Mid-successional Stands Research Articles

Seed rain and establishment in successional forests in Chiapas, Mexico

Background and Aims: Agriculture in the tropics is decreasing, fragmenting and altering forests and forest landscapes. We hypothesized differences in species richness and dominance of life forms in the seed rain and in richness and survival in the recruit assemblages among mature forests, mid-successional forests, early successional forests, pastures and milpa fields (arable lands with maize) surrounded by natural and human disturbed habitats. Methods: Samples of seeds and plants were collected during a year in Lacanjá-Chansayab and Bonampak-Bethel, in the buffer zone of the Montes Azules Biosphere Reserve, Selva Lacandona, Chiapas, Mexico. We deployed 14 seed traps in 12 sites representing a gradient of vegetation succession (2 sites × 6 habitats × 14 traps; n = 168 traps). Independently, to assess changes in recruitment and early survival, we established 15 quadrats (0.5 × 2.0 m) in each of the studied forests (2 sites × 3 habitats × 15 quadrats; n = 90 quadrats). Key results: We collected ~13,600 seeds of 144 species from 48 botanical families. Mature forests had the highest seed rain species richness (60-61) and pastures (14-11) the lowest. We observed a decline in species richness and a change in dominance of life forms in the seed rain from less disturbed to most perturbed habitats. Mature forests included seeds of diverse tree species while the assemblage in pastures was dominated by seeds of few grass species. Intensive traditional milpa fields showed homogeneous seed assemblages. For the new recruits, we recorded ~3,416 individuals (<0.5 m height) of 238 morphospecies in 42 families, 129 were identified to species level. The largest number of species occurred in mature and mid-successional stands compared to early forests. Annual survival of recruits was higher in mid- and late successional forests than in early ones.Conclusions: We document species loss and widespread simplification and homogenization in community composition due to pervasive effect of humans on remnant tropical lowland forests.

The relationship between tree biodiversity and biomass dynamics changes with tropical forest succession.

Theory predicts shifts in the magnitude and direction of biodiversity effects on ecosystem function (BEF) over succession, but this theory remains largely untested. We studied the relationship between aboveground tree biomass dynamics (Δbiomass) and multiple dimensions of biodiversity over 8-16years in eight successional rainforests. We tested whether successional changes in diversity-Δbiomass correlations reflect predictions of niche theories. Diversity-Δbiomass correlations were positive early but weak later in succession, suggesting saturation of niche space with increasing diversity. Early in succession, phylogenetic diversity and functional diversity in two leaf traits exhibited the strongest positive correlations with Δbiomass, indicating complementarity or positive selection effects. In mid-successional stands, high biodiversity was associated with greater mortality-driven biomass loss, i.e. negative selection effects, suggesting successional niche trade-offs and loss of fast-growing pioneer species. Our results demonstrate that BEF relationships are dynamic across succession, thus successional context is essential to understanding BEF in a given system.

Stand-level Attributes of Snowshoe Hare (<em>Lepus americanus</em>) Habitat in a Post-Fire Trembling Aspen (<em>Populus tremuloides</em>) Chronosequence in Central Yukon

Large-scale fires are anticipated to increase in frequency in the boreal forest under global climate warming scenarios. To understand concomitant responses by wildlife to fire-induced habitat changes, fecal pellet counts were used to assess Snowshoe Hare (Lepus americanus) use of four age-classes of boreal forest after fire in central Yukon, Canada. Use of stands across a chronosequence of 8–177 years was bimodal. Post-fire Trembling Aspen (Populus tremuloides) stands ≤20 years old had greater densities of Snowshoe Hare pellets (median 156 pellets/dam2) than Trembling Aspen stands 21–70 years old, mixedwood stands 71–120 years old (<17 pellets/dam2), or Western White Spruce (Picea albertiana) -dominated stands 121–170 years old (71 pellets/dam2). Forty stand-level compositional and structural variables were assessed as possible predictors of Snowshoe Hare pellet densities. Multidimensional scaling was used to identify variables (n = 10) that were most strongly related to pellet densities and was followed by multiple regression. Canopy cover of Trembling Aspen <50 cm tall and Western White Spruce ≤1 m tall, and deadfall depth, in combination, were the best estimators of Snowshoe Hare pellet densities among stands in the chronosequence (P <0.001, 64.5% variance explained). Although Trembling Aspen <50 cm tall explained the most variance, its canopy cover did not exceed 10%. More Trembling Aspen cover <50 cm tall and greater deadfall depths within the chronosequence were associated with stands ≤20 years old. Peak Snowshoe Hare use occurred in early (≤20 years old) rather than mid-successional (21–120 years old) stands, contrary to use patterns reported elsewhere. The lack of tall understory shrubs likely limited the use of mid-successional stands.

Repeated burning of eastern tallgrass prairie increases richness and diversity, stabilizing late successional vegetation

Understanding temporal effects of fire frequency on plant species diversity and vegetation structure is critical for managing tallgrass prairie (TGP), which occupies a mid-continental longitudinal precipitation and productivity gradient. Eastern TGP has contributed little information toward understanding whether vegetation-fire interactions are uniform or change across this biome. We resampled 34 fire-managed mid- and late-successional ungrazed TGP remnants occurring across a dry to wet-mesic moisture gradient in the Chicago region of Illinois, USA. We compared hypotheses that burning acts either as a stabilizing force or causes change in diversity and structure, depending upon fire frequency and successional stage. Based on western TGP, we expected a unimodal species richness distribution across a cover-productivity gradient, variable functional group responses to fire frequency, and a negative relationship between fire frequency and species richness. Species diversity was unimodal across the cover gradient and was more strongly humpbacked in stands with greater fire frequency. In support of a stabilizing hypothesis, temporal similarity of late-successional vegetation had a logarithmic relationship with increasing fire frequency, while richness and evenness remained stable. Temporal similarity within mid-successional stands was not correlated with fire frequency, while richness increased and evenness decreased over time. Functional group responses to fire frequency were variable. Summer forb richness increased under high fire frequency, while C4 grasses, spring forbs, and nitrogen-fixing species decreased with fire exclusion. On mesic and wet-mesic sites, vegetation structure measured by the ratio of woody to graminoid species was negatively correlated with abundance of forbs and with fire frequency. Our findings that species richness responds unimodally to an environmental-productivity gradient, and that fire exclusion increases woody vegetation and leads to loss of C4 and N-fixing species, suggest that these processes are uniform across the TGP biome and not affected by its rainfall-productivity gradient. However, increasing fire frequency in eastern TGP appears to increase richness of summer forbs and stabilize late-successional vegetation in the absence of grazing, and these processes may differ across the longitudinal axis of TGP. Managing species diversity in ungrazed eastern TGP may be dependent upon high fire frequency that removes woody vegetation and prevents biomass accumulation.

Complex effects of site preparation and harvest on snowshoe hare abundance across a patchy forest landscape

Maintaining forest biodiversity requires an understanding of how forest-dependent species respond to a variety of forestry activities. This is particularly true for mammals, which often act as keystone species or focal species for conservation efforts in forest ecosystems. Snowshoe hares (Lepus americanus) serve as an important herbivore and prey resource throughout their range in the boreal forest. We examined the response of hares to partial and clearcut harvest, site preparation activities (post-harvest burning or chemical/mechanical preparation for regrowth), and precommercial thinning, using pellet plots on 359 forest stands across an expansive landscape in northern Idaho over a 5-year period. Hares initially responded negatively to harvest, with clearcut harvest having a greater negative impact than partial harvest, and this response was exacerbated by site preparation activities. Mid-successional stands (15–40years old) that had been clear cut with site preparation treatments applied or partially harvested with no site preparations applied had greater snowshoe hare pellet counts than mature or recently harvested stands. Pre-commercial thinning had no detectable effect on hares in this landscape. Our findings suggest an interaction between site preparation and type of harvest (clear-cut vs. partial), which suggests that both the initial negative impacts and subsequent positive response to site preparation as stands age are attenuated somewhat in partially harvested vs. clear-cut stands. Our results likely relate to the influence of management activities on vegetative cover, which is strongly related to hare abundance in a variety of systems. They suggest that silvicultural activities such as site preparation could be damaging to hare populations when applied widely across landscapes, particularly when young age classes predominate. However, because the effects of harvest and site preparation are largely transitory, use of these techniques may not substantially depress hare populations if enough stands can be maintained in the 15–40year age class.

Carbon Accumulation Patterns During Post-Fire Succession in Cajander Larch (Larix cajanderi) Forests of Siberia

Increased fire activity within boreal forests could affect global terrestrial carbon (C) stocks by decreasing stand age or altering tree recruitment, leading to patterns of forest regrowth that differ from those of pre-fire stands. To improve our understanding of post-fire C accumulation patterns within boreal forests, we evaluated above- and belowground C pools within 17 Cajander larch (Larix cajanderi) stands of northeastern Siberia that varied in both years since fire and stand density. Early-successional stands ( 70-year old), aboveground larch biomass, ANPPtree, and soil organic layer C pools increased with stand age. These stands were low density and multi-aged, containing both mature trees and new recruits. The rapid accumulation of aboveground larch biomass in high-density, mid-successional stands allowed them to obtain C stocks similar to those in much older low-density stands (~8,000 g C m−2). If fire frequency increases without altering stand density, landscape-level C storage could decline, but if larch density also increases, large aboveground C pools within high-density stands could compensate for a shorter successional cycle.

Impacts of postfire salvage logging on understory plant communities of the boreal mixedwood forest 2 and 34 years after disturbance

We compared understory vegetation composition and richness in aspen-dominated boreal mixedwood forest stands in Alberta, Canada, that had been burned by wildfire with those that burned and were subsequently salvage logged. Stands were examined at early and midsuccessional (2 and 34 years after disturbance(s), respectively) developmental stages. In comparison with wildfire stands, understory communities of early successional salvage-logged stands were characterized by greater species richness, weedy species presence, higher shrub abundance, and lower abundances of fire-specialist seed bank species. In constrained ordination, the understory community of early successional wildfire stands was related to greater canopy cover, sapling density, and moss depth, whereas that of salvage-logged stands was related to greater light, volume of downed deadwood, and litter and organic matter. Longer term effects of salvage logging on the understory community were minimal and, instead, reflected the influence of forest canopy redevelopment. In midsuccessional stands, understory composition was related to conifer density, litter cover, soil moisture, organic layer depth, tall shrub density, and bryophyte-covered microsite cover. Postfire salvage logging can have substantial short-term effects on the postfire understory plant community; in the longer term, effects will depend to a large extent on the influence of harvesting and subsequent management on canopy redevelopment.

Linking forest structure and composition: avian diversity in successional forests of Chiloé Island, Chile

We show how Chilean forest bird species richness, abundance and guild structure changes as a function of structural properties of forest stands. We surveyed bird assemblages in two old-growth (>200 years), two mid-successional (30–60 years), and two early-successional forest stands (4–20 years), from November 1999 to September 2000, on Chiloé Island, southern Chile (42°S). Birds were grouped into four habitat-use guilds: large-tree users, vertical-profile generalists, understory species, and shrub-users that occasionally use forests. We recorded a total of 24 bird species: 21 in old-growth, 14 in mid-successional and 16 in early-successional stands. Large-tree users and understory birds were most abundant in old-growth stands, vertical-profile generalists were common in both old-growth and mid-successional stands, and shrub-users were only common in early-successional stands. For nine bird species we found significant relationships between their local abundance and forest structural elements. Higher bird densities in old-growth forests were associated with greater availability of canopy emergent trees, snags, logs and understory bamboo cover in this habitat. Accordingly, bird species diversity in forest stands can be predicted by the presence of these structural elements, and forests should be managed to conserve structural elements that create favorable habitat for bird species in order to prevent future species losses due to logging practices.

Long-term effects of precommercial thinning on small mammals in northern Maine

Precommercial thinning (PCT) is being practiced increasingly throughout the Acadian forest of eastern North America to meet silvicultural objectives; however, effects of this practice on wildlife, both immediately and several years post-treatment are not well understood. Forest dependent small mammals have ecological roles as prey for numerous avian and mammalian predators, dispersers of seeds, fruit, and spores, and contribute to nutrient cycling. Researchers in the northwestern USA have suggested that thinning of young, regenerating clearcuts may increase the abundance and diversity of some forest-dependent small mammals by increasing rates of forest development and enhancing the ecological representation of mid-successional stands across managed landscapes. We examined the effects of PCT within conifer-dominated forest stands 1-, 6-, 11-, and 16-years post-treatment, on abundances of mice, voles, and shrews, and on within-stand structure in the commercially managed, Acadian forests of northern Maine. We live-trapped small mammals on 24 herbicide-treated clearcuts treated with PCT and on 13 similar, unthinned stands during summers of 2000 and 2001. Thinning of mid-successional conifer stands resulted in increased abundances, (red-backed voles, Clethrionomys gapperi, P = 0.008; masked shrews, Sorex cinereus, P < 0.001) or had no detectable effect on (deer mice, Peromyscus maniculatus, P = 0.544; short-tailed shrews, Blarina brevicauda, P = 0.517) the 4 most common species of Muridae and Soricidae in northern Maine. In general, abundance of deer mice responded more positively to increasing development class and to the number of years since thinning than other species of small mammals. Several within-stand habitat characteristics associated with stand maturity, such as larger stem diameters and a partially open canopy, occurred in thinned stands. Thus, PCT may accelerate the development of habitat attributes typical of mid-successional conifer stands in intensively managed stands within the Acadian Forest. PCT may increase abundances of small mammal species associated with mid-seral forest conditions at the scale of the forest stand.

Regeneration microsites and tree species coexistence in temperate rain forests of Chiloé Island, Chile

Summary 1 We studied the importance of fallen logs as recruitment sites for tree species, their role in species coexistence, and also the influence of canopy openness and litter depth on tree species establishment in mid-successional and old-growth temperate rain forests of Chiloe Island, southern Chile. 2 Old-growth (OG) stands showed significantly more fallen logs than mid-successional (MS) stands. Concomitantly, the proportion of seedlings and saplings established on logs was significantly greater in OG than MS stands. 3 Of 13 tree species found at our study sites, eight showed a significant bias towards establishment on logs, especially those in advanced stages of decomposition. 4 In some stands, all seedlings of Eucryphia cordifolia , Laureliopsis philippiana , Nothofagus nitida , Tepualia stipularis and Weinmannia trichosperma occurred on decaying logs, whereas all Podocarpus nubigena seedlings were found on undisturbed soil sites. 5 Small-seeded species were more common on logs, whereas large-seeded trees occurred on soil. 6 On soil, litter depth negatively affected local abundance of log-dependent seedlings, suggesting that variation in litter accumulation influences species distributions across the forest floor mosaic. 7 The density of shade-intolerant seedlings was more enhanced by the presence of fallen logs under closed canopy than by the occurrence of canopy gaps over soil sites. 8 Seed size plays an important role in successful establishment of species across the mosaic of fallen logs and different litter depth on the forest floor. We suggest that this mosaic of microsites is an important factor for species coexistence.

Coarse woody debris biomass in successional and primary temperate forests in Chiloé Island, Chile

Coarse woody residues (snags and logs) are a massive and functionally important component of high-latitude forest ecosystems, both natural and managed. Decaying logs and snags provide habitat for many organisms and participate in biogeochemical element fluxes within the forest ecosystem. Because of their large mass and slow decay rates, they may also play a significant role in the global carbon cycle. For these reasons, it is important to understand the dynamics of coarse woody debris (CWD) during forest succession. Here, we estimated the biomass of CWD (defined as logs or snags >10 cm stem diameter) in nine forest stands, both primary and secondary, on Chiloé Island (42°S), southern Chile. The stands represented a successional chronosequence following non-catastrophic human disturbance by fire and logging. We characterized logs in terms of their decay class, total carbon content, moisture content, volume, and dry biomass. CWD biomass ranged from 18 to 413 Mg ha −1, with a recently disturbed and old-growth forests having the largest values. Early- and mid-successional stands had the lowest values. Decay classes of logs differed in density, moisture contents and total carbon content. Carbon stored in logs and snags was nearly 10 times higher in old-growth and primary forests than in young-successional forests. CWD biomass is substantially reduced in young growing stands with disrupting effects on ecological functions and biodiversity.

Spatial and temporal patterns of carbon storage and species richness in three South Carolina coastal plain riparian forests

The distribution of organic matter within a floodplain is a controlling factor affecting water quality, habitat, and food webs. Accordingly, development of vegetation in the riparian zone can be expected to influence ecosystem functions, and organic matter storage patterns are believed to be indicators of functional recovery in disturbed riparian zones. Our objective was to compare the distribution and allocation of organic matter among microsites within the floodplain and with temporal changes (successional status) associated with community development. Three third order streams in the upper coastal plain of South Carolina were selected. Measurement transects were established across three floodplains of varying successional status, Meyer's branch; a mature riparian hardwood forest; Fourmile branch; a mid-successional riparian forest; and Pen Branch, an early successional riparian forest. Overall, measurements of aboveground biomass, soil carbon, and stand structure indicate that the early and mid successional stands are becoming more similar to the mature stand and that microsite differences within the braided, riparian stream systems are small.

Forest floor microbial biomass across a northern hardwood successional sequence

Microbial dynamics partially control forest productivity, yet are poorly understood in the northern hardwood forest. Following forest cutting, microbial biomass may reflect changes in the forest floor environment. Using direct counts, forest floor microbial biomass was studied in a sequence of northern hardwood forest stands ranging in age since clearcutting from 3 to more than 120 yr. The organic horizon was collected 3 times in each of 1994 and 1995, in June, August and October. Samples were analyzed for active fungi, active bacteria, pH, forest floor moisture content and organic fraction. Forest floor temperature was measured in the field with each collection; forest floor mass was measured once in each stand. In 1995, forest floor samples were also analyzed for total N, nitrate and ammonium. Amounts of active microbial biomass (fungi plus bacteria) ranged from 38 to 103 μg g −1 organic matter; active microbial biomass per unit area ranged from 40 to 800 mg m −2 forest floor organic matter. Microbial biomass varied significantly among sampling periods in 1994, but not in 1995, and was greater in early- and late-successional stands than in mid-successional stands. Microbial biomass was not very responsive to the environmental factors measured in this study. Moisture content was the environmental variable that most often contributed to variation in microbial biomass, and together with additional factors explained only 23% of the variation in 1994, and only 27% in 1995. Lower microbial biomass in mid-successional stands suggests that microbial dynamics in this forest are not controlled by factors directly related to forest harvesting.