Question
What are the differences between plant communities recognised using supervised versus un‐supervised methods?
Location
Northeastern Australia.
Methods
Two classifications of savanna plant ...communities were formed independently with two different approaches: supervised and un‐supervised (using agglomerative hierarchical clustering). Each approach used the same vegetation datasets and, importantly, classification criteria. The communities occur on two different landscapes, with differing environmental gradients, covering an area of 53,500 km2. We compared the internal characteristics of plant communities between approaches and landscapes using four evaluation criteria: identifiability, distinctiveness, similarity of internal heterogeneity and predictability of species foliage cover. Additionally, we compared the central floristic concepts and compositional boundaries of communities identified by each approach.
Results
Supervised and un‐supervised approaches recognised similar floristic community concepts. Compositional boundaries between communities were similar on the landscape with steeper environmental gradients but significantly different on the landscape with gradual environmental gradients. However, communities distinguished using supervised methods were significantly less distinct and identifiable, worse at predicting species foliage cover and significantly more variable in species composition than those identified using un‐supervised methods.
Conclusions
Using supervised rather than un‐supervised approaches to distinguish plant communities can result in less recognisable communities, possibly reducing their usefulness for land management planning. Importantly, we found a large disparity between the two approaches in delineating compositional boundaries between communities on landscapes with gradual environmental gradients. This is particularly relevant to communities in biomes such as the savanna which comprises 20% of the Earth's landmass. Ecologists can be more confident using a supervised approach on landscapes with steep environmental gradients but should target landscapes with gradual environmental gradients for un‐supervised classification.
Supervised and un‐supervised classification approaches are common. We tested composition attributes of plant communities recognised by each approach in savanna vegetation, northeastern Australia. Communities from the un‐supervised approach were significantly more recognisable, identifiable and useful for land‐management planning. This was especially true for landscapes with broad environmental gradients. Understanding these implications is important when deciding which classification approach to use. Photograph by M.R. Newton
Growing evidence suggests that liana competition with trees is threatening the global carbon sink by slowing the recovery of forests following disturbance. A recent theory based on local and regional ...evidence further proposes that the competitive success of lianas over trees is driven by interactions between forest disturbance and climate. We present the first global assessment of liana–tree relative performance in response to forest disturbance and climate drivers. Using an unprecedented dataset, we analysed 651 vegetation samples representing 26,538 lianas and 82,802 trees from 556 unique locations worldwide, derived from 83 publications. Results show that lianas perform better relative to trees (increasing liana‐to‐tree ratio) when forests are disturbed, under warmer temperatures and lower precipitation and towards the tropical lowlands. We also found that lianas can be a critical factor hindering forest recovery in disturbed forests experiencing liana‐favourable climates, as chronosequence data show that high competitive success of lianas over trees can persist for decades following disturbances, especially when the annual mean temperature exceeds 27.8°C, precipitation is less than 1614 mm and climatic water deficit is more than 829 mm. These findings reveal that degraded tropical forests with environmental conditions favouring lianas are disproportionately more vulnerable to liana dominance and thus can potentially stall succession, with important implications for the global carbon sink, and hence should be the highest priority to consider for restoration management.
Des preuves de plus en plus nombreuses suggèrent que la competition entre lianes et les arbres menace le puits de carbone mondial en ralentissant la récupération des forêts après une perturbation. Une théorie récente, fondée sur des observations locales et régionales, propose en outre que le succès compétitif des lianes sur les arbres est dû aux interactions entre la perturbation forestière et le climat. Nous présentons la première évaluation mondiale de la performance relative des lianes par rapport aux arbres en réponse aux perturbations forestières et aux facteurs climatiques. En utilisant un ensemble de données sans précédent, nous avons analysé 651 échantillons de végétation représentant 26,538 lianes et 82,802 arbres, issus de 556 emplacements uniques dans le monde entier, tirés de 83 publications. Les résultats montrent que les lianes ont de meilleure performances par rapport aux arbres (augmentation du ratio liane‐arbre) lorsque les forêts sont perturbées, sous des zones chaudes aves précipitations faibles, et vers les basses altitudes tropicales. Nous avons également constaté que les lianes peuvent être un facteur critique entravant la récupération des forêts dans les forêts perturbées connaissant des climats favorables aux lianes, car les données de chronoséquence montrent que le succès compétitif élevé des lianes sur les arbres peut persister pendant des décennies après les perturbations, surtout lorsque la température annuelle moyenne dépasse 27.8°C, que les précipitations sont inférieures à 1614 mm et que le déficit hydrique climatique est supérieur à 829 mm. Ces découvertes révèlent que les forêts tropicales dégradées avec des conditions environnementales favorables aux lianes sont disproportionnellement plus vulnérables à la dominance des lianes, et peuvent ainsi potentiellement entraver la succession, avec d'importantes implications pour le puits de carbone mondial et devraient donc être la plus haute priorité à considérer pour la gestion de la restauration.
In a meta‐analysis, we use an unprecedented dataset, representing 556 unique locations worldwide, distributed across 44 countries and six continents to show for the first time that lianas (woody vines) thrive relatively better than trees when forests are disturbed, temperature increase, precipitation decrease, and particularly in tropical lowlands. We demonstrate that liana dominance can persist for decades post‐disturbance and hinder the recovery of disturbed forests, especially when climate favours lianas. With implications for the global carbon sink, our findings suggest that degraded tropical forests with environmental conditions favouring lianas should be the highest priority to consider for restoration management.
AIM: To examine variation in the phylogenetic diversity (PD) of tree communities across geographical and environmental gradients in Amazonia. LOCATION: Two hundred and eighty‐three c. 1 ha forest ...inventory plots from across Amazonia. METHODS: We evaluated PD as the total phylogenetic branch length across species in each plot (PDss), the mean pairwise phylogenetic distance between species (MPD), the mean nearest taxon distance (MNTD) and their equivalents standardized for species richness (ses.PDss, ses.MPD, ses.MNTD). We compared PD of tree communities growing (1) on substrates of varying geological age; and (2) in environments with varying ecophysiological barriers to growth and survival. RESULTS: PDss is strongly positively correlated with species richness (SR), whereas MNTD has a negative correlation. Communities on geologically young‐ and intermediate‐aged substrates (western and central Amazonia respectively) have the highest SR, and therefore the highest PDss and the lowest MNTD. We find that the youngest and oldest substrates (the latter on the Brazilian and Guiana Shields) have the highest ses.PDss and ses.MNTD. MPD and ses.MPD are strongly correlated with how evenly taxa are distributed among the three principal angiosperm clades and are both highest in western Amazonia. Meanwhile, seasonally dry tropical forest (SDTF) and forests on white sands have low PD, as evaluated by any metric. MAIN CONCLUSIONS: High ses.PDss and ses.MNTD reflect greater lineage diversity in communities. We suggest that high ses.PDss and ses.MNTD in western Amazonia results from its favourable, easy‐to‐colonize environment, whereas high values in the Brazilian and Guianan Shields may be due to accumulation of lineages over a longer period of time. White‐sand forests and SDTF are dominated by close relatives from fewer lineages, perhaps reflecting ecophysiological barriers that are difficult to surmount evolutionarily. Because MPD and ses.MPD do not reflect lineage diversity per se, we suggest that PDss, ses.PDss and ses.MNTD may be the most useful diversity metrics for setting large‐scale conservation priorities.
The image on the left-hand side shows a shoot sample burning on the flammability testing device. The right-hand side graph shows the post-hoc plot burnt biomass results for growth form (top) and ...family (bottom) with leaf dry matter content. The green squares represent low flammability groups and red squares represent high flammability groups.
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•Rainforest trees are higher in flammability compared to shrubs and vines.•Plant families Sapindaceae, Proteaceae, Fabaceae, and Lauraceae are especially high in flammability while Moraceae is very low.•Plant functional trait leaf dry matter content is significantly positively associated with flammability.•Restoration plantings in the Wet Tropics of Australia include large proportions of highly flammable plant species.
Canopy fires are increasing globally with anthropogenic climate and land-use changes, even in fire-sensitive rainforest ecosystems. Identifying the ecological drivers that may be aiding canopy fires, such as species or growth form flammability, is crucial to recognising and mitigating fire risks. To address this, we quantified the shoot-flammability of 124 rainforest plant species using an experimental approach. We compared three flammability measures (burnt biomass, total burn time and maximum temperature reached) with plant functional traits across seven different growth forms (i.e., canopy, pioneer, and understory trees; pioneer, understory and invasive shrubs, and vines) and nine common plant families and other higher-level clades, such as conifers, hereafter abbreviated to families. From burning > 600 sun-exposed shoots, we found trees were higher in flammability than shrubs and vines, and the plant families: Sapindaceae, Proteaceae, Fabaceae, and Lauraceae, had especially high flammability, whereas Moraceae was very low. Of the functional traits examined, leaf dry matter content was consistently and significantly positively associated with species flammability. Invasive shrubs as a group were not particularly flammable, although there were exceptions, e.g., wild tobacco (Solanum mauritianum) was highly flammable. This study has two important implications for the management of fire in rainforests. First, we have demonstrated that many tropical rainforest trees may readily burn under severe fire conditions if fire were to reach the rainforest canopy. Second, a large proportion of the > 1 million rainforest trees planted in the Wet Tropics under restoration planting schemes are from our most flammable rainforest plant families, as these families are often recommended for their carbon sequestration potential. Hence, these plantings may be highly vulnerable to fire and if planted along the borders of primary forest they may carry fire into their canopies. Therefore, where fire risk is high, we recommend planting species with low flammability along borders of plantings and forests to act as ‘green firebreaks’ to reduce the risk of fire incursions.
► Certification schemes offer sustainable agricultural products from the tropics. ► High Conservation Value concept provides insufficient protection to biodiversity. ► We discuss six critical areas ...for future research, which will benefit biodiversity. ► National-level development of sustainable agricultural and Biobanks are key. ► Understanding the costs of certification will attract more business support.
The expansion of agricultural plantations at the expense of forest drives dramatic losses of biodiversity and carbon. Consumers are now demanding sustainability in tropical agriculture and producers are responding with questionable certification standards. Many certification schemes—including those for oil palm, soy, sugar cane and cacao—rely upon the High Conservation Value (HCV) concept to prevent unacceptable losses of biodiversity to agricultural conversion. This concept protects very rare species or habitats, exceptional concentrations of wildlife, or large landscape-level areas of forest. Yet much biodiversity persists below these thresholds yielding the spectre of unsustainable conversion of forest to certified plantation crops under a green label. To meet more rigorous standards of sustainability, tropical plantations would have to retain large patches of native forests in the matrix. We highlight six critical areas in need of consideration by conservation scientists, practitioners and certification processes. In particular, the application of HCV to sustainable agricultural development at the national-level, the use of Imperata grasslands and abandoned agriculture, the creation of Biobanks, and increased price premiums for certified crops could redound to the long-term protection of tropical biodiversity.
The phylogenetic structure of communities can reveal forces shaping community assembly, but the vast majority of work on phylogenetic community structure has been conducted in mature ecosystems. ...Here, we present an analysis of the phylogenetic structure of three Neotropical rain forest communities undergoing succession. In each site, the net relatedness of the community is initially high and consistently declines during succession. This pattern is evident both when comparing plots of different age classes and when comparing stem size classes within each plot: the oldest plots and the youngest stem cohorts, representing the most advanced stages of succession, have the lowest relatedness. Our results suggest that succession leaves a distinct signature in the phylogenetic structure of communities, which may reflect an increasing role of biotic interactions in community assembly during succession. We discuss theoretical explanations for the decline in community phylogenetic relatedness during succession, and suggest directions for future study.
We used data from a long-term (14–18 years) demographic study to infer the maximum longevity for populations of 93 relatively abundant tree species in central Amazonia. We also assessed the influence ...of several life-history features (wood density, growth form, mortality rate, recruitment rate, stem diameter, growth increment, population density) on tree longevity. Data on 3159 individual trees were collected in 24 permanent, 1
ha plots in undisturbed forest arrayed across a large (ca. 1000
km
2) study area. For each species, three estimates of longevity were generated (by dividing the stem diameter of the largest tree by the median, upper quartile, and upper decile of observed diameter-growth rates), and the mean of these three values was used as a longevity estimate. Longevity values ranged from 48 years in the pioneer
Pourouma bicolor (Cecropiaceae) to 981 years for the canopy tree
Pouteria manaosensis (Sapotaceae), with an overall mean of 336±196 years. These growth-based estimates of maximum tree age were concordant with those derived from analyses of mean mortality rates. Tree longevity was positively correlated with wood density, maximum stem diameter, and population density, and negatively correlated with annual mortality, recruitment, and growth rates. On average, pioneer species had much lower longevity than did non-pioneers, whereas among old-growth trees, emergent species had greater longevity than did canopy species. Our results are consistent with radiocarbon-based studies that suggest that Amazonian trees can occasionally exceed 1000 years of age.
In the lowland tropical rain forest of central Amazonia, I investigated the responses of understory bird communities to forest edges bordering a narrow (<40 m width), unpaved road. From 3681 mist-net ...captures, bird abundance and composition were examined at three distances (10, 70, and 130 m) from forest edge. The degree to which regrowth vegetation along forest borders reduced edge effects was also assessed by contrasting bird communities near roads with varying levels of regrowth. Forests near road margins had reduced canopy cover and height and higher numbers of fallen trees than did forest interiors. Bird communities near forest edges also differed markedly from those in forest interiors. Total bird captures and captures of insectivores increased with distance from edge, whereas captures of frugivores and nectarivores did not vary significantly. Mature regrowth on edges significantly increased the captures of insectivores. When insectivorous birds were divided into feeding guilds, birds in six of eight guilds showed significant responses to distance from edge; five of these had fewer captures near road margins whereas one guild (edge/gap specialists) had higher captures. For most guilds, significant alterations in abundance and species composition were evident within 10-70 m of road margins. My results suggest that the majority of Amazonian understory birds respond negatively to artificial edges created by a narrow, unpaved road. Forest regrowth along road margins alleviated negative edge effects for only a limited subset of forest-dependent species.
Much of the lowland tropical forests in the Solomon Islands have been heavily logged. However, little is known about the recovery status of these forests. We examined factors that influenced the ...recovery of forest structural attributes within 50 years after selective logging on Kolombangara Island in the western Solomon Islands. Twelve study sites—six logged and six unlogged—were identified across the Island, with two logged sites in each of three recovery-time classes: 10, 30, and 50 years after logging. Within each study site, 12 0.1-ha plots were randomly established, and a series of forest attributes measured in each plot. Our results revealed that local logging intensity and soil attributes have stronger influence on forest-structural recovery than do site attributes such as local topography or tree architecture. Furthermore, half a century of regeneration following logging is insufficient to permit full recovery of forest structure. We conclude that logged forests on Kolombangara and possibly across the Solomon Islands may not fully recover structurally before the next logging cycle, in the absence of a policy on re-entry harvesting. The development of such a policy coupled with robust forest-management measures is pivotal to facilitating sustainable logging while supporting biodiversity conservation in the Solomon Islands. This may be the last best hope for saving lowland forests and their biodiversity on this unique tropical archipelago.
► We identify policies to improve forest conservation in Papua New Guinea (PNG). ► PNG should reduce its annual timber harvests and curtail raw-log exports. ► It should allow traditional communities ...to sue corporations for environmental damage. ► PNG should expand and better manage its meager network of protected areas. ► The country must stabilize its administration of international carbon-trading funds.
The native forests of Papua New Guinea (PNG) are some of the most biologically and culturally diverse ecosystems on Earth, and are a major stock of terrestrial carbon. These forests are being altered at a rapid rate, especially by industrial logging and swidden farming. Wildfires, expanding oil palm plantations, and industrial mining are also causing forest loss. At current rates of exploitation, most industrially accessible forests in PNG are projected to be logged or cleared in just 1–2 decades. Much of the country’s timber is exported as raw logs, mainly to China, providing only limited income and employment for local communities. Despite growing exploitation of its natural resources, many social indicators in PNG, such as per-capita income, literacy, and the Human Development Index, remain alarmingly low. We highlight a range of near-term policy measures designed to improve forest conservation and sustainability in PNG. These include: (1) reducing annual timber harvests to a more-sustainable level, (2) curtailing raw-log exports while promoting expansion of domestic wood-processing industries, (3) combating serious corruption in the forestry sector, (4) reinstating the rights of traditional communities in PNG to sue corporations for past or anticipated environmental damage, (5) expanding and improving the management of protected areas, and (6) stabilizing the administration of international carbon-trading funds designed to reduce emissions from forest conversion. We also recommend that educational and family-planning support be improved for younger women to help reduce the destabilizing effects of rapid population growth. These measures will help to reduce environmental damage and slow the exhaustion of timber supplies, while improving societal benefits for PNG citizens.
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