Diversity and composition of tropical secondary forests recovering from large-scale clearing: results from the 1990 inventory in Puerto Rico

The extensive recovery from agricultural clearing of Puerto Rican forests over the past half-century provides a good opportunity to study tropical forest recovery on a landscape scale. Using ordination and regression techniques, we analyzed forest inventory data from across Puerto Rico’s moist and wet secondary forests to evaluate their species composition and whether the landscape structure of older forest affected tree species composition of recovering forests at this scale. Our results support conclusions from studies conducted in Puerto Rico at smaller scales and temperate forests at larger scales that timing of abandonment and land use history are of overwhelming importance in determining the species composition of recovering forests. Forest recovery is recent enough in Puerto Rico that previous land use is clearly evident in current species composition, and creates new forest communities. As demonstrated in other work, physical factors such as elevation and substrate co-vary with land use history, so that the species composition of the forest landscape results from the interplay between biophysical and socioeconomic forces over time. Our results also indicate that increasing the distance to the largest forest patches occurring in the landscape 12 years previous had a small negative impact on species richness but not species diversity or community composition. We conclude that land use history has as much influence in species composition as biophysical variables and that, at the scale of this study, there is no large influence of forest landscape structure on species diversity or composition.     

The role of land tenure in conservation of tree and shrub species diversity in miombo woodlands of southern Malawi

Biodiversity is declining throughout southern African miombo woodlands due to poor land use practices that are detrimental to soil, vegetation and habitat. We aimed at examining tree and shrub species diversity and distribution in Uapaca kirkiana-dominated woodlands under three land tenure types; forest reserves, customary and leasehold land. The study was conducted at different forest sites within similar agroecological zone in southern Malawi. Study sites were located at a range of altitudes between 900 and 1,200 m a.s.l. Three circular plots each with 32.60 m radius were established at each study site to sample tree and shrub species composition, structure and distribution under three land tenure types. We found that forest reserves had higher species diversity with an average of 16 tree families, 27 genera and 34 species as compared to only 10 tree families, 6 genera and 6 species on customary forest lands. Comparisons of diameter at breast height size class distributions showed that customary land had significantly low numbers of small (5.0–10.0 cm) and very large (≥30 cm) diameter classes suggesting lower levels of regeneration and high rate of anthropogenic activities. The high species diversity and richness in forest reserves and leasehold land indicate high potential for protected lands to restore tree species diversity. It is concluded that levels of human activities as influenced by land tenure type reduce tree species diversity, composition and distribution at the different sites, and this confirms the hypothesis that open access lands are not compatible with conservation of tree and shrub species diversity because of high anthropogenic activities.     

Plant and bird diversity in rubber agroforests in the lowlands of Sumatra,Indonesia

Plant and bird diversity in the Indonesian jungle rubber agroforestry system was compared to that in primary forest and rubber plantations by integrating new and existing data from a lowland rain forest area in Sumatra. Jungle rubber gardens are low-input rubber (Hevea brasiliensis) agroforests that structurally resemble secondary forest and in which wild species are tolerated by the farmer. As primary forests have almost completely disappeared from the lowlands of the Sumatra peneplain, our aim was to assess the contribution of jungle rubber as a land use type to the conservation of plant and bird species, especially those that are associated with the forest interior of primary and old secondary forest. Species-accumulation curves were compiled for terrestrial and epiphytic pteridophytes, trees and birds, and for subsets of ‘forest species’ of terrestrial pteridophytes and birds. Comparing jungle rubber and primary forest, groups differed in relative species richness patterns. Species richness in jungle rubber was slightly higher (terrestrial pteridophytes), similar (birds) or lower (epiphytic pteridophytes, trees, vascular plants as a whole) than in primary forest. For subsets of ‘forest species’ of terrestrial pteridophytes and birds, species richness in jungle rubber was lower than in primary forest. For all groups, species richness in jungle rubber was generally higher than in rubber plantations. Although species conservation in jungle rubber is limited by management practices and by a slash-and-burn cycle for replanting of about 40 years, this forest-like land use does support species diversity in an impoverished landscape increasingly dominated by monoculture plantations.     

半干旱黄土丘陵区不同土地利用类型植物物种多样性研究

【目的】为了更好地利用半干旱黄土丘陵区的土地资源,促进该区域植被恢复,提高生物多样性,使区域生态环境有所改善。【方法】以半干旱黄土丘陵区龙滩流域的植被为研究对象,对该流域11种典型的土地利用类型(2种农地、自然荒草地、人工牧草地、撂荒草地、灌木林地、5种乔木林地)的植被物种组成和多样性进行了调查研究。【结果】流域内共调查到种子植物56科166属254种,其中裸子植物门3科7属13种,占流域总科、总属、总种的5.36%、4.22%、5.12%;被子植物门53科159属241种,占流域总科、总属、总种的94.64%、98.78%、94.88%。在被子植物门中,双子叶植物纲有50科134属205种,单子叶植物纲有3科25属36种,两纲物种分别占流域总种数的80.71%和14.17%,占该门物种总种数的85.06%和14.94%。不同土地利用类型植被多样性分析结果表明,物种丰富度表现为农地物种丰富度低,人工牧草地、撂荒草地和灌木柠条林地居中,乔木林地(山杏、山毛桃、油松、侧柏、青杨)和天然荒草地高;物种多样性表现为马铃薯农地的物种多样性低,覆膜农地、人工牧草地、撂荒草地及灌木柠条林地居中,自然荒草地和乔木林地比较高;物种均匀度表现为农地、撂荒草地、人工牧草地及灌木柠条林地的均匀度高,自然荒草地居中,乔木林地低。【结论】不同土地利用方式和植被演替恢复时间差异是产生植被多样性差异的基础,也是该区域植被恢复和物种多样性保护需要考虑的重要要素。     

Understorey vegetation in young naturally regenerated and planted birch (Betula spp.) stands on abandoned agricultural land

The abandonment of agricultural lands in Northern and Eastern Europe increases the area covered by first generation forests, which are either formed as an outcome of secondary succession or established as plantations. However, questions remain as to how these new stands develop and what kind of species they favour, which in turn has impacts on their ecological and economical value. Our aim was to compare understorey vascular plant and bryophyte vegetation characteristics between naturally regenerated and planted birch stands on abandoned agricultural sites in Estonia, focusing on the aspects of species richness and forest understorey recovery. Species richness and diversity of vascular plants were similar in both stand types but the number of forest vascular plant species was significantly higher in naturally regenerated stands. The bryophyte layer of naturally regenerated stands had a higher species richness, diversity, and number of forest bryophyte species. The higher number of forest vascular plant and bryophyte species in naturally regenerated stands can be explained by the longer undisturbed succession period. The recovery of the forest understorey was unaffected by former agricultural land use (crop field or grassland). The influence of soil properties on the recovery of the forest understorey was not detected, but the number of vascular plant species that grow in forests as well as in grasslands was negatively correlated with distance from forest. Overall, understorey vegetation of natural and planted birch stands did not reveal substantial differences. However, in the case of vigorous natural birch regeneration in the vicinity of forest land, unassisted reforestation should be favoured.     

Understory plant communities of boreal mixedwood forests in western Canada: Natural patterns and response to variable-retention harvesting

We examined patterns of variation in richness, diversity, and composition of understory vascular plant communities in mixedwood boreal forests of varying composition (broadleaf, mixedwood, conifer) in Alberta, Canada, before and for 2 years following variable-retention harvesting (clearcut, 20 and 75% dispersed green tree retention, control). Broadleaf-dominated forests differed from mixedwood or conifer-dominated forests in that they had greater canopy cover, litter depth, soil nitrogen, warmer soils, as well as greater shrub cover, herb and shrub richness and diversity (plot scale). In contrast, conifer, and to a lesser extent mixedwood, forest had greater β diversity than broadleaf forest. Overall, mixedwood and conifer forests were similar to one another, both differed from broadleaf forest. Several species were found to be significant indicators of broadleaf forest but most of these also occurred in the other forest types. Understory composition was related to canopy composition and edaphic conditions. Variable-retention harvesting had little effect on understory cover, richness, or diversity but resulted in reduced richness and β diversity at a larger scale. The clearcut and 20% treatments affected composition in all forest types. Early successional species and those common in disturbed sites were indicators of harvesting while evergreen, shade-tolerant understory herbs were indicators of the control forest and 75% retention harvest. We conclude that it is important to maintain a range of variation in canopy composition of mixedwood forests in order to conserve the associated understory communities. The presence of conifers in these forests has a particularly important influence on understory communities. The threshold for a lifeboat effect of variable-retention harvesting is between 20 and 75% retention. Examination of richness and β diversity at a variety of scales can provide interesting information on effects of harvesting on spatial reorganization and homogenization of understory plant communities.     

桑植石漠化林地植物多样性及群落数量特征分析

以湖南桑植县石漠化区为研究对象,采用样方调查法对3个不同石漠化等级样地进行植被调查,分析不同石漠化等级植物物种组成结构及植物多样性指数差异,并根据Jaccard群落相似性原理进行不同石漠化等级植物群落间的相似度评价分析.结果表明:调查区共出现草本植物23科39种、木本植物26科38种,优势科为菊科、蔷薇科、百合科、禾本...     

Tropical reptiles in pine forests: Assemblage responses to plantations and plantation management by burning

Worldwide, the land area devoted to timber plantations is expanding rapidly, especially in the tropics, where reptile diversity is high. The impacts of plantation forestry and its management on native species are poorly known, but are important, because plantation management goals often include protecting biodiversity. We examined the impact of pine (Pinus caribaea) plantations, and their management by fire, on the abundance and richness of reptiles, a significant proportion of the native biodiversity in tropical northern Australia, by (i) comparing abundance and diversity of reptiles among pine plantations (on land cleared specifically for plantation establishment), and two adjacent native forest types, eucalypt and Melaleuca woodlands, and (ii) comparing reptile abundance and richness in pine forest burnt one year prior to the study to remove understorey vegetation with pine forest burnt two years prior to the study. We also examined the influence of fire on reptile assemblages in native vegetation, by comparing eucalypt woodland burnt two years prior to the study and unburnt for eight years. To quantify mechanisms driving differences in reptile richness and abundance among forest types and management regimes, we measured forest structure, the temperatures used by reptiles (operative temperature) and solar radiation, at replicate sites in all forest types and management regimes. Compared to native forests, pine forests had taller trees, lower shrub cover in the understorey, more and deeper exotic litter (other than pine), and were cooler and shadier. Reptile assemblages in pine forests were as rich as those in native forests, but pine assemblages were composed mainly of species that typically use closed-canopy rainforest and prefer cooler, shadier habitats. Burning did not appear to influence the assemblage structure of reptiles in native forest, but burning under pine was associated with increased skink abundance and species richness. Burned pine was not warmer or sunnier than unburned pine, a common driver of reptile abundance, so the shift in lizard use after burning may have been driven by structural differences in understorey vegetation, especially amounts of non-native litter, which were reduced by burning. Thus, burning for management under pine increased the abundance and richness of lizard assemblages using pine. Pine plantations do not support the snake diversity common to sclerophyllous native forests, but pine may have the potential to complement rainforest lizard diversity if appropriately managed.     

Woody vegetation structure and composition along a protection gradient in a miombo ecosystem of western Tanzania

Fully protected areas such as National Parks are often assumed to be the best way to conserve plant diversity and maintain intact forest composition and structure. To evaluate this assertion, we sampled trees in areas with four different levels of protection: a National Park, where the protection level is very high, a Game Controlled Area which allows tourist hunting of big game animals, a Forest Reserve which allows selective harvest of trees, and an Open Area where human populations have unrestricted access to forest resources. All four land management areas had healthy size-class distributions with greater numbers of juvenile trees (2–10 cm DBH) than adults. Surprisingly, mean stem density of trees was highest (947 stems ha⁻¹) in the Game Controlled Area but was lowest (635 stems ha⁻¹) in the National Park. The former had the highest basal area value (24 m² ha⁻¹) while the human-inhabited Open Area had the lowest (11 m² ha⁻¹). Species richness in the Forest Reserve and Game Controlled Area was significantly higher than in the other areas. The total number of plots with unique species not found anywhere else was lowest in the National. Our measures of forest structure and composition therefore show that fully protected National Parks do not necessarily conserve the greatest diversity of tree species or unique species, indicating that a suite of different types of protection strategies may be the key to conservation of trees in these African dry tropical forests.     

The effects of wildfires on wood-eating beetles in deciduous forests on the southern slope of the Swiss Alps

The effect of fires on Cerambycidae, Buprestidae and Lucanidae were studied at 23 sites within a chestnut forest in southern Switzerland. We compared six unburnt sites, two freshly burnt sites, eight sites which burned once at different times in the last 30 years, and seven sites where fires occurred repeatedly in the last 30 years. The diversity and the species composition of the three xylobiont families were related to various ecological variables at two levels of spatial scale, a small scale of 0.25 ha and a large scale of 6.25 ha. These variables were: fire frequency, time since the last fire, clear cutting after the fire, forest structure, amount of dead wood, and habitat mosaic. The fire does not have a direct effect on the xylobiont beetles community at small scale; however, fire has an indirect effect by maintaining a relatively open forest structure. The mosaic of forest areas burnt with different frequencies and at different times was an important factor influencing species richness and species composition at the large spatial scale.Data presented here supports the strategy to conserve the diversity and includes species composition of xylobiont fauna in deciduous forests: (i) at small spatial scale, to maintain highly structured and relatively open stands with large amounts of dead wood and big oak trees; (ii) at large spatial scale, to favour a mosaic of different forest habitats and successional stages. A forest offering a good structural diversity is important for maintaining landscape complexity and thus a high species richness of xylophagous beetles.     

Assemblages and species diversity of wood destroying termites in different land use systems in Western Ghat, India

Spatial distribution, abundance and assemblage of termites depend mainly on the local conditions and habitats in which they thrive. Striking differences are observed in the species richness, number of clades and functional diversity of termite assemblage between different habitats. This study aim was to examine effect of human interference in the diversity of wood destroying termite in forest areas as well as managed and unmanaged plantations in South India. Termites attacking trees and wooden logs were collected from forest areas, managed plantations and unmanaged plantations. The termites collected were identified and compared for species abundance, richness and species diversity. Results show that the species composition and species diversity of the wood destroying termites vary according to the conditions. The species diversity measures revealed that there is a significance variation among the forest fauna and plantations. The forest areas have the highest species composition compared to plantations. Species richness is high forest areas. Even though the abundance of termites are more in unmanaged plantations, there is no significant difference related to species diversity among the managed and unmanaged plantations.     

Changes in forest vegetation and arbuscular mycorrhizae along a steep elevation gradient in Arizona

We assessed species composition, richness and abundance of understory vegetation, as well as arbuscular mycorrhizal (AM) inoculum potential on the San Francisco Peaks, tallest mountains in Arizona, crossing a steep, south-facing elevational gradient. These mountains have a high conservation value due to their rare habitats but previous vegetation studies have been limited. Because mature trees in the Pinaceae do not form associations with AM fungi, there may be more variation in plant community and AM fungal associations in coniferous forest than in ecosystems where all species associate with AM fungi. Differences in species composition between forest types reflected differences in the historical disturbance regimes. Species richness was highest in ponderosa pine forest (32.6 ± 1.4 per 1000 m² plot), although plant abundance was highest in aspen forest (49.4 ± 3.8%). Ponderosa pine and bristlecone pine forest were both high in species richness and contained species which were tolerant of frequent, low-intensity fire. Exotic species richness and abundance were highest in the lower elevations, which were also areas of high species richness and greater anthropogenic disturbance. Arbuscular mycorrhizal inoculum potential varied widely (1.2–80.1%), decreasing with increases in tree cover. We suggest indicator species that may be of use in monitoring these forests under changing climate and fire regimes.     

Towards practices favourable to plant diversity in hybrid poplar plantations

Much research effort is being devoted to developing forest management practices with limited impacts on biodiversity. While the impact of poplar Populus sp. plantations on biodiversity is relatively well-known at the landscape scale, the impact of alternative management practices at the plantation scale has received much less attention. Yet biodiversity is likely to be impacted by the choice of the poplar clone, stem density at plantation, type and duration of the understory control, and age at which the poplars are harvested. In this study, we investigated the impact of these factors on herbaceous plant communities with data from plant surveys conducted in 85 young (2–5 years) and 96 mature (11–17 years) hybrid poplar high-forest plantations in northern France. On average, ruderal or generalist plants contributed to 40.5% of the plot species richness; tall herbs (60.2%), forest (26%) and meadow plants (13.8%) contributed to the remaining 59.5% more specialised species. Soil moisture and soil nitrogen were major determinants of plant communities: wet soils were favourable to tall herbs, while meadow and forest species preferred moist soils; a significantly lower diversity of the three species groups was reported in the nutrient richer soils (in mature plantations only for forest plants). Mean species richness decreased with plantation age except for forest species. Plant communities in young plantations showed little differences in composition according to the type of understory control (chemical, mechanical or both). The development of a shrubby layer in mature plantations was restricted to the drier soils and was detrimental to both meadow plants and tall herb species. Effects of previous land use on forest and tall herb species were found only in young plantations, suggesting a rapid reset of plant communities for these two groups. This may not be the case for meadow species as the influence of previous land use was significant in mature plantations only. Finally, clone type and stem density at plantation had no significant impact on plant communities. Adjusting age at which the poplars are harvested seems the only effective way to drive plant communities in high-forest poplar plantations: delaying poplar harvest (probably beyond 15–20 years) would benefit forest plants, while advancing poplar harvest (about 10 years) would benefit tall herbs, especially in wet soil conditions.     

Breeding bird community composition in different successional vegetation in the montane coniferous forests zone of Taiwan

To examine the relationship between forest succession following fire and the composition of bird communities, we investigated the vegetation structure, bird population density, foraging behavior and guild structure in bamboo grasslands (11 years since the last fire), pine savanna (41 years), pine woodland (58 years), old-growth hemlock forest (never burned), and old-growth spruce forest (never burned) in the Tatachia area of central Taiwan. Canopy height, total foliage cover, tree density, total basal area of tree, total basal area of snags, foliage height diversity, and tree species richness all increased with successional age. However, shrub cover peaked in intermediate successional stages. The vertical profile of foliage cover was more diverse in later successional forests, which had more breeding bird species and ecological guilds. All the breeding bird species recorded in early and intermediate stages were also found distributed in the late successional forests. Because Taiwan has high precipitation and humidity, and most forest fires in Taiwan are caused by human activities, forest fires and large areas of early successional vegetation were probably rare in the mountain areas of Taiwan prior to the arrival of humans. Therefore, bird species have not had enough time to adapt to areas with early or intermediate successional vegetation. Moreover, late successional forests host all the major plant species found in the early and intermediate stages and have higher foliage height diversity index, which was positively correlated with the bird species richness and bird species diversity index in this study. As a result, all breeding bird species and guilds in the area can be found in late successional forests. Efforts for conserving avian diversity in Taiwan should focus on protecting the remaining native old-growth forests.     

Do the size and landscape context of forest openings influence the abundance and breeding success of shrubland songbirds in southern New England?

Early successional birds have declined in the northeastern United States due to the regeneration of forest on abandoned farm fields and the suppression of natural disturbances that once provided appropriate habitat. These species have become increasingly dependent on early successional habitats generated by such activities as timber harvesting. Recent approaches of timber harvesting, which range from single-tree harvesting to clearcutting, create forest openings of different sizes and configurations embedded in landscapes with different land use patterns. To assess the importance of forest openings created by timber harvesting for shrubland birds, we surveyed birds on 50 m radius plots in 34 harvest sites (0.5–21 ha). We collected data on multi-scaled habitat variables ranging from plot-level vegetation characteristics to land use patterns within 1 km of each study site. We also monitored mating and nesting success of Blue-winged Warblers (Vermivora pinus) in 10 forest openings.

The abundance of most shrubland species was influenced by plot-level habitat variables, such as tree density and vegetation height, rather than shrubland area or the composition of land uses in the surrounding landscape. Only Eastern Towhees (Pipilo erythrophthalmus) were more frequent in survey plots in larger forest openings. In contrast, neither abundance nor reproductive activity of Blue-winged Warblers was correlated with the size of the forest opening. Their abundance was negatively related to vegetation height, however. Only 54% of the territorial male Blue-winged Warblers in forest openings were mated. We documented relatively low nest success rates of 21.1% during the egg laying and incubation nest stages, but increased success rates during the later stages of nesting.

Our results indicate that even small forest openings with low vegetation provide habitat for Blue-winged Warblers and other shrubland birds. The overall reproductive rate of territorial male Blue-winged Warblers in forests openings was low during the 2 years of the study, however. Further studies are needed to assess the long-term value of this type of habitat for sustaining shrubland bird populations.     

Abundance and diversity of soil invertebrates in annual crops, agroforestry and forest ecosystems in the Nilgiri biosphere reserve of Western Ghats, India

Biologically mediated soil processes rely on soil biota to provide vital ecosystem services in natural and managed ecosystems. However, land use changes continue to impact on assemblages of soil biota and the ecosystem services they provide. The objective of the present study was to assess the effect of land use intensification on the distribution and abundance of soil invertebrate communities in the Nilgiri, a human-dominated biosphere reserve of international importance. Soil invertebrates were sampled in 15 land use practices ranging from simple and intensively managed annual crop fields and monoculture tree plantations through less intensively managed agroforestry and pristine forest ecosystems. The lowest taxonomic richness was found in annual crops and coconut monoculture plantations, while the highest was in moist-deciduous and semi-evergreen forests. With 21 ant species, agroforestry systems had the highest diversity of ants followed by forest ecosystems (12 species). Earthworms and millipedes were significantly more abundant in agroforestry systems, plantations and forest ecosystems than in annual crop fields. Ants, termites, beetles, centipedes, crickets and spiders were more abundant in forest ecosystems than in other ecosystems. It is concluded that annual cropping systems have lower diversity and abundance of soil invertebrates than agroforestry and natural forest ecosystems. These results and the literature from other regions highlight the potential role that agroforestry practices can play in biodiversity conservation in an era of ever-increasing land use intensification and habitat loss.     

福建茫荡山自然保护区昆虫多样性的初步研究

对茫荡山自然保护区的5个不同生境进行了昆虫多样性调查,并分别用物种丰富度S、个体数N、多样性指数H’、均匀度指数R和优势度指数D对不同的群落进行了统计分析。结果表明在不同生境中物种丰富度、个体教、多样性指数、均匀度指数和优势度指数皆不相同．其中常绿阔叶林内昆虫的丰富度、多样性指数和均匀度均为最高,针阔混交林次之,毛竹林为最低,但是毛竹林物种的优势度却是最高的。表明昆虫的多样性与其生境的植物种类、丰富程度密切相关。     

Ant abundance increases with clearing size

Ants are an important part of biodiversity and are useful bioindicators. Our aim was to determine whether ant species richness and composition differ among large clear-cuts (ca. 3000 m²), small forest gaps (ca. 400 m²), and areas of mature forest. The research was conducted in a large plantation of Picea abies in the Jizerske Mountains in the Czech Republic at an elevation of ca. 900 m a.s.l. Ants were sampled using pitfall traps in three areas, each of which had a large clear-cut, mature forest, and gaps; a randomized complete block design was used. Species richness and composition of ant assemblages differed significantly among plots in the three treatments. The results showed that ant abundance and diversity were similarly low in small gaps and in closed-canopy mature stands; in comparison, large clear-cuts supported significantly higher species richness, more complex species composition, and a higher abundance of ants. Six species were found in large clear-cuts, but only one and two species were collected in small gaps and mature forests, respectively. Our findings suggest that small-scale forest management is not suitable for maintaining ant diversity.     

Effects of Different Management Practices on Stand Composition and Species Diversity in Subtropical Forests in Nepal: Implications of Community Participation in Biodiversity Conservation

In this article, we compared the structure, composition, and diversity of trees, shrubs and saplings, seedlings and herbaceous species of community- and government-managed forests in the lowlands of eastern Nepal. Results suggest that among the trees, the community forest was dominated by a single species, Shorea robusta. However, Shorea robusta and Terminalia myriocarpa were codominant in the government forest. Tree density and basal area were higher in the government forest, but shrub/sapling density and basal area were higher in the community forest, suggesting a positive effect of community management on tree regeneration. Overstory species assemblages showed an obvious compositional difference between the forests, but understory species assemblages were less obvious. Plot-level tree and shrub/sapling species richness was higher in the government forest than the community forest. However, seedling-herbaceous species richness was higher in the community forest. The dominance of Shorea robusta trees in the community forest suggests that people involved in managing forests may be more interested in a limited number of economically valuable species while removing less important trees. Such preferential management practices may increase resource heterogeneity within a forest and maintain species diversity in the understory. Thus, community participation in forest management should be encouraged, with guided management techniques and exercises, to achieve maximum forest recovery, provide sustainable ecosystem services, and maintain forest diversity.     

Assessing the relationships between stand development and understory vegetation using a 420-year chronosequence

The large-scale conversion of old forests to tree plantations has made it increasingly important to understand how understory vegetation responds to such landscape changes. For instance, in some forest types a reduction in understory richness and cover is thought to result from the development of canopy closure in plantations, although there is a paucity of empirical data demonstrating this relationship. We used a 420-year forest chronosequence as a case study to assess the relationship between stand age, tree canopy cover and understory vascular plant richness and composition in the Siskiyou Mountains of Oregon. The chronosequence consisted of six young managed (age 7–44) and nine older unmanaged (age 90–427) stands. All stands were similar in underlying geology, slope, elevation, and aspect. We found a non-linear relationship between stand age and richness, in which richness was highest in the youngest stands, reached a low in mid-aged stands (∼55 years), then increased in the oldest stands. We also found that percent tree canopy cover was correlated with total understory cover, richness, diversity, and species composition. In general, young stands were characterized by high shrub and graminoid cover and old stands were characterized by an abundant herb layer. Our work suggests that a major component of our study landscape is currently entering the forest stage (canopy closure) characterized by low levels of vascular plant species richness and cover. We use our results to discuss the potential effects of future forest management on understory plants.