The hydraulic limitation hypothesis revisited

We proposed the hydraulic limitation hypothesis (HLH) as a mechanism to explain universal patterns in tree height, and tree and stand biomass growth: height growth slows down as trees grow taller, maximum height is lower for trees of the same species on resource-poor sites and annual wood production declines after canopy closure for even-aged forests. Our review of 51 studies that measured one or more of the components necessary for testing the hypothesis showed that taller trees differ physiologically from shorter, younger trees. Stomatal conductance to water vapour (g(s)), photosynthesis (A) and leaf-specific hydraulic conductance (K L) are often, but not always, lower in taller trees. Additionally, leaf mass per area is often greater in taller trees, and leaf area:sapwood area ratio changes with tree height. We conclude that hydraulic limitation of gas exchange with increasing tree size is common, but not universal. Where hydraulic limitations to A do occur, no evidence supports the original expectation that hydraulic limitation of carbon assimilation is sufficient to explain observed declines in wood production. Any limit to height or height growth does not appear to be related to the so-called age-related decline in wood production of forests after canopy closure. Future work on this problem should explicitly link leaf or canopy gas exchange with tree and stand growth, and consider a more fundamental assumption: whether tree biomass growth is limited by carbon availability.     

Habitat suitability in muskrats: a test of the food limitation hypothesis

Data on habitat selection, overwinter survival, and litter survival rate of muskrats indicate that fitness differs among habitats in marsh environments. We measured nine nutrient-dependent phenotypic variables of muskrats and one independent assessment of forage quality during the ice-free period (May-October) from 1992 to 1994. Data were used to test the hypothesis that differences in food resources limit the demographic performance of muskrats in marginal versus prime habitats (food limitation hypothesis, FLH). None of the null predictions relating habitat quality with reproductive parameters could be rejected statistically. Adult body mass and length were greater in prime habitats, but fat content did not differ, which suggested that the difference was associated with larger structural size of dominant animals in prime habitats. Growth rate of weaned juveniles was not different among habitats, which represented the strongest evidence for rejecting the FLH. Faecal crude protein content, which was used as an index of quality of food ingested, was marginally different between the two most suitable habitats, but the direction of the difference did not support our prediction. Overall, the temporal variation in faecal crude protein content was similar among habitats. We conclude that food is not a key factor limiting the demographic performance of muskrats among habitats during the ice-free season.     

Bioherbicide Potential of Eucalyptus saligna Leaf Litter Essential Oil

We aimed to evaluate the bioherbicide potential of Eucalyptus saligna leaf litter essential oil, its fractions and major compounds. Six essential oil fractions were obtained by preparative thin‐layer chromatography and analyzed by gas chromatography/mass spectrometry. Effects of the oil, its fractions and major compounds (1,8‐cineole and/or α‐pinene) diluted in aqueous solution were assessed on germination and seedling growth. Recipient species were Lactuca sativa (model), Amaranthus viridis (weed), Eragrostis plana (weed), and Paspalum notatum (forage). The essential oil was more phytotoxic on A. viridis and L. sativa, followed by E. plana, and caused no effects on P. notatum. Amaranthus viridis was inhibited by all treatments, mainly the essential oil, α‐pinene and fraction 6, whereas E. plana was more affected by the oil fractions. Results revealed the species‐specific phytotoxic effects of E. saligna essential oil, indicating its potential use for controlling A. viridis and E. plana. Phytotoxic effects of essential oils or their components should not be generalized, as effects may change according to recipient species.     

Ploidy evolution in the yeast Saccharomyces cerevisiae: a test of the nutrient limitation hypothesis

The nutrient limitation hypothesis provides a nongenetic explanation for the evolution of life cycles that retain both haploid and diploid phases: differences in nutrient requirements and uptake allow haploids to override the potential genetic advantages provided by diploidy under certain nutrient limiting conditions. The relative fitness of an isogenic series of haploid, diploid and tetraploid yeast cells (Saccharomyces cerevisiae), which were also equivalent at the mating type locus, was measured. Fitness was measured both by growth rate against a common competitor and by intrinsic growth rate in isolated cultures, under four environmental conditions: (1) rich medium (YPD) at the preferred growth temperature (30 °C); (2) nutrient poor medium (MM) at 30 °C; (3) YPD at a nonpreferred temperature (37 °C); and (4) MM at 37 °C. In contrast to the predictions of the nutrient limitation hypothesis, haploids grew significantly faster than diploids under nutrient rich conditions, but there were no apparent differences between them when fitness was determined by relative competitive ability. In addition, temperature affected the relative growth of haploids and diploids, with haploids growing proportionately faster at higher temperatures. Tetraploids performed very poorly under all conditions compared. Cell geometric parameters were not consistent predictors of fitness under the conditions measured.     

Chlorophyll fluorescence of the trunk rather than leaves indicates visual vitality in Eucalyptus saligna

Leaf chlorophyll fluorescence has been used to assess physiological stress effects in trees for at least 30?years. This paper describes a novel method for indicating tree vitality in Eucalyptus saligna Sm using bark chlorophyll fluorescence. A visual vitality index was successfully verified by comparing it with growth measurements such as total leaf area and above ground biomass. Bark and leaf chlorophyll fluorescence were then compared with the visual vitality index in spring, summer and autumn. The relationship between leaf chlorophyll fluorescence and the visual vitality index was weak, even with the one parameter that did show a relationship. There was good evidence for a statistical relationship between bark chlorophyll fluorescence and visual vitality, which may become a useful tool for tree vitality assessments in this species and possibly other tree species.     

Constraints on treeline advance in a warming climate: a test of the reproduction limitation hypothesis

Aims Several mechanistic hypotheses have been developed to explain the existence of alpine treelines worldwide. The reproduction limitation hypothesis (RLH) postulates that reproductive processes such as seed production and dispersal are restricted in treeline environments, thereby limiting the establishment of new individuals in advance of extant treelines. Despite its popularity, no study has tested this hypothesis in a comprehensive fashion. In this experiment, we attempted to answer whether there are enough viable seeds being dispersed beyond treeline for sexually mediated treeline advance, and what the implications of climate change might be on these processes.Methods We established 30 plots across two aspects (north vs. south) and three elevational habitats (forest, treeline and tundra) in a white spruce (Picea glauca) boreal forest-alpine tundra ecotone in southwest Yukon, Canada. In each plot, tree characteristics, seed production and predispersal damage were measured. Additionally, eight dispersal trays were positioned in each plot to measure seed rain, and germination trials with and without predation exclosures were constructed in a subset of plots to quantify dispersal and germination success.Important findings Results were highly variable both temporally and spatially. In 2014, a mast year, 69% of adult trees produced cones compared to 0.4% in the following year. Higher density of trees in forest plots compared to treeline and tundra resulted in greater seed production at lower elevations. Across all plots, 88% of seeds were damaged before dispersal or were not viable. Treeline plots had significantly greater predispersal damage. Seed rain was greater in south-facing plots than north-facing plots. Less than 2% of seeds produced on the landscape were dispersed into Tundra plots, located 50 m above treeline. There was a net movement of seeds from the north-facing slope to the south at our study site, likely due to prevailing winds during the dispersal period. Germination counts were more than double on north-facing slopes and one-third higher inside exclosures. Cumulatively, the results provide some evidence for the RLH. Collectively, the high amount of predispersal damage and non-viable seeds, variability associated with dispersal and significant seed predation can functionally influence treeline dynamics. These findings suggest that global treeline distribution models, which rely largely on temperature, may not be entirely accurate for predicting treeline advance—at least at finer temporal scales. Many stochastic factors need to align temporally for successful advance, which is likely to result in a lag of many decades between the period of temperature amelioration and an increased number of trees beyond extant treelines.     

Effect of Eucalyptus saligna and Albizia falcataria on soil processes and nitrogen supply in Hawaii

This study investigated the differences between two fast-growing tropical tree species on soil N flux and availability. The work was conducted in the island of Hawaii and included three sites located along the Hamakua coast on the northeastern side of the island. Within each site pure stands of Eucalyptus saligna (Sm.)␣and the N2-fixing Albizia falcataria (L.) Fosberg [=Paraserianthes falcataria (L.) Nielsen] were arranged in four randomized complete blocks. For most of the variables considered in this study, the species effects were usually strong and the site effects were significant in some cases. After 13 years, soils under the Albizia stand contained larger pools of total soil C and N, and larger pools of inorganic N. Soil N availability indexed by ion exchange resin bags revealed a strong pattern of species and site effect on N availability; soils under Albizia showed a 2.6–9 fold increase in N availability (P < 0.01). Potential net rates of N transformation (10- and 30-day aerobic incubations) were more than twice as high for soils under the Albizia than under the Eucalytus stands. Nitrogen mineralization during anaerobic incubations were about 10% greater on Albizia soils. Gross microbial mineralization and immobilization were determined by estimating the gross rates of N transformation by the ¹⁵N-isotope pool dilution techniques. Across species and sites, a strong linear positive relationship was obtained for gross immobilization and gross mineralization indicating faster gross immobilization as gross mineralization increases. Soil microbial biomass on Albizia soils contained larger proportion of it as bacterial biomass, while larger proportion of fungi biomass comprised the microbial biomass under Eucalyptus soils. This study clearly showed that the presence of Albizia increased total N pools and N supply to the ecosystem. The overall effect on soil fertility will need to be characterized by the effect of the N₂-fixer on other nutrients, especially the effect on phosphorus. Received: 28 February 1997 / Accepted: 22 September 1997     

A canopy-scale test of the optimal water-use hypothesis

Common empirical models of stomatal conductivity often incorporate a sensitivity of stomata to the rate of leaf photosynthesis. Such a sensitivity has been predicted on theoretical terms by Cowan and Farquhar, who postulated that stomata should adjust dynamically to maximize photosynthesis for a given water loss.
In this study, we implemented the Cowan and Farquhar hypothesis of optimal stomatal conductivity into a canopy gas exchange model, and predicted the diurnal and daily variability of transpiration for a savanna site in the wet–dry tropics of northern Australia. The predicted transpiration dynamics were then compared with observations at the site using the eddy covariance technique. The observations were also used to evaluate two alternative approaches: constant conductivity and a tuned empirical model.
The model based on the optimal water-use hypothesis performed better than the one based on constant stomatal conductivity, and at least as well as the tuned empirical model. This suggests that the optimal water-use hypothesis is useful for modelling canopy gas exchange, and that it can reduce the need for model parameterization.     

Carotenoid limitation and mate preference evolution: a test of the indicator hypothesis in guppies (Poecilia reticulata)

Under the indicator models of mate choice, female preferences evolve to exploit the condition-dependence or "indicator value" of male traits, which in turn may cause these traits to evolve to elaborate extremes. If the indicator value of a male trait changes, the payoff function of the female preference for that trait should change and the preference should evolve to a new optimum. I tested this prediction in the guppy, Poecilia reticulata, a species in which the indicator value of a sexually selected male trait, carotenoid coloration, varies geographically. Carotenoid coloration is thought to be an indicator of foraging ability and health because animals must obtain carotenoid pigments from their diet. The primary dietary source of carotenoids for guppies is unicellular algae, the abundance of which varies among natural streams because of variation in forest canopy cover. Carotenoid availability limits male coloration to a greater extent in streams with greater forest canopy cover. Thus, the indicator value of male coloration covaries positively with canopy cover. To test the indicator model prediction, I measured genetic divergence in the strength of female preferences for carotenoid coloration between high- and low-carotenoid availability streams in each of three river drainages. Second-generation laboratory-born females were given a choice between full-sib males raised on three different dietary levels of carotenoids. For all six populations, male attractiveness (as determined from the responses of females to male courtship displays) increased with dietary carotenoid levels. However, the strength of female preferences differed between populations in the predicted direction in only one of three river drainages. These results fail to support a crucial prediction of the indicator model. More studies taking an interpopulation approach to studying mate preference evolution are needed before the explanatory value of the indicator models can be rigorously assessed.     

Stonefly predation along a hydraulic gradient: a field test of the harsh—benign hypothesis

SUMMARY. 1. Microhabitat preferences of predatory stoneflies and four prey taxa were assessed by taking benthic samples along a hydraulic gradient in a Black Forest stream in West Germany. Densities of predator and prey species were estimated at twenty-one hydraulic regimes.
2. Enclosures containing the stonefly, Dinocras cephalotes , and control cages with no predators were placed in the substrate at hydraulic regimes favourable and unfavourable to predators. Cages received initial prey communities that were obtained from benthic samples taken at hydraulic regimes matching those intended for each cage.
3. Population densities of the two most numerically important prey taxa, the mayfly. Baetis rhodani , and the Chironomidae, were reduced in the presence of Dinocras , but only when enclosures were placed in the hydraulic regimes favourable to the predator. Thus, predation effects increased as the hydraulic regime became more benign to the predators.
4. Densities of two other prey species rare in the diets of Dinocras ( Hydropsyche instabilis and Gammarus fossarum ) were generally unaffected by predators regardless of the hydraulic regime.
5. These data provide support for the hypothesis that perception of the abiotic regime as harsh or benign to predators is a good predictor of predator impact on densities of preferred prey species. In harsher abiotic regimes, impact will be low, while impact will be high in benign abiotic regimes.     

A test of the air-seeding hypothesis using sphagnum hyalocysts

“Air-seeding” is a proposed mechanism for the initiation of water stress embolism in dead plant cells. During air-seeding, external air is drawn into the lumen of a dead plant cell through a pore or crack in the cell wall. The resulting bubble may expand to fill the lumen, thus embolizing the cell. The data presented confirm that Sphagnum hyalocysts can embolize by air-seeding when the pressure difference across the air-water meniscus is given by ΔP = 0.3/D (derived from the capillary equation), where ΔP is the pressure difference across the meniscus (megapascal), and D is the diameter (micrometer) of the pore through which the air bubble enters.     

A test of the good-genes-as-heterozygosity hypothesis using red-winged blackbirds

Brown recently proposed that the "good genes" that females pursuewhen choosing mates may be individual heterozygosity becausemore heterozygous mates sire offspring with higher fitness.Further, because heterozygosity might enhance developmentalstability, males with more heterozygosity are recognized bythe reduced fluctuating asymmetry (FA) of their bilaterallypaired traits. We used a point sample of 67 male red-winged blackbirds(Agelaius phoeniceus) to test two predictions of this hypothesis:(1) males with more heterozygosity have higher fitness, and(2) males with more heterozygosity have lower FA. We identified7 polymorphic loci from an initial screening of 16 enzymes;32 individuals were completely homozygous, and 35 individualswere heterozygous at at least 1 locus. Larger and older malesrealized higher mating success in this population, but neither sizenor age was related to heterozygosity. Heterozygous males werenot in better condition than homozygous males, nor were theyless infected by hematozoa, lice, or mites. Among 1-year oldmales, epaulet length did not differ between homozygotes andheterozygotes, but among older males, heterozygotes did havelonger epaulets. Homozygotes and heterozygotes did not differin their mean FA scores for nine individual characters. Althoughthe two groups of males did differ in composite FA, heterozygousmales were less symmetrical. Interestingly, this differencewas attributable to a single allele at the PGM-3 locus. Combinedwith previous results showing that FA was generally unrelatedto male health, viability, parental care, social dominance,or mating success, the present results indicate that Brown's hypothesisdoes not explain mate choice or male quality in this populationof red-winged blackbirds.     

The effect of fertilization on sap flux and canopy conductance in a Eucalyptus saligna experimental forest

Land devoted to plantation forestry (50 million ha) has been increasing worldwide and the genus Eucalyptus is a popular plantation species (14 million ha) for its rapid growth and ability to grow well on a wide range of sites. Fertilization is a common silvicultural tool to improve tree growth with potential effects on stand water use, but the relationship between wood growth and water use in response to fertilization remains poorly quantified. Our objectives in this study were to determine the extent, timing and longevity of fertilization effects on water use and wood growth in a non‐water limited Eucalyptus saligna experimental forest near Hilo, HI. We evaluated the short‐ and long‐term effects of fertilization on water use by measuring sap flux per unit sapwood area, canopy conductance, transpiration per unit leaf area and water‐use efficiency in control and fertilized stands. Short‐term effects were assessed by comparing sap flux before and after fertilizer application. Long‐term effects were assessed by comparing control plots and plots that had received nutrient additions for 5 years. For the short‐term response, total water use in fertilized plots increased from 265 to 487 mm yr^?1 during the 5 months following fertilization. The increase was driven by an increase in stand leaf area accompanied by an increase in sap flux per unit sapwood area. Sap flux per unit leaf area and canopy conductance did not differ during the 5 months following fertilizer additions. For the last 2 months of our short‐term measurements, fertilized trees used less water per unit carbon gain (361 compared with 751 kg H₂O kg C^?1 in control stands). Trees with 5 years of fertilization also used significantly more water than controls (401 vs. 302 mm yr^?1) because of greater leaf area in the fertilized stands. Sap flux per unit sapwood area, sap flux per unit leaf area, and canopy conductance did not differ between control and fertilized trees in the long‐term plots. In contrast to the short‐term response, the long‐term response of water use per unit wood growth was not significant. Overall, fertilization of E. saligna at our site increased stand water use by increasing leaf area. Fertilized trees grew more wood and used more water, but fertilization did not change wood growth per unit water use.     

A test of the hypothesis of pheromone attraction in salmonid migration

Synopsis We tested the hypothesis that anadromous salmonids are guided on their homeward migration by population-specific pheromones. Our findings do not support the hypothesis. Wild migrant Arctic charr,Salvelinus alpinus, from Ikarut River, Labrador were transferred and held in a tributary previously uninhabited by anadromous fish. None of the charr migrating up Ikarut River entered the tributary after fish were transferred. Similarly, migrant charr, which were caught in Ikarut River and released in the tributary below the captive fish, did not remain in the tributary. We re-evaluated the data which have been used to uphold the concept of pheromone attraction in salmonid migration and concluded that support for the hypothesis is unsubstantiated.     

Migrants in tropical bird communities: the balanced breeding limitation hypothesis

Explanations for the ecological integration of migratory and non-migratory (resident) insectivorous birds in the tropics have been complicated by the paradox that arthropod abundances are low when bird abundances reach their annual peak. The breeding currency hypothesis and the nest predation hypothesis both account for this paradox by postulating that residents are held below the non-breeding season carrying capacity, which frees resources available for migratory insectivores. The breeding currency hypothesis suggests residents are limited by food suitable for nestlings, whereas the nest predation hypothesis emphasizes the primacy of high rates of nest predation. However, theoretical arguments suggest that food availability and predation risk interact strongly to limit breeding birds. We use graphical analyses to extend the breeding currency hypothesis to incorporate effects of nest predation. This yields a more synthetic and realistic model for the integration of migrant and resident insectivores in the tropics – the balanced breeding limitation hypothesis.     

A test of the heterozygote-advantage hypothesis in cystic fibrosis carriers.

We report a test of the hypothesis that the high frequency of cystic fibrosis (CF) in Caucasian populations is due to a fertility advantage in CF carriers. One hundred forty-three grandparent couples of Utah CF cases were compared with 20 replicate sets of matched control couples drawn from the Utah Genealogical Database. Ascertainment correction, which has not been applied in previous studies of CF carrier fertility, was applied to these data. Before ascertainment correction was applied, CF carriers appeared to manifest a significant fertility advantage over controls. After the correction formula was applied, this difference disappeared. Carriers and controls were also compared in terms of the length of intervals between births. Again, no significant differences were found. It was concluded that fertility differences are unlikely to account for the observed Caucasian CF gene frequency. Other mechanisms, particularly a past selective event or random genetic drift, are more likely to be responsible.