Kinetics of ammonium, nitrate and phosphorus uptake by Canna indica and Schoenoplectus validus

Canna indica L. is an upright perennial rhizomatous herb, and Schoenoplectus validus (Vahl) A. Löve and D. Löve is a tall, perennial, herbaceous sedge. The nutrient uptake kinetics of C. indica and S. validus were investigated using the modified depletion method after plants were grown for 4 weeks in simulated secondary-treated wastewater. The maximum uptake rate (I_max) and Michaelis–Menten constant (K_m) were estimated by iterative curve fitting. The I_max for NH₄N (623 μmol g⁻¹ dry root weight h⁻¹) was significantly higher than that for NO₃N (338 μmol g⁻¹ dry root weight h⁻¹) in S. validus. In contrast, no difference was observed in C. indica. The I_max values for NO₃N and NH₄N were higher in S. validus than in C. indica. A significantly lower K_m was detected for NO₃N uptake in C. indica (385 μmol L⁻¹) compared to that in S. validus (1908 μmol L⁻¹). The I_max for PO₄P did not differ between the plant species. The K_m for PO₄P was significantly higher in C. indica (157 μmol L⁻¹) than in S. validus (60 μmol L⁻¹). In conclusion, we found that S. validus preferred NH₄N over NO₃N, had greater capacity for N uptake and higher affinity for PO₄P, but C. indica had greater affinity for NO₃N. Nutrient uptake capacity is likely related to habitat preference, and is influenced by the structure of roots and rhizomes.     

Interactive effects of N and P on growth but not on resource allocation of Canna indica in wetland microcosms

The interactive effects of three levels of N (mM) (low 0.36, medium 2.1 and high 6.4) and two levels of P (mM) (low 0.10 and high 0.48) on growth and resource allocation of Canna indica Linn. were studied in wetland microcosms. After 91 days of plant growth, there was a significant interactive effect of N and P on plant growth, but not on resource allocation (except for allocation of N to leaves and allocation of P to the stems). The plant growth positively responded to the relatively higher nutrient availability (taller plants with more stems, leaves and flowers), but the growth performance was not significantly different between the medium N-low P and high N-low P treatments. At high P, the total biomass in the high N was about 51% higher than that in the medium N and about 348% higher than that in the low N. The growth performance was related to the physiological responses. The photochemical efficiency (F_v/F_m) increased from 0.843 to 0.855 with an increase in N additions. The photosynthetic rate increased from 13 to 16 μmol m⁻² s⁻¹ in the low P levels and from 14 to 20 μmol m⁻² s⁻¹ in the high P levels with an increase in N applications, but significant difference was only between the low and medium N levels, regardless of the P levels. The tissue concentrations of N increased with an increase in N applications and decreased with an increase in P additions, whereas reverse was true for tissue concentrations of P. The highest concentrations of N and P in leaves were 30.8 g N kg⁻¹ in the high N-low P treatment and 4.9 g P kg⁻¹ in the low N-high P treatment. The percent biomass allocation to aboveground tissues in the high N was nearly twice that in the low N treatments. The N allocation to aboveground tissues was slightly larger in high N than in low N treatments, whereas the P allocation to aboveground tissues increased with an increase in the N addition. Although some patterns of biomass allocation were similar to those of nutrient allocation, they did not totally reflect the nutrient allocation. These results imply that in order to enhance the treatment performance, appropriately high nutrient availability of N and P are required to stimulate the growth of C. indica in constructed wetlands.     

Growth-rate adaptation of Phycomyces sporangiophores to partial depletion of oxygen

The growth rate of Phycomyces blakesleeanus sporangiophores was found to be very sensitive to sudden changes in the oxygen concentration. A change from 20% to 15% oxygen elicits a transient decrease in the growth rate which returns to normal 10 min after altering the concentration. After a step change to 10% oxygen, the growth rate shows two minima at 6–8 and 30–35 min and it reaches about 80% of its original value 50 min after this change. A threshold curve for this negative growth response shows that sporangiophores begin to sense a decrease in the oxygen concentration from 20% to 17%. Seven phototropically abnormal mutants with defects in the genes madA to madG were tested for the oxygen response. Two strains, C149madD120 and C316madF48, were found to have recoveries different from those of the wild type after step changes from 20% to 10% oxygen.     

Trophic relationships and seasonal utilization of salt-marsh creeks by zooplanktivorous fishes

Synopsis In a high salinity estuary at North Inlet, South Carolina, co-occurrence and possible competition among adults of four dominant zooplanktivorous fishes were minimized by seasonal adjustments in lateral and vertical distributions as well as in dietary preferences. In winter, Atlantic silversides, Menidia menidia, occupied the entire water column while other planktivores were rare or absent from the estuary, and they consumed large prey such as mysid shrimps and fish larvae. An immigration of bay anchovies, Anchoa mitchilli, in the spring resulted in a redistribution of species with Atlantic silversides shifting to the surface waters and bay anchovies dominating the lower half of the water column. Both fishes consumed mostly copepods in the spring, but each favored a different species. There was little similarity in the large prey items consumed by the two fishes. Striped anchovies, Anchoa hepsetus, arrived in mid-summer and were most abundant at the surface while bay anchovies continued to dominate the bottom waters. Atlantic silversides were rare in all summer collections. The diets of the two anchovies were similar, but vertical separation during the period of maximum zooplankton abundance probably minimized competition. Rough silversides, Membras martinica, which were obligate surface dwellers, shared the upper water column with striped anchovies, but the two species had very different diets during their period of co-occurrence. Although seasonal changes in fish diets reflected shifts in zooplankton composition and all fishes consumed a variety of prey types, preferences for some prey taxa and total avoidance of others were indicated. Electivity indices indicated an especially strong selection for fiddler crab megalopae by all fishes in the summer and fall. All fishes, except rough silversides, which fed almost exclusively on copepods and crab zoeae, consumed large prey items when they were available. Fine scale partitioning of the food resources was apparent in the selection of different copepod and insect species by the fishes. Spatial and temporal separation in the distribution and/or dietary preferences of the zooplanktivores fishes probably reduces the potential for resource competition. Given the high abundances and selectivity of the planktivores, significant impacts on some zooplankton populations probably result.     

Growth and allocation of the arctic sedges Eriohorum angustifolium and E. vaginatum: effects of variable soil oxygen and nutrient availability

In arctic tundra soil, oxygen depletion associated with soil flooding may control plant growth either directly through anoxia or indirectly through effects on nutrient availability. This study was designed to evaluate whether plant growth and physiology of two arctic sedge species are more strongly controlled by the direct or indirect effects of decreased soil aeration. Eriophorum angustifolium and E. vaginatum, which originate from flooded and well-drained habitats, respectively, were grown in an in situ transplant garden at two levels of soil oxygen, nitrogen, and phosphorus availability over two growing seasons. In both species, N addition had a stronger effect on growth and biomass allocation than P addition or soil oxygen depletion. Net photosynthesis and carbohydrate concentrations were relatively insensitive to changes in these factors. Biomass reallocated from shoots to below-ground parts in response to limited N supply was equally divided between roots (nutrient acquisition) and perennating rhizomes (storage tissue formation) in E. angustifolium. E. Vaginatum only increased its allocation to rhizomes. In the flood-tolerant E. angustifolium, growth was improved by soil anoxia and biomass allocation among plant parts was not significantly affected. Contrary to our initial hypothesis, whole-plant growth in E. vaginatum improved in flooded soils; however, it only did so when N availability was high. Under low N availability growth in flooded soils was reduced by 20% compared to growth in the aerobic environment. Reduced biomass allocation to rhizomes and thus to storage potential under anaerobic conditions may reduce long-term survival of E. vaginatum in flooded habitats.     

Fruit maturation patterns of Carya spp. (Juglandaceae): an intra-crown analysis of growth and reproduction

Summary Fruit survival patterns, from fertilization to maturation, were examined for Carya ovata and C. tomentosa in a New Jersey USA forest. We observed fruiting and shoot growth characteristics over a 3-yr period to determine: (1) the patterns of fruit survivorship (from initiation to maturity) within and among years, (2) the relationships between shoot growth, fruit initiation, and fruit survival to maturity, and (3) the influence of phytophagous insects on fruit survival. We found that within years, smaller infructescences (1–2 fruits) exhibited greater relative survivorship than larger ones (3–4 fruits); however, absolute nut production was greatest for mid-sized infructescences (2–3 fruits). Among years, fruit survivorship varied considerably within populations. Across the 3-yr period we observed average fruit survivorship to be convex, linear, and concave, respectively. Likewise, shoot characteristics (length, width, number of leaves) varied concomitantly (decreasing fruit survivorship was accompanied by decreasing shoot length and number of leaves). Within years, we found no strong relationship between shoot characteristics and infructescence size and survival. The patterns of tree-to-tree variation suggested a strong genetic basis to shoot growth and fruit maturation. However, patterns of variation within and among years also indicated a strong environmental influence on these traits as well. Natural phytophagy by insects was observed to be low (<5%); however, shoot defoliations of 10–25% were not uncommon. Experimental defoliations (ambient, 10–15%, 20–40%, and 75–100%) did not result in reduced survival to maturity. Collectively, the data suggest that year-to-year variability in shoot growth has a greater influence on fruit maturation patterns than within year fruit-shoot relations.     

Effects of dilution on dissolved oxygen depletion and microbial populations in the biochemical oxygen demand determination

The biochemical oxygen demand (BOD) value is still a key parameter that can determine the level of organics, particularly the content of biodegradable organics in water. In this work, the effects of sample dilution, which should be done inevitably to get appropriate dissolved oxygen (DO) depletion, on the measurement of 5-day BOD (BOD₅), was investigated with and without seeding using natural and synthetic water. The dilution effects were also evaluated for water samples taken in different seasons such as summer and winter because water temperature can cause a change in the types of microbial species, thus leading to different oxygen depletion profiles during BOD testing. The predation phenomenon between microbial cells was found to be dependent on the inorganic nutrients and carbon sources, showing a change in cell populations according to cell size after 5-day incubation. The dilution of water samples for BOD determination was linked to changes in the environment for microbial growth such as nutrition. The predation phenomenon between microbial cells was more important with less dilution. BOD₅ increased with the specific amount of inorganic nutrient per microbial mass when the natural water was diluted. When seeding was done for synthetic water samples, the seed volume also affected BOD due to the rate of organic uptake by microbes. BOD₅ increased with the specific bacterial population per organic source supplied at the beginning of BOD measurement. For more accurate BOD measurements, specific guidelines on dilution should be established.     

Sagebrush carbon allocation patterns and grasshopper nutrition: the influence of CO2 enrichment and soil mineral limitation

Summary Artemisia tridentata seedlings were grown under carbon dioxide concentrations of 350 and 650 l l^–1 and two levels of soil nutrition. In the high nutrient treatment, increasing CO₂ led to a doubling of shoot mass, whereas nutrient limitation completely constrained the response to elevated CO₂. Root biomass was unaffected by any treatment. Plant root/shoot ratios declined under carbon dioxide enrichment but increased under low nutrient availability, thus the ratio was apparently controlled by changes in carbon allocation to shoot mass alone. Growth under CO₂ enrichment increased the starch concentrations of leaves grown under both nutrient regimes, while increased CO₂ and low nutrient availability acted in concert to reduce leaf nitrogen concentration and water content. Carbon dioxide enrichment and soil nutrient limitation both acted to increase the balance of leaf storage carbohydrate versus nitrogen (C/N). The two treatment effects were significantly interactive in that nutrient limitation slightly reduced the C/N balance among the high-CO₂ plants. Leaf volatile terpene concentration increased only in the nutrient limited plants and did not follow the overall increase in leaf C/N ratio. Grasshopper consumption was significantly greater on host leaves grown under CO₂ enrichment but was reduced on leaves grown under low nutrient availability. An overall negative relationship of consumption versus leaf volatile concentration suggests that terpenes may have been one of several important leaf characteristics limiting consumption of the low nutrient hosts. Digestibility of host leaves grown under the high CO₂ treatment was significantly increased and was related to high leaf starch content. Grasshopper growth efficiency (ECI) was significantly reduced by the nutrient limitation treatment but co-varied with leaf water content.     

Competition between two congeneric stream gobies for habitat in southwestern Shikoku, Japan

Competition between two congeneric gobies, Rhinogobius sp. LD (large-dark type) and CB (cross-band type), for habitat was studied in a tributary of the Shimanto River, southwestern Shikoku, Japan. Habitat use by CB, measured by water depth, current velocity, and substrate, was compared between before and after the removal of LD. After the removal of LD, CB shifted their habitat use to coarser substrates than previously, suggesting a release from effects of LD. This result supports a hypothesis from our previous study that habitat partitioning between CB and LD is a consequence of interference effects of LD on CB.     

Establishment and growth pattern of <Emphasis Type="Italic">Pinus pumila</Emphasis> under a forest canopy in central Kamchatka

We investigated the spatial distribution and growth of the Siberian dwarf pine (Pinus pumila) in a valley–foothill larch–birch (Larix cajanderi–Betula platyphylla as canopy trees) mixed forest of fire origin located in central Kamchatka with the aim of elucidating the ecological features of P. pumila when it is an undergrowth species in a forest. The spatial distribution of all individuals of all tree species was clumped, and the spatial distribution of the two canopy tree species did not repulsively affect that of P. pumila (i.e., its establishment site). These results suggest that the regeneration of P. pumila does not depend on canopy gaps. However, the analysis using a growth model indicated that the canopy trees negatively affected the growth of P. pumila and that the negative effect of L. cajanderi on P. pumila growth was stronger than that of B. platyphylla. The direction of the crown extension of P. pumila was weakly related to the open-space direction. Our results suggest that, although the spatial pattern of establishment of P. pumila is not repulsed by the distribution of canopy trees, the crown can spread horizontally toward the more sparsely populated areas of the canopy trees where they may have higher growth rates.     

Reproductive seasonality and nest-site differentiation in three closely related armoured catfishes (Siluriformes: Callichthyidae)

Synopsis Three sympatric, closely related armoured catfishes showed a similar, bimodal breeding season in coastal plain swamps in Suriname (South America). The bimodal pattern of reproduction inCallichthys callichthys,Hoplosternum littorale andHoplosternum thoracatum was correlated to the annual distribution of rainfall. Floating bubble nests were constructed throughout the period of swamp inundation (rainy season). Close inspection of the timing of reproduction ofH. littorale in northern South America revealed the relationship between reproduction and rainfall, but also unexpected differences in the length of the breeding season. Strong interspecific competition among the three species was probably avoided through differentiation of nest sites with respect to water depth, distance to the nearest tree, distance to the edge of the swamp, and cover above the nest. Nests ofH. littorale were built in herbaceous swamps, whileC. callichthys andH. thoracatum build their nests in swamp-forest. Nests ofC. callichthys were observed in extremely shallow water or in holes. Few nests of the three species were observed in canals. Over large parts of their geographical rangeC. callichthys andH. thoracatum are found in small rainforest streams. Several aspects of the reproductive ecology and behaviour of both species are probably related to the unpredictability of the stream habitat.     

Interspecific competition between the principal larval parasitoids of the pine sawfly,Neodiprion sertifer (Geoff.) (Hym.: Diprionidae)

Summary Interspecific competition between the 3 principal larval parasitoids of the pine sawfly, Neodiprion sertifer, is of common occurrence when total larval parasitism, and hence multiple attack rates, are high. At the intrinsic level, the ectoparasitoid Exenterus abruptorius is superior to the 2 endoparasitoids, Lophyroplectus luteator and Lamachus eques, respectively, whereas L. luteator is superior to L. eques only. During mass outbreaks of the host competition between Exenterus and Lophyroplectus is most intense, whilst Lamachus fails to build up in the presence of its 2 competitors (Fig. 1). However, due to its superior host finding ability, Lamachus dominates in areas of low host densities where the 2 other species are less efficient (Table 1). L. luteator frequently sustains heavy losses in direct competition with E. abruptorius, but these can be largely compensated in the next generation because its fertility is about 5-times higher than that of Exenterus.     

Effects of various environmental conditions on growth and reproduction of the sea hareAplysia oculifera (Adams and Reeve, 1850)

We studied the influences of food type, food quantity, water currents, starvation and light on growth and reproduction of the sea hareaplysia oculifera (Adams and Reeve, 1850) under laboratory conditions. Out of five species of algae served as food,Enteromorpha intestinalis promoted the fastest growth ofA. oculifera, Ulva spp. slower growth,Cladophora sp. allowed maintenance spp. slower growth,Cladophora sp. allowed maintenance of steady body mass, and the brown algaeColpomenia sp. andPadina pavonia were rejected by the sea hares. When sea hares were exposed to four levels of water currents, growth rates decreased as water currents increased. Sea hares fed on 50% ration grew slower than those fed on 100% ration (ad libitum). During 10 days of starvation sea hares lost weight, but when subsequently fed 100% ration they recovered and grew at a rate similar to those fed continuously with 100% ration. Under shade and under natural sunlight sea hares grew at the same rates. Whenever growth rates decreased, sea hares began to spawn at a smaller body size.A. oculifera demonstrated physiological plasticity that adapted them to varied and unpredictable environmental conditions. At different conditions of food availability they applied different tactics of resource allocation between growth and reproduction.     

Light reduction predicts widespread patterns of dominance between asters and goldenrods

Here we investigate the long-cited pattern that throughout the eastern United States, Solidago species (goldenrods), and in particular S. canadensis displace Aster species and dominate old-field communities. Theory predicts that such a ubiquitous pattern of repeated dominance should be linked to competitive ability for a limiting resource. However, no one has investigated this possibility in old-fields, representing a potentially significant gap in our understanding of a common human-altered environment. We tested the hypothesis that S. canadensis is the superior competitor for light compared to other common co-occurring goldenrod species, and that the goldenrods in general are the superior competitors for light compared to coexisting aster species, which are typically less abundant. We tested this hypothesis by comparing the light attenuation abilities of four goldenrod species, S. canadensis, S. rugosa, S. gigantea, and Euthamia graminifolia, and three aster species, Aster novae-angliae, A. pilosus, and A. prenanthoides. Consistent with our hypothesis, S. canadensis had a greater ability to attenuate light than any of the other goldenrods at higher densities, and the goldenrods overall had a greater ability to attenuate light than the asters. By conducting a census in our study area, we verified that S. canadensis is locally the most abundant goldenrod and that goldenrods are more locally abundant than asters. Furthermore, by conducting a literature survey we found evidence that S. canadensis replaces A. pilosus through time. Thus we found a close correspondence between relative abundance in the field and light attenuation ability in field experiments. These results are consistent with theory predicting that competition for limiting resources, in this case light, explains patterns of dominance and relative abundance in old-field plant communities.     

Predicting extinctions: interspecific competition,predation and population variability in experimental Daphnia populations

We examine how interspecific competition and two types of size-selective predation affect population density, variability and persistence in laboratory cultures of two species of Daphnia, D. magna and D. longispina. When both species were analysed together, and for D. longispina alone, there were weak negative relationships between mean population density and population variability. Interspecific competition resulted in lower population densities and higher population variability. Extinct populations had lower densities and were also more variable than persisting ones. There was still an effect of population variability on extinction probability after the effect of density on population variability had been accounted for. Hence, the effects of population density and variability on population persistence were partly independent of each other. The effects of size-selective predation on population persistence were more species-specific and not directly related to density or variability. Since the effects of species interactions on persistence were large, we suggest that it is likely that population vulnerability analyses not incorporating effects of interspecific interactions are often misleading.     

Colony-structure variation and interspecific competitive ability in the invasive Argentine ant

The success of some invasive species may depend on phenotypic changes that occur following introduction. In Argentine ants ( Linepithema humile) introduced populations typically lack intraspecific aggression, but native populations display such behavior commonly. We employ three approaches to examine how this behavioral shift might influence interspecific competitive ability. In a laboratory experiment, we reared colonies of Forelius mccooki with pairs of Argentine ant colonies that either did or did not exhibit intraspecific aggression. F. mccooki reared with intraspecifically non-aggressive pairs of Argentine ants produced fewer eggs, foraged less actively, and supported fewer living workers than those reared with intraspecifically aggressive pairs. At natural contact zones between competing colonies of L. humile and F. mccooki, the introduction of experimental Argentine ant colonies that fought with conspecific field colonies caused L. humile to abandon baits in the presence of F. mccooki, whereas the introduction of colonies that did not fight with field colonies of Argentine ants resulted in L. humile retaining possession of baits. Additional evidence for the potential importance of colony- structure variation comes from the Argentine ants native range. At a site along the Rio de la Plata in Argentina, we found an inverse relationship between ant richness and density of L. humile (apparently a function of local differences in colony structure) in two different years of sampling.     

Growth and survival performance of the gametophyte of Gigartina skottsbergii (Rhodophyta, Gigartinales) under defined nutrient conditions in laboratory culture

Gigartina skottsbergii is a commercially important carrageenan producer that has been suffering severe extraction pressure in Chile’s Magellan Region and Cape Horn Archipelago since 1998. In order to create baseline information for its cultivation and repopulation, we studied the effects of agricultural fertilizers on growth of G. skottsbergii early developmental stages. The culture media utilized were: a) seawater + Bayfoland, b) seawater + Superphosphate, c) seawater + Urea, d) seawater + Provasoli and e) seawater as a control. The culture conditions were: a) 12L:12D photoperiod; b) temperature 8 ± 1°C and c) irradiance at 45 μmol photons m⁻² s⁻¹. After 60 days, higher relative growth rates between treatments were observed; the treatments that included Bayfoland and Provasoli showed greater growth (382 ± 55 and 378 ± 50 μm, respectively,) compared to Superphosphate (88 ± 16 μm), control (78 ± 10 μm) and Urea (70 ± 11 μm) treatments, after 81 days. The Urea treatment and the control had inhibitory effects on G. skottsbergii germlings growth and survival, as evidenced by progressive loss of pigmentation and death after 60 days. These results showed that Bayfoland was an excellent alternative to develop cultures.     

水肥胁迫对尾叶桉无性系生长及叶片变异的影响研究

为选择尾叶桉(Eucalyptus urophylla)优良无性系,以20个尾叶桉无性系为材料,采用2因素(水分、养分)3水平完全随机区组设计,对水肥胁迫半年后尾叶桉苗木生长和叶片性状变异进行分析和评估。结果表明,尾叶桉的叶尖角、叶长、叶宽、叶周长和叶面积在不同水肥处理下差异显著,表明水分和养分会影响叶片的大小。苗木生长和叶片性状间大多数呈显著或极显著正相关,少数呈显著负相关或一定的负相关性。13种性状可综合为4个主成分,其累计贡献率达98%,说明这些性状具有极强的代表性。除地径的水分×养分互作不显著外,生长性状在水分、养分及水分×养分互作上达显著和极显著的水平。无性系在树高、地径和冠幅上的方差分量为2.19~149.59,重复力为0.16~0.45。以20%入选率,采用基因型值筛选出4个优良无性系:LDUA10、ZQUA9、ZQUA8和LDUA24,他们的树高、地径和冠幅比对照分别高出5.0%、12.8%和14.5%,为后续骨干亲本选择及优良无性系推广提供依据。     

Effects of filamentous algae and deposited matter on the survival of Fucus vesiculosus L. germlings in the Baltic Sea

As a result of increased nutrient levels in the Baltic Sea during thepast 50 years, mass developments of filamentous algae have become a commonfeature along the Swedish east coast and deposition of organic matter has alsoincreased. To test whether these two factors have any effects on the early lifestages of Fucus vesiculosus a number of laboratory andfield studies were conducted. The amount of epilithic and epiphytic filamentousalgae on F. vesiculosus and the amount of deposited matterin the littoral zone were quantified during the two reproductive periods ofF. vesiculosus, early summer (May–June) and lateautumn (September–October). Both filamentous algae (Cladophoraglomerata) and deposited matter (introduced either before or aftersettlement of fertilized eggs) were shown to significantly decrease the numberof surviving germlings. The survival of germlings seeded on stones withfilamentous algae, or seeded on culture dishes concurrently with the lowestconcentration of deposited matter (0.1 g dm^–2),was 5% or less. In the field, the amount of filamentous algae was significantlyhigher during F. vesiculosus summer reproduction, whereasthe amount of deposited matter collected in traps was significantly higherduring the period of autumn reproduction. The greatest biomass of filamentousalgae was observed at sheltered sites. Based on the negative effects offilamentous algae and deposited matter on Fucusrecruitmentand the observation of local and seasonal differences in abundance offilamentous algae and deposition, we suggest that the prerequisites for thesurvival of either summer or autumn-reproducing populations of F.vesiculosus in the Baltic Sea may differ locally.     

Water flux and canopy conductance of natural versus planted forests in Patagonia, South America

In NW Patagonia, South America, natural shrublands and mixed forests of short Nothofagus antarctica (G. Forst.) Oerst. trees are currently being replaced by plantations with Pseudotsuga menziesii (Mirb) Franco. This land use change is controversial because the region is prone to drought, and replacement of native vegetation by planted forests may increase vegetation water use. The goal of this study was to examine the physiological differences, especially the response of water flux and canopy conductance to microclimate, that lead to greater water use by exotic trees compared to native trees. Meteorological variables and sapflow density of P. menziesii and four native woody species were measured in the growing season 2005–2006. Canopy conductance (gc) was estimated for both the exotic (monoculture) and native (multi-species) systems, including the individual contributions of each species of the native forest. Sapflow density, stand-level transpiration and gc were related to leaf-to-air vapor pressure difference (VPD). All native species had different magnitudes and diurnal patterns of sapflow density compared to P. menziesii, which could be explained by the different gc responses to VPD. Stomatal sensitivity to VPD suggested that all native species have a stronger stomatal control of leaf water potential and transpiration due to hydraulic limitations compared to P. menziesii. In conclusion, differences in water use between a P. menziesii plantation and a contiguous native mixed forest of similar basal area could be explained by different gc responses to VPD between species (higher sensitivity in the native species), in addition to particular characteristics of the native forest structure.