Doria R. Gordon

EFFECTS OF INVASIVE, NON‐INDIGENOUS PLANT SPECIES ON ECOSYSTEM PROCESSES: LESSONS FROM FLORIDA

Individual plant species that modify ecosystem properties have traditionally been thought to be uncommon in natural systems. I hypothesize that many invasive non- indigenous species do alter these properties at several scales. The non-indigenous plant species in Florida considered the most invasive by the Florida Exotic Pest Plant Council are examined for this capability through review of the available literature. Out of 31 species total, 12-20 (39-64%) potentially alter the ecosystem properties of geomorphology, hy- drology, biogeochemistry, and disturbance. When population-level properties that indicate superior competitive ability of the invading species are examined, 13-24 (42-77%) of the species are included, with the majority of species showing traits capable of modifying natural systems at both ecosystem and community/population scales. This review suggests that ecosystem alteration may be relatively common among invasive non-indigenous spe- cies. However, much of the current information is anecdotal. Empirical studies directly examining the effects of species on ecosystem and smaller-scale processes are necessary, and highly invasive species may be particularly appropriate for such research. Further, as non-indigenous species homogenize the global flora, they may also homogenize the local flora by increasing the representation of ruderal species. Where ecosystem processes have been altered, site restoration likely will require both control of the invader(s) and recovery of processes.

Deliberate Introductions of Species: Research Needs Benefits can be reaped, but risks are high

The silent invasion of Hawaii by insects, disease organisms, snakes, weeds and other pests is the single greatest threat to Hawaii’s economy and natural environment.... Even one new pest-like the brown tree snake--could forever change the character of our islands. (Coordinating Group on Alien Pest Species 1996, P. 1). Reforestation in the tropics is so vastly behind deforestation that we cannot wait to fully appraise all the potential negative elements of domestication. Weediness is of consequence perhaps in Honolulu, but not in Addis or Delhi. (James Brewbaker, quoted by Hughes 1994, p. 244 ).

Science Priorities for Reducing the Threat of Invasive Species to Sustainable Forestry

Abstract Invasive species pose a major, yet poorly addressed, threat to sustainable forestry. Here we set forth an interdisciplinary science strategy of research, development, and applications to reduce this threat. To spur action by public and private entities that too often are slow, reluctant, or unable to act, we recommend (a) better integrating invasive species into sustainable forestry frameworks such as the Montréal Process and forest certification programs; (b) developing improved cost estimates to inform choices about international trade and pest suppression efforts; and (c) building distributed information systems that deliver information on risks, identification, and response strategies. To enhance the success of prevention and management actions, we recommend (a) advancing technologies for molecular identification, expert systems, and remote sensing; (b) evolving approaches for ecosystem and landscape management; and (c) better anticipating interactions between species invasions and other global change processes.

Competitive effects of grassland annuals on soil water and blue oak (Quercus douglasii) seedlings

Four California annual grassland species were used to examine the hypothesis that different plant species have equivalent competitive effects. We investigated the effects of the annuals on soil water availability and the growth responses of blue oak (Quercus douglasii Hook & Arn.) to neighbor—induced water depletion. Neighborhoods of annuals were composed of species from California annual grassland with differing phenology and morphology that were hypothesized to show non—equivalent competitive effects on both a per—individual— and a per—unit—tissue basis. Three introduced species of winter annuals were sown at each of three densities (10, 30, or 100 seeds/dm2) around a single oak acorn. The grass Bromus diandrus Roth. and the forb Erodium botrys Cav. flower in early spring, while the grass B. mollis L. flowers slightly later. A native summer annual forb, Hemizonia luzulaefolia DC., was sown at only the intermediate density. Plants were grown outdoors in 15 cm diameter X 1 m deep tubes filled with soil from an oak woodland site. Identity and density of annual species had independent and interactive effects on the water resource level. Soil water potential was negatively density—dependent, remaining highest when oaks were grown without competitors. At the intermediate density soil water potentials decreased in the following order: B. mollis (slowest), B. diandrus, E. botrys, and H. luzulaefolia (most rapid). This order followed the pattern of root length developed in the neighborhoods, which was species— and density—dependent. The relationship between sowing density and neighborhood dry mass also differed among the annual species. Correlations between root length and soil water potential were stronger for the forbs than for the grasses. A given root length of the summer annual, H. Luzulaefolia, depleted soil water to a greater extent than did the same root length of any species. Oak seedling growth, gas exchange rates, and survivorship were negatively related to the rate and extent of soil water depletion. Additionally, the relationship between annual root length and oak seedling height or root length differed by neighborhood species. Thus, neighborhood species were non—equivalent in competitive effect on both a per—individual and a per—unit—root—length basis. Phenologically different species may represent different functional groups of competitors.

Overstory tree mortality resulting from reintroducing fire to long-unburned longleaf pine forests: the importance of duff moisture

In forests historically maintained by frequent fire, reintroducing fire after decades of exclusion often causes widespread overstory mortality. To better understand this phenomenon, we subjected 16 fire-excluded (ca. 40 years since fire) 10 ha longleaf pine (Pinus palustris Mill.) stands to one of four replicated burning treatments based on volumetric duff moisture content (VDMC): wet (115% VDMC); moist (85% VDMC); dry (55% VDMC); and a no-burn control. Dur- ing the first 2 years postfire, overstory pines in the dry burns suffered the greatest mortality (mean 20.5%); pine mortality in the wet and moist treatments did not differ from the control treatment. Duff reduction was greatest in the dry burns (mean 46.5%), with minimal reduction in the moist and wet burns (14.5% and 5%, respectively). Nested logistic regression using trees from all treatments revealed that the best predictors of individual pine mortality were duff consumption and crown scorch (P < 0.001; R 2 = 0.34). Crown scorch was significant only in dry burns, whereas duff consumption was sig- nificant across all treatments. Duff consumption was related to moisture content in lower duff (Oa; R 2 = 0.78, P < 0.001). Restoring fire to long-unburned forests will require development of burn prescriptions that include the effects of duff con- sumption, an often overlooked fire effect.

Reproductive biology of three sympatric endangered plants endemic to Florida scrub

We investigated the reproductive biology of three plants endemic to rosemary scrub habitats on the Lake Wales Ridge of Florida, USA. We used hand-pollination experiments and observations of flowers and their insect visitors to determine their mating systems and pollination. Fruit or seed set after self pollination was 94, 97, and 8% of fruit or seed set after cross pollination in Eryngium cuneifolium (Apiaceae), Hypericum cumulicola (Hypericaceae), and Liatris ohlingerae (Asteraceae) respectively, indicating that the first two are self-compatible and the last is obligately outcrossing. All three depend on insects for seed production (4–7% fruit or seed set without insects). Diverse insects visit flowers of E. cuneifolium (101 species recorded), whereas L. ohlingerae is visited predominantly by butterflies and H. cumulicola by one genus of bees (Dialictus, Halictidae). Our data indicate pollinator visitation does not currently limit seed production in E. cuneifolium or H. cumulicola, but does in L. ohlingerae. Despite the features they share (habit, habitat, disturbance regime), we found unique aspects of these species’ reproductive biology yielding unique risks to population viability. We suggest that multispecies recovery plans must consider several aspects of the biology of species with superficial similarities to be successful. # 2003 Elsevier Science Ltd. All rights reserved.

Predicting invasive plants in Florida using the Australian weed risk assessment.

Abstract Screening tools that effectively predict which nonnative species are likely to become invasive are necessary because of the disproportionate ecological and economic costs associated with invaders. We tested the effectiveness of the Australian Weed Risk Assessment system (WRA) in distinguishing plant species that are major invaders, minor invaders, and noninvaders in Florida. The test included 158 annuals and perennials in six growth forms from 52 families in 27 orders. The WRA with a secondary screen met all hypothesized accuracy levels: it correctly rejected 92% of test species that have been documented to be invasive in Florida and correctly accepted 73% of the noninvaders. The incorrect rejection of noninvaders was 8% with the remaining 19% of noninvaders falling into the “evaluate further” outcome. Only 10% of the 158 species required further evaluation. Invaders of natural areas and agricultural systems were identified with equal accuracy. Receiver operating characteristic analysis demonstrated high separation of invaders from noninvaders. The degree to which the WRA is precautionary may be adjusted by altering the cutoff scores that define the “accept, evaluate further,” and “reject” outcomes. This approach could be adopted in Florida as a screening mechanism to reduce importation of new invaders.

Genetic diversity and reproductive biology in Warea carteri (Brassicaceae), a narrowly endemic Florida scrub annual

Carters mustard (Warea carteri) is an endangered, fire-stimulated annual endemic of the Lake Wales Ridge, Florida, USA. This species is characterized by seed banks and large fluctuations in plant numbers, with increases occurring in postdisturbance habitat. We investigated the mating system, patterns of isozyme variation, and effective population sizes of W. carteri to better understand its population biology and to comment on reserve designs and management proposals relevant to this species. Warea carteri is self-compatible and autogamous, and probably largely selfing. Measures of genetic variation in W. carteri were lower than values reported for species with similar ecological and life history traits (6.6% of loci polymorphic within populations, 1.87 alleles per polymorphic locus, and 0.026 and 0.018 expected and observed heterozygosity, respectively). The high average value for Neis genetic identity (0.989) reflects the paucity of genetic diversity. Genetic variation within populations was not correlated with aboveground population size, effective population size estimates (N(e)), or recent disturbance history. Much of the diversity detected was found among populations (F(ST) = 0.304). A significant cline in allele frequencies at one locus and a significant negative correlation between geographic distance and Neis genetic identity also point to spatial organization of genetic diversity. As a result we propose that reserve design should include the entire geographic range of W. carteri. We also recommend that the natural fire regime be mimicked.

Capture and allocation of nitrogen byQuercus douglasii seedlings in competition with annual and perennial grasses

SummaryThe spatial overlap of woody plant root systems and that of annual or perennial grasses promotes competition for soil-derived resources. In this study we examined competition for soil nitrogen between blue oak seedlings and either the annual grassBromus mollis or the perennial grassStipa pulchra under controlled outdoor conditions. Short-term nitrogen competition was quantified by injecting15N at 30 cm depth in a plane horizontal to oak seedling roots and that of their neighbors, and calculating15N uptake rates, pool sizes and15N allocation patterns 24 h after labelling. Simultaneously, integrative nitrogen competition was quantified by examining total nitrogen capture, total nitrogen pools and total nitrogen allocation.Stipa neighbors reduced inorganic soil nitrogen content to a greater extent than didBromus plants. Blue oak seedlings responded to lower soil nitrogen content by allocating lower amounts of nitrogen per unit of biomass producing higher root length densities and reducing the nitrogen content of root tissue. In addition, blue oak seedlings growing with the perennial grass exhibited greater rates of15N uptake, on a root mass basis, compensating for higher soil nitrogen competition inStipa neighborhoods. Our findings suggest that while oak seedlings have lower rates of nitrogen capture than herbaceous neighbors, oak seedlings exhibit significant changes in nitrogen allocation and nitrogen uptake rates which may offset the competitive effect annual or perennial grasses have on soil nitrogen content.

Breeding bird response to midstory hardwood reduction in Florida sandhill longleaf pine forests

The dramatic loss and degradation of longleaf pine (Pinus palustris) forests in the southeastern United States have contributed to the declines of several bird species associated with this ecosystem. We examined breeding bird response to habitat restoration in longleaf pine sandhill forests of northwest Florida, USA. We compared habitat variables and abundance of breeding birds among 3 hardwood reduction techniques (prescribed spring burning, herbicide application, mechanical felling-girdling) and maintenance of fire exclusion (control). We also examined abundance of breeding birds in fire-maintained, non-experimental reference sites as a means to gauge management success based on similarity analysis. All 3 hardwood reduction methods decreased midstory hardwoods and canopy cover, and increased herbaceous cover compared with fire-suppressed control plots, but had little effect on longleaf pine basal area. Midstory hardwood reduction methods increased the similarity of the breeding bird species assemblages to those found in the reference plots, compared with the fire-excluded control plots. Species that most strongly contributed to the similarity between the hardwood reduction and the reference plots were, in decreasing order of importance: red-cockaded woodpecker (Picoides borealis), red-headed woodpecker (Melanerpes erythrocephalus), tufted titmouse (Baeolophus bicolor), and brown-headed nuthatch (Sitta pusilla). Depending on the similarity index used, additional but weaker contributors to the similarity between the hardwood reduction and the reference plots were northern cardinal (Cardinalis cardinalis), Bachmans sparrow (Aimophila aestivalis), and Carolina chickadee (Poecile carolinensis). Species associated with hardwoods (e.g., tufted titmouse, northern cardinal, Carolina chickadee) contributed to greater similarity of the reference condition by decreasing in abundance after hardwood reduction, whereas open-habitat species increased in numbers, compared with birds on the fire-excluded control plots. Only the detection rates of cardinals and Carolina chickadees were not different among hardwood reduction methods. Two notable species of concern that increased in all hardwood reduction plots compared with the control, but were not strong contributors of the similarity to the reference condition, were northern bobwhite (Colinus virginianus) and southeastern American kestrel (Falco sparverius paulus). Our results suggest that these management methods were beneficial to landbirds characteristic of fire-maintained longleaf pine sandhills in northwest Florida. However, northern bobwhites and Bachmans sparrows may require more intensive management than red-cockaded woodpeckers, such as shorter burn intervals, as these species depend directly on ground cover vegetation. We recommend prescribed spring burning as the most economical and ecologically desirable method for managing breeding bird habitat in longleaf pine sandhill forests.