Aboveground net primary production dynamics in a northern Chihuahuan Desert ecosystem

Aboveground net primary production (ANPP) dynamics are a key element in the understanding of ecosystem processes. For semiarid environments, the pulse-reserve framework links ANPP to variable and unpredictable precipitation events contingent on surficial hydrology, soil moisture dynamics, biodiversity structure, trophic dynamics, and landscape context. Consequently, ANPP may be decoupled periodically from processes such as decomposition and may be subjected to complex feedbacks and thresholds at broader scales. As currently formulated, the pulse-reserve framework may not encompass the breadth of ANPP response to seasonal patterns of precipitation and heat inputs. Accordingly, we examined a 6-year (1999–2004), seasonal record of ANPP with respect to precipitation, soil moisture dynamics, and functional groups in a black grama (Bouteloua eriopoda) grassland and a creosotebush (Larrea tridentata) shrubland in the northern Chihuahuan Desert. Annual ANPP was similar in the grassland (51.1 g/m²) and shrubland (59.2 g/m²) and positively correlated with annual precipitation. ANPP differed among communities with respect to life forms and functional groups and responses to abiotic drivers. In keeping with the pulse-reserve model, ANPP in black grama grassland was dominated by warm-season C₄ grasses and subshrubs that responded to large, transient summer storms and associated soil moisture in the upper 30 cm. In contrast, ANPP in creosotebush shrubland occasionally responded to summer moisture, but the predominant pattern was slower, non-pulsed growth of cool-season C₃ shrubs during spring, in response to winter soil moisture accumulation and the breaking of cold dormancy. Overall, production in this Chihuahuan Desert ecosystem reflected a mix of warm-temperate arid land pulse dynamics during the summer monsoon and non-pulsed dynamics in spring driven by winter soil moisture accumulation similar to that of cool-temperate regions. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of long-term rainfall variability on evapotranspiration and soil water distribution in the Chihuahuan Desert: A modeling analysis

We used the patch arid land simulator (PALS-FT) – a simple, mechanistic ecosystem model – to explore long-term variation in evapotranspiration (ET) as a function of variability in rainfall and plant functional type (FT) at a warm desert site in southern New Mexico. PALS-FT predicts soil evaporation and plant transpiration of a canopy composed of five principal plant FTs: annuals, perennial forbs, C₄ grasses, sub-shrubs, and evergreen shrubs. For each FT, the fractional contribution to transpiration depends upon phenological activity and cover as well as daily leaf stomatal conductance, which is a function of plant water potential, calculated from root-weighted soil water potential in six soil layers. Simulations of water loss from two plant community types (grass- vs. shrub-dominated) were carried out for the Jornada Basin, New Mexico, using 100 years of daily precipitation data (1891–1990). In order to emphasize variability associated with rainfall and fundamental differences in FT composition between communities, the seasonal patterns cover of perennials were held constant from year to year. Because the relative amount of year to year cover of winter and summer annual species is highly variable in this ecosystem, we examined their influence on model predictions of ET by allowing their cover to be variable, fixed, or absent.Over the entire 100-yr period, total annual ET is highly correlated with total annual rainfall in both community types, although T and E alone are less strongly correlated with rainfall, and variation in transpiration is nearly 3 times greater than evaporation and 2 times greater than variation in rainfall (CV of rainfall = 35%). Water use shows a relatively high similarity between the grass- and shrub-dominated communities, with a 100-yr average T/ET of 34% for both communities. However, based on a year-by-year comparison between communities, T/ET was significantly greater in the grass-dominated community, reflecting the fact that over the long term more than half of the rain occurs in the summer and is used slightly more efficiently (T¿E) by the C₄-grass community than the shrub community, although we found some rainfall patterns that resulted in much greater T/ET in the shrub community in a given year. Percent of water lost as transpiration (T/ET) suggests that while there is a general trend toward increased T/ET with rainfall in both community types, T/ET is extremely variable over the 100-yr simulation, especially for normal and below normal amounts of rainfall (T/ET values range from 1 to 58% for the grass-dominated site and 6 to 60% for the shrub-dominated site).These predictions suggest that because of the relatively shallow distribution of soil water, there is little opportunity for vertical partitioning of the soil water resource by differential rooting depths of the plant FTs, in contrast to the two-layer hypothesis of Walter (1971). However, functional types may avoid competition by keying on particular `windows' of moisture availability via differences in phenologies. We found very little differences in average, long-term model predictions of T, E, and ET when annual plant cover was variable, fixed, or absent. The results of our simulations help reconcile some of the disparate conclusions drawn from experimental studies about the relative contribution of transpiration vs. evaporation to total evapotranspiration, primarily by revealing the great year-to-year variability that is possible.     

Plant regulation of soil nutrient distribution in the northern Chihuahuan Desert

Vegetation throughout the southwestern United States has changed from perennial grassland to woody shrubland over the past century. Previous studies on the development of islands of fertility focused primarily on only the most limiting, plant-essential element, soil nitrogen (N). The research presented here addressed the question of whether other plant-essential elements, namely phosphorus (P) and potassium (K), showed similar concentration gradients under the desert shrub Larrea tridentata, creosotebush. It also examined whether the spatial distribution of N, P, and K differed from that of essential, but non-limiting nutrients, namely calcium (Ca), magnesium (Mg), and sulfur (S), and non-essential elements, namely sodium (Na), chloride (Cl), and fluoride (F). Within adjacent grassland and shrubland plots, surface soils were collected under and between vegetation and analyzed for a suite of soil nutrients. Soil nutrient distribution followed a uniform pattern that mirrored the spatial homogeneity of bunchgrasses in the grassland, but followed a patchy distribution that mirrored the spatial heterogeneity of individual shrubs in the shrubland. The main differences were that in the grassland, all elements were uniformly distributed, but in the shrubland the plant-essential elements, nitrogen, phosphorus, and potassium, were concentrated under the shrub canopy, and the non-limiting and non-essential elements were either concentrated in the intershrub spaces or were equally concentrated under shrubs and in the interspaces. Our results show how vegetation shifts from grassland to shrubland contribute to long-term, widespread change in the structure and function of desert ecosystems.     

Genetic change following fire in populations of a seed-banking perennial plant

Disturbances such as fire have the potential to remove genetic variation, but seed banks may counter this loss by restoring alleles through a reservoir effect. We used allozyme analysis to characterize genetic change in two populations of the perennial Hypericum cumulicola, an endemic of the fire-prone Florida scrub. We assessed genetic variation before and 1, 2, and 3 years after fire that killed nearly all aboveground plants. Populations increased in size following fire, with most seedlings likely recruited from a persistent seed bank. Four of five loci were variable. Most alleles were present in low frequencies, but our large sample sizes allowed detection of significant trends. Expected heterozygosity increased, and allele presence and allele frequencies showed marked shifts following fire. The post-fire seedling cohort contained new alleles to the study and one new allele to the species. Population differentiation between the two study sites did not change. Our study is the first to directly documents genetic changes following fire, a dominant ecological disturbance worldwide, and is also one of the few to consider shifts in a naturally recruiting post-disturbance seedling cohort. We demonstrate the potential of seed banks to restore genetic variation lost between disturbances. Our study demonstrates that rapid genetic change can occur with disturbance and that fire can have positive effects on the genetics of rare species.     

Historical shrub–grass transitions in the northern Chihuahuan Desert: modeling the effects of shifting rainfall seasonality and event size over a landscape gradient

We use a spatially explicit landscape model to investigate the potential role of rainfall on shrub–grass transitions in the Jornada Basin of southern New Mexico during the past century. In long‐term simulations (1915–1998) along a 2700 m transect running from a dry lake bed to the foothills of a small mountain, we test two hypotheses: (i) that wetter winters and drier summers may have facilitated shrub encroachment in grasslands, and (ii) that increases in large precipitation events may have increased soil water recharge at deeper layers, thus favoring shrub establishment and growth. Our model simulations generally support the hypothesis that wetter winters and drier summers may have played a key role, but we are unable to reproduce the major shifts from grass‐ to shrub‐domination that occurred in this landscape during the early part of the 1900s; furthermore, the positive shrub response to wetter winters and drier summers was only realized subsequent to the drought of 1951–1956, which was a relatively short ‘window of opportunity’ for increased shrub establishment and growth. Our simulations also generally support the hypothesis that an increase in the number of large precipitation events may also have favored shrub establishment and growth, although these results are equivocal, depending upon what constitutes a ‘large’ event and the timing of such events. We found complex interactions among (i) the amount/seasonality of rainfall, (ii) its redistribution in the landscape via run‐on and runoff, (iii) the depth of the soil water recharge, and (iv) subsequent water availability for the growth and reproduction of shrubs vs. herbaceous plants at various landscape positions. Our results suggest that only a mechanistic understanding of these interactions, plus the role of domestic cattle grazing, will enable us to elucidate fully the relative importance of biotic vs. abiotic factors in vegetation dynamics in this semiarid landscape.     

Pollination ecology ofOxalis violacea (Oxalidaceae) following a controlled grass fire

Vernal grass fires may encourage profuse flowering in clonal, colonies ofOxalis violacea. Long-styled colonies appear to be more floriferous than short-styled colonies and set a greater number of capsules. Individual flowers of both morphs live one or two days, change position on their respective pedicels and advertise nectar concealed at the base of the floral throat. AlthoughDiptera, Hymenoptera, andLepidoptera forage for nectar, bees (Andrenidae,Anthophoridae, Halictidae, andMegachilidae) probably make the only effective pollen transfers between the two morphs. Both male and female bees may transport pollen of both morphs and short-tongued bees (e.g.,Augochlorella spp.,Dialictus spp.) may be more common but as effective as pollinators as long-tongued bees (e.g.,Calliopsis andreniformis andHoplitis spp.). The conversion rate of flowers into capsules is only 13–17%. The spreading style in the short-styled morph is interpreted as an adaptation restricting insect-mediated, self-pollination but encouraging bee-stigma contact during nectar foraging.     

Differences in above- and below-ground responses to ozone between two populations of a perennial grass

Our study examined the influence of elevated ozone levels on the growth and mycorrhizal colonization of two populations of Elymus glaucus L. (blue wildrye). We hypothesized that ozone would reduce carbon availability to the plants, particularly below ground, and would affect mycorrhizal colonization. Because of the wide geographic range of E. glaucus, two populations of plants were selected from areas of contrasting ozone histories to examine intraspecies variation in response to ozone. Two populations of E. glaucus (southern California versus northern California) exposed in a factorial experiment involving ozone, mycorrhizal inoculation with Glomus intraradices Schenck and Smith, and plant source population. Ozone had a subtle effect on leaf area and number of tillers but did not affect overall root:shoot ratio in either population. The impact of ozone on above-ground growth characteristics was most pronounced in the southern population that came from a high-ozone environment, while below-ground responses such as reduced arbuscular colonization was most pronounced in the northern population which originated in a low-ozone environment. Further analysis of soil characteristics from the northern population of plants revealed a significant reduction in active soil bacterial biomass and an increase in total fungi per gram dry weight soil, suggesting a possible role for ozone in altering soil processes. Whether or not population differences in response to ozone were due to genetic shifts resulting from prior ozone remains to be determined. However, these subtle but important differences in population response to ozone above- and below-ground have significant implications in any attempt to generalize plant response, even within a species. Future research efforts need to include better characterization of intraspecific variation in response to ozone as well as possible adaptive strategies that may result from chronic ozone exposure.     

Vertebrate frugivory and seed dispersal of a Chihuahuan Desert cactus

Vertebrate frugivory of fleshy-fruited plants may be very important for the recruitment of sexually derived seedlings if it represents the main mechanism of primary seed-dispersal.Opuntia rastrera produces fleshy fruits rich in water and sugars that are attractive to vertebrate frugivores. However, there is a very low rate of seedling recruitment in natural conditions. One of the causes that can influence this low recruitment is an insufficient seed dispersal due to a low fruit removal even under different resource (fruits) availability. To test this prediction, we studied the production and consumption of fruits in two consecutive years in two vegetation types: nopaleras (dense Opuntia-dominated scrublands) and grasslands with sparse populations of O. rastrera. Plant cover, fruit production and removal, and frugivore identity were recorded within each vegetation type in four randomly selected circular plots (7854 m²). Fruit production per area was higher in nopaleras in both years whereas per cladode production did not differ between vegetation types but differed between years in response to variation in precipitation. Fruit consumption by vertebrates was high (100%) and independent of spatial and temporal fruit availability. The intensity of fruit removal was inversely related to resource availability: it was faster in the less dense community (grassland) and in the driest year. Contrary to other studies with similar cacti, fruit removal by small mammals was insignificant whereas main consumers were birds and large mammals. Vertebrate frugivory represents the only mechanism of primary dispersal of seeds as all fruits are removed in about one month. Despite the high quantity of viable seeds (more than one million per ha in nopaleras and a tenth of that in grasslands) that are dispersed by frugivores after the consumption of about 300 kg of fruit per ha in nopaleras and a tenth of that in grasslands, the rare establishment of seedlings (about one seedling per three million of seeds produced) reported in the literature indicates that the interaction between O. rastrera and the disperser guild is indeed very asymmetrical. We speculate that the harsh conditions for cactus establishment found in this ecosystem demand a high investment in disperser rewards (fleshy fruits) to allow a very modest rate of sexually-derived seedling establishment.     

Resprouting vigour of two mediterranean shrub species after experimental fire treatments

Experimental fire treatments were carried out by applying a propane torch flame to individual stumps ofArbutus unedo andErica arborea, two dominant ericaceous shrubs living in relatively moist maquis of the western Mediterranean Basin. No mortality was observed in either species. Individual plant size, measured as the individual stump area, was the most important factor determining both number and biomass of resprouts for all fire treatments. The number of resprouts at 3 months was less at the higher temperature, but no difference could not be detected at 18 months. Duration of flame application had no effect on resprouting success. This effect was not statistically significant 18 months after treatments were applied. Biomass of resprouts was not clearly affected by different fire treatments either 3 months or 18 months after the start of the resprouting process.     

Synergistic influences of introduced herbivores and fire on vegetation change in northern Patagonia,Argentina

Question: We investigated how cattle and European hares, the two most widespread exotic herbivores in Patagonia, affect species composition, life‐form composition and community structure during the first 6 years of vegetation recovery following severe burning of fire‐resistant subalpine forests and fire‐prone tall shrublands. We asked how the effects of introduced herbivores on post‐fire plant community attributes affect flammability of the vegetation. Location: Nahuel Huapi National Park, northwest Patagonia, Argentina Methods: We installed fenced plots to exclude livestock and European hares from severely burned subalpine forests of Nothofagus pumilio and adjacent tall shrublands of N. antarctica. The former is an obligate seed reproducer, whereas the latter and all other woody dominants of the shrubland vigorously resprout after burning. Results: Repeated measures ANOVA of annual measurements over the 2001‐2006 period indicate that cattle and hare exclusion had significant but complex effects on the cover of graminoids, forbs, climber species and woody species in the two burned community types. Significant interactions between the effects of cattle and hares varied by plant life forms between the two communities, which implies that their synergistic effects are community dependent. Conclusions: Following severe fires, the combined effects of cattle and hares inhibit forest recovery and favour transition to shrublands dominated by resprouting woody species. This herbivore‐induced trend in vegetation structure is consistent with the hypothesis that the effects of exotic herbivores at recently burned sites contribute to an increase in the overall flammability of the Patagonian landscape.     

Responses of Mediterranean Plant Species to different fire frequencies in Garraf Natural Park (Catalonia,Spain): field observations and modelling predictions.

Dynamics of the coexisting Mediterranean species Pinus halepensis, Quercus coccifera, Erica multiflora, Rosmarinus officinalis, Cistus albidus, C. salviifolius and Ampelodesmos mauritanica, with contrasted life history traits have been studied under different fire scenarios, following two approaches: a) field survey in areas with three different fire histories (unburned for the last 31 years, once burned in 1982, and twice burned in 1982 and 1994), and b) simulations with different fire recurrence using the FATE vegetation model. We compared observed abundance in the field survey to simulation outputs obtained from fire scenarios that mimicked field fire histories. Substantial mismatching did not occur between field survey and simulations. Higher fire recurrences were associated with an increase in the resprouting Ampelodesmos grass, together with a decrease in Pinus abundance. Resprouting shrubs did not show contrasting changes, but trends of increase in Quercus and decrease in Erica were observed. The seeders Rosmarinus and Cistus achieved maximum abundance at intermediate fire recurrence. We also performed ten 200 year simulations of increasing fire recurrence with average times between fires of 100, 40, 20, 10, and 5 years. A scenario without fire was also simulated. High fire recurrence produces an increase in Ampelodesmos, a grass which is becoming dominant in the area, and a small increase in Erica, but Quercus abundance decreases and Pinus disappears. Rosmarinus and Cistus abundance peaks at intermediate fire frequencies. When comparing these simulations to those in which Ampelodesmos was excluded, we found that the absence of the grass only increased Cistus occurrence in the community, this effect being more important at frequent fire recurrence. The study suggests that simple models based on life history traits may be useful in interpreting plant community dynamics in Mediterranean ecosystems that are greatly influenced by differences in the fire regime.     

Cactus species turnover and diversity along a latitudinal transect in the Chihuahuan Desert Region

A standardized sampling method was used to evaluate turnover (β diversity) among cactus species assemblages along a 798 km long latitudinal megatransect across the Chihuahuan Desert Region, from north-central Mexico to southern Texas. A total of 71 cactus species were found along the megatransect, 66.2% of which appeared at low frequencies, mostly as a consequence of their highly discontinuous distribution pattern. At the scale the study was conducted, there was always species turnover among cactus assemblages. The rate of turnover among contiguous sites primarily fluctuated from low to medium, but when all site combinations were considered (contiguous and non-contiguous), medium β diversity values were predominant (β = 0.331–0.66); however, 25.4% of the site pair combinations registered high values (β = 0.661–1.0). Our results showed that turnover among cactus species assemblages in the CDR does not consist for the most part of a process of species succession in the geographic space. Instead, we concluded that the continuous spatial changes in cactus species composition are primarily explained by the commonly intermittent distribution patterns of the species, by the presence in the megatransect of species at the margin of their distribution range, and, to a lesser extent, by the existence of narrowly endemic species.     

Neighborhood interactions in a natural population of the perennial bunchgrassBouteloua gracilis

We analyzed neighborhood interactions in a natural population of the perennial bunchgrass blue grama (Bouteloua gracilis). Space occupation by individual plants was characterized in terms of neighborhood size. Neighborhood size was defined as the area potentially ‘available’ to an individual, which included the basal area of the plant and the bare area closer to the edges of the plant than to any others. Geographic Information Systems (GIS) were used to describe space partitioning. Growing season performance was evaluated as a function of neighborhood area and neighbor size, controlling for focal plant size. The area of the neighborhood was significant in explaining the remaining variation of allometric relationships between basal area and current vegetative and reproductive performance. In contrast, current performance of focal individuals was not related to the average basal area or the sum of basal areas of adjacent neighbors. Growing season performance was apparently affected by plant spacing, suggesting that competition for spatially distributed resources occurs. The presence of relatively small plants in neighborhoods with a high proportion of bare soil is consistent with the view of a community composed of patches undergoing their own successional dynamics. Competition and disturbances seem to play an important role in this semiarid grassland.     

Nitrogen translocation in a clonal dune perennial,Hydrocotyle bonariensis

Summary Hydrocotyle bonariensis, a common rhizomatous perennial of coastal North Carolina, forms extensive clones in dune systems characterized by a patchy nitrogen distribution. An experiment was conducted in which Hydrocotyle clones were grown across artificially created soil nitrogen gradients to determine: (1) the effect of soil nitrogen availability and nitrogen translocation on clonal structure and (2) the costs versus benefits of nitrogen translocation as measured by sexual and clonal reproduction. Acropetal translocation of nitrogen resulted in highly significant benefits to clones growing from areas of high N to areas of low N. Limited basipetal translocation was also demonstrated. Hydrocotyle ramets responded to increased nitrogen availability, from either intraclonal translocation or immediate uptake from the soil, by producing branches. Nitrogen level, however, had no effect on internode distances. Clonal integration of nitrogen, in tandem with a plastic morphology, allow Hydrocotyle clones to expand across a nitrogen-limited dune environment and to locally exploit nitrogen patches when they are encountered.     

Water relations and growth of shrubs before and after fire in a semi-arid woodland

Summary Plant water relations and shoot growth rate of shrubs resprouting after fire or unburnt were measured in a semi-arid poplar box (Eucalyptus populnea) shrub woodland of eastern Australia. In vegetation unburnt for about 60 years, the dawn xylem water potential (_x) of the dominant shrub species was about-1.0 MPa when the soil was wet and-8.0 MPa when the soil was very dry. At any one time, the dominant shrub species,Eremophila mitchellii, E. sturtii, Geijera parviflora andCassia nemophila, were similar in _x butAcacia aneura andDodonaea viscosa were consistently higher in _x than this group when the soil was moist and lower when the soil was dry. The dominant tree species,Eucalyptus populnea andE. intertexta, appeared to have access to additional water beneath the hardpan which is located 60–80 cm below the surface. When shrubs were under extreme water stress (_x of-8 MPa), the trees had a _x of-3 to-3.6 MPa. Following a fire, both _x and leaf stomatal conductance (g _s) of resprouting shrubs were higher for about 5 years than comparable-aged unburnt vegetation, with relative differences in _x increasing with drought stress. Elongation rate of resprouts was positively linked to prefire shrub height in 3 of 4 species. However, shrubs resprouting after high intensity fires had substantially higher rates of shoot elongation than after low intensity fires which were in turn higher than for foliar expansion of unburnt shrubs. It is concluded that the growth rate of resprouting shrubs is primarily determined by physiological/ morphological factors associated with plant size but is also assisted by greater availability of water and possibly nutrients for a period after fire.     

Juggling carbon: allocation patterns of a dominant tree in a fire-prone savanna

In frequently burnt mesic savannas, trees can get trapped into a cycle of surviving fire-induced stem death (i.e. topkill) by resprouting, only to be topkilled again a year or two later. The ability of savanna saplings to resprout repeatedly after fire is a key component of recent models of tree–grass coexistence in savannas. This study investigated the carbon allocation and biomass partitioning patterns that enable a dominant savanna tree, Acacia karroo, to survive frequent and repeated topkill. Root starch depletion and replenishment, foliage recovery and photosynthesis of burnt and unburnt plants were compared over the first year after a burn. The concentration of starch in the roots of the burnt plants (0.08 ± 0.01 g g⁻¹) was half that of the unburnt plant (0.16 ± 0.01 g g⁻¹) at the end of the first growing season after topkill. However, root starch reserves of the burnt plants were replenished over the dry season and matched that of unburnt plants within 1 year after topkill. The leaf area of resprouting plants recovered to match that of unburnt plants within 4–5 months after topkill. Shoot growth of resprouting plants was restricted to the first few months of the wet season, whereas photosynthetic rates remained high into the dry season, allowing replenishment of root starch reserves. ¹⁴C labeling showed that reserves were initially utilized for shoot growth after topkill. The rapid foliage recovery and the replenishment of reserves within a single year after topkill implies that A. karroo is well adapted to survive recurrent topkill and is poised to take advantage of unusually long fire-free intervals to grow into adults. This paper provides some of the first empirical evidence to explain how savanna trees in frequently burnt savannas are able to withstand frequent burning as juveniles and survive to become adults. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Growth and nutrient content of perennial grasslands following burning in semi-arid,sub-tropical Australia

Fire often increases the productivity of perennial tussock grasslands inmesic environments but can reduce growth for one or more growing seasons inaridand semi-arid environments. We examined effects of single-burns on growth andnutrient content of grasslands in sub-tropical, northwestern Australia. Thesegrasslands were dominated by Themeda triandra, a speciesoften managed by regular burning in wetter temperate and tropical zones. Burnswere in the late dry season and were replicated using small plots (5 ×5-m) within fenced areas at two sites.Total projective cover and aboveground biomass were significantly less in burntplots relative to controls for 2.5 years after burning despite four growingseasons, including the first summer, of above-average rainfall. Recovery ofburnt plots was hindered by an extended dry period in the second year,demonstrating that rainfall in subsequent seasons can be as important asrainfall in the first season in determining post-burn productivity ofgrasslandsin semi-arid environments. Greater decreases in grass cover in burnt plotsduring the extended dry period may have been due to less standing dead andlitter than controls, and therefore less insulation from extreme summertemperatures, although relationships between cover changes and cover at thestart of the period were weak. With the exception of increased pH near grasstussocks, burning had little effect on chemical characteristics of surfacesoilsin the first week. Concentrations of N, and particularly P, in abovegroundplantmaterial were greater in burnt plots four months after burning, followingsummerrains, but were either less than or similar to those in controls withincreasingly dry conditions. Significantly lower concentrations of P in greenfoliage from burnt plots during dry seasons, when uptake from soil pools wouldbe minimal, indicated that burning decreased P retranslocation from plantstores. However, we found no evidence that single-burns increased nutrientlimitations to growth because plant contents of N and P were comparable inburntand control plots during periods of adequate water supply. Our data supportprevious generalizations that prescribed burning of perennial tussockgrasslandsin semi-arid environments is mostly unnecessary because putative benefits ofincreased productivity and forage quality, characteristic of more mesicenvironments, were not realized.     

Influence of grazing and fire on breeding birds and perennial plants in Cyprus scrub and forest systems

Grazing and browsing by sheep and goats has been an important anthropogenic influence on ecosystems in the Mediterranean region for centuries. This influence has changed significantly in recent decades, with a general shift from range grazing to the penning of animals. The intermediate disturbance hypothesis (IDH) proposes that perturbation - including anthropogenic disturbance - is the norm for Mediterranean ecosystems, and that higher species diversity is found under conditions of continuous, moderate disturbance. Here we test the IDH as it relates to grazing of scrub and open forest habitats in Cyprus, while also testing for the effects of fire. We carried out surveys of breeding birds and vegetation at 48 study sites in scrub and open woodland across Cyprus. We estimated relative grazing pressure (past and present) and fire history at these sites, and tested for associations between these factors, breeding bird species richness and perennial vegetation diversity. We found moderate current grazing reduces density of lower and middle level scrub, and a higher level of diversity of perennial vegetation at moderately over intensively grazed sites. We found that moderate historical grazing pressure had a positive influence on richness of breeding bird species in lower scrub habitat such as phrygana, but that this effect was weaker the taller and denser the habitat. By contrast, current grazing pressure had a negative effect on species richness in lower scrub, but species richness in grazed habitats was higher in denser, taller scrub habitat such as garrigue and maquis. Our study suggests grazing plays an important role in maintaining habitat heterogeneity but the impact on avian species richness depends on the density and height of the vegetation.     

The effects of neighbors on the growth and survival of shrub seedlings following fire

Where plant species vie for limited resources, disturbances might preclude competition by releasing a flush of nutrients, or by reducing biomass and thereby diminishing the consumption of resources. However, if new seedlings colonize in clumps, they may still deplete resources within the local aggregations, which may then reduce their growth and survivorship. We investigated competition among seedlings in a burned area by examining the relationship between the performance of newly established shrub seedlings of Ceanothus impressus and (1) the proximity and (2) the identity of their near neighbors. We also investigated the relationship between neighbor proximity and the availability of water. Both survivorship and growth of C. impressus were positively associated with increasing distance to near neighbors, in a manner consistent with resource competition. The availability of water (as determined by pre-dawn xylem pressure potentials) tended to be greater when neighbors were farther away, providing evidence that water was a resource for which plants were competing. This conclusion is reinforced by the finding that the effects of neighbors were stronger in drought years, suggesting that yearly variation in the availability of an important resource (water) can affect the strength of competitive interactions. This suggests that after disturbances, when some resources are apparently abundant on a large scale, competition may be important in determining the small scale patterns of seedling growth and survival.     

Environmental relationships and vegetation associates of columnar cacti in the northern Sonoran Desert

The environmental distribution, habitat segregation, and vegetation associates of the columnar cacti Carnegiea gigantea, Stenocereus thurberi, and Lophocereus schottii were examined in Organ Pipe Cactus National Monument, Arizona. Three primary environmental gradients were identified with principal components analysis of environmental data: soil texture, elevation/nutrients, and xericness (based on slope aspect and angle). Environmental influents of spatial variation in density were modeled with ordinary least squares regression analysis, and common associates were identified with two-way indicator species analysis for each cactus. Of the three cacti, Carnegiea gigantea occurred over the broadest ecological range of habitats, but was densest on coarse, granitically derived alluvial soils of flat upper bajadas and basin floors, where it was associated with Larrea tridentata, Ambrosia deltoidea, and Opuntia fulgida. Stenocereus thurberi reached its maximum densities on coarse sandy soils of steep, south-facing granitic slopes, with Encelia farinosa, Jatropha cuneata, and Opuntia bigelovii as associates. Lophocereus schottii was restricted to very coarse, granitically derived alluvial soils in the southern part of the monument, where it occurred along wash banks with Beloperone californica, Hymenoclea salsola, Acacia greggii, and Opuntia arbuscula.Abbreviations DCA Detrended correspondence analysis - OPCNM Organ Pipe Cactus National Monument - OLS Ordinary least squares - PCA Principal components analysis - RA Reciprocal averaging - TWINSPAN Two-way indicator species analysis