Forest regeneration following ungulate removal in a montane Hawaiian wet forest

Most Hawaiian forests lack resiliency following disturbance due to the presence of non‐native and invasive plant and animal species. The montane wet forest within Hakalau Forest National Wildlife Refuge on Hawai'i island has a long history of ungulate disturbance but portions of the refuge were fenced and most ungulates excluded by the early 1990s. We examined patterns of regeneration within two 100 ha study sites in this forest following the removal of ungulates and in the absence of invasive woody tree species to determine, in part, if passive restoration techniques can be successful under these conditions. We characterized growth, mortality, and basal area (BA) changes for approximately 7,100 marked individuals of all native tree species present in two surveys over a 17–18‐year period within two hundred 30 m diameter forest plots. Considerable recruitment within plots of new trees of all species significantly changed size class distributions and erased deficits in small‐sized trees observed during the first survey, particularly for the codominant canopy tree, koa (Acacia koa). Overall, growth of established dominant 'ōhi'a trees (Metrosideros polymorpha) and recruitment of mid‐canopy trees contributed to increases in BA while high levels of mortality for large A. koa trees contributed to decreased BA. This resulted in a slight increase in BA between the two surveys (+1.9%). This study demonstrates that fencing and ungulate removal may have rescued the A. koa population by facilitating the first real pulse in recruitment in over a century, and that passive restoration can be a successful management strategy in this forest.     

Effects of invasive alien kahili ginger (Hedychium gardnerianum) on native plant species regeneration in a Hawaiian rainforest

Questions: Does the invasive alien Hedychium gardnerianum (1) replace native understory species, (2) suppress natural regeneration of native plant species, (3) increase the invasiveness of other non‐native plants and (4) are native forests are able to recover after removal of H. gardnerianum. Location: A mature rainforest in Hawai'i Volcanoes National Park on the island of Hawai'i (about 1200 m a.s.l.; precipitation approximately 2770 mm yr^?1). Study sites included natural plots without effects of alien plants, ginger plots with a H. gardnerianum‐dominated herb layer and cleared plots treated with herbicide to remove alien plants. Methods: Counting mature trees, saplings and seedlings of native and alien plant species. Using non‐parametric H‐tests to compare impact of H. gardnerianum on the structure of different sites. Results: Results confirmed the hypothesis that H. gardnerianum has negative effects on natural forest dynamics. Lower numbers of native tree seedlings and saplings were found on ginger‐dominated plots. Furthermore, H. gardnerianum did not show negative effects on the invasive alien tree species Psidium cattleianum. Conclusions: This study reveals that where dominance of H. gardnerianum persists, regeneration of the forest by native species will be inhibited. Furthermore, these areas might experience invasion by P. cattleianum, resulting in displacement of native canopy species in the future, leading to a change in forest structure and loss of other species dependent on natural rainforest, such as endemic birds. However, if H. gardnerianum is removed the native Hawaiian forest is likely to regenerate and regain its natural structure.     

Assessing the conservation value of a human-dominated island landscape: Plant diversity in Hawaii

The conversion of native habitats to pasture and other working lands, unbuilt lands modified by humans for production, is one of the greatest threats to biodiversity. While some human-dominated landscapes on continents support relatively high native biodiversity, this capacity is little studied in oceanic island systems characterized by high endemism and vulnerability to invasion. Using Hawaii as a case study, we assessed the conservation value of working landscapes on an oceanic island by surveying native and non-native plant diversity in mature native forest and in the three dominant land covers/uses to which it has been converted: native, Acacia koa timber plantations, wooded pasture, and open pasture. As expected, native plant diversity (richness and abundance) was significantly higher and non-native abundance significantly lower in mature native forests than any other site type. A. koa plantations and wooded pasture supported four and three times greater, respectively, species richness of native understory plants than open pasture. Also, A. koa plantations and wooded pasture supported similar species communities with about 75% species in common. Conservation and restoration of mature native forest in Hawaii is essential for the protection of native, rare species and limiting the spread of non-native species. A. koa plantations and wooded pasture, however, may help harmonize production and conservation by supporting livelihoods, more biodiversity than open pasture, and some connectivity between native forest remnants important for sustaining landscape-level conservation value into the future.     

Community Comparison and Determinant Analysis of Understory Vegetation in Six Plantations in South China

Plantations cover large areas in many countries, and the enhancement of plantation biodiversity is an increasingly important ecological concern. Many studies have demonstrated that overstory composition is important because it influences understory regeneration. To compare the understory vegetation and analyze its determinant factors, six typical plantations in South China were investigated: Acacia mangium plantation, Schima superba plantation, Eucalyptus citriodora plantation, E. exserta plantation, mixed‐coniferous plantation, and mixed native species plantation. The results show that native species plantations shaded out more grasses and herbs than exotic species plantations, mixed‐species plantations recruited more understory species than monoculture plantations, the leguminous species plantation had higher soil nitrogen than nonleguminous species plantations, and understory vegetation in the mixed‐coniferous plantation was similar to that of mixed, native broadleaf species plantation. Although light is the crucial environmental factor affecting the understory community and diversity among the 14 measured factors, other environmental variables such as soil nutrients and soil moisture are also important.     

Regeneration of indigenous trees in Mgahinga Gorilla National Park, Uganda

This study examines the regeneration of indigenous tree species in the formerly encroached area in Mgahinga Gorilla National Park (MGNP), south‐western Uganda. Before gazetting in 1992, MGNP had basically been agricultural land for well over 50 years. The distribution of exotic vegetation was established using a Geographical Positioning System receiver and indigenous vegetation was sampled by establishment of quadrats along transect lines. Observations indicated that approximately 2% of the old cropland was covered by exotic woodlots. Black wattle (Acacia mearnsii) and Eucalyptus trees were found to be the most widely distributed and Pinus patula the least distributed species in the park. Species numbers of indigenous trees (n = 26) were high in the old cropland, compared with twelve species observed in exotic woodlots. The natural forest supported the highest (75%) stem density and the lowest (4%) stem density was recorded in exotic woodlots. Seedling class (< 2 cm, d.b.h.) accounted for the majority of juveniles, with the lowest stem density (1350 seedlings ha^?1) recorded in exotic woodlots compared with 6609 seedlings ha^?1 in the old cropland and 24,625 seedlings ha^?1 in the natural forest. The levels of tree diversity and stocking characteristics recorded under the exotic species suggest that a low diverse community of native species may exploit this environment.     

Composition and distribution of indigenous trees and shrubs as possible criteria for indicating adapted species in semi‐arid rangelands

This study assessed the composition and natural distribution of indigenous trees and shrubs as possible criteria for selecting suitable species for rehabilitation of degraded sites in semi‐arid rangelands. Study sites were identified at Nthangu, Kathonzweni and Kibwezi forests of Makueni County, Kenya using existing vegetation, agro‐climatic maps and Landsat imageries. The sites had mean annual rainfalls of 974 mm, 700 mm and 616 mm, respectively, and moisture indices of 49%, 35% and 32%. Data were collected by establishing sample plots and assessing species counts and diameters at breast height (DBH). Basal area, relative dominance, relative abundance, relative frequency and important value indices (IVIs) were computed for individual families and species at each site. The number of families, genera and species declined from Nthangu (33, 60, 77) through Kibwezi (30, 48, 70) to Kathonzweni (28, 42, 69). Corresponding mean basal areas were 16.7 m² ha^?1, 76.8 m² ha^?1 and 19.3 m² ha^?1. The families Combretaceae, Burseraceae and Mimosaceae were the most important and widely distributed. Based on ecological importance values, candidate species for rehabilitation of degraded sites at Nthangu, Kathonzweni and Kibwezi were Combretum molle and Acacia hockii; Combretum collinum, Commiphora campestris and Acacia tortilis; and Commiphora africana and A. tortilis, respectively.     

Relationships among the seed rain,seed bank and vegetation of a Hawaiian forest

Abstract. Hawaiian ecosystems are prone to invasion by alien plant species. I compared the seed rain, seed bank, and vegetation of a native Hawaiian forest to examine the potential role that seed ecology plays in allowing alien species to invade native forest. Absolute cover of seed plants in the forest was 126 %, annual seed rain was 5 713 seeds m^-2 yr^-1, and the mean density of seedlings emerging from the seed bank averaged across four seasons was 1 020/m². The endemic tree Metrosideros polymorpha was the most abundant species in the vegetation, seed rain and winter seed bank. Overall, native seed plants comprised 95 % of the relative cover in the vegetation and 99 % of the seeds in the seed rain, but alien species comprised 67 % of the seeds in the seed bank. Alien species tended to form persistent seed banks while native species formed transient or pseudo-persistent seed banks. Dominance of the seed bank by alien species with persistent seed banks suggests that aliens are favorably placed to increase in abundance in the vegetation if the forest is disturbed.     

Altering Light Availability to Restore Invaded Forest: The Predictive Role of Plant Traits

Many studies have demonstrated that reduced light availability, which can be manipulated at local scales by planting or seeding canopy species, can curtail the growth of invasive species and promote the growth of native species. Species differences in functional traits, such as light use and stress tolerance, may be used to determine how native and invasive species will respond to these resource manipulations. We altered light availability in a mesic Hawaiian forest understory and found that low light levels reduced the biomass and growth of two invasive grasses (Pennisetum clandestinum and Ehrharta stipoides) relative to two native shrubs (Pipturus albidus and Coprosma rhynchocarpa) and two native canopy species (Metrosideros polymorpha and Acacia koa). Native species generally displayed traits associated with shade tolerance (high quantum yield, chlorophyll content, and leaf mass per area), whereas the two invasive grasses displayed traits associated with shade intolerance (high photosynthetic rate and growth rate). Several key traits pertaining to light acquisition and shade tolerance (quantum yield, chlorophyll content, and leaf mass per area) predicted seedling survival in low‐light treatments. Our data suggest that differences in light use among native and invasive species can help to determine the utility of resource manipulation as a restoration tool and, more specifically, to predict which native species will be optimal for restoration efforts that manipulate light availability.     

On some rust fungi (Uredinales) collected in an Acacia koa–Metrosideros polymorpha woodland, Mauna Loa Road, Big Island, Hawaii

Nine species of rust fungi (Uredinales) were found during a 1-day field study in an Acacia koa–Metrosideros polymorpha woodland in Volcanoes National Park on Big Island, Hawaii. Two species, both found on Acacia koa, are native (endemic) rusts whereas 7 species are nonnative on nonnative hosts, highlighting the high proportion of introduced species in the Hawaiian flora even in more or less natural habitats. One species, Uromyces linearis on Panicum repens, constitutes the first record of this rust for the Hawaiian archipelago, bringing the total to 93 species on the islands, 70 (75.3%) of which are introduced. The species records are annotated with emphasis on the geographic origin of each taxon. In addition, the study has led to the molecular reevaluation of the genus Racospermyces, indicating that it is synonymous with Endoraecium, and six new combinations are proposed for the species previously placed in Racospermyces. The high number of introduced species in Hawaii coupled with the paucity of native species when compared to other global regions is discussed.     

Structure of a unique inland mangrove forest assemblage in fossil lagoons on the Caribbean Coast of Mexico

Scrub mangrove wetlands colonize the intertidal zone of fossil lagoons located in carbonate continental margins along the Yucatan Peninsula of Mexico. These unique ecological types were investigated in October, 1994, by locating transects in several mangrove forests along the Caribbean coast of the peninsula. Four species of mangrove occurred at these sites including Rhizophora mangle, Avicennia germinans, Laguncularia racemosa, Conocarpus erecta. This is one of the first examples of a species rich scrub forest. The mangroves fell into three height categories: short scrub less than 1.5 m, tall scrub to 3.0 m, and basin forests between 4.5 and 6 m. Average height, diameter (dbh), basal area, and complexity index generally increased from short scrub to basin forests. Basal area, ranged from 0.16 m² ha^–1 in a short scrub forest intermixed with Cladium jamaicense to 12.9 m² ha^–1 in a basin forest. Density ranged from 1520 trees ha^–1 to over 25,000 trees ha^–1 in a short scrub forest dominated by R. mangle. The complexity index ranged from 0.01 to 8.3. Height, dbh, basal area, and complexity index were positively related. A number of trees were growing as sprouts from larger downed trunks, suggesting that hurricanes, such as Gilbert that occurred in 1988, are important in controlling the structure of these forests. These forests appear isolated from the sea, but are influenced by groundwater exchange occurring at the land-margin zone.     

Moderating Night Radiative Cooling Reduces Frost Damage to Metrosideros polymorpha Seedlings Used for Forest Restoration in Hawaii

Winter frosts caused by radiative cooling were hypothesized to limit successful reintroduction of Hawaiian plants other than Acacia koa to alien‐dominated grasslands above 1700 m elevation. We determined, in the laboratory, the temperature at which irreversible tissue damage occurred to Metrosideros polymorpha leaves. We also conducted a field study of this species to determine if (1) leaf damage was correlated with sub‐zero leaf temperatures, (2) radiative cooling could be moderated by canopies of A. koa, and (3) low soil temperatures contributed to seedling damage. The last was evaluated by thermally buffering seedlings with water‐filled bladders placed at their base to keep roots warm, or by installing a radiation shield to reduce early morning transpiration when water uptake from cold soils would be least. Leaf temperatures were monitored between midnight and 7:00 a.m. using fine‐wire thermocouples, and leaf damage was recorded monthly. In the laboratory, supercooling protected leaves from mild sub‐zero temperatures; irreversible tissue damage occurred at about ?8°C. In the field, leaf damage was strongly correlated with degree‐hours below freezing. Unprotected seedlings suffered the greatest leaf damage. Those sheltered under A. koa trees rarely experienced temperatures below ?3°C, and damage was minimal. Shaded and thermally buffered seedlings suffered less damage than unprotected plants, probably due to elevated leaf temperatures rather than improved water relations. Using A. koa or artificial devices to reduce radiative cooling during winter nights should enhance establishment of M. polymorpha in high‐elevation rangeland.     

Contrasting Cloud Forest Restoration Potential Between Plantations of Different Exotic Tree Species

The role of exotic tree plantations for biodiversity conservation is contested. Such plantations nevertheless offer various ecosystem service benefits, which include carbon storage and facilitation of indigenous tree species regeneration. To assess forest restoration potential in tropical exotic tree plantations, we assessed native cloud forest tree regeneration in 166 plots in ca. 50‐year‐old plantations of five timber species that are widely used in tropical plantations (Pinus patula, Eucalyptus saligna, Cupressus lusitanica, Grevillea robusta and Acacia mearnsii). Differences in species abundance, diversity and composition were compared among plantations, and between plantations and disturbed and undisturbed indigenous Afromontane cloud forest (southeast Kenya) relicts after controlling for environmental variation between plots (i.e. altitude, distance to indigenous forest, soil depth, slope, aspect) and for environmental and stand structural variation (i.e. dominant tree height and basal area). Regenerating trees were mostly early‐successional species. Indigenous tree species regeneration was significantly higher in Grevillea plantations, where the seedling community also included late‐successional tree species. Regeneration under Eucalyptus was particularly poor. Acacia had a strong invasive nature, reducing its potential role and usefulness in indigenous forest restoration. Our study underlined that exotic tree plantations have differential effects on native tree species regeneration, with high potential for Grevillea plantations and low potential for invasive exotic species.     

Regeneration of cleared Acacia zanzibarica bushland in Kenya

Abstract. Woody biomass production in natural forests of arid and semi-arid regions is low. The fuelwood demand of settlements often exceeds the sustained yield and regeneration capacity of natural forests, which results in deforestation. Regeneration and woody biomass development was studied in cleared Acacia zanzibarica bushland in Bura, eastern Kenya. The area was cleared in 1982 and studied in 1988. The site had been colonized primarily by Acacia zanzibarica and A. reficiens. Mean density was 1333 trees/ha, mean total woody biomass (dry weight) 1954 kg/ha, equal to 2.53 m³/ha. Mean annual increment was 293 kg/ha, or 0.3 8m³/ha. Expressed as rain use efficiency, the natural dry matter productivity of the woody component equals 0.83 kg ha^-1 yr^-1 mm^-1. The regeneration potential and some management implications are discussed.     

Diversity of understory plants in undisturbed and disturbed tropical lowland forests of Little Andaman Island, India

Species richness and density of understory plants were investigated in eight 1 ha plots, distributed one each in undisturbed and disturbed tropical evergreen, semi-evergreen, deciduous and littoral forests of Little Andaman island, India, which falls under one of the eight hottest hotspots of Biodiversity in the world viz. the Indo-Burma. One hundred 1 m⁻² quadrats were established in each 1 ha plot, in which all the understory plants (that include herbs, undershrubs, shrubs and herbaceous climbers) were enumerated. The total density of understory plants was 6,812 individuals (851 ha⁻¹) and species richness was 108 species, representing 104 genera and 50 families. Across the four forest types and eight study plots, the species richness ranged from 10 to 39 species ha⁻¹. All the disturbed sites harbored greater number of species than their undisturbed counterparts. Herbs dominated by species (63%) and density (4,259 individuals). The grass Eragrostis tenella (1,860 individuals; IVI 40), the invasive climber Mikania cordata (803; IVI 20) and the shrub Anaxagorea luzonensis (481; IVI 17.5) were the most abundant species. Poaceae, Asteraceae, Acanthaceae, Orchidaceae and Euphorbiaceae constituted the species-rich families represented by 6 species each. The species-area curves attained an asymptote at 0.8 ha level except in sites DD and DL, indicating 1 ha plot is not sufficient to capture all the understory species in disturbed forests. The alien weeds formed about one-fourth of the species richness (31 species; 28%) and density (1,926 individuals; 28.3%) in the study sites, indicating the extent of weed invasion and the attention required for effective conservation of the native biodiversity of the fragile island forest ecosystem.     

Nitrogen aboveground turnover and soil stocks to 8 m depth in primary and selectively logged forest in southern Amazonia

Extensive areas of Amazonia undergo selective logging, modifying forest structure and nutrient cycles. Anthropogenic‐accelerated rates of nitrogen (N) turnover could increase N loss and affect regeneration, carbon sequestration and timber production. We quantified leaf area reduction, canopy opening and downed biomass and resultant N flux from reduced impact logging (RIL) activities. We compared canopy reduction, surface soil moisture and nitrate to 8 m depth between logged gaps and intact primary forest to determine if logging activities increase subsoil nitrate. To test long‐term logging effects, we evaluated surface N stocks along a 12‐year postlogging chronosequence. At the harvest rate of 2.6 trees ha^?1, total N additions in logging gaps, including leaves and wood from felled crowns (24.8 kg N ha^?1) and other killed trees (41.9 kg N ha^?1), accounted for over 80% of the total N addition to aboveground necromass from all logging activities (81.9 kg N ha^?1). Despite this N turnover by logging, belowground nitrate storage to 8 m depth did not differ between logging gaps and primary forest at the low harvest rate and disturbance intensity of this study. Soil water depletion also did not differ between gaps and primary forest over 1 year, indicating the impact on belowground inorganic N was low. Compared with primary forest, nitrate concentrations to 8 m depth in logging gaps were only significantly higher at 60–100 cm, suggesting some N redistribution beyond the bulk of the fine roots in logging gaps. Extrapolated to the Amazon Basin scale, we provide a conservative estimate that logging damage and bole export under RIL would turn over 0.14 ± 0.07 to 0.23 ± 0.12 Tg N yr^?1 based on 1999–2002 selective logging rates. Greater damage during conventional selective logging would cause higher N turnover throughout the Amazon Basin than our results based on RIL.     

High competitiveness of a resource demanding invasive acacia under low resource supply

Mechanisms controlling the successful invasion of resource demanding species into low-resource environments are still poorly understood. Well-adapted native species are often considered superior competitors under stressful conditions. Here we investigate the competitive ability of the resource demanding alien Acacia longifolia, which invades nutrient-poor Mediterranean sand dunes such as in coastal areas of Portugal. We explore the hypothesis that drought may limit invasion in a factorial competition experiment of the alien invasive versus two native species of different functional groups (Halimium halimifolium, Pinus pinea), under well-watered and drought conditions. Changes in biomass, allocation pattern, and N-uptake-efficiency (via ¹⁵N-labeling) indicated a marked drought sensitivity of the invader. However, highly efficient drought adaptations of the native species did not provide a competitive advantage under water limiting conditions. The competitive strength of H. halimifolium towards the alien invader under well-watered conditions turned into a positive interaction between both species under drought. Further, low resource utilization by native species benefited A. longifolia by permitting continued high nitrogen uptake under drought. Hence, the N-fixing invader expresses low plasticity by continuous high resource utilization, even under low resource conditions. The introduction of novel traits into a community like N-fixation and high resource use may promote A. longifolia invasiveness through changes in the physical environment, i.e., the water and nutrient cycle of the invaded sand dune system, thereby potentially disrupting the co-evolved interactions within the native plant community.     

Stand-level management of plantations to improve biodiversity values

As conservation reserves expand, the likelihood that they will capture areas degraded by previous land use increases. Ecological restoration of such areas will therefore play an increasing role in biodiversity conservation. On the New South Wales North Coast, recent expansion in the conservation estate has captured over 300 softwood and hardwood plantations, many with understoreys dominated by exotic weeds. Here we present an overview of the practices we have adopted in managing flooded gum (Eucalyptus grandis) plantations infested with lantana (Lantana camara) to enhance their biodiversity value. Experiments designed to overcome barriers limiting regeneration of native forest in conjunction with measurement of soil and plant responses yielded insights into the management of former timber plantations for biodiversity. Canonical Correspondence Analysis indicated that the level of canopy retention (or logging intensity) within sites consistently explained the greatest amount of variation in plant community composition (32–38% post-treatment). Thinning and burning stimulated regeneration of native species. Retained canopy cover was proportional to the richness or abundance of native woody shrubs, understorey trees and native perennial herbs, indicating that management intensity can be varied to promote a range of conservation values. A state-and-transition model summarising purported management actions and likely outcomes for these plantations is presented. This is the first time plantations have been managed solely for biodiversity. Logging income means that plantation restoration can be cost-neutral, and the positive influence of a cover crop of trees means that plantation management may generally be manipulated to promote biodiversity conservation.     

Recruitment dynamics of two low-density neotropical multiple-use tree species

In the present study, we describe the temporal and spatial variability in recruitment, growth, and mortality rates of seedlings and saplings of two low-density neotropical tree species, Dipteryx odorata and Copaifera reticulata in Eastern Amazonia, Brazil. As both species have important timber and non-timber uses, for each species we compare regeneration parameters among different management scenarios (sites used for timber logging, non-timber product extraction, and undisturbed forests). Results suggest that both species share similar natural regeneration characteristics. These include temporally and spatially asynchronous germination, existence of individuals that have more abundant and frequent fruit production than the average of the population and a positive influence of the mother tree crown on seedling and sapling density. The management activities analyzed did not influence the regeneration parameters of both species, which suggests that timber logging the way it was performed and current rates of D. odorata seed gathering and C. reticulata tapping at the study site are not sufficiently intense to threaten species population. However, some species characteristics, such as their reproductive strategies, light-demanding syndromes, low-dispersal ranges, and high-mortality rates of seedlings make both species vulnerable to exploitation.     

Using plant functional types to compare vegetation structure of alien-invaded and uninvaded Acacia nilotica savannas

Four plant functional types (PFTs) were used to compare the vegetation structure of an alien-invaded Acacia nilotica savanna with one of negligible invasions. Heights, canopy covers and species richness of three native PFTs (woody plants, grasses and herbs) and one alien PFT (woody plants) were measured in 14, 1-m² quadrats sampled in a stratified-random pattern in a 400-m² plot demarcated in each savanna. In the uninvaded plot, mean heights of native PFTs were stratified. In the invaded plot, the mean height of aliens extended into the native woody stratum with the lower range of native woody PFT heights reduced to the grass stratum. Discriminant analysis of canopy covers and species richness of the four PFTs revealed significant differences in composition between plots with the alien PFT being the most important variable correlated with these differences. Univariate analysis confirmed the dominance of alien woody plants in the invaded plot but also showed significant reductions in the canopy covers and species richness of native herbs and grasses compared to those in the uninvaded plot. These results suggest that PFTs can rapidly measure small-scale, spatial differences in the physiognomy, composition and species richness of A. nilotica savannas when invaded by alien woody plants.