Laura Foster Huenneke

Biological Feedbacks in Global Desertification

Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.

EFFECTS OF SOIL RESOURCES ON PLANT INVASION AND COMMUNITY STRUCTURE IN CALIFORNIAN SERPENTINE GRASSLAND

Non-native annual grasses dominate most Californian mediterranean-cli- mate grasslands today. However, native Californian grassland flora persists in grasslands on serpentine-derived soils. We manipulated soil nutrient resources to explore the links between nutrient availability, plant productivity, and the relative abundances of native and non-native species in serpentine grassland. Factorial combinations of nitrogen, phos- phorus, and other components of a nutritionally complete formula were added to field plots over two growing seasons. Fertilization with nitrogen and phosphorus increased biomass of the resident vegetation substantially in the first season, and within two years allowed the invasion and dominance of non-native annual grasses in patches originally dominated by native annual forbs. Species richness declined with fertilization, as the increased biomass production by invaders sup- pressed some native forbs. Increased macronutrient availability can increase production on serpentine-derived soil, even when other serpentine characteristics (such as low Ca/Mg ratios and high heavy-metal concentrations) have not been mitigated. Observed changes in community structure and composition demonstrate that the invasibility of plant com- munities may be directly influenced by nutrient availability, independent of physical dis- turbance.

Functional diversity revealed by removal experiments

The dominant protocol to study the effects of plant diversity on ecosystem functioning has involved synthetically assembled communities, in which the experimental design determines species composition. By contrast, the composition of naturally assembled communities is determined by environmental filters, species recruitment and dispersal, and other assembly processes. Consequently, natural communities and ecosystems can differ from synthetic systems in their reaction to changes in diversity. Removal experiments, in which the diversity of naturally assembled communities is manipulated by removing various components, complement synthetic-assemblage experiments in exploring the relationship between diversity and ecosystem functioning. Results of recent removal experiments suggest that they are more useful for understanding the ecosystem effects of local, nonrandom extinctions, changes in the natural abundance of species, and complex interspecific interactions. This makes removal experiments a promising avenue for progress in ecological theory and an important source of information for those involved in making land-use and conservation decisions. Current extinction rates caused by human activities are orders of magnitude higher than natural background levels [1], and it is crucial that we understand the functional consequences of such extinctions. Terrestrial plants provide the basis for many fundamental ecosystem processes and services; therefore, many initiatives have been launched in the past decade to address this issue by documenting the possible effects of terrestrial plant diversity on ecosystem processes. Most of these studies are based on experiments using synthetic communities, in

Long-term and large-scale perspectives on the relationship between biodiversity and ecosystem functioning

Abstract In a growing body of literature from a variety of ecosystems is strong evidence that various components of biodiversity have significant impacts on ecosystem functioning. However, much of this evidence comes from short-term, small-scale experiments in which communities are synthesized from relatively small species pools and conditions are highly controlled. Extrapolation of the results of such experiments to longer time scales and larger spatial scales—those of whole ecosystems—is difficult because the experiments do not incorporate natural processes such as recruitment limitation and colonization of new species. We show how long-term study of planned and accidental changes in species richness and composition suggests that the effects of biodiversity on ecosystem functioning will vary over time and space. More important, we also highlight areas of uncertainty that need to be addressed through coordinated cross-scale and cross-site research.

Microsite abundance and distribution of woody seedlings in a South Carolina cypress-tupelo swamp

At least 16 types of microsites or substrates for vascular plant seedlings can be distinguished in bald cypress-water tupelo (Taxodium distichum-Nyssa aquatica) swamps. We measured the relative abundances of these microsite types, and the distribution of woody seedlings on them, in two riverine swamp forests on the Savannah River floodplain, South Carolina. Microsite abundances in a little disturbed forest differed significantly from those in a more open stand which had experienced much recent sediment deposition from upstream erosion, as well as higher water temperatures. Woody seedlings were distributed nonrandomly among microsite types (i.e., not in proportion to the abundance of a given microsite type). There were significant differences in microsite distribution patterns among growth forms (tree spp. vs. shrubs vs. vines) and among species within growth form. Many human activities may alter substrate nature and abundance in a wetland, thus indirectly altering the abundance and species composition of seedling recruitment.

Seedling and clonal recruitment of the invasive tree Psidium cattleianum: Implications for management of native Hawaiian forests

Abstract Non-native plants present serious management problems in many preserves. Strawberry guava Psidium cattleianum (Myrtaceae), a small tree and aggressive invader of tropical areas, is rapidly spreading through many Hawaiian forests including those of the two US national parks in Hawaii. Feral pigs and non-native birds disperse Psidium seeds; pigs also create soil disturbances that may enhance the trees spread. Our study of guavas reproductive biology focussed on its dependence on non-native animals. We found that the abundantly produced seed germinated rapidly under a wide range of conditions, without scarification. Psidium seedlings occur on the same substrates as do native seedlings, usually on undisturbed sites. Both seedlings and clonally produced suckers are common, but suckers contribute greater leaf areas. Guavas clonal growth may partially explain its success in dominating native forests. Apparently germination and establishment do not depend on animal dispersal, or on disturbances created by pigs; thus, control of the plant cannot rest entirely on control of non-native animals.

Stem Dynamics of the Shrub Alnus Incana SSP. Rugosa: Transition Matrix Models

We constructed transition matrices from observed rates of sprout production and stem growth in two New York state populations of a clonal shrub, Alnus incana ssp. rugosa. The largest real eigenvalue of the transition matrix, X, summarized current pop- ulation rates of growth. An old-field alder thicket showed slow population decline over all 3 yr of study. The other population, in an undisturbed natural swamp, showed decline in each year but overall increase when data from all 3 yr were combined. Sensitivity matrices were calculated from the stable size distributions and reproductive value vectors for each 3 yr matrix. Sensitivity of population growth rates to specific tran- sitions was illustrated by changing values of the matrix elements and calculating resultant X values. Population growth rates were least affected by changes in survivorship of small stems, and most affected by altering sprout production rates and large stem survivorship. Annual variations in demographic rates, and comparisons between actual stem size class distributions and the stable distributions predicted by the matrix models, confirmed the inadequacy of assuming constant transition probabilities. In such a case, transition matrix models are best viewed not as projections of long-term population behavior, but as indi- cators of current demographic trends.

Gopher mound soil reduces growth and affects ion uptake of two annual grassland species

SummaryPortions of an annual serpentine grassland community in California are subject to frequent gopher mound formation. Consequently, studies were undertaken to characterize the effects of mound soils on plant growth and ion uptake. For two of the dominant annual species (Bromus mollis L. and Plantago erecta Morris), growth was reduced in gopher mound soil relative to that in inter-mound soil. A similar reduction in growth was found for plants grown in soils collected at a depth corresponding to the depth of gopher burrowing. This reduction in growth was associated with lower total P and N contents of the soil which were reflected in lower shoot contents of N and P. Additional experiments, however, showed that reduced N and P availabilities in mound soil were not entirely responsible for the growth reduction. Similarly, shoot Ca/Mg ratios were reduced in mound soil but additions of Ca improved the Ca/Mg ratio without improving growth. Growth reductions were associated with altered shoot concentrations of microelements, particularly elevated levels of Mn. A competition experiment between Plantago and Bromus showed that Bromus was more competitive than Plantago in mound and inter-mound soils and that soil type had only small affects on the nature of the interaction between the two species.

Understory Response to Gaps Caused by the Death of Ulmus americana in Central New York

HUENNEKE, L. F. (Ecology & Systematics, Cornell University, Ithaca, N.Y. 14853). Understory response to gaps caused by the death of Ulmus americana in central New York. Bull. Torrey Bot. Club 110: 170-175. 1983.-Circular plots of 250 m2 area centered on standing dead stems of Ulmus americana were used to sample understory vegetation in canopy gaps created by the death of elms. A total of 37 plots were sampled in 28 sites representing a variety of wetland habitats. Shrub species typical of open habitats were abundant in gaps created by the death of 2 or more neighboring elms (multiple-tree gaps) but not in single-tree gaps. Multipletree gaps contained fewer seedlings and saplings of canopy tree species than did single-tree gaps. The patchiness of occurrence of the open-canopy shrubs and their relationship to multiple-tree gaps were not reflected in conventional large-scale (0.1 ha) vegetation samples of the same stands. The establishment of dense patches of shrubs may inhibit tree regeneration within the multiple-tree gaps.

Effect of reproductive modes and environmental heterogeneity in the population dynamics of a geographically widespread clonal desert cactus

The dynamics of plant populations in arid environments are largely affected by the unpredictable environmental conditions and are fine-tuned by biotic factors, such as modes of recruitment. A single species must cope with both spatial and temporal heterogeneity that trigger pulses of sexual and clonal establishment throughout its distributional range. We studied two populations of the clonal, purple prickly pear cactus, Opuntia macrocentra, in order to contrast the factors responsible for the population dynamics of a common, widely distributed species. The study sites were located in protected areas that correspond to extreme latitudinal locations for this species within the Chihuahuan Desert. We studied both populations for four consecutive years and determined the demographic consequences of environmental variability and the mode of reproduction using matrix population models, life table response experiments (LTREs), and loop and perturbation analyses. Although both populations seemed fairly stable (population growth rate, λ∼1), different demographic parameters and different life cycle routes were responsible for this stability in each population. In the southernmost population (MBR) LTRE and loop and elasticity analyses showed that stasis is the demographic process with the highest contributions to λ, followed by sexual reproduction, and clonal propagation contributed the least. The northern population (CR) had both higher elasticities and larger contributions of stasis, followed by clonal propagation and sexual recruitment. Loop analysis also showed that individuals in CR have more paths to complete a life cycle than those in MBR. As a consequence, each population differed in life history traits (e.g., size class structure, size at sexual maturity, and reproductive value). Numerical perturbation analyses showed a small effect of the seed bank on the λ of both populations, while the transition from seeds to seedlings had an important effect mainly in the northern population. Clonal propagation (higher survival and higher contributions to vital rates) seems to be more important for maintaining populations over long time periods than sexual reproduction.