Human impacts in pine forests: past, present, and future

Pines (genus Pinus) form the dominant tree cover over large parts of the Northern Hemisphere. Human activities have affected the distribution, composition, and structure of pine forests for millennia. Different human-mediated factors have affected different pine species in different ways in different regions. The most important factors affecting pine forests are altered fire regimes, altered grazing/browsing regimes, various harvesting/construction activities, land clearance and abandonment, purposeful planting and other manipulations of natural ecosystems, alteration of biotas through species reshuffling, and pollution. These changes are occurring against a backdrop of natural and anthropogenically driven climate change. We review past and current influence of humans in pine forests, seeking broad generalizations. These insights are combined with perspectives from paleoecology to suggest probable trajectories in the face of escalating human pressure. The immense scale of impacts and the complex synergies between agents of change calls for urgent and multifaceted action.Centre of Excellence for Invasion Biolog

Comparative patterns of phenology and growth form diversity in two winter rainfall deserts: The Succulent Karoo and Mojave Desert ecosystems

A comparative study of community structure and seasonal growth dynamics in the arid winter rainfall regions of the Succulent Karoo in South Africa and the Mojave Desert of the United States suggests that remarkably divergent patterns of resource use and resultant growth form diversity exist in regions with outwardly similar climatic regimes. An understanding of these divergent patterns in the two winter rainfall deserts allows predictions to be made on vegetation response to global change. Above-ground plant growth in the Succulent Karoo begins with the first significant rains in late summer and continues through winter because moderate minimum temperatures allow continued growth. These communities have low structural diversity above-ground, but also below-ground, where root systems commonly do not exceed 20 cm in depth. These shallow root systems harvest water from upper soil horizons soon after rain falls, and growth declines as rainfall decreases in late spring. In contrast, low temperatures during winter inhibit growth in the Mojave Desert until early spring at a time when a mean 74% of the hydrologic year precipitation (July-June) has already occurred. Thus species in this structurally diverse system rely on deeper stores of water for growth in spring and early summer. A global change scenario of a 2 to 4 °C increase in mean annual temperature and increased summer rainfall in the Mojave desert would be expected to produce similar conditions in the Mojave Desert to those that exist in the Succulent Karoo today. Assuming no genetic constraints on phenotypic plasticity, this would suggest increased species diversity and a decline in structural diversity in the Mojave Desert over evolutionary time. Increased summer rainfall in the Succulent Karoo would be expected to lead to invasions of grasses and thus increased competitive pressure reducing community diversity.Articl

Ecological and phylogenetic patterns of carbon isotope discrimination in the winter-rainfall flora of the Richtersveld, South Africa

Leaf tissues of vascular plant species from the arid Richtersveld of northern Namaqualand (Succulent Karoo), South Africa, show a great range of variation in carbon isotope discrimination. Analyses of A values for species with obligate C3 metabolism demonstrated that the water use efficiency indicated by these values varies between species and within a species population both between seedlings and mature shrubs and between sites along an aridity gradient. An expected pattern of increasing water use efficiency with greater longevity in these species was not present, however. Only among ephemerals were there taxa with high Δ values associated with low water use efficiency, but these high values were balanced by other ephemeral species with low Δ values comparable to those typical of moderate to long-lived shrubs. The extent to which putative CAM species in the succulent flora rely entirely on nocturnal carbon fixation is highly variable. Species of Crassulaceae and Aloe (Asphodelaceae) relied almost exclusively on CAM photosynthesis, although Crassulaceae from more mesic environments may be CAM-flexible. Succulent-leaved species of Senecio and leaves of stem-succulent Euphorbia were CAM-flexible in their Δ values, while shrubby species of Ceraria (Portulacaceae) appeared to rely primarily on C3 photosynthesis. Variable patterns of reliance on CAM photosynthesis were present in the ecologically dominant Mesembryanthema of the Aizoaceae. Shrubby species of the subfamily Mesembryanthemoideae, although capable of CAM-flexibility in less xeric habitats, relied largely on CAM fixation of carbon in our field sites. Within the subfamily Ruschioideae, there was a wide range of A values indicating species with CAM, CAM flexibility, and primary reliance on C3 photosynthesis. Low Δ values indicative of typical CAM fixation were associated with species with greater longevity, suggesting that a positive correlation may exist between the degree of reliance on CAM photosynthesis and plant life span.Articl

Tree invasions into treeless areas: mechanisms and ecosystem processes

Non-native tree invasions occur not only in woodland or forest vegetation, but also into areas with little or no native tree presence. Limiting factors for tree establishment and survival include seasonal or annual drought, low nutrient availability, cold temperature extremes, fire, and other abiotic conditions to which trees are poorly adapted as well as biotic conditions such as herbivory and lack of soil mutualist inoculum. Tree invasions of grasslands and semi-arid riparian areas in particular are now widespread and frequently result in the rapid conversion of these habitats to woodlands or forests. In some cases, these invasions are the result of a change in extrinsic conditions such as climate, fire, and/or grazing that remove what have been previous barriers to tree establishment. However, in other cases, tree species with particular life-history and dispersal traits fill open niches or outcompete native species. Significant examples of tree invasion into treeless areas can be seen with invasions of Pinus species into temperate grasslands and fynbos shrublands, Melaleuca quinquenervia and Triadica sebifera into grassy wetlands, Prosopis and Tamarix species into semi-arid riparian zones, and Acacia and Morella invasions into nutrient-poor shrublands and barrens. The establishment of trees into treeless areas may have strong impacts on ecosystem processes, influencing biogeochemical cycling, carbon sequestration and cycling, and ecohydrology, as well possible edaphic legacies that persist even if trees are removed

Namaqualand, South Africa - An overview of a unique winter-rainfall desert ecosystem

Namaqualand is a winter-rainfall desert of some 50 000 km2, located in north-western South Africa. For a desert ecosystem, the region is characterized by a unique selective regime, namely highly predictable annual rainfall and a moderate temperature regime throughout the year. This selective regime is responsible for the unique plant ecological features of Namaqualand. These include: the dominance of communities by dwarf to low, shallow-rooted, short-lived and drought-sensitive leaf succulent shrubs; high abundance and diversity of geophytes; winter growth phenology; regular and spectacular spring flowering displays; rapid population turnover of perennials; weak competitive interactions; and high local and regional plant diversity. This paper provides a physiographical, biogeographical and ecological overview as background to this special issue. First, we outline the physiography and climate of Namaqualand, emphasizing unique components of its selective regime. Next we discuss the region's phytogeographical and floristic characteristics. Then we summarize what is known about plant form and function, focusing on growth form mix, phenology, carbon gain, water relations, and reproduction in vegetation dominated by leaf-succulent shrubs. Finally, we interpret Namaqualand's uniquely rich and dynamic plant communities in terms of plant functional processes. These plant ecological patterns and processes are discussed in terms of the predictions of a simple conceptual model for succulent-rich communities.Articl