Jacob Bendix

Hydrological and geomorphological impacts on riparian plant communities.

Riparian vegetation is affected by both flood processes and the characteristics of landforms that are shaped by floods. In many instances, species occurrence can be linked directly to specific fluvial landforms. These spatial relationships are largely due to the role of floods in the differential destruction of vegetation, in the determination of substrate characteristics, and in the transport of propagules. Major floods may, depending on the climatic context, allow for the establishment of stands of vegetation, or restart processes of plant community change. Disturbance by floods can also affect biodiversity: species richness in some watersheds is greatest where steep valley floor gradients allow for high-energy floods. The recognition and analysis of hydrogeomorphological influences on riparian vegetation are complicated by multiple scales of environmental interactions, by the covariance of some environmental variables, and by feedbacks between vegetation and flood regimes. Copyright

Stream power influence on southern Californian riparian vegetation

Abstract. Mechanical damage by floodwaters is frequently invoked to explain the distribution of riparian plant species, but data have been lacking to relate vegetation to specific estimates of flood damage potential. This research uses detailed estimates of unit stream power (an appropriate measure of the potential for mechanical damage) in conjunction with vegetation cover data to test this relationship at 37 valley-bottom sites in the Transverse Ranges of Southern California. A computer program, HEC-2, was used to model the slope and the variation in flow depth and velocity of the 20-yr flood across the sites. Regression models tested the influence of stream power (and of height above the water table) on the woody species composition of 393 4-m cross-section segments of the valley-bottom sites. Results indicate that unit stream power does have a significant effect on the riparian vegetation, but that the amount of that influence and its importance relative to the influence of height above the water table varies between watersheds. Some species are found primarily in locations of high stream power, while others are limited to portions of the valley bottom that experience only low stream power.

Riparian vegetation research in Mediterranean-climate regions: common patterns, ecological processes, and considerations for management

Riparian corridors in Mediterranean-climate regions (med-regions) are resource-rich habitats within water-limited, larger landscapes. However, little is known about how their plant communities compare functionally and compositionally across med-regions. In recent decades, research on these ecosystems has expanded in both geographic scope and disciplinary depth. We reviewed 286 riparian-vegetation studies across the five med-regions, and identified common themes, including: (1) high levels of plant biodiversity, structural complexity, and cross-region species introductions; (2) strong physical controls on plant demographics and community structure; and (3) intensive human impacts. European and Californian ecosystems were the most represented among the studies reviewed, but Australia, South Africa, and Chile had the greatest proportional increases in articles published since 2000. All med-regions support distinct riparian flora, although many genera have invaded across regions. Plant species in all regions are adapted to multiple abiotic stressors, including dynamic flooding and sediment regimes, seasonal water shortage, and fire. The most severe human impacts are from land-use conversion to agriculture, streamflow regulation, nutrient enrichment, and climate change. Current knowledge gaps and subjects for future research include cumulative impacts to small, ephemeral streams and large, regulated rivers, as well as understudied ecosystems in North Africa, the western Mediterranean basin, and Chile.

Among-site variation in riparian vegetation of the southern California transverse ranges

-Data from 37 sites in the Transverse Ranges in southern California were used to assess the influence of a variety of environmental variables on among-site variation in riparian vegetation. Ordination and regression analyses indicated that elevation, unit stream power, fire history and valley width all have significant impacts on the composition of this vegetation. TWINSPAN classification generated four species groups: a Baccharis glutinosa/Salix laevigata group that appears related to low elevations, high stream power and recent fire; a Populusfremontii/Alnus rhombifolia group associated with high stream power, but longer time since burning and higher elevations; a Salix lasiolepis/Rosa californica group typically found at high elevation sites with low stream power; and a Platanus racemosa/Adenostoma fasciculatum group that did not show a clear environmental preference.

LANDSCAPE-SCALE GEOMORPHIC INFLUENCES ON VEGETATION PATTERNS IN FOUR ENVIRONMENTS

This paper reviews and synthesizes research about geomorphic influences on vegetation patterns apparent at the landscape scale. After an overview of the effects that landforms and geomorphic processes have on plant distributions, these relationships are discussed in more detail for each of four distinct physical settings: temperate riparian environments, slopes affected by landslides and other forms of mass movement, desert alluvial fans, and nonmountainous glaciated landscapes. These four landscapes were selected because they encompass a broad range of temporal and spatial scales at which the geomorphic processes most strongly linked with vegetation patterns operate; furthermore, they collectively illustrate some of the more prominent themes in recent research on this topic. Finally, we identify four topics that particularly merit future research on geomorphic-biogeographic interactions: (1) feedback between vegetation and landforms, (2) distinctions between landform characteristics and the associated ge...

Old-Growth Forests on Network News: News Sources and the Framing of An Environmental Controversy:

The old-growth forest debate involves two sides (“procut” and “prosave”) presenting competing views of the issue. Television news stories may reflect one or the other of these frames through (1) choice of sources, (2) choice of visuals, and (3) reporters summary remarks. We examined four years of coverage on ABC, CBS, and NBC, and found that while the distribution of visuals was inconclusive, source use and reporter wrap-ups predominantly reflected the procut frame. This may be because the procut frame emphasized an unambiguous conflict that was more amenable to brief explanations.

IMPACT OF A FLOOD ON SOUTHERN CALIFORNIA RIPARIAN VEGETATION

This study assesses flood impacts on riparian vegetation in two watersheds within the Transverse Ranges. Data collected in 1993 were compared to baseline data from 1990 to measure the effect of a 1992 flood. T-tests were used to test for significant post-flood changes in overall vegetation characteristics, and vegetation change was regressed on drainage area to test for spatial variation in flood impacts. Results of means comparisons for the overall data set suggest that the impacts of the flood were insignificant. However, these results in part are artifacts of data agglomeration, as regression results suggest that changes in total cover and diversity are spatially varied, with profound impacts at some downstream sites. The relative cover of most species remained constant, reflecting an environment in which frequent floods help to maintain a vegetation assemblage that is not entirely flood resistant, but at least uniform across species in its degree of resistance. [Key words: riparian vegetation, floods,...

Disturbance and riparian tree establishment in the Sespe Wilderness, California, USA

The riparian forests of southern California are subject to disturbance by both fire and flood. These agents are capable of causing pulses of mortality and recruitment, but it remains unclear how they interact to determine patterns of stand development. We use dendrochronology to identify establishment dates for stems of major riparian tree species in the Sespe Creek watershed, in order to examine their relationship to regional flooding and fire history. Our 11 study sites were burned by major fires in 1932 and 2002, with a smaller 1975 fire affecting only two sites; major floods were concentrated within the second half of the 1933–2009 streamflow record, with the largest floods occurring in 1969, 1978, and 1983. Three periods of stand development are evident: (1) the oldest alder (Alnus), cottonwood (Populus), and oak (Quercus) stems became established soon after the 1932 Matilija Fire, (2) minimal stem establishment between the 1940s and mid-1960s, and (3) continued, although irregular, recruitment of alder and cottonwood since the late 1960s. These patterns show episodes both of regeneration following a catastrophic site-clearing event (Matilija Fire) and of more localized stem replacement during the recent period of increased flood magnitude, with implications for changes in the composition of these forests.

Intermediate disturbance and patterns of species richness

Abstract The intermediate disturbance hypothesis (IDH) predicts highest species diversity in environments experiencing intermediate intensity disturbance, after an intermediate timespan. Because many landscapes comprise mosaics with complex disturbance histories, the theory implies that each patch in those mosaics should have a distinct level of diversity reflecting the magnitude of disturbance and the time since it occurred. We model changing patterns of species richness across a landscape experiencing varied scenarios of simulated disturbance in order to predict first the variation of richness through time in individual patches, based on their disturbance histories, and then the changing patterns of richness across the landscape through time, representing the cumulative impact of changing richness within the individual patches. Model outputs show that individual landscape patches have highly variable species richness through time, with the trajectory reflecting the timing, intensity and sequence of disturbances. When the results are mapped across the landscape, the resulting temporal and spatial complexity reveals a distribution of biodiversity that is strikingly contingent on the details of disturbance history. These results illustrate the danger of generalization (in either data interpretation or management decisions), as IDH actually imposes a highly variable pattern of diversity.

Clay (a poem)

A brief reflection, in which the smallest discrete part of our physical world is elevated (or more accurately, translocated) into the leading role.