern stands might be decreasing in radial growth.     

Composition and structure of a chronosequence of young, mixed-species forests in southeastern Ohio, USA

Clearcutting, a commonly used silvicultural practice in southeastern Ohio, often results in a forest stand with a different species composition than the parent stand. The time frames during which shifts in species composition occur on different sites are unclear. While some studies have documented species composition at specific points in time, none have attempted to examine differences throughout the first decades of stand development. This study focused on the early successional dynamics of young, mixed-species forests of southeastern Ohio. Species compositions were examined across a chronosequence of sixteen stands that developed following clearcutting. Stand ages ranged from six to 26 years. The sample was limited to dry-mesic hardwood forests on southerly aspects and on soils derived from residuum or colluvium. Across the chronosequence, stand density ranged from 17 636 stems ha^-1 at age 6 to 2759 stems ha^-1 at age 26, and basal area ranged from 8.2 m² ha^-1 to 22.1 m² ha^-1. Clumps comprised a substantial portion of the total stand density and basal area. At age 6–8 years after clearcutting, clumped stems accounted for 35.1% of the density and 48.2% of the basal area. At age 26 years, these proportions were 25.7% and 29.4%, respectively. Clumped stems were significantly larger (p<0.05) than non-clumped stems at each age group except 26 years. Total Quercus spp. density was greatest at age 6–8 years (3386 stems ha^-1), and least at age 26 years (581 stems ha^-1). When considered as a proportion of the total stand, however, the proportion was relatively stable, averaging 21.3%. However, importance value (IV=[relative density + relative basal area]/2) of Quercus in the upper canopy (dominant and codominant crown classes) was twice as much (72%) at age 26 years compared to age 6–8 years (35%). Quercus prinus L. was the major species across the chronosequence. For all age groups except 18–20 years. Q. prinus IV was the highest of any individual species in the upper canopy, and it ranged from 27 in the youngest stands (6–8 years) to 69 in the oldest stand (26 years). Within the intermediate crown class, the IV of Q. prinus equaled or exceeded those of all other species, except for the 18–20 year age group where it was second to A. rubrum. Quercus alba L. and Quercus velutina Lam. were minor components at age 26 years, although they dominated a comparison sample of six mature stands of the same ecosystem type. Liriodendron tulipifera L. was abundant 6–8 years after clearcutting, but nearly absent at age 26 years. Acer rubrum L. was the major species in both the intermediate and overtopped crown classes throughout the chronosequence. As gaps in the canopy occur. A. rubrum may become a more common species within the dominant-codominant crown class.     

Free-air exposure systems to scale up ozone research to mature trees

Because seedlings and mature trees do not necessarily respond similarly to O(3) stress, it is critically important that exposure systems be developed that allow exposure of seedlings through to mature trees. Here we describe three different O(3) Free-Air Exposure Systems that have been used successfully for exposure at all growth stages. These systems of spatially uniform O(3) release have been shown to provide reliable O(3) exposure with minimal, if any, impact on the microclimate. This methodology offers a welcome alternative to chamber studies which had severe space constraints precluding stand or community-level studies and substantial chamber effects on the microclimate and, hence physiological tree performance.     

Estimation of tree replacement patterns in an Appalachian Picea-Abies forest

Abstract. Patterns of tree species replacement in a Picea-Abies forest, determined by several different methods, are compared and the methods are assessed. Methods are grouped as either understory-based or gap-based estimates of replacement. The understory-based methods characterize canopy-understory interactions with spatial statistics, sapling density measurement, sapling frequency measurement, and successor sapling identification beneath live canopy trees. The gap-based methods include sapling density measurement, sapling frequency measurement, and successor sapling identification in tree-fall gaps. Methods except those based on frequency indicate a strong trend of replacement of all canopy species by Abies. Understory-based methods may underestimate canopy recruitment of intolerant trees, while gap-based methods relying on sapling density or frequency may overestimate recruitment of intolerant trees. Estimates based on the selection of successor saplings in the understory or in gaps are reliable. Gap successor estimates consider the process of gap capture and are useful in analyses of forest dynamics.     

Factors determining plant species richness in Alaskan arctic tundra

Abstract. We studied the relationship between plant N:P ratio, soil characteristics and species richness in wet sedge and tussock tundra in northern Alaska at seven sites. We also collected data on soil characteristics, above‐ground biomass, species richness and composition. The N:P ratio of the vegetation did not show any relationship with species richness. The N:P ratio of the soil was related with species richness for both vegetation types. Species richness in the tussock tundra was most strongly correlated with soil calcium content and soil pH, with a strong correlation between these two factors. N:P ratio of the soil was also correlated with soil pH. Other factors correlated with species richness were soil moisture and Sphagnum cover. Organic matter content was the factor most strongly correlated with species richness in the wet sedge vegetation. N:P ratio of the soil was strongly correlated with organic matter content. We conclude that N:P ratio in the vegetation is not an important factor determining species richness in arctic tundra and that species richness in arctic tundra is mainly determined by pH and flooding. In tussock tundra the pH, declining with soil age, in combination with Sphagnum growth strongly decreases species richness, while in wet sedge communities flooding over long periods of time creates less favourable conditions for species richness.     

Conifer seedling recruitment in a southeastern Canadian boreal forest: the importance of substrate

Abstract. In order to explain conifer species recruitment in Canada's southeastern boreal forest, we characterized conifer regeneration microsites and determined how these microsites vary in abundance during succession. Microsite abundance was evaluated in deciduous, mixed and coniferous stands along a 234-yr postfire chronosequence. Conifers were most often found in relatively well-illuminated microsites, devoid of litter, especially broad-leaf litter, and with a reduced cover of lower vegetation (< 50 cm tall). Although associated with moss-rich forest floor substrates, Abies balsamea was the most ubiquitously distributed species. Picea glauca and especially Thuja occidentalis seedlings were frequently found on rotten logs. Light measurements did not show differences among seedling species nor between stand types. The percentage cover of broad-leaf litter decreased significantly during succession. Also, rotten logs covered with moss occupied a significantly larger area in the mid-successionnal stands than in early successional deciduous or late successional coniferous stands. The results suggest that the presence of specific forest floor substrate types is a factor explaining low conifer recruitment under deciduous stands, conifer codominance in the mid-successional stage, and delayed Thuja recolonization after fire. Results also suggest that some facilitation mechanism is responsible for the observed directional succession.     

Long-term growth dynamics of natural forests in Hokkaido,northern Japan

Abstract. The long-term growth dynamics of natural forest stands on the island of Hokkaido were described on the basis of an analysis of data from 38 permanent plots spanning 15–22 yr. Stand structure was characterized by basal area, stem density and tree size variability. To detect trends in stand structure, regression models for recruitment rate (per ha per yr), mortality rate and the rate of change in stem density and tree size variability were developed by a stepwise method using initial basal area, stem density, tree size variability, species composition summarized by LNMDS ordination, altitude, annual mean temperature, annual precipitation, type of understorey vegetation, topography and slope aspect as candidates for predictor variables. The same analyses were conducted for basal area increment (net growth) and its components: survivor growth = basal area gain by growth of surviving individuals and mortality = basal area loss by death of individuals. Stem density remained generally unchanged; recruitment was relatively low even in very sparse stands. Stand basal area generally increased as survivor growth was approximately double the mortality. Recruitment rate was strongly affected by the presence of dwarf bamboo (Sasa spp.) vegetation on the forest floor which inhibited tree regeneration. Mortality rate was density-dependent; dense stands had higher mortality than sparse stands. Density change rate (recruitment rate - mortality rate) was, therefore, determined by both the type of understorey vegetation and stem density. Survivor growth was high in stands with high stem density and basal area. Mortality was dependent on basal area and altitude. Net basal area increment (net growth) was dependent only on stem density with other factors that influenced survivor growth and mortality omitted. Tree size variability decreased in stands with high tree size variability whereas it increased in stands with low size variability. Based on the obtained models for density change rate and net basal area increment, trajectories of stands were illustrated on a log-log diagram of stem density and basal area. The predicted differences in trajectories as affected by the understorey vegetation type indicated the importance of dwarf bamboo vegetation for forest dynamics on Hokkaido.     

Dynamics of Salix caprea L. populations during forest regeneration after strong herbivore pressure

Abstract. Broadleaved forest communities degenerated through strong pressure from large herbivores. Relief of this pressure led to regeneration, in particular of Salix caprea and other light-seeded pioneer trees: Populus tremula, Betula pendula and B. pubescens. This regeneration proceeded following conservation protection of degenerate stands in a nature reserve and later in Biaowiéza National Park. The emergence and development of the Salix caprea population proceeded following the expansion of Picea abies, which coincided with the period of enhanced animal pressure on the broadleaved forest. Salix caprea filled all the gaps in the tree stand after the destruction of trees and undergrowth by herbivores (in the years 1892–1915). The species also appeared abundantly in old, at the time unforested, clearings and felled areas. Here, S. caprea developed large populations with certain trees in good condition, with a growth form typical of forest trees and attaining considerable heights. The majority of trees were 50 - 60 yr old at the time of death, although some individuals reached 74 yr of age. The process of extinction of the Salix population — observed over 19 yr on permanent plots with marked trees — proceeded very quickly, especially in the first decade of observation. It led to the almost complete disappearance of S. caprea from the forest communities of Biaowiéza National Park. The death of individual trees is preceded by impairment of their health and reduced annual increments in the last 4–9 years of their life. The development of populations of permanent constituents of the forest, notably Carpinus betulus, Tilia cordata, Acer platanoides and Ulmus glabra, under the canopy of light-seeded trees, and the absence of a new generation of pioneer trees points to the end of the process of regeneration in the forest communities of BiaHwiéza National Park.     

Dispersal,diversity and distribution patterns in pioneer vegetation: The role of river‐floodplain connectivity

Question: Are there changes in dispersal patterns in floodplain pioneer vegetation with effects on seedling number, species richness and species composition along a gradient of declining river‐floodplain connectivity? Location: Middle Elbe river floodplain, Germany. Methods: An experiment with five treatments was set up along a gradient of declining river‐floodplain connectivity, partitioning seedlings into three groups: (1) emerging solely from water dispersed seeds, (2) from wind/animal dispersed seeds and (3) from the soil diaspore bank. Two controls were established: without any manipulation and exclusion of all seeds. The results were compared with those of vegetation and soil sampling to evaluate the representativeness of the experimental sites in terms of species composition, diversity, seedling number and soil parameters. Results: Water dispersal and the soil diaspore bank were the major dispersal strategies shaping floodplain pioneer vegetation at the Middle Elbe river. The number of seedlings, species richness and the variation in species composition in these habitats depend on the degree of connectivity. The seedling number and species richness is highest in sites of permanent or almost permanent exchange with the main channel, where water dispersal additionally contributes to the number of seedlings grown from the soil seed bank. Conclusion: The results underline the importance of river‐floodplain ecotones as sink habitats for water‐dispersed seeds. Considering the strongly reduced river‐floodplain interactions due to dykes and other engineering structures, management strategies are necessary to improve connectivity and the renewal of fluvial land forms.     

Secondary succession in disturbed Pinus-Quercus forests in the highlands of Chiapas,Mexico

We present floristic and structural data on serai plant communities (Old-Field, Grassland, Shrubland, and Early Successional, Mid-Successional, and Mature Forest) resulting from the current land use pattern in the Pinus-Quercus forests in the highlands of Chiapas, Mexico. The number of species ranged from 24 in the Old-Field stage up to 100 in the Mature Forest, and the shrub layer was richest in all the forested stands. An almost complete floristic replacement exists between the open and the forested communities. Quercus and Pinus dominate the canopy of old-growth stands, but their recruitment does not occur or is very low in the shade. Vigorous seedlings and saplings of these species are found only in the open serai stages or in forest gaps. Demographic and phytosociological data are used to indicate the successional role of dominant species in the forested stages. Some implications of the regional land use patterns for conservation and succession are discussed.     

Vegetation and soil seed bank of successional stages in tropical lowland deciduous forest

Abstract. We studied vegetation structure and soil seed bank composition in different successional stages of secondary lowland tropical deciduous forest in Yucatán, Mexico. The series of study sites includes: slashed (S), slashed-and-burned (SB), and regenerating for 1, 6, 10, 15, 30, 40 and 100 yr. Species richness (S = 42 - 65), evenness (E = 0.32 - 0.38), and diversity (H' = 1.2 - 1.6) do not vary much as the forest grows older. 20 species of shrubs and trees were present in at least six of the seven regrowth years studied; 10 of these account for more than 50% of the total density values per regrowth year. These species dominate the vegetation due to their capacity to withstand repeated fire and felling. One third of the individuals sampled had regenerated from coppiced shoots. Species composition little resembles that in earlier accounts. The area is now largely covered by young regrowth stages (1 - 20 yr). Species constituting the original woody structure of the mature forest are rare or absent due to the lack of seed sources and failure of dispersal (which is due to limited dispersal capacities), lack of dispersal agents, or long distances. Herbs were the most important life form in the soil seed banks; only one tree species was found. The number of viable seeds varied between sampled areas: 70/m² in the 40 yr-old, to 1 815/m² in the slashed-and-burned (SB). The vegetation of S- and SB-areas was the same, but the number of viable seeds germinating in SB was twice the number in S; the number of species in the seed bank is the same for both areas. We speculate that fire modifies species dominance early in succession, allowing seeds of some species to germinate in great numbers.