Predicting which species are probable invaders has been a long-standing goal of ecologists, but only recently have quantitative methods been used to achieve such a goal. Although restricted to few taxa, these studies reveal clear relationships between the characteristics of releases and the species involved, and the successful establishment and spread of invaders. For example, the probability of bird establishment increases with the number of individuals released and the number of release events. Also, the probability of plant invasiveness increases if the species has a history of invasion and reproduces vegetatively. These promising quantitative approaches should be more widely applied to allow us to predict patterns of invading species more successfully.

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Progress in invasion biology: predicting invaders

Understanding and predicting ecosystem functioning (e.g., carbon and water fluxes) and the role of soils in carbon storage requires an accurate assessment of plant rooting distributions. Here, in a comprehensive literature synthesis, we analyze rooting patterns for terrestrial biomes and compare distributions for various plant functional groups. We compiled a database of 250 root studies, subdividing suitable results into 11 biomes, and fitted the depth coefficient β to the data for each biome (Gale and Grigal 1987). β is a simple numerical index of rooting distribution based on the asymptotic equation Y=1-βd, where d = depth and Y = the proportion of roots from the surface to depth d. High values of β correspond to a greater proportion of roots with depth. Tundra, boreal forest, and temperate grasslands showed the shallowest rooting profiles (β=0.913, 0.943, and 0.943, respectively), with 80-90% of roots in the top 30 cm of soil; deserts and temperate coniferous forests showed the deepest profiles (β=0.975 and 0.976, respectively) and had only 50% of their roots in the upper 30 cm. Standing root biomass varied by over an order of magnitude across biomes, from approximately 0.2 to 5 kg m-2. Tropical evergreen forests had the highest root biomass (5 kg m-2), but other forest biomes and sclerophyllous shrublands were of similar magnitude. Root biomass for croplands, deserts, tundra and grasslands was below 1.5 kg m-2. Root/shoot (R/S) ratios were highest for tundra, grasslands, and cold deserts (ranging from 4 to 7); forest ecosystems and croplands had the lowest R/S ratios (approximately 0.1 to 0.5). Comparing data across biomes for plant functional groups, grasses had 44% of their roots in the top 10 cm of soil. (β=0.952), while shrubs had only 21% in the same depth increment (β=0.978). The rooting distribution of all temperate and tropical trees was β=0.970 with 26% of roots in the top 10 cm and 60% in the top 30 cm. Overall, the globally averaged root distribution for all ecosystems was β=0.966 (r 2=0.89) with approximately 30%, 50%, and 75% of roots in the top 10 cm, 20 cm, and 40 cm, respectively. We discuss the merits and possible shortcomings of our analysis in the context of root biomass and root functioning.

A global analysis of root distributions for terrestrial biomes

Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geographically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.

The Ecology of Interfaces: Riparian Zones

Savannas occur where trees and grasses interact to create a biome that is neither grassland nor forest. Woody and gramineous plants interact by many mechanisms, some negative (competition) and some positive (facilitation). The strength and sign of the interaction varies in both time and space, allowing a rich array of possible outcomes but no universal predictive model. Simple models of coexistence of trees and grasses, based on separation in rooting depth, are theoretically and experimentally inadequate. Explanation of the widely observed increase in tree biomass following introduction of commercial ranching into savannas requires inclusion of interactions among browsers, grazers, and fires, and their effects on tree recruitment. Prediction of the consequences of manipulating tree biomass through clearing further requires an understanding of how trees modify light, water, and nutrient environments of grasses. Understanding the nature of coexistence between trees and grass, which under other circumstances...

Tree-grass interactions in Savannas

There has been a dramatic growth in research on biological invasions over the past 20 years, but a mature understanding of the field has been hampered because invasion biologists concerned with different taxa and different environments have largely adopted different model frameworks for the invasion process, resulting in a confusing range of concepts, terms and definitions. In this review, we propose a unified framework for biological invasions that reconciles and integrates the key features of the most commonly used invasion frameworks into a single conceptual model that can be applied to all human-mediated invasions. The unified framework combines previous stage-based and barrier models, and provides a terminology and categorisation for populations at different points in the invasion process.

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A proposed unified framework for biological invasions.

. Much confusion exists in the English-language literature on plant invasions concerning the terms ‘naturalized’ and ‘invasive’ and their associated concepts. Several authors have used these terms in proposing schemes for conceptualizing the sequence of events from introduction to invasion, but often imprecisely, erroneously or in contradictory ways. This greatly complicates the formulation of robust generalizations in invasion ecology.

Based on an extensive and critical survey of the literature we defined a minimum set of key terms related to a graphic scheme which conceptualizes the naturalization/invasion process. Introduction means that the plant (or its propagule) has been transported by humans across a major geographical barrier. Naturalization starts when abiotic and biotic barriers to survival are surmounted and when various barriers to regular reproduction are overcome. Invasion further requires that introduced plants produce reproductive offspring in areas distant from sites of introduction (approximate scales: > 100 m over  6 m/3 years for taxa spreading by roots, rhizomes, stolons or creeping stems). Taxa that can cope with the abiotic environment and biota in the general area may invade disturbed, seminatural communities. Invasion of successionally mature, undisturbed communities usually requires that the alien taxon overcomes a different category of barriers.

We propose that the term ‘invasive’ should be used without any inference to environmental or economic impact. Terms like ‘pests’ and ‘weeds’ are suitable labels for the 50–80% of invaders that have harmful effects. About 10% of invasive plants that change the character, condition, form, or nature of ecosystems over substantial areas may be termed ‘transformers’.

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Naturalization and invasion of alien plants: concepts and definitions

Terrestrial plant ecology , Terrestrial plant ecology , مرکز فناوری اطلاعات و اطلاع رسانی کشاورزی

Terrestrial Plant Ecology

Sierra Nevada forests have high understory species richness yet we do not know which site factors influence herb and shrub distribution or abundance. We examined the understory of an old-growth mixed-conifer Sierran forest and its distribution in relation to microsite conditions. The forest has high species richness (98 species sampled), most of which are herbs with sparse cover and relatively equal abundance. Shrub cover is highly concentrated in discrete patches. Using overstory tree cover and microsite environmental conditions, four habitats were identified; tree cluster, partial canopy, gap, and rock/shallow soil. Herb and shrub species were strongly linked with habitats. Soil moisture, litter depth and diffuse light were the most significant environmental gradients influencing understory plant distribution. Herb cover was most strongly influenced by soil moisture. Shrub cover is associated with more diffuse light, less direct light, and sites with lower soil moisture. Herb richness is most affected by conditions which influence soil moisture. Richness is positively correlated with litter depth, and negatively correlated with direct light and shrub cover. Disturbance or management practices which change forest floor conditions, shallow soil moisture and direct light are likely to have the strongest effect on Sierran understory abundance and richness.

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Influence of light and soil moisture on Sierran mixed-conifer understory communities

We revisited 68 plots of forest vegetation in the San Bernardino Mountains that had been quantitatively described in 1929-1935, from the California Vegetation Type Map (VTM) Survey. By using the same sampling methods, we documented changes—over approximately 60 years and during a period of fire suppression management—in tree density by both species and size class. In general, we found increasing stand densities, a transformation from old-growth age structure to young growth, and a compositional shift from Pinus ponderosa and P. jeffreyi to Abies concolor and Calocedrus decurrens. Density of trees of more than 12 cm diameter at breast height (dbh) increased by 79%, including three to ten-fold increases in the youngest cohorts 12–66 cm dbh. The magnitude of change depended upon initial forest composition and local annual precipitation. Monotypic stands of P. jeffreyi or those initially dominated by Abies concolor showed the least change in species composition; the most xeric stands of P. jeffreyi showed the least gain in density; and mesic mixed P. ponderosa stands showed the most dramatic change in composition and density. We compared these data to records of past and present forests in the Sierra Nevada and found parallel trends, but magnified by the increased precipitation of the Sierra Nevada. We also compared VTM data from the San Bernardino Mountains to mixed conifer forests in the Sierra San Pedro Martir of Baja California. These Mexican sites and forests are ecologically similar to those in California, but they still experience unmanaged fire regimes. Californian forests of 60 years ago are remarkably similar to modern forests in the Sierra San Pedro Martir. Thus, we conclude that forest changes in the San Bernardino Mountains are primarily due to lengthening fire intervals. Forest changes as a result of fire suppression have important conservation consequences for bird species diversity in general and for Spotted Owl and Neotropical migrants in particular.



Volvimos a visitar 68 parcelas de vegetacion boscosa en las Montanas de San Bernardino, las cuales habian sido descritas cuantitativamente en 1929–1935, a partir del relevamiento del mapa de tipos de vegetacion de California. Utilizando los mismos metodos de muestreo, documentamos los cambios (ocurridos durante aproximadamente 60 anos y durante un periodo bajo manejo de supresion de incendios) en la densidad de arboles tanto por especie como por clase de tamano. Encontramos, en general, un incremento en la densidad de las parcelas, una transformacion de una estructura de edades caracteristica de bosques de crecimiento antiguo a una caracteristica de bosques jovenes y un cambio en la composicion que paso de Pinus ponderosa y P. Jeffreyi a Abies concolor y Calocedrus decurrens. Las densidades de los arboles con un area basal mayor de 12cm aumento en un 79%, incluyendo incrementos de 3 a 10 veces en las cohortes mas jovenes (dbh 12–66cm). La magnitud del cambio dependio de la composicion original del bosque y de la precipitacion anual local. Los rodales monotipicos de P. jeffreyi o aquellos dominados inicialmente por Abies concolor mostraron el cambio menos pronunciado en composicion de especies; los rodales mas xerofilos de P. jeffreyi mostraron el menor incremento en densidad; y los rodales mixtos mesicos de P. ponderosa mostraron el cambio mas dramatico en composicion y densidad. Comparamos estos datos con registros pasados y presentes de los bosques de las Montanas de la Sierra Nevada y encontramos tendencias similares, pero amplificadas por el efecto de la mayor precipitacion de la Sierra Nevada. Tambien comparamos los mapas de tipos de vegetacion de las Montanas de San Bernardino con los bosques mixtos de coniferas de la Sierra de San Pedro Martir en Baja California. Estos sitios y bosques de Mexico son ecologicamente similares a aquellos de California, peroaun se encuentran bajo un regimen de incendios no controlado. Los bosques de California 60 anos atras son increiblemente similares a los bosques actuales de la Sierra de San Pedro Martir. Concluimos por lo tanto que los cambios en los bosques de las Montanas de San Bernardino se deben principalmente a la ampliacion de los intervalos entre incendios. Los cambios en et bosque, resultantes de la supresion de incendios tienen consecuencias importantes a nivel de conservacion para la diversidad del especies de aves en general, y para el buho moteado y los migrantes neotropicales en particular.

Sixty Years of Change in Californian Conifer Forests of the San Bernardino Mountains

The influence of fire and climate events on age structure of different species was examined in old-growth mixed conifer in the southern Sierra Nevada. Within a 48-ha stem-mapped sample area, after a mechanical thinning, all stumps were examined for fire scars and 526 stumps were cut to ground level and aged. Before 1865, which was the last widespread fire event, the mean interval between scars for an individual tree was 17.3 years and the mean fire return interval for the period with the greatest number of recording trees was 11.4 years. A significantly greater than expected number of fires occurred in dry La Nina years, but these fires were not significantly larger in size than fires in other years. The response of mixed-conifer recruitment to climate and fire events varied by species. Before 1865, Jeffrey pine and sugar pine recruitment were correlated with wet El Nino years, but only sugar pine establishment was associated with fire. Red fir recruitment did not follow fire events but was associated with El Nino years before and after 1865. Most white fir and incense cedar (84%), including many large-diameter trees (76 cm dbh), recruited after the last widespread fire in 1865. Although tree distribution is clustered in the southern Sierra Nevada, mixed-conifer groups are not age cohorts because species have different recruitment patterns relative to climate and fire events. In mixed conifer, top-down effects of fire and weather on recruitment are mediated by different species responses to these effects and within-stand differences in where species are located. FOR .S CI. 51(3):187-197.

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Michael G. Barbour

Papers

Naturalization and invasion of alien plants: concepts and definitions

Terrestrial Plant Ecology

Influence of light and soil moisture on Sierran mixed-conifer understory communities

Sixty Years of Change in Californian Conifer Forests of the San Bernardino Mountains

Influence of fire and El Nino on tree recruitment varies by species in Sierran mixed conifer