Mixed-species Plots Research Articles

Linking tree biodiversity to belowground process in a young tropical plantation: Impacts on soil CO 2 flux

This study examined the effect of tree species identity and diversity on soil respiration in a 3-year-old tropical tree biodiversity plantation in Central Panamá. We hypothesized that tree pairs in mixed-species plots would have higher soil respiration rates than those in monoculture plots as a result of increased primary productivity and complementarity leading to greater root and microbial biomass and soil respiration. In addition to soil respiration, we measured potential controls including root, tree, and microbial biomass, soil moisture, surface temperature, bulk density. Over the course of the wet season, soil respiration decreased from the June highs (7.2 ± 3.5 μmol CO 2/(m 2 s −1) to a low of 2.3 ± 1.9 μmol CO 2/(m 2 s −1) in the last 2 weeks of October. The lowest rates of soil respiration were at the peak of the dry season (1.0 ± 0.7 μmol CO 2/(m 2 s −1)). Contrary to our hypothesis, soil respiration was 19–31% higher in monoculture than in pairs and plots with higher diversity in the dry and rainy seasons. Although tree biomass was significantly higher in pairs and plots with higher diversity, there were no significant differences in either root or microbial biomass between monoculture and two-species pairs. Path analyses allow the comparison of different pathways relating soil respiration to either biotic or abiotic controls factors. The path linking crown volume to soil temperature then respiration has the highest correlation, with a value of 0.560, suggesting that canopy controls on soil climate may drive soil respiration.

Biodiversity enhances individual performance but does not affect survivorship in tropical trees

We developed an analytical method that quantifies the relative contributions of mortality and individual growth to ecosystem function and analysed the results from the first biodiversity experiment conducted in a tropical tree plantation. In Sardinilla, central Panama, over 5000 tree seedlings were planted in monoculture and mixed-species plots. After 5 years of growth, mixed-species plots yielded, on average, 30-58% higher summed tree basal area than did monocultures. Simulation models revealed that the increased yield of mixed-species plots was due mostly to enhancement of individual tree growth. Although c. 1500 trees died during the experiment, mortality was highly species-specific and did not differ consistently between biodiversity treatments. Our results show that the effects of biodiversity on growth and mortality are uncoupled and that biodiversity affects total biomass and potentially self-thinning. The Sardinilla experiment suggests that mixed-species plantings may be a viable strategy for increasing timber yields and preserving biodiversity in tropical tree plantations.

Effects of multiple perturbations on the survivorship of fragments of three coral species

Fist-sized fragments of Porites cylindrica, Porites rus and Pavona frondifera were deployed in single-species ( P. cylindrica) and mixed-species (all three) plots in a shallow reef area in the northwestern Philippines. After 6 months, the corals in half of the plots were broken into smaller pieces to simulate an episodic physical disturbance. The survival of all corals was monitored from March 2000 to July 2001 during which the corals experienced 2 typhoons and episodes of algal overgrowth. For both intact and broken treatments, there was significantly higher survival in the mixed-species plots than in the single-species treatments. Fragment mortality varied between disturbances of varying frequencies and magnitudes, namely: one-time fragmentation stress, seasonal overgrowth by cyanobacteria and macroalgae, short-term (1 day) and long-term (more than 1 week) burial. The mixed-species assemblages had higher fragment survivorship than the monospecific assemblages during small-scale perturbations (e.g., algal overgrowth), but not in the face of subsequent, larger scale disturbances. This study emphasizes that coral responses to disturbance are both species- and context-specific.

Floral diversity and the facilitation of pollination

Summary 1 Multiple-species floral displays have been hypothesized to facilitate pollination by attracting a greater number and/or diversity of pollinators. Here I present experimental confirmation of pollination facilitation among coflowering plants that have morphologically distinct flowers. 2 Pollinator visits to Raphanus raphanistrum, a self-incompatible herbaceous plant, increased when it occurred with one or a combination of Cirsium arvense, Hypericum perforatum and Solidago canadensis than when it occurred alone. 3 Enhanced visitation to R. raphanistrum in mixed species plots was reflected by increased seed production. 4 Facilitative effects in pollination were conditional on the density and evenness of the floral mixture and graded into competition as the relative abundance of R. raphanistrum declined in a two-species mixture. 5 Previously proposed mechanisms for facilitative interactions cannot explain facilitation among florally distinct plant displays. An alternative mechanism of differential but complementary floral rewards is proposed to explain facilitative attraction of pollinators. 6 Facilitation of, and competition for, pollination has implications for regeneration by seed of rare or isolated plants, and of mitigating Allee effects that afflict such populations.

Coral responses in single- and mixed-species plots to nutrient disturbance

Space is a limiting resource in coral reef communities causing actively growing coral colonies to come in proximity and interact with each other. Although most contact interactions among corals have been studied extensively, very few non-contact and non-aggressive interactions have been documented so far. We present results from a 3 yr field study of coral communities showing that, under unperturbed conditions, the reef-building coral Porites cylindrica exhibits significantly higher growth when transplanted together with 2 other species (P. rus and Pavona frondifera) than when grown in monoculture. However, the introduction of a chronic disturbance (nutrient enrichment) adversely affected its growth rates and survival, thus overturning the earlier trend. Furthermore, the 3 species used in the experiment exhibited different responses to the perturbation (negative, no effect, positive). Our results show that the presence of other species can enhance performance at the colony level, while differential species responses potentially provide buffering effects at the community level that may contribute to the maintenance of community structure and function during periods of disturbance.

POLLINATOR PREFERENCES FOR NICOTIANA ALATA, N. FORGETIANA, AND THEIR F1HYBRIDS

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed-species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.

POLLINATOR PREFERENCES FOR NICOTIANA ALATA, N. FORGETIANA, AND THEIR F1 HYBRIDS

The role of pollinators in plant speciation and maintenance of species boundaries is dubious because most plant species are visited by several types of pollinators, and most pollinator species visit several species of plants. We investigated pollinator preferences and their efficacy as ethological isolation mechanisms between two interfertile species, Nicotiana alata and N. forgetiana and their F1 hybrids. Hawkmoths pollinate N. alata, while primarily hummingbirds and occasionally small hawkmoths visit N. forgetiana. F1 hybrids are easily produced in the greenhouse and although the species grow in similar habitats, hybrids have not been found in nature. In Rio Grande do Sul, Brazil, near where both species are found, experimental plots were studied containing both species, and both species plus F1 hybrids. In the mixed-species plots, hawkmoths showed a strong preference for N. alata. Hummingbirds were less common and only visited N. forgetiana. Hybrid seed was produced but plants made significantly fewer hybrid offspring than predicted by the frequency of interspecific pollinator movements. Nicotiana forgetiana was the seed parent of 97% of the F1 offspring, suggesting an asymmetry in pollen delivery or postpollination processes. In plots containing F1 hybrids plus both parental species, hawkmoths preferred N. alata and undervisited the other two phenotypes, except that in the third plot they visited hybrids in proportion to the hybrid frequency. Hummingbirds strongly preferred N. forgetiana in all plots but also visited F1 hybrids in proportion to their frequency in the third plot. Overall, F1 hybrids were well pollinated and were frequently visited immediately before or after one of the parental species. Thus hybrids could facilitate gene flow between the parental species. We conclude that pollinator discrimination among species is strong but is an imperfect isolation mechanism, especially if hybrids are present.

A model of ecosystem health and its application

Aimed at the special background of arid region Shapotou in China, the paper presented a measure of ecosystem health, which was based on three criteria proposed by Costanza: vigor, organization, and resilience, then modified Costanza’s model, and calculated. The calculated results show that the mixed species plots had the highest overall health, this means that the monitoring and managing modes of mixed species plots are more suitable. The conclusion provide quantitative basis for restoration and rehabilitation of artificial vegetation in arid and semi-arid regions.

Twenty years of stand development in pure and mixed stands of Eucalyptus saligna and nitrogen-fixing Facaltaria moluccana

Abstract The vast majority of tropical tree plantations are monocultures, and only a few case studies have examined growth of mixed-species plantations. Mixtures that contain nitrogen (N)-fixing species may have the potential to increase overall plantation yields, but too few case studies are available to provide general insights. We extended previous measurements of 10 years of stand development in a mixed-species experiment in Hawaii through age 20 years. The plots had a constant total number of stems initially (2500 ha−1), but varying ratios of Eucalyptus saligna (Sm.) to nitrogen-fixing Facaltaria moluccana (Miquel) Barneby & Grimes (=Albizia Facaltaria (L.) Fosberg, and Paraserianthes Facaltaria (L.) I. Nielsen). Eucalyptus stem growth declined strongly in monocultures after age of 7 years, but showed no decline in mixtures. After 10 years of stand development, Eucalyptus stem mass was greatest in the monoculture, averaging about 155 Mg/ha (versus 140 Mg/ha in the 0.5 Facaltaria treatment); total (combined) stem mass for both species was almost twice as great in mixtures as in monocultures. By the age of 20 years, Eucalyptus stem mass was within a 20% range across treatments (260 Mg/ha), and total stem mass ranged up to 430 Mg/ha in the 0.75 Facaltaria treatment. Losses of stem mass from mortality were moderate for Eucalyptus between 18 and 20 years, averaging 20–25% of gross stem growth in mixed-species treatments, but 60% in the Eucalyptus monoculture. Mortality losses of stem mass were extremely high for Facaltaria in all plots, matching or exceeding gross stem growth from age 18 to 20 years. The size of individual Eucalyptus trees was much greater in the mixed plots than in the monoculture plots, so any short-term economic gain from the mixed-species plots may derive from larger tree sizes rather than from more wood mass per hectare. The longer term benefits of mixed plots may include the effects of Facaltaria on soil fertility and community diversity. Overall, these very large changes in ecosystem production in mixtures underscores the need for a large number of additional studies to examine other species combinations, sites, and silvicultural systems.

An evaluation of direct seeding for reforestation of degraded lands in central São Paulo state, Brazil

As part of a larger study evaluating several silvicultural techniques for restoring tropical moist forests on abandoned agricultural lands in southeastern Brazil, direct seeding with five early-successional Atlantic forest species was tested at three degraded sites, characterized by different soil types and land-use histories, within the Environmental Protection Area at Botucatu, SP. The species used in this study were Chorisia speciosa, Croton floribundus, Enterolobium contorstisiliquum, Mimosa scabrella, and Schizolobium parahyba. Scarified seeds of each of these species were sown in prepared seed spots in replicated, 0.25 ha mixed-species plots at an initial espacement of 1 m×1 m at each site. Of the five species planted, only two, Enterolobium and Schizolobium, showed good seed germination, seedling survival, and early growth rates, averaging 4.1–4.6 cm stem diameter and 1.5–1.7 m height growth during the first 2 years after sowing. These two species constituted 88–100% of the total stand density, which ranged from 1050 to 1790 stems ha −1 at 2 years. Despite the poor performance of the other species tested, we observed that the natural regeneration of native forest species originating from remnant forests in the general vicinity of our study sites was significantly greater within the direct-seeded plots than in unplanted control plots that were protected from fire and other disturbances.

Effects of water table, clipping, and species interactions on Carex nebrascensis and Poa pratensis in riparian meadows

In this study, we hypothesized that the primary variable determining species responses and interactions within wet/mesic riparian meadows in central Nevada, USA was the water table but that the direct and indirect effects of livestock grazing modified both species responses and interactions. We tested this hypothesis for two widespread riparian species, Carex nebrascensis and Poa pratensis ssp. pratensis, that co-occur but have maximum expression at different water tables. Carex nebrascensis has widely spread tillers typical of ‘guerilla’ plant architecture, while P. pratensis has closely spaced and compact tillers typical of ‘phalanx’ plant architecture. Individuals of both species were grown at mid-and low water tables with or without neighbors and were either clipped or not clipped at the end of the first growing season. For the study site with the most complete record, mean water-table depth during the growing season (May through August) on the mid sites was −32 cm in year 1 and −7cm in year 2. Water-table depth on the low sites was −69 cm in year 1 and −31 cm in year 2. Plant survival, tillering, biomass, and seed production over a 2-yr period were used to quantify the species responses. Water table had no effect on tillering or biomass of C. nebrascensis, indicating that it is adapted for growth and persistence over the range of water tables examined. In contrast, growth and tillering of P. pratensis was severely restricted at more shallow water tables. Poa pratensis had about 50% fewer tillers and lower biomass (9.0 vs 46.5 g) for neighbor-removed plots on mid- than low-water-table sites at one of the study meadows. Further, tiller numbers of P. pratensis increased over time on the low-water-table plots, but decreased on the mid-water-table plots. Clipping had no effect on the survival, tillering, or growth of either species. The clipping treatment may not have removed sufficient leaf mass or may have been applied too near the end of the growing season to elicit a response. Neighborhood removal resulted in a 3- to 10-fold increase in tillering and higher plant mass (1.3 vs. 9.5 g) for C. nebrascensis. Poa pratensis showed an even greater response to neighbor removal. Tillering was 6-to 100-fold greater and mass was 15 to 50 times greater on neighbor removed than neighbor-intact plots. Comparisons between single species and mixed species plots indicated that there was an interaction between the species that limited tiller production in C. nebrascensis. These results indicate that P. pratensis, the ‘phalanx’ species, is capable of more rapidly responding to disturbances that remove neighbors and increase available space than C. nebrascensis, the ‘guerilla’ species. Contrary to previous studies, the interactions between the two species do not seem to be related to plant architecture and can be best attributed to generally greater growth rates and increased competitive ability for P. pratensis at lower water tables. Grazing may further alter the relative competitive ability of the two species in favor of P. pratensis.

Changes in dinitrogen fixation in maturing stands of Casuarinaequisetifolia and Leucaenaleucocephala

Biological dinitrogen fixation in Casuarinaequisetifolia J.R. & G. Forst. and Leucaenaleucocephala (Lam.) de Wit was evaluated using the 15N-enrichment technique under field conditions in single-species and mixed-species plantings (with a nonfixing reference species, Eucalyptus ×robusta J.E. Smith) between 1.0 and 3.5 years of age in Puerto Rico. Following periodic labelling of trenched and untrenched plantation quadrats with 15N-enriched ammonium sulfate, analyses of foliar and whole-tree (weighted average) N-isotopic ratios and total biomass N were used to estimate the proportion of nitrogen derived from biological dinitrogen fixation (PNDFA) and total nitrogen derived from fixation (TNDFA) in C. equisetifolia and L. leucocephala. The 15N-enrichment technique yielded accurate estimates of dinitrogen fixation in maturing stands of these two tree species provided the reference species (Eucalyptus) was grown in close proximity to the N-fixing species in trenched, mixed-species plots. Changes in the 15N/14N ratio of soil-available nitrogen in single-species plots of the N-fixing and reference were found to yield inaccurate estimates of dinitrogen fixation in the single-species plots of C. equisetifolia and L. leucocephala after 2 years of age. The results confirm earlier findings that foliar sampling is a useful nondestructive alternative to whole-tree biomass sampling for the 15N-enrichment protocol. Between 1.0 and 3.5 years after plantation establishment, PNDFA in C. equisetifolia remained relatively constant between 50 and 60%, while PNDFA in L. leucocephala declined from nearly 100% at 1 year to less than 40% at 3.5 years. The rate of dinitrogen fixation (kg•ha−1•year−1) did not decline as the stands matured. Cumulative dinitrogen fixation (TNDFA) estimates at 3.5 years were very similar between species: 73 in C. equisetifolia and 74 kg N•ha−1•year−1 in L. leucocephala.

Paper 3. impacts and responses to climate change in forests of the mink region

The adaptability of forests in the U.S. midwest to a changing climate is assessed. The forests of Missouri are simulated with a ‘forest-gap’ model, a stochastic model of the annual growth and mortality of trees within mixed-species forest plots. The development of representative forest plots under an analog climate like that of the 1930s is compared to development under baseline climate conditions. With no management response, average forest biomass in the region declines by 11% within ten years, primarily due to moisture-stress induced mortality. Longer term declines in forest productivity on the order of 30% are simulated. A variety of possible management responses through planting or harvesting practices were evaluated. None of these adaptations appear to be practical, although the salvage harvest of stressed trees would offset the economifc losses associated with the early mortality. An investigation of anticipated trends in the broader forest products sector suggests that opportunities for further adaptation to offset the decline in primary productivity of this region's forest are quite limited. However, a shift to wood powered electrical generation in the region might justify a level of management that would allow some adaptation to the analog climate change.

Influence of Patch Plant Density on Herbivory Levels by Etiella zinckenella (Lepidoptera: Pyralidae) on Glycine max and Crotalaria pallida

Experiments were conducted to determine the influence of the density and size of patches of soybeans ( Glycine max (L.)) on the extent of pod damage by the lima bean pod-borer Etiella zinckenella (Treitschke). In addition, an evaluation was made of changes in rates of parasitism as a function of changes in patch characteristics. Low-density plots had a greater proportion of pod damage than did high-density plots. Plot size did not influence the proportion of pods attacked per plant. Larval parasitoids of E. zinckenella , in general, were not influenced by plant density but were influenced by host herbivore density. Other experiments were conducted to determine if mixtures of soybeans with a feral host, Crotalaria pallida Ait, a species in the Fabaceae, would influence damage by the pod borer or parasitism. No differences were observed between mixed species plots and pure C. pallida or pure soybean plots, in the number of E. zinckenella larvae per plant raceme nor in the level of parasitism. The abundance of pods produced by plants as a function of cultivar type, patch characteristics and time of season were also recorded.

Comparative evaluation of white clover cultivars in spaced plant and small mixed species plot trials

Abstract The ability to predict the growth of 124 white clover cultivars in a grass sward under sheep grazing from performance in a spaced plant block was examined. Over a 3-year period the extent of variation in cultivar yield in grass swards explained by growth as spaced plants decreased from 45% in Year 1 to 10% in Year 3. Spaced plants gave a better prediction of performance of large-leaved than smallleaved cultivars. However, lines which had highest scores (i.e., when in the top 10) when grown as spaced plants in Years 2 and 3, were generally the best when grown in a grass sward in Years 2 and 3. This would suggest that selection of genotypes from the best lines on the basis of spaced plant data would provide material of reliable agronomic potential when grown in grass swards that are to be grazed.

Host plant growth form and diversity: Effects on abundance and feeding preference of a specialist herbivore, Acalymma vittata (Coleoptera: Chrysomelidae)

Abundances of the specialist herbivore, Acalymma vittata (Fab.) (Coleoptera: Chrysomelidae), were assessed in small experimental plots with three levels of plant diversity (cucumber monoculture, cucumber/corn, and cucumber/tomato) and two levels of host plant growth form (horizontal on the ground and vertical, staked up or growing up other plant species). Host plant growth form more strongly affected beetle abundances than did plant diversity; greater numbers were found on vertically growing than on horizontally growing cucumber plants. The combination of cucumber monoculture and vertical growth form supported significantly greater herbivore abundances than did any other type of plot, emphasizing a strong interaction between diversity and growth form. Beetles were not more common in monocultures with horizontal growth forms than in mixed species plots, and beetles did not respond differently to plots with corn and plots with tomatoes.Feeding experiments demonstrated that the plant diversity under which a host plant is grown strongly influenced herbivore feeding preference. Beetles given a choice of cucumber leaves grown in monoculture and in plots with tomatoes exhibited individual differences in their food selection behavior, however, a significantly greater number of beetles preferred monoculture leaves. Those individuals preferring monoculture leaves and those individuals preferring leaves from plots with tomatoes did not differ in either absolute or relative amounts of feeding damage per leaf.Neither plant size nor the date on which plots were colonized by beetles explained the differences in herbivore abundance. It is suggested that differences in movement patterns and plant quality contributed to the greater numbers of beetles on plants growing vertically in monocultures.