Shrubs are an important component of many ecosystems, contributing to spatial and resource heterogeneity and adding to life form and species diversity. Many shrub species have adapted to regular disturbances such as fire by resprouting after stem removal. The relative vigor with which shrubs resprout influences post-disturbance size and density of individuals and thus, can alter community structure. However, little is known about how disturbance frequency and seasonality influence resprouting. We studied resprout vigor of the native shrub American hazel (Corylus americana Walter) in oak savanna in Minnesota, USA. We measured resprout growth of individual shrubs in response to the frequency and timing of clipping in relatively open and shaded conditions. We hypothesized that resprouting of C. americana would be negatively related to clip frequency, due to more rapid depletion of stored resources necessary for resprouting, and positively related to light availability. In 12 weeks following a single clipping, shrubs recovered 82% of the lost stem biomass in open savanna, but only 17% in shaded forest. In both open and closed sites, shrubs clipped three times or more resprouted only 10-15% as much biomass as those clipped once. Moreover, the timing of clipping had as large an effect on resprout potential as the number of clipping events, and small differences in timing were important. Plants clipped once in mid or late June or in July regrew 57, 17, and 8% as much biomass, respectively in the six weeks following clipping as those clipped in early June. These results illustrate that both the timing and number of disturbance events within a growing season can strongly influence shrub growth, which can have important implications for ecosystem structure, function, and management. 相似文献
Despite the critical role of biological soil crusts (BSCs) in arid and semi-arid ecosystem function, few studies are found concerning the most important environmental variables affecting their distribution and physiology. This study seeks to determine soil and microenvironmental factors affecting the spatial distribution and pigment production of BSC-forming lichens and mosses in open patches of a semi-arid Mediterranean kermes oak thicket. We measured late-successional BSC cover, shrub cover, distance to nearest kermes oak (to test for effects of kermes oak thicket microenvironment on BSC), and pigment concentration of one lichen (Cladonia foliacea) and one moss (Pleurochaete squarrosa) species in the Nature Reserve El Regajal-Mar de Ontígola (Central Spain). At the macroscale (>0.5 m), results showed that BSC distribution and pigments were tightly coupled to a suite of soil properties, in particular soil pH, Fe, and Ca. Specifically, soil pH had a positive relationship with the cover of five individual BSC-forming lichen species and was negatively related to pigment production in C. foliacea. When pH was excluded from the analysis, Ca appeared as the main soil variable and was correlated with total BSC cover and total lichen cover. The micronutrient Fe had a significant positive relationship with the concentration of eight pigments in P. squarrosa and was also coupled with the cover of two BSC-forming lichens. Manganese, previously proposed as a key limiting micronutrient for BSCs, affected lichen diversity in a negative way. At the microscale (∼0.5 m), kermes oak microenvironment, shrub cover, and moss cover were determinants of BSC distribution, and total lichen and total BSC cover were overrepresented on N and E-facing shrub microsites. Our findings suggest that soil chemical variability and microsite diversity created by neighbouring vegetation affect BSC distribution in complex and essential ways and that studies aiming to explore BSC-environment relationships should be conducted at various spatial scales. Studies based on species- or group-specific responses are, thus, inadequate to unveil the main factors determining the distribution of the diverse organisms that constitute BSCs and/or to propose potential tools aiming to restore BSC in arid and semiarid ecosystems. 相似文献
Shrub encroachment is a worldwide phenomenon with implications for desertification and global change. We evaluated its effects on the activities of urease, phosphatase and β-glucosidase in Mediterranean semiarid grasslands dominated by Stipa tenacissima by sampling 12 sites with and without resprouting shrubs along a climatic gradient. The presence of shrubs affected the evaluated enzymes at different spatial scales. Soils under S. tenacissima tussocks and in bare ground areas devoid of vascular plants had higher values of phosphatase and urease when the shrubs were present. For the β-glucosidase, this effect was site-specific. At the scale of whole plots (30 m × 30 m), shrubs increased soil enzyme activities between 2% (β-glucosidase) and 22% (urease), albeit these differences were significant only in the later case. Our results indicate that shrub encroachment does not reduce the activity of extracellular soil enzymes in S. tenacissima grasslands. 相似文献
In the Iberian Peninsula Mediterranean oak forests have been transformed into a mosaic landscape of four main patch-types:
forests, savannas, shrublands and grasslands. We used aerial photographs over a period of 45 years (1958–2002) to quantify
the persistence and rates of transitions between vegetation patch-types in southern Portugal, where cork oak is the dominant
tree species. We used logistic regression to relate vegetation changes with topographical features and wildfire history. Over
the 45 years, shrublands have been the most persistent patch-type (59%), and have been expanding; forests are also persistent
(55%) but have been decreasing since 1985; savannas and grasslands were less persistent (33% and 15%, respectively). Shrublands
persistence was significantly correlated with wildfire occurrence, particularly on southern exposures after 1995. In contrast,
forest persistence decreased with wildfire occurrence, and forests were more likely to change into shrublands where wildfire
had occurred after 1995. 相似文献
A rainwater harvesting system on slopes using contour furrows with plastic-covered transverse ridges designed to be used in small rainfall dominated areas of the semiarid loess region of China has been tested from 2001 to 2004. The system consisted of constructing contour furrows on the loess slope at a distance of 5 m with plastic-covered transverse ridges built in the furrows between shrubs of Caragana korshinskii. There were three treatments in the study: (1) plastic-covered ridge with gravel-mulched furrows (T1), (2) plastic-covered ridge with bare furrows (T2), and (3) control (no ridge and no contour furrow) (T3).
The experimental results indicated that runoff from the natural loess slope was small and variable, and only produced from a few rainfall events with high intensity. Runoff efficiency averaged 13.8, 4.5, 1.4, and 0.4% in 2001, 2002, 2003, and 2004, respectively. However, the plastic-covered ridges accumulated runoff from most rainfall events, particularly from the light rains less than 5 mm. So the natural loess slope between the furrows and the plastic-covered ridges in the furrows can complement each other, i.e., the plastic-covered ridges induce runoff from small rainfall to the planted area, and the natural loess slope between the furrows concentrate runoff from heavy rainfall, thus improving rain use efficiency. The total runoff collected from both the natural loess slope and the plastic-covered ridges to the planted area in the furrows was 231, 143, 88, and 59 mm in 2001, 2002, 2003, and 2004, respectively. Soil moisture storage in the 200-cm deep soil layer was obviously higher for T1 and T2 than for T3, and C. korshinskii showed a significant improvement in growth for the T1 and T2 treatments. Therefore the combination of contour furrows and plastic-covered ridges as rainwater harvesting system may have a great potential development in the small rainfall dominated arid regions of China. 相似文献
Runoff yield and sediment delivery are compared in three shrub species (Medicago arborea, Atriplex nummularia, and Psoralea bituminosa) with natural (matorral) vegetation in an experimental plot set. The planted species are arranged in discontinuous rows perpendicular to the slope, acting as barriers to the direction of the surface water flow. The study was carried out from 1988 to 1995, in which 56 erosive rain events were monitored in runoff production and in soil losses, together with their intrinsic characteristics (duration, intensity and rainfall). Growth of the three shrub species (height, vegetation cover, and stem diameter) and the influence on soil properties during this period were recorded. Because of certain characteristics of the study area (shallow soils, high levels of carbonates, stoniness) and the climate (an especially intense period of drought since 1990), the growth of these plants was very irregular. M. arborea gave the best protection against water erosion reducing sediment production by more than 58% compared to the bare soil. However, the protection afforded by this species never reached that obtained with natural vegetation, which reduced sediment production by almost 75%. 相似文献