Showing papers in "Perspectives in Plant Ecology Evolution and Systematics in 2003"

Tropical forest recovery: legacies of human impact and natural disturbances

Abstract: Land-use history interacts with natural forces to influence the severity of disturbance events and the rate and nature of recovery processes in tropical forests. Although we are far from an integrated view of forest recovery processes, some generalizations can be made. Recovery of forest structure and composition is relatively rapid following disturbances that primarily impact forest canopies, such as hurricanes. Recovery is considerably slower following disturbances that heavily impact soils as well as aboveground vegetation, such as bulldozing, heavy or long-term grazing, and severe fires, often with long-lasting effects on species composition. The landscape matrix plays a critical role in local recovery processes. Proximity of disturbed areas to remnant forest patches promotes more rapid recovery, which depends heavily on seed dispersal. Recovery of aboveground biomass is constrained by soil fertility and texture across regions as well as across soil types within a region. Restoration of soil fertility may be a prerequisite for forest recovery on sites with severely degraded soils. Despite evidence of rapid forest recovery following large-scale deforestation, many degraded areas of today’s tropics will require human assistance to recover forest structure, species composition, and species interactions typical of mature tropical forests.

Plants in a warmer world

Abstract: Climate is a major determinant for the phenology, physiology, distribution and interactions of plants. The world's recent climate has shown a substantial increase in average temperature which is changing these processes in a perceptible way. The following review compiles and discusses studies reporting recently observed changes in the behaviour, ranges and interactions of species which are thought to be associated with climate change. The multitude of recently published studies providing evidence for the ecological impacts of climate change on many different continents strongly suggests that the last 30 years of warmer temperatures have had a substantial influence on both seasonal patterns, and altitudinal and poleward shifts in vegetation. Common features of change, but also some discrepancies in the response of plants to climate change, are discussed, as well as implications for biodiversity, higher level impacts on community structure and trophic interactions, and some ecosystem consequences.

Interplate dispersal paths for megathermal angiosperms

Abstract: The dispersal of megathermal angiosperms between tectonic plates is reviewed on the basis of fossil evidence for the Cretaceous and Tertiary periods, since the radiation of the angiosperms, and the period of break-up of Gondwana. The combination of tectonic plate disassembly and redistribution, coupled with phases of global warming followed by pronounced cooling, has resulted in the formation of intermittent dispersal opportunities for frost-intolerant plants, and has been a major factor in determining the direction of angiosperm diversification. The Early Cretaceous radiation of angiosperms seems to show little relationship to the formation of Tethys. However, for the Late Cretaceous and Tertiary nine relevant dispersal routes can be differentiated that can be divided into two distinct categories: routes which formed following the break-up of Gondwana during the Late Cretaceous and Earlier Tertiary, when warm climates encouraged dispersal of megathermal elements globally, and routes which formed since the Middle Eocene, following phases of plate collision, as global climates were cooling down, inhibiting such dispersal. Most inter-plate dispersal of megathermal angiosperms took place in the Late Cretaceous and Early Tertiary at a time when global climates were markedly different from those of today, and the global area of megathermal vegetation several times greater than at present. Under such a scenario, it is likely than opportunities for speciation were much higher than for present-day megathermal plants.

The myriad consequences of hunting for vertebrates and plants in tropical forests

Abstract: Humans hunt forest vertebrates throughout the tropics. Many preferred game species consume flowers, fruit, seeds and/or leaves, and these interactions will cause their harvest to ramify through forests. Three related issues will determine how severely the harvest of forest vertebrates influences the plant community. First, the types of species selected by hunters and the intensity of the harvest will determine which vertebrates are removed and which remain. Second, the possible presence of ecologically similar, non-game species able to expand their activities to fulfill the ecological role of heavily exploited species will determine how severely the harvest disrupts ecological relationships between the community of forest vertebrates and the community of forest plants. Finally, hunters will alter plant species composition if the harvest of vertebrates differentially affects mutualists or pests of particular plant species. Hunters will also alter plant diversity if the harvest of vertebrates disrupts ecological mechanisms that permit plant species to coexist. I examine hunter selectivity, the intensity of the hunt, possible compensation by non-game species, and the types and strengths of interactions among game species and plants for tropical forests to determine when and where these outcomes occur.

Vegetation and soil and plant chemistry on ultramafic rocks in the tropical Far East

Abstract: The tropical Far East has many outcrops of ultramafic rock including very large areas in Sulawesi (c. 8000 km2) and New Caledonia (c. 5500 km2). The outcrops occur under several different climates, and give rise to a range of soils, the characteristics of which are reviewed. The vegetation on them is very varied. Under the same climate one can find grassland, scrub, and both short and tall rain forests. The variation in species richness on the ultramafics is difficult to explain. The degree of endemism varies too; it is probably less dependent on soil characteristics than on historical factors. The causes of the various unusual types of vegetation on ultramafic outcrops are discussed. It is possible that the somewhat dwarfed forests result from a shortage of one or more major nutrients or from very high soil Mg/Ca quotients or high Ni concentrations. The distinct ‘maquis’ vegetation of New Caledonia, and probably ultramafic scrub elsewhere, has evolved in relation to not only the soil chemical factors just listed but also periodic fire and varying degrees of drought. Fires are certainly more important than was once thought and the adverse soil factors may have a role in delaying recolonisation. The plant chemistry is notable for the presence of species which hyperaccumulate certain elements, notably Ni. This phenomenon is discussed in relation to its ecological importance, which may be protection of the hyperaccumulators against herbivores. The need for a conservation policy for the ultramafic areas is stressed, and mention is made of the restoration work on sites damaged by nickel mining in New Caledonia.

Variation in tropical forest growth rates: combined effects of functional group composition and resource availability

Abstract: Rates of tree growth in tropical forests reflect variation in life history strategies, contribute to the determination of species' distributional limits, set limits to timber harvesting and control the carbon balance of the stands. Here, we review the resources that limit tree growth at different temporal and spatial scales, and the different growth rates and responses of functional groups defined on the basis of regeneration strategy, maximum size, and species' associations with particular edaphic and climatic conditions. Variation in soil water availability determines intra- and inter-annual patterns of growth within seasonal forests, whereas irradiance may have a more important role in aseasonal forests. Nutrient supply limits growth rates in montane forests and may determine spatial variation in growth of individual species in lowland forests. However, its role in determining spatial variation in stand-level growth rates is unclear. In terms of growth rate, we propose a functional classification of tropical tree species which contrasts inherently fast-growing, responsive species (pioneer, large-statured species), from slow-growing species that are less responsive to increasing resource availability (shade-bearers, small-statured species). In a semi-deciduous forest in Ghana, pioneers associated with high-rainfall forests with less fertile soils, had significantly lower growth rates than pioneers that are more abundant in low-rainfall forests with more fertile soils. These results match patterns found in seedling trials and suggest for pioneers that species' associations with particular environmental conditions are useful indicators of maximum growth rate. The effects of variation in resource availability and of inherent differences between species on stand-level patterns of growth will not be independent if the functional group composition of tropical forests varies along resource gradients. We find that there is increasing evidence of such spatial shifts at both small and large scales in tropical forests. Quantifying these gradients is important for understanding spatial patterns in forest growth rates.

Does artificial snow production affect soil and vegetation of ski pistes? A review

Abstract: The production of artificial snow and the use of snow additives in ski resorts have increased considerably during the last 20 years. Their ecological consequences are the subject of environmental concerns. This review compiles studies about the ecological implications of ski pistes preparation in general and of artificial snow production. The main direct impacts of ski piste preparation on the vegetation are related to the compaction of the snow cover, namely the induction of soil frost, the formation of ice layers, mechanical damage and a delay in plant development. The vegetation reacts with changes in species composition and a decrease in biodiversity. Artificial snowing modifies some of these impacts: The soil frost is mitigated due to an increased insulation of the snowpack, whereas the formation of ice layers is not considerably changed. The mechanical impacts of snow-grooming vehicles are mitigated due to the deeper snow cover. The delay of the vegetation development is enhanced by a considerably postponed snowmelt. Furthermore, artificial snowing induces new impacts to the alpine environment. Snowing increases the input of water and ions to ski pistes, which can have a fertilising effect and hence change the plant species composition. Increasingly, snow additives, made of potentially phytopathogenic bacteria, are used for snow production. They enhance ice crystal formation due to their ice nucleation activity. Although sterilised, additives affected the growth of some alpine plant species in laboratory experiments. Salts are applied not only but preferably on snowed pistes to improve the snow quality for ski races. The environmental impacts of most salts have not yet been investigated, but a commonly used nitrate salt has intense fertilising properties. Although snowing mitigates some of the negative impacts of ski piste preparation in general, new impacts induced by snowing could be non-beneficial to the vegetation, which, however, has yet to be clarified.

Floristic zonation of tree communities on wet tropical mountains revisited

Abstract: The altitudes of the ecotones between the main vegetation-zones up mountains of the humid tropics are reviewed, with emphasis on floristics and those areas with which the author is personally familiar. A detailed comparison is made of the zonation on Mt Kinabalu (c. 6°N) and the Himalaya of Bhutan (c. 27°N), and is supported by information from other parts of SE Asia, and to a decreasing extent South America and Africa. The relationship between forest-types based on structure and physiognomy and forest-types based on floristics is complicated. In SE Asia lower montane forests may be dominated by dipterocarps or an ‘oak-laurel’ assemblage. The dominant leaf size in both is notophyll, but the oak-laurel assemblage shifts to the microphyll size-class in China where the transition from tropical lower montane to warm temperate lowland forest begins. The opposite trend is seen where the lower montane flora of Papua New Guinea corresponds to the lowland flora in NE Australia. There are only modest changes between the Equator and the Tropic in the altitudes at which major transitions occur. It is concluded that soil changes, mediated only in part by temperature and associated climatic factors, may play a more direct and diverse role than has previously been indicated. The soil factors emphasized are the increase in organic matter content in lower montane soils as opposed to lowland, accompanied by a change from termite-dominance to earthworm-dominance, and the frequent occurrence of a dense moss layer and mor humus in upper montane soils. The case is made particularly by reference to east Asian mountains, which have been the most studied.

Seed paternity in flowering plants: an evolutionary perspective

Abstract: The ultimate importance of paternal contributions to fitness and of post-pollination selection in flowering plants have remained elusive, largely because of the technical difficulty of assigning paternity. I review empirical studies that use heritable markers to investigate per-fruit seed paternity in natural populations and after experimental multiple-donor pollination. Thirty-one studies covering 23 species from 16 plant families document that in natural populations seeds from a single fruit are often fathered by multiple pollen donors (5 species from 5 families), that donors can differ significantly in seed-siring success (8 species from 6 families), that variation in pollen tube growth rates can be heritable (n = 1 out of 4 studies), that donor and recipient genotypes can simultaneously affect paternity (n = 2), and that temporal order of pollen deposition (n = 1) and environmental effects(n = 2) affect the outcome of pollen competition. These studies also investigate the role of post-pollination selection in the avoidance of inbreeding and for species boundaries. Most studies of male reproductive success in plants to date base on isozyme electrophoresis. The availability in the last decade of highly polymorphic molecular markers such as microsatellite DNA has been expected to open new possibilities to investigate competition and selection during the gametophytic phase. Yet, to date, there is still need for greater data wealth on seed paternity to test theories of sex allocation and to gain deeper understanding of floral trait evolution and of the evolutionary consequences of post-pollination selection in flowering plants.

The ecology, silviculture and biogeography of mahogany (Swietenia macrophylla): a critical review of the evidence

Abstract: Recent studies of Swietenia macrophylla King argue that natural regeneration of this highly valued tree is episodic and occurs only after infrequent catastrophic disturbance. An analysis of 14 population size-distributions in natural forests shows that population structure varies across this species' range. In forests that are in successional transition from open deciduous forest to evergreen forest there is often prolific regeneration. We show that in this type of forest in southern Para (Brazil) patches of even-sized trees are small. Large-scale disturbance may be necessary for regeneration only in evergreen forests where little light penetrates through the canopy. The transition zone between evergreen and deciduous forest in Para is moving south, and we propose that mahogany populations to the north are in the process of competitive exclusion. Sustainable harvesting of mahogany must take account of the successional stage of the forest.

Interpreting some outstanding features of the flora and vegetation of Madagascar

Abstract: Six features are covered. (1) The high endemism, which is not discussed in detail, is all-pervasive, and has resulted from the isolation of Madagascar from Africa some 125 million years ago and their present separation by 430 km. (2) The great richness in plant species (especially relative to Africa), seen particularly in the families of woody species in the wetter vegetation-types, involves both sympatry and allopatry within genera, and is explicable in terms of much less extreme drying out than in Africa during the Pleistocene and effective ‘species-pumping’ rather than mass extinctions during that period. (3) The abundance and species-richness of palms, pandans, tree-ferns, bamboos, and certain families of dicot trees (notably Lauraceae, Monimiaceae, Myrsinaceae and Myristicaceae) in the lowland rain forests also appears to be a result of both past and present wetness of the climate, while it is hypothesized that the low stature of most lowland rain forests, paucity of large-girth trees, and small size and sparsity of broad-leaved herbs, are a result of most rain forest soils being old and relatively nutrient-poor. (4) Within the dry evergreen forest region where rainfall is moderate (900–1600 mm yr −1 ) a sub-set of trees with fire-resistant bark seems to have evolved at sites prone to frequent ground fires, some perhaps spreading out of adjacent palm savanna on seasonally flooded sites. (5) Both the evolution of thicket rather than grassy woodland in the driest areas (300–600 mm yr −1 ), and the abundance of evergreen trees and shrubs on ordinary soils – not confined to run-on sites – are explicable in terms of there being a finite chance of rain throughout the year rather than one short wet season, coupled with relatively high values for air humidity throughout the year. The same factors probably explain the abundance and variety of succulents in the thicket; they are found throughout and not just on rocks. (6) Concerning physical defence against herbivores, the rain forests, dry evergreen forests and deciduous forests all show a complete lack of plants with physiognomic features plausibly related to browsing by extinct giant birds (a strong contrast with New Zealand), but in the semi-deciduous thicket there are many tiny-leaved, mostly non-spiny shrubs and small trees, whose dense branching and impenetrability have plausibly evolved as a defence against browsing by elephant birds. The Didiereaceae of the thicket are spiny (unlike members of the same family in Africa), and are giant analogues of the ‘ocotillo’ ( Fouquieria splendens ) in western North America rather than of Cactaceae; their spines appear to be protecting the leaves more than the stems against arboreal primates, spine length paralleling leaf length.

Reproductive modes in the epixylic hepatic Anastrophyllum hellerianum

Abstract: The frequency and dynamics of sexual and asexual reproduction were investigated in a dioecious epixylic hepatic, Anastrophyllum hellerianum , which has declined in recent decades in Finland as a consequence of forestry practices. In our investigation asexual reproduction by gemmae was the dominant mode of reproduction and specialised gemmiparous shoots were present in all colonies studied. The proportions of dead shoots were considerably higher among sex-expressing than among non-sexexpressing shoots. Our results suggest that lower reproductive investment is required for asexual than for sexual reproduction. For instance, no trade-off is detected between asexual reproduction and survival of the gemmiparous shoots in A. hellerianum . Sexual reproduction occurred only in 12% of the colonies and it was promoted by the following factors: medium shoot density, high proportion of sex-expressing shoots, an even sex ratio and very short distances between individuals representing opposite sexes. The ratio of dead males to dead females was significantly female-biased, which suggests higher mortality among female shoots. At the level of individual shoots, more spores than gemmae were produced. However, as a consequence of the low frequency of sporophyte-bearing shoots, gemma production highly exceeded spore production at the colony level. Furthermore, cultivation tests of the propagules showed that gemmae germinate faster than spores.

Herbivore-deterring secondary compounds in heterophyllous woody species of the Mascarene Islands

Abstract: Heterophylly (juvenile-adult leaf dimorphism) is widespread among native woody species of the Mascarene Islands, but the causes to this phenomenon have so far not been fully explained. The absence of mammals and dominance of now extinct giant tortoises and flightless birds are characteristics of the original Mascarene fauna. The present study investigates the hypothesis that the distinct morphology and colouration of juvenile leaves signalled unpalatability to browsing giant tortoises or birds. Juvenile and adult leaves of 28 heterophyllous woody species endemic to the islands of Mauritius and Rodrigues were screened for possible chemical defences against herbivory. The screening comprised the following classes of secondary compounds: alkaloids, cyanogenic constituents, saponins, simple phenolics, tannins and anthocyanins. The screening showed that there are no consistent significant differences between juvenile and adult leaves regarding any of the studied secondary compound classes, with the exception of the level of anthocyanins, which was significantly higher in juvenile leaves. This difference was also clearly reflected in a generally high degree of red patterning of juvenile leaves. Based on comparisons with floras of other oceanic islands once inhabited by tortoises and birds, and an evaluation of feeding habits of these herbivores, we suggest that the reduced lamina and partly red colouration of juvenile leaves may be an evolutionary response to selective herbivory by birds.

Analysis of biodiversity across levels of biological organization: a problem of defining traits

Abstract: Biodiversity is a term that comprises the appearance, structure and function of all levels of biological organization, including genes, species and ecosystems. The vast majority of measures of biodiversity (usually termed ‘diversity indices’) considers only number, proportion and distribution of species which belong to a specified group and exist in a defined area or ecosystem. Genetic diversity as a part of biodiversity within species (or populations) was either not regarded in this respect or was treated (by geneticists) as a separate entity of diversity quantified with separate measures. Little attention has been given to the integration of both types of diversity, within and among species, in a single measurement (termed ‘transspecific’ diversity). In order to attain this integration on a general basis, an operational trait concept is developed which allows the determination of variation in traits observable in members not only of the same species but also of different species. The concept rests on methods of investigation that can be adapted to a broader range of organisms without modification of their characteristics. Once a trait is specified on this basis, any meaningful measure of diversity can be applied to assess biodiversity across levels of biological organization. The utility of the concept is demonstrated by application to the results of an earlier study on associations between species and genetic diversity in a forest tree community. Attributes of isozymes which are visible in electrophoresis are used as a transspecific genetic trait.

Factors controlling resource allocation in mountain birch

Abstract: We present a comprehensive analysis of factors affecting resource allocation and crown formation in a subarctic birch tree, Betula pubescens ssp. czerepanovii (Orlova) Hamet-Ahti. Using biomass measurements and digitized data on tree architecture, we investigated several hypotheses on various factors that may modify plant growth. We also analyzed the extent to which different mechanisms operate at different scales, ranging from individual shoots to the whole branches or trees. Different factors affected allocation at different levels of organization. Stem age had a minor effect, suggesting that similar control mechanisms operate at all stages of development. Fates of individual shoots were affected by their local growing conditions as indicated, for example, by the dependence of long shoot production on light. Buds formed in the current long shoots were likely to become new long shoots. In the innermost crown parts, radial growth had priority compared to long shoot production. Elongation of individual long shoots was controlled by two conflicting factors. Long distance from the roots suppressed growth, probably indicating costs associated with resource transportation, whereas a high level of light augmented growth. In contrast, growth of entire branches was not so clearly related to the availability of resources, but showed limitation due to allometric scaling. This set a relationship between the maximum long shoot number and the overall branch size, and may indicate allometric constraints to the way a tree is constructed. Strict allometric relationships existed also between other structural traits of mountain birch, most of them similar at all levels of branching hierarchy. However, despite the upper level restrictions set by allometry, source-sink interactions and localized responses of individual shoots operated as local processes that directed allocation towards the most favourable positions. This may be a mechanism for achieving efficient tree architecture in terms of resource intake and costs of transportation.