Gopher Disturbance Research Articles

Pocket gopher disturbance slows soil carbon accumulation in abandoned agricultural lands.

Soil carbon (C) sequestration rates vary widely in abandoned agricultural lands, and factors determining this variation, beyond climate, soil type, and productivity, are poorly understood. One such factor is soil disturbance by burrowing mammals. Despite being ubiquitous in all grasslands, the impact of burrowing mammals on soil C dynamics is not well understood. We quantified the major ecosystem processes that are influenced by one such burrowing mammal, plains pocket gophers (Geomys bursarius), in old field ecosystems located in east-central Minnesota, USA. We found that pocket gopher abundance varied among old fields and that newly formed gopher mounds covered up to 6% of the soil surface annually. We first measured short-term C pool and flux changes induced by gopher activities. Soil N mineralization did not differ between the soil in gopher mounds and undisturbed soil. However, for the soil under gopher mounds, N mineralization was 30% lower compared with the undisturbed soil. We developed a process model to simulate the long-term gopher disturbance impact on old field soil C accumulation. This simulation showed that pocket gophers reduced both the rate of soil C accumulation and the total C pool. This reduction is primarily driven by reduced plant C input due to the time it requires for the vegetation to recolonize gopher mounds. Soil organic matter (SOM) decomposition changes had only a minor impact. The process model showed that the depth from which burrowing mammals redistribute soil to the surface is a key factor in determining the overall impact on SOM. In total, our study indicated that soil disturbance by burrowing animals could significantly reduce C storage in old field ecosystems when the mammals are mostly active at the surface soil and can be a significant factor in decreasing overall C sequestration after land abandonment. However, at our study site, gopher abundance decreased with abandonment age, which was likely to have been cause by successional vegetation changes, therefore the gopher disturbance-induced reduction in soil C is transient and decreases with abandonment age.

Micro‐scale geography of synchrony in a serpentine plant community

Abstract Fluctuations in population abundances are often correlated through time across multiple locations, a phenomenon known as spatial synchrony. Spatial synchrony can exhibit complex spatial structures, termed ‘geographies of synchrony’, that can reveal mechanisms underlying population fluctuations. However, most studies have focused on spatial extents of 10s to 100s of kilometres, making it unclear how synchrony concepts and approaches should apply to dynamics at finer spatial scales. We used network analyses, multiple regression on similarity matrices, and wavelet coherence analyses to examine micro‐scale synchrony and geographies of synchrony, over distances up to 30 m, in a serpentine grassland plant community. We found that species' populations exhibited a geography of synchrony even over such short distances. Often, well‐synchronized populations were geographically separate, a spatial structure that was shaped mainly by gopher disturbance and dispersal limitation, and to a lesser extent by relationships with other plant species. Precipitation was a significant driver of site‐ and community‐wide temporal dynamics. Gopher disturbance appeared to drive synchrony on 2‐ to 6‐year timescales, and we detected coherent fluctuations among pairs of focal plant taxa. Synthesis. Micro‐geographies of synchrony are an intriguing phenomenon that may also help us better understand community dynamics. Additionally, the related geographies of synchrony and coherent temporal dynamics among some species pairs indicate that incorporating interspecific interactions can improve understanding of population spatial synchrony.

Long‐term propagule pressure overwhelms initial community determination of invader success

AbstractThe role of plant diversity in reducing invasions has generated decades of debate. Diverse communities might be more resistant to invasion because the communities contain resident species that are functionally similar to the invader (limiting similarity), or multiple species use the range of available resources more effectively (complementarity) than single species. However, the correlation of native and exotic diversity often reverses, becoming positive, with increasing spatial and temporal scale, in a phenomenon called the invasion paradox. We addressed two groups of hypotheses related to this paradox, broadly that (1) functional diversity and identity resist invasion initially, via complementarity or limiting similarity; and (2) disturbance and propagule pressure weaken the effects of functional diversity and identity on invader success through time. Using long‐term data from experimental serpentine grassland assemblages in California, we examined how the abundance of a high impact invader, yellow starthistle (Centaurea solstitialis), related to functional diversity, functional dissimilarity, pocket gopher disturbance, and propagule pressure. We also conducted a single‐season experiment in which we seeded disturbed and undisturbed areas and quantified invader success the following year. Neither diversity, nor dissimilarity, nor disturbance significantly impacted the success of C. solstitialis during the years of this study. Instead, propagule pressure was the single most important predictor of C. solstitialis abundance. We consolidated these findings into a novel conceptual model of invader success to illustrate how propagule input may outweigh community resistance through time, and what implications these dynamics have for the invasion paradox.

Pocket gopher (Thomomys talpoides) soil disturbance peaks at mid-elevation and is associated with air temperature, forb cover, and plant diversity

ABSTRACT Burrowing mammals can be ecosystem engineers by increasing soil aeration and erosion and altering the structure of plant communities. Studies that characterize the constraints on the distributions of fossorial mammal disturbances to soil can help predict changes in ecosystem engineering under future climates. We quantified the density of soil disturbances caused by Thomomys talpoides (northern pocket gopher) over replicate elevation gradients spanning 2,700–4,000 m a.s.l. in the Upper Gunnison Basin, Colorado, USA. As a conceptual framework for predicting biogeographic variation in soil disturbance, we used the abundant center hypothesis (ACH), which proposes that species abundance declines monotonically away from the most abundant location in its distribution, with the assumption that ecosystem engineering scales with gopher abundance. We also evaluated the relative importance of abiotic and biotic variables as correlates of soil disturbance. Gopher disturbance peaked at mid elevations (~3,150 m), supporting the ACH. The best model for predicting gopher-caused soil disturbance contained both abiotic and biotic variables, with increased soil disturbance where mean annual temperature, forb cover, and plant diversity were greatest. Results suggest that mountain ecosystems may experience increases in gopher-caused soil disturbance as climate warms, possibly accompanied by increases in plant diversity and forb cover.

Pathways for effects of small‐scale disturbances on a rare plant: How Mimulus angustatus benefits from gopher mounds

AbstractSmall‐scale soil disturbances such as soil mounds produced by gophers are known to influence local plant communities. A variety of mechanisms might account for the influence of gopher disturbances on individual plant success, but understanding of these mechanisms is not well developed. Disturbances are often assumed to affect plants through changing competition or the abiotic environment, but disturbances might also influence plant size, which in turn influences other biotic interactions, including pollination. In this study, we tested for effects of soil disturbance on the density, flower size, and reproduction of an annual plant, Mimulus angustatus. We used soil disturbance manipulations, observational data, and a pollination experiment to examine two possible pathways for effects of disturbance on M. angustatus: competition and pollination. We also considered how effects of gopher mounds change with years since initial disturbance. We found that disturbance strongly increased the local density and flower size of M. angustatus, but the strength of these effects decays quickly. We found no support for these effects being mediated by competition with other plant species. We also found that M. angustatus with larger flowers receive more natural pollination. Our pollination experiment suggests that M. angustatus benefits from pollination (seed set increases with hand pollination or access to pollinators), and suggests that pollen limitation may occur. Taken together, our results indicate that increased pollination and seed set is an alternative response to disturbance that should receive further consideration. Regardless of the exact mechanism, it appears that in the absence of frequent small‐scale soil disturbance, M. angustatus would not persist at our field site, as a large majority of plant recruitment and seed set occurs on recent disturbances.

How Does Simulated Gopher Disturbance Affect the Establishment ofHolcus lanatusL. (Poaceae) in Calfornia Coastal Prairie?

Abstract Disturbance often has the net effect of promoting invasive plant establishment, but the precise nature of the relationship between disturbance and invasion can depend on community context. We used simulated gopher mounds in bare and monoculture plots of three California native grasses (Bromus carinatus var. maritimus [Piper] C. L. Hitch., Festuca rubra L., and Calamagrostis nutkaensis [J. Presl] Steudel) to test the effects of mounds on seedling establishment and survival of the European perennial grass Holcus lanatus L. Soil disturbance treatments were crossed with manipulations of the plant canopy (shade cages in bare and Bromus plots vs. pinning back grass leaves in Festuca and Calamagrostis plots) to separate some of the positive and negative effects of natural mounds. Mean PAR measured in February at the soil surface varied from 11 to 969 µm/m2/s. As predicted, shade structures that decreased light availability but also increased soil moisture generally increased Holcus seedling establishmen...

Contributions of gopher mound and casting disturbances to plant community structure in a Cascade Range meadow complex

Pocket gophers (Geomyidae) are major agents of disturbance in North American grasslands. Gopher mounds bury existing plants and influence community structure through various mechanisms. However, in mountain meadows that experience winter snowpack, gophers also create winter castings, smaller tube-shaped deposits, previously ignored in studies of plant–gopher disturbance relationships. We studied the influences of the Mazama pocket gopher (Thomomys mazama Merriam, 1897) in montane meadows of the Oregon Cascades, quantifying community patterns at larger spatial scales than previously studied in this system and considering, for the first time, effects of both mounds and castings. We measured cover of disturbance and individual plant species along twenty 5 m transects in each of four plots with differing species composition. Total plant cover was negatively correlated with mounds and castings. However, only mounds influenced growth-form dominance, reducing graminoid cover and increasing the forb–graminoid ratio — effects attributable to the greater volume and longevity of mounds. Forb-disturbance relationships were variable among plots, likely due to differences in species’ tolerance of burial. Transect-scale richness and heterogeneity (variation in composition within transects) increased with disturbance. We conclude that frequent, small-scale disturbances create a shifting mosaic of vegetation states, reducing graminoid dominance and enhancing species richness and heterogeneity at larger spatial scales.

Rock Outcrops Harbor Native Perennials in Type-Converted Coastal Scrub

Type-conversion of coastal scrub to exotic annual grassland has been extensive in certain parts of southern California, especially in drier inland locales. Field observations suggest that rock outcrops harbor native perennials associated with coastal scrub vegetation after conversion to exotic annual grassland has occurred. Surveys were conducted to test this observation. In addition, measures of fossorial mammal disturbance, soil depth, exotic annual plant abundance, and soil moisture were collected and used to infer potential mechanisms that may influence patterns of native and exotic vegetation. Results showed that larger outcrops are associated with more native perennial species than smaller outcrops, consistent with predictions based on species-area relationships. In addition, the edges of rock outcrops were shown to have greater native perennial species richness than the surrounding exotic annual grassland matrix. Higher soil moisture, lower soil depth, and less disturbance by Botta's pocket gopher (Thomomys bottae) were also found at the edge of rock outcrops compared to the surrounding grassland matrix. Furthermore, soil depth was positively correlated with pocket gopher disturbance and with exotic annual grass cover. Rock outcrops serve as refuge habitat for native coastal scrub perennials in areas where exotic annual grasses have become dominant. Rock outcrops may become increasingly important for conservation efforts if remaining coastal scrub vegetation continues to deteriorate.

Pocket Gophers and the Invasion and Restoration of Native Bunchgrass Communities

More than 7 million hectares of California native plant communities are now dominated by exotic annual species, a biological invasion that has made native bunchgrass ecosystems in this region one of the most endangered ecosystems in North America. Many land use and environmental factors have contributed to the conversion of bunchgrass areas to annual grassland, but the role of gopher disturbance remains understudied. Here I report observational evidence that suggests gopher foraging is nonrandomly concentrated in the open spaces between clumps of bunchgrass and as a result may inhibit the recruitment of bunchgrass into these areas. Understanding patterns of direct gopher impact and coincident soil disturbance in bunchgrass versus annual grassland is important for successful restoration of bunchgrass habitats. While the prospect of excluding gophers is daunting, even short-term reductions in gopher populations would help to promote a restored community structure with a mature native bunchgrass and annual forb association.

LONG-TERM DATA REVEAL COMPLEX DYNAMICS IN GRASSLAND IN RELATION TO CLIMATE AND DISTURBANCE

We conducted a long-term experiment in grassland with the aim of elucidating grassland dynamics in relation to variations in rainfall amount and spatial and temporal variation in disturbance. In particular we aimed to increase our understanding of ecosystem dynamics and function, species redundancy, invasion biology, and other related topics. We studied the dynamics of serpentine annual grassland in northern California over the period 1983-2002 in a replicated series of experimental plots comprising controls, gopher exclosures, and aboveground herbivore exclosures. Annual rainfall amount varied greatly during the study period, which included two major El Nino events and a period of prolonged below-average rainfall. Gopher disturbance was highly variable both spatially and temporally but was positively correlated with soil depth. Disturbance was reduced but not eliminated from the gopher exclosures and was significantly increased in the aboveground herbivore exclosures. Grassland dynamics were driven by rainfall amounts and distributions that had the most pronounced effects on the dominant plant species, while gopher disturbance had additional effects on the rarer species. Effects of excluding aboveground herbivores were swamped by a large increase in gopher disturbance within aboveground exclosures. Overall species numbers were reduced during a period of below-average rainfall but recovered in subsequent years. There was a large array of different responses of individual plant species to both rainfall and disturbance. Our results provide support for the insurance hypothesis, which suggests that biodiversity buffers ecosystem processes against environmental changes because different species (or phenotypes) respond differently to these changes, leading to functional compensations among species. Here, a species that was at very low abundance levels at the start of the study (Microseris douglasii) temporarily increased in abundance to become one of the dominant species in the grassland following a period of prolonged below-average rainfall. We also observed the repeated invasion of the serpentine grassland by the nonnative grass Bromus hordeaceus, which increased greatly in abundance following both of the major El Nino events. The results emphasize the importance of long-term observations in providing a context for shorter-term studies and allowing analysis of plant community responses to climate variation and disturbance, particularly in the face of ongoing global change.

Selective gopher disturbance influences plant species effects on nitrogen cycling

Plant species effects on ecosystem processes are mediated by traits such as litter quality and exudation. These same traits also influence the activity and distribution of animals that play key roles in regulating ecosystem dynamics. We planted monocultures of eight plant species commonly found in California grasslands to investigate the relative importance of plant species direct effects on nitrogen cycling, versus their indirect effects mediated by plant interactions with gophers. Plant species differed in their litter C:N ratio, which closely related to species effects on rates of net mineralization and nitrification in undisturbed soil. However, the effect of selective gopher disturbance on N cycling greatly altered these species effects.Plant species differed in their effects on the type and timing of gopher disturbance. Small feeding holes were formed in late spring in plots containing species with high tissue quality. These feeding holes minimally disturbed the soil and did not alter N cycling rates over the short term. Large gopher mounds were formed in the winter and early spring, primarily in plots containing the grass, Aegilops triuncialis, and to a lesser extent in plots containing Avena barbata. These large mounds significantly disturbed the soil and greatly increased net nitrification rates, but had no consistent effects on net N mineralization. In undisturbed soil, Aegilops had the highest litter C:N ratio and one of the lowest rates of net nitrification. However, gophers preferentially built large mounds in Aegilops plots. Once the effects of gopher burrowing were considered, Aegilops had one of the highest rates of net nitrification, indicating that the indirect effects of plant species on N cycling can be more important than the direct effects alone. This experiment indicates that it is vital to consider interactions between plants and other organisms in order to predict the ecosystem effects of plant communities.

Temporal patterns in seedling establishment on pocket gopher disturbances.

Disturbances often facilitate seedling establishment, and can change the species composition of a community by increasing recruitment of disturbance-adapted species. To understand the effects of pocket gopher disturbances on alpine seedling dynamics, we examined the gopher disturbances' effects on seedling emergence and survival on gopher disturbances 0 to 5 years old. In contrast to results from most other ecosystems, these recently created gopher mounds had lower seedling emergence and survival rates than undisturbed areas. A lack of correlation between species' abundances on gopher mounds and undisturbed sites in one of the two communities studied suggested that a suite of disturbance-adapted species recruited onto the mounds. To explain low seedling emergence on recent gopher mounds, we quantified gopher mound seed banks and studied recruitment in a site with mounds that ranged from 0 to >20 years old. Seed numbers in first-year gopher mound soils were extremely low relative to undisturbed soils, and this pattern was mirrored in seedling establishment patterns over the long term. Gopher disturbance depressed seedling emergence density for the first 5 years. Subsequently, emergence density increased until at least 20 years following the disturbance. Emergence on disturbances more than 20 years old was higher than on undisturbed sites. Therefore, gopher disturbances probably facilitate seedling establishment in alpine dry and moist meadow; however, this process takes place over decades.

Components of spatial patterning in a serpentine grassland

We analyzed the components of spatial patterning in species abundance in a diverse grassland dominated by early season, annual forbs. Species abundance, soil depth and gopher disturbances were measured by means of a nested spatial design on two 8 m × 20 m plots that differed in the amount of larger scale variability in soil depth. Species distributions at the scale of the 10 cm × 10 cm quadrat were highly clumped, with a decline in clumping with mean abundance. Even with the effect of abundance on clumping removed, species showed differences in the degree of clumping and these differences were consistent between the two plots. The abundance of the species and the frequency of disturbance correlated weakly to moderately with soil depth. Semivariance analyses indicated that the most common species all showed complex spatial patterning at a range of scales. Some of this variation corresponded to patterns of soil depth variation and patterns of gopher disturbance; however, a large amount of the spatial patterning in species abundance remains unexplained.

GOPHER–PLANT–FUNGAL INTERACTIONS AFFECT ESTABLISHMENT OF AN INVASIVE GRASS

Many attempts have been made to link invasions of exotic plants to specific plant traits and key attributes of invaded ecosystems. While these factors play a role in determining the potential for invasion, they are often inadequate in predicting the success of a specific invasion. We show that interactions of an invasive grass with other members of the community determine the local pattern of invasion. A fungus, Ulocladium atrum, aids the establishment of barbed goatgrass (Aegilops triuncialis) by weakening the grass's tough seed head, thereby accelerating germination and seedling establishment. In contrast, gophers, Thomomys bottae, decrease establishment of this invader by selectively burying patches of goatgrass seedlings under mounds. Plants that survive these gopher disturbances produce seeds that are uninfected by Ulocladium atrum, which may further decrease the establishment of the next generation of goatgrass. A field survey indicated that goatgrass achieves dominance in areas with minimal gopher disturbance, but has limited establishment in pastures with high gopher activity, indicating that the landscape pattern of gopher activity influences patterns of goatgrass invasion by manipulating gopher-plant-fungal interactions.

SHRUB EXPANSION IN MONTANE MEADOWS: THE INTERACTION OF LOCAL-SCALE DISTURBANCE AND SITE ARIDITY

Montane meadows in the Sierra Nevada of California have experienced dra- matic expansion of shrubs (Artemisia rothrockii) and reduction in herbaceous species cover since the introduction of livestock in the late 1800s. Increases in meadow aridity due to livestock use has been proposed as the primary factor facilitating sagebrush dominance in these areas. However, our data suggest that sagebrush can readily expand into moist meadow areas where the water table is shallow. We explored how the relative importance of local processes influencing seedling establishment vary with changes in site aridity. We quantified patterns of sagebrush abundance in relation to water table depth and surface soil moisture and sagebrush seedling occurrence relative to distance from reproductive sagebrush plants and the presence of gopher disturbance. We tested the independent and interactive effects of vegetation clipping and surface soil disturbances on sagebrush germination, survival, and growth using experiments established in four vegetation types that differed in water table depth, surface soil moisture, and herbaceous species cover. Experiments were con- ducted over two growing seasons that differed in water availability. Our results suggest that small ( , 1m 2 ) soil disturbances promote the germination and growth of sagebrush seedlings in intact, ungrazed, moist, herbaceous meadow areas. In the absence of distur- bance, dense herbs, whether clipped or not, prevented germination. The effects of distur- bance were strongest in sites with moist surface soil that support a dense herbaceous canopy and were less important in sites with lower surface-soil moisture, where seedling estab- lishment rates were low despite abundant exposed soil. The spatial distribution of sagebrush seedlings is consistent with these experimental results. Sagebrush seedling density decreased dramatically with distance from reproductive shrubs, and seedlings were almost always preferentially associated with gopher mounds in moist herbaceous areas. Clipping above- ground biomass of herbs on a relatively small scale (4 m 2 ) had no effect on sagebrush germination or early seedling growth; however, the growth and survival of larger trans- planted seedlings was enhanced by clipping. We conclude that, while sagebrush expansion is traditionally associated with increased meadow aridity, it exhibits the greatest potential for seedling germination, growth, and survival in mesic, rather than xeric, sites. Realization of this potential is dependent on the confluence of exposed soil, a nearby seed source, and reduction of aboveground herb biomass.

Effects of Plains Pocket Gopher (Geomys bursarius) Disturbances on Tallgrass-prairie Plant Community Structure

Abstract Our objective was to evaluate the effects of soil disturbances created by the plains pocket gopher (Geomys bursarius) on the structure of a tallgrass-prairie plant community. We predicted that soil mounds and burrows would provide sites for the establishment of subordinate plant species, thereby increasing regional plant community richness in this ecosystem that is highly dominated by perennial C4 tallgrasses. Our results, however, revealed that plant species richness and biomass were temporarily decreased or unaffected in areas disturbed by gophers. Moreover, the species found locally on mounds and burrows were a subset of the dominant plants present in the undisturbed plant community and graminoids were more frequent on disturbances than forbs. Our results indicate that perennial graminoids predominate in the rapid recovery of vegetation on pocket gopher mounds and burrows. This preempts the establishment of less frequent forbs and, contrary to our predictions, decreases plant community richness.

Spatial relationships between plant litter, gopher disturbance and vegetation at different stages of old‐field succession

Abstract. Fine‐scale spatial patterns of small mammal disturbances and local accumulation of plant litter were studied together with the spatial pattern of vegetation in different stages of old‐field succession at Cedar Creek Natural History Area, Minnesota, USA. Seven stands from one to 66 years old were sampled. Presence of living plant species, local soil disturbances by pocket gophers (Geomys bursarius) and plant litter accumulation were recorded in 10 cm × 10 cm contiguous microquadrats along elliptical transects. Spatial patterns and associations were analyzed using information theory models. Dominant grasses were spatially independent, while subordinate functional groups were strongly dependent on the existing patchwork of dominant species, plant litter and gopher disturbances. Litter had consistent negative associations with subordinate functional groups in all but the initial years. Gopher disturbances were abundant but had weak and variable associations with vegetation. These results suggest that gopher disturbance does not facilitate the colonization of native prairie species and that diversity can be improved by controlling litter accumulation in Minnesota old‐fields.

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.

Analysis of fine-scale spatial pattern of a grassland from remotely-sensed imagery and field collected data

An important practical problem in the analysis of spatial pattern in ecological systems is that requires spatially-intensive data, with both fine resolution and large extent. Such information is often difficult to obtain from field-measured variables. Digital imagery can offer a valuable, alternative source of information in the analysis of ecological pattern. In the present paper, we use remotely-sensed imagery to provide a link between field-based information and spatially-explicit modeling of ecological processes. We analyzed one digitized color infrared aerial photograph of a serpentine grassland to develop a detailed digital map of land cover categories (31.24 m × 50.04 m of extent and 135 mm of resolution), and an image of vegetation index (proportional to the amount of green biomass cover in the field). We conducted a variogram analysis of the spatial pattern of both field-measured (microtopography, soil depth) and image-derived (land cover map, vegetation index, gopher disturbance) landscape variables, and used a statistical simulation method to produce random realizations of the image of vegetation index based upon our characterization of its spatial structure. The analysis revealed strong relationships in the spatial distribution of the ecological variables (e.g., gopher mounds and perennial grasses are found primarily on deeper soils) and a non-fractal nested spatial pattern in the distribution of green biomass as measured by the vegetation index. The spatial pattern of the vegetation index was composed of three basic components: an exponential trend from 0 m to 4 m, which is related to local ecological processes, a linear trend at broader scales, which is related to a general change in topography across the study site, and a superimposed periodic structure, which is related to the regular spacing of deeper soils within the study site. Simulations of the image of vegetation index confirmed our interpretation of the variograms. The simulations also illustrated the limits of statistical analysis and interpolations based solely on the semivariogram, because they cannot adequately characterize spatial discontinuities.

Spatial and temporal variability in California annual grassland: results from a long‐term study

Abstract. We present data from the first 11 years of a longterm study of the dynamics of an annual grassland on serpentine soil in Jasper Ridge Biological Preserve, Northern California. Annual rainfall amounts and distributions varied greatly over the period 1982‐1993, as did the amount and distribution of gopher disturbance. Temporal variation in gopher disturbance showed no relationship with rainfall, but spatial variation in disturbance frequency was related to soil depth. The disturbance regime experienced by the grassland is complex, both spatially and temporally, and most of the area is disturbed at least once every 3‐5 years. Plant species abundances showed a variety of responses to climate variation and disturbance. Abundances of individual species in any given year could not be linked directly to rainfall amount due to hysteresis effects and other interactions. The grassland composition changed markedly over the study. Exclusion of gophers suggested that changing abundances of several species were linked to gopher disturbance. In particular, perennial species' abundances increased greatly in the years following exclosure, but then subsequently declined. Data on plant densities on gopher mounds disturbed at different times of year and in different years indicate that the local species composition remains distinct for a number of years following disturbance. Disturbance history is hence a major factor controlling local community variation. Changing species importances, a complex disturbance regime and the importance of disturbance history make prediction and modelling of this system difficult. It is suggested that the same is probably true for many plant communities, and that long‐term studies must be an essential part of ecological research programs. This study illustrates the practical problems inherent in maintaining long‐term field experiments and in analyzing complex time series data which suffer from inadvertent deviations from the original experimental design.