Vegetation Patch Types Research Articles

Are plant functional types relevant to describe degradation in arid, southern Tunisian steppes?

Abstract. In the Tunisian arid zone disturbances (e.g. overgrazing and agriculture) and stresses (e.g. aridity, low fertility) drive changes in the structure and functioning of rangelands, with a decrease in perennial plant cover, changes in floristic composition and erosion. Long‐term monitoring requires (1) an understanding of the dynamics of vegetation change and associated ecological processes and (2) identification of relevant indicators. Using data from the arid zone of southern Tunisia we tested the hypothesis that plant functional response types could be used to address these two goals. We identified plant functional response types in response to a gradient of soil and vegetation types characterized by changes in perennial plant cover, dominant species and associated soil types. Vegetation samples were stratified by contrasted vegetation patch types with varying perennial plant cover (1.6 to 22%). We focused our analysis of trait responses within dwarf–shrubs, which are the dominants in typical steppe ecosystems of south Tunisia. Available trait data concerned morphology (plant height, leaf type), regeneration (dispersal mode, phenology and regeneration mode) and grazing value. Although we found it difficult to recognize ‘indicator response types’ that could be used directly to monitor changes in community composition, we were able to identify plant response syndromes that are relevant to long‐term vegetation changes, and in particular degradation processes, in the region. Two main response types were identified: the decreaser type, made up of small or medium chamaephytes with high grazing palatability and the increaser type with medium to tall chamaephytes and low grazing palatability. These response types are proposed as key elements in a state‐and‐transition model of vegetation dynamics in the context of agropastoral disturbances and climatic and edaphic stresses.

Are plant functional types relevant to describe degradation in arid, southern Tunisian steppes?

Abstract In the Tunisian arid zone disturbances (e.g. overgrazing and agriculture) and stresses (e.g. aridity, low fertility) drive changes in the structure and functioning of rangelands, with a decrease in perennial plant cover, changes in floristic composition and erosion. Long-term monitoring requires (1) an understanding of the dynamics of vegetation change and associated ecological processes and (2) identification of relevant indicators. Using data from the arid zone of southern Tunisia we tested the hypothesis that plant functional response types could be used to address these two goals. We identified plant functional response types in response to a gradient of soil and vegetation types characterized by changes in perennial plant cover, dominant species and associated soil types. Vegetation samples were stratified by contrasted vegetation patch types with varying perennial plant cover (1.6 to 22%). We focused our analysis of trait responses within dwarf–shrubs, which are the dominants in typical ste...

An explanation of patterns of breeding bird species richness and density following clearcutting in northeastern USA forests

We surveyed breeding birds for an average of 4 years on 23 primarily even-aged successional stands ranging from 2 to 120 years old to observe changes in species richness and density following clearcutting in central New York. An initial sharp increase in richness and density from post-cut years 2–6 preceded a steep decline by year 25. Cubic functions best fit this ( P<0.0007) and a second, smaller increase and decline in richness and density between years 25 and 120 ( P<0.03). The rapid initial increases in richness and density were highly correlated with leaf area which, on plots that were heavily stocked prior to cutting, returned to precut levels by year 6 or 7. Species richness on 6-year-old clearcuts was more than twice that of any mature stand sampled. Similarly, 6-year-old clearcuts exhibited a mean avian density (501 pr/40 ha), more than 2.5 times that of any mature stand (184 pr/40 ha). The decline in avian populations from years 7 to 25 accompanied canopy closure and was likely associated with the loss of foraging and nesting sites near ground level. The second, less pronounced increase in avian richness and density from years 25 to 100 was associated with (1) the redistribution of foliage to all vertical layers of the forest resulting from the increasing size and duration of canopy gaps caused by tree deaths; and (2) the increasing size and bark complexity of tree boles, which provided new foraging and nesting substrates. Overall, avian successional trends appear to be determined by the development and decline of vegetation patch types appropriate to specific guilds of birds. We suggest that, while lacking the vertical heterogeneity and bole-related substrates traditionally cited as correlates of avian richness and density in older forests, early successional stages of northeastern forests support significantly higher avifaunal richness and density due to a combination of high productivity of leaves and insects along with increased horizontal structural complexity. Even-aged stand management, carefully implemented through clearcutting, has the potential to increase the species richness of forested landscapes and provide important habitat for early successional Neotropical migrants, while effectively meeting demands for marketable timber.

Runoff and Erosion in a Piñon–Juniper Woodland Influence of Vegetation Patches

In many semiarid regions, runoff and erosion differ according to vegetation patch type. These differences, although hypothesized to fundamentally affect ecological processes, have been poorly quantified. In a semiarid piñon–juniper woodland [Pinus edulis Engelm. and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico, we measured runoff and erosion from the three patch types that compose these woodlands: Canopy patches (those beneath woody plants), vegetated patches in intercanopy areas, and bare patches in intercanopy areas. The bare intercanopy patches exhibited the highest rates, followed by vegetated intercanopy patches and then by canopy patches. Large convective summer storms, though relatively infrequent, generated much of the runoff and most of the sediment; prolonged frontal storms were capable of generating considerable runoff but little sediment. A portion of the runoff and most of the sediment generated from bare intercanopy patches was redistributed downslope, probably to adjacent vegetated intercanopy patches, demonstrating connectivity between these two patch types. Our results indicate that there are significant and important differences in runoff and sediment production from the three patch types; that bare intercanopy patches act as sources of both water and sediment for the vegetated intercanopy patches; and that the transfer of water and sediment at small scales is both frequent enough and substantial enough to be considered ecologically significant.

Grazing response groups among understorey plants in arid rangelands

Abstract.We analysed attributes of the understorey flora in different plant communities along two grazing gradients in arid Australian rangelands. We aimed to determine if there were patterns among species in response to grazing, and easily recognizable ‘indicator response types’ for monitoring grazing‐induced change in community composition. Measurements were stratified by vegetation patch type, in woody groves and open patches. Trait selection and analyses followed a hierarchical approach which searched for patterns within major plant life forms. Patch type exerted a dominant influence on both life forms and species attributes, but interacted with grazing. Grazing was associated with loss in differentiation of species composition between patch types, rather than loss in numbers of species overall. Heavy grazing was variously associated with small size, prostrate habit, low meristems, small leaves, coated leaves, high regrowth potential, plasticity in response to grazing, and high fecundity; and light grazing with opposing attributes. Many attributes tended to vary independently of each other and grazing‐related attribute syndromes were recognisable only among grasses. This could be because the environmental filters acting on the communities have given rise to many different species with different natural attribute combinations, few of which are closely associated with grazing resistance. For grassy communities ‘large, erect tussocks branching above‐ground’ and ‘small, sprawling basal tussocks’ may have potential as response types indicative of light and heavy grazing respectively, but no response types could be identified for herbaceous communities.

Spatial patterns of the germinable soil seed bank in northern Patagonia

AbstractThe spatial variation of the soil seed bank in the steppe ofLarrea divaricataandStipaspp. was evaluated in relation to the spatial pattern of the established vegetation, over two years. Plant distribution was expected to affect the spatial pattern of the soil seed bank. It was hypothesized that patches of bare soil are seed-limited, while soil beneath the external crown of vegetation patches contains greater numbers of seeds. The soil seed bank of the steppe ofLarrea divaricataandStipaspp. displayed a patchy distribution associated with the spatial pattern of above-ground vegetation. Seed banks in areas of bare soil showed lower numbers of seeds than those beneath vegetation. The soil seed bank was dominated by annuals, both in open areas of soil and under vegetation canopies. The relative composition of the soil seed bank varied spatially. Annuals (dicots and grasses) were more abundant in grass-shrub patches (GSP) and grass patches (GP) than in incipient grass-shrub patches (IGSP) and bare soil (BS). Seeds of perennial grasses were more abundant under GP, less abundant under grass-shrub mixed canopies (IGSP and GSP) and practically absent in BS. In addition, seeds of perennial dicots were more frequent in grass-shrub mixed canopies, scarcely represented in GP and absent in BS. The concentration of seeds of perennial grasses in GP and GSP indicates that these are essential for the maintenance of the size of grass populations. Sustainable management should be focused to maintain the structure and function of these types of vegetation patches.

Marsh development after large floods in an alluvial, arid-land river

Large expanses of riverine marsh are rare in the desert Southwest, given the dry surface of many floodplain soils. Along the Hassayampa River, riverine marsh underwent a 5-fold increase (from 2% to 9% of the floodplain-channel area) after a large winter flood in 1993. Flood waters eroded terraces that had aggraded during frequent, smaller floods, widened the channel from about 3 to 50 m, and recharged the floodplain aquifer. The net effect of these changes was a lowering of the floodplain surface relative to the water table, a variable of critical importance to riparian plant composition in arid-land rivers. Olney’s bulrush (Scirpus americanus Pers.), southern cattail (Typha domingensis Pers.), jointed rush (Juncus articulatus L.), and other obligate wetland species were abundant in 1993 and 1994 on areas with saturated surface soils or shallow water tables and often were intermixed with seedlings of early-seral tree species, including Fremont cottonwood (Populus fremontii S. Watson), Goodding willow (Salix gooddingii Ball), and salt cedar (Tamarix chinensis Loureiro and related species). The gain in riverine marsh and young cottonwood-willow stands occurred at the expense of mature cottonwood-willow forests and deep-rooted, velvet mesquite (Prosopis velutina Woot.) woodlands. Another large flood in 1995 scoured the channel of most existing vegetation and aggraded the 1993 flood channel. Early-seral tree species again established in moist soils exposed by the slowly receding flood waters. However, redevelopment of extensive marsh habitat was precluded by sediment deposition that increased the elevation of the floodplain surface relative to the water table. These changes highlight the transitory nature of riverine marsh and other vegetation patch types in the dynamic floodplains of alluvial, arid-land rivers and underscore the importance of maintaining flood flows of varying magnitude to maintain patch type diversity.

Spatial patterns of nitrogen availability, mineralization, and immobilization in Northern Patagonia, Argentina

The spatial variation of nitrogen availability, mineralization, and immobilization was studied in heavily grazed areas of the Larrea divaricata and Stipa spp. steppe in northern Patagonia in order to identify the interactions between the nitrogen pool and vegetation. For this purpose, four of the most predominant types of vegetation patches and neighboring bare soil patches were selected for study. Soil samples were taken, during 1 year, in the external crown of each patch type, and inorganic N, in situ mineralization, and N in microbial biomass were analyzed. Nitrogen stores in the soil were related to canopy characteristics. Soil beneath undisturbed patches of vegetation held higher stores of available N and N retained in microbial biomass than bare soil. Small patches with incipient vegetation cover represented an intermediate situation between undisturbed patches and bare soil. In situ N mineralization rates were low, and no significant differences were detected among patches. This was attributed to t...

Effects of Litter, Distance, Density and Vegetation Patch Type on Postdispersal Tree Seed Predation in Old Fields

Because the fate of seed rain is critical to understanding the invasion of old fields by trees, we investigated spatial and temporal variation and difference among six tree species in seed predation. Very few Juniperus virginiana seeds were lost to predation but predation for all other test species was reduced by additions of Quercus litter and solidago litter. Carya tomentosa seed predation was also reduced by increasing distance from the forest edge. In addition to litter and distance effects, differing starting densities of Acer rubrum seeds significantly affected predation rates but Fraxinus americana rates were not affected (...)

Factors Controlling Denitrification in a Chihuahuan Desert Ecosystem

AbstractDenitrification may be an important pathway for N loss from desert ecosystems. Few studies, however, have investigated the factors limiting this process in a desert environment. A factorial experiment was conducted to determine the factors controlling denitrification in the northern Chihuahuan Desert. Specifically, we measured the response of denitrification to additions of water, C, N, and C + N. Soil cores were collected from four vegetation zones along an alluvial piedmont. Dry cores were subjected to five treatments: (i) water; (ii) water + NO3; (iii) water + C; (iv) water + NO3 + C; and (v) a control (no additions). When denitrification rates were averaged across vegetation zones and patch types (between or under vegetation), the following treatment effects were significantly different: water + NO3 + C &gt; &gt; &gt; water + NO3 = water » water + C &gt; control. These results indicate that denitrification at this site is limited by the availability of water. In wet soil cores, C additions immobilized available NO3 and suppressed denitrification. When water + NO3 + C was added (C/N = 22), however, denitrification was significantly greater than when water + NO3 were added. This result indicates that C and N interact to control denitrification in wet desert soils. No evidence for an overall NO3 limitation in moist cores was found. Surprisingly, denitrification rates in wet cores of nutrient‐poor desert soils (≈32.9 ng N cm−2 h−1) were similar to those measured in the nutrient‐rich soils of temperate and tropical forests. When extrapolated to an annual rate, denitrification for this site is 7.22 kg N ha−1 yr−1. Extreme drying‐wetting cycles common in desert ecosystems may account for the high rates observed.