Extractable nitrogen and microbial community structure respond to grassland restoration regardless of historical context and soil composition

Grasslands have a long history of invasion by exotic annuals, which may alter microbial communities and nutrient cycling through changes in litter quality and biomass turnover rates. We compared plant community composition, soil chemical and microbial community composition, potential soil respiration and nitrogen (N) turnover rates between invaded and restored plots in inland and coastal grasslands. Restoration increased microbial biomass and fungal : bacterial (F : B) ratios, but sampling season had a greater influence on the F : B ratio than did restoration. Microbial community composition assessed by phospholipid fatty acid was altered by restoration, but also varied by season and by site. Total soil carbon (C) and N and potential soil respiration did not differ between treatments, but N mineralization decreased while extractable nitrate and nitrification and N immobilization rate increased in restored compared with unrestored sites. The differences in soil chemistry and microbial community composition between unrestored and restored sites indicate that these soils are responsive, and therefore not resistant to feedbacks caused by changes in vegetation type. The resilience, or recovery, of these soils is difficult to assess in the absence of uninvaded control grasslands. However, the rapid changes in microbial and N cycling characteristics following removal of invasives in both grassland sites suggest that the soils are resilient to invasion. The lack of change in total C and N pools may provide a buffer that promotes resilience of labile pools and microbial community structure.

Contrasting physiological responses of ozone-tolerant Phaseolus vulgaris and Nicotiana tobaccum varieties to ozone and nitric acid

Ozone (O3) and nitric acid (HNO3) are synthesized by the same atmospheric photochemical processes and are almost always co-pollutants. Effects of O3 on plants have been well-elucidated, yet less is known about the effects of HNO3 on plants. We investigated the physiological effects of experimental O3 and HNO3 fumigation on Phaseolus vulgaris (snap bean) and Nicotiana tobaccum (tobacco) varieties with known sensitivity to O3, but unknown responses to HNO3. Responses were measured as leaf absorptance, aboveground plant biomass, and photosynthetic CO2-response curve parameters. Our results demonstrate that O3 reduced absorptance, stomatal conductance and plant biomass in both species, and maximum photosynthetic rate in P. vulgaris, whereas the main effect of HNO3 was an increase in mesophyll conductance. Overall, the results suggest that HNO3 affects mesophyll conductance through increased nitrogen absorbed by leaves during HNO3 deposition which in turn increases photosynthetic demand for CO2, or that damage to epicuticular waxes on leaves increased diffusion of CO2 to sites of carboxylation.

Coordination of stem and leaf hydraulic conductance in southern California shrubs: a test of the hydraulic segmentation hypothesis

Coordination of water movement among plant organs is important for understanding plant water use strategies. The hydraulic segmentation hypothesis (HSH) proposes that hydraulic conductance in shorter lived, 'expendable' organs such as leaves and longer lived, more 'expensive' organs such as stems may be decoupled, with resistance in leaves acting as a bottleneck or 'safety valve'. We tested the HSH in woody species from a Mediterranean-type ecosystem by measuring leaf hydraulic conductance (Kleaf) and stem hydraulic conductivity (KS). We also investigated whether leaves function as safety valves by relating Kleaf and the hydraulic safety margin (stem water potential minus the water potential at which 50% of conductivity is lost (Ψstem-Ψ50)). We also examined related plant traits including the operating range of water potentials, wood density, leaf mass per area, and leaf area to sapwood area ratio to provide insight into whole-plant water use strategies. For hydrated shoots, Kleaf was negatively correlated with KS , supporting the HSH. Additionally, Kleaf was positively correlated with the hydraulic safety margin and negatively correlated with the leaf area to sapwood area ratio. Consistent with the HSH, our data indicate that leaves may act as control valves for species with high KS , or a low safety margin. This critical role of leaves appears to contribute importantly to plant ecological specialization in a drought-prone environment.

Determinants of change in subtropical tree diameter growth with ontogenetic stage

We evaluated the degree to which relative growth rate (RGR) of saplings and large trees is related to seven functional traits that describe physiological behavior and soil environmental factors related to topography and fertility for 57 subtropical tree species in Dinghushan, China. The mean values of functional traits and soil environmental factors for each species that were related to RGR varied with ontogenetic stage. Sapling RGR showed greater relationships with functional traits than large-tree RGR, whereas large-tree RGR was more associated with soil environment than was sapling RGR. The strongest single predictors of RGR were wood density for saplings and slope aspect for large trees. The stepwise regression model for large trees accounted for a larger proportion of variability (R(2) = 0.95) in RGR than the model for saplings (R(2) = 0.55). Functional diversity analysis revealed that the process of habitat filtering likely contributes to the substantial changes in regulation of RGR as communities transition from saplings to large trees.

Can vessel dimension explain tolerance toward fungal vascular wilt diseases in woody plants? Lessons from Dutch elm disease and esca disease in grapevine

This review illuminates key findings in our understanding of grapevine xylem resistance to fungal vascular wilt diseases. Grapevine (Vitis spp.) vascular diseases such as esca, botryosphaeria dieback, and eutypa dieback, are caused by a set of taxonomically unrelated ascomycete fungi. Fungal colonization of the vascular system leads to a decline of the plant host because of a loss of the xylem function and subsequent decrease in hydraulic conductivity. Fungal vascular pathogens use different colonization strategies to invade and kill their host. Vitis vinifera cultivars display different levels of tolerance toward vascular diseases caused by fungi, but the plant defense mechanisms underlying those observations have not been completely elucidated. In this review, we establish a parallel between two vascular diseases, grapevine esca disease and Dutch elm disease, and argue that the former should be viewed as a vascular wilt disease. Plant genotypes exhibit differences in xylem morphology and resistance to fungal pathogens causing vascular wilt diseases. We provide evidence that the susceptibility of three commercial V. vinifera cultivars to esca disease is correlated to large vessel diameter. Additionally, we explore how xylem morphological traits related to water transport are influenced by abiotic factors, and how these might impact host tolerance of vascular wilt fungi. Finally, we explore the utility of this concept for predicting which V. vinifera cultivars are most vulnerable of fungal vascular wilt diseases and propose new strategies for disease management.

PrometheusWiki Gold Leaf Protocol: gas exchange using LI-COR 6400

This leaf gas exchange protocol enables light or CO2 response curves using a LI-COR LI-6400 portable photosynthesis system. The protocol originates in PrometheusWiki (http://prometheuswiki.publish.csiro.au/) where it has been tested and verified, and has received favourable user reviews. This reformatted and non-editable version is published as a Gold Leaf Protocol. For the most current version, including any user-commentary updates, readers may view the live version of the protocol at http://bit.ly/PWLicorGold.

Plant water status and hydraulic conductance during flowering in the southern California coastal sage shrub Salvia mellifera (Lamiaceae)

PREMISE OF THE STUDY

Plant water status during flowering is important for plant reproduction, but the physiology of floral water use is not well understood. We investigated plant water status in relation to leaf and floral physiology in naturally occurring individuals of a semiarid shrub, Salvia mellifera E. Greene.

METHODS

We measured stomatal (g(s)) and corolla (g(c)) conductance to water vapor, transpiration from leaves (E(leaf)) and corollas (E(corolla)), leaf-specific hydraulic conductance (K(H)), bulk shoot water potential (Ψ(shoot)), and shoot water content on irrigated and control plants to analyze whether water was limiting to leaf and floral water use.

KEY RESULTS

Experimental irrigation caused a 203% increase in soil moisture content, a 20% increase in predawn Ψ(shoot), a 29% increase in midday Ψ(shoot), and a 92% increase in K(H). Floral and leaf gas exchange did not respond significantly to water addition, indicating that rates were at seasonal maxima and not limited by water availability. Total daily water use by corollas was ∼20% of total shoot water use. There were no significant differences in total daily shoot water use with water addition. Mean shoot water content (5.07 g) was close to mean daily shoot water use (6.71 g), indicating that the equivalent of total shoot water content turned over every 0.76 d.

CONCLUSIONS

These results demonstrate that although irrigation improved whole-plant hydraulic conductance, gas exchange was not limited by water availability. Additionally, the high water use of flowers in this species might limit future flowering and reproductive success during dry years.

Environmental regulation of carbon isotope composition and crassulacean acid metabolism in three plant communities along a water availability gradient

Expression of crassulacean acid metabolism (CAM) is characterized by extreme variability within and between taxa and its sensitivity to environmental variation. In this study, we determined seasonal fluctuations in CAM photosynthesis with measurements of nocturnal tissue acidification and carbon isotopic composition (δ(13)C) of bulk tissue and extracted sugars in three plant communities along a precipitation gradient (500, 700, and 1,000 mm year(-1)) on the Yucatan Peninsula. We also related the degree of CAM to light habitat and relative abundance of species in the three sites. For all species, the greatest tissue acid accumulation occurred during the rainy season. In the 500 mm site, tissue acidification was greater for the species growing at 30% of daily total photon flux density (PFD) than species growing at 80% PFD. Whereas in the two wetter sites, the species growing at 80% total PFD had greater tissue acidification. All species had values of bulk tissue δ(13)C less negative than -20‰, indicating strong CAM activity. The bulk tissue δ(13)C values in plants from the 500 mm site were 2‰ less negative than in plants from the wetter sites, and the only species growing in the three communities, Acanthocereus tetragonus (Cactaceae), showed a significant negative relationship between both bulk tissue and sugar δ(13)C values and annual rainfall, consistent with greater CO(2) assimilation through the CAM pathway with decreasing water availability. Overall, variation in the use of CAM photosynthesis was related to water and light availability and CAM appeared to be more ecologically important in the tropical dry forests than in the coastal dune.

Water relations of evergreen and drought-deciduous trees along a seasonally dry tropical forest chronosequence

Seasonally dry tropical forests (SDTF) are characterized by pronounced seasonality in rainfall, and as a result trees in these forests must endure seasonal variation in soil water availability. Furthermore, SDTF on the northern Yucatan Peninsula, Mexico, have a legacy of disturbances, thereby creating a patchy mosaic of different seral stages undergoing secondary succession. We examined the water status of six canopy tree species, representing contrasting leaf phenology (evergreen vs. drought-deciduous) at three seral stages along a fire chronosequence in order to better understand strategies that trees use to overcome seasonal water limitations. The early-seral forest was characterized by high soil water evaporation and low soil moisture, and consequently early-seral trees exhibited lower midday bulk leaf water potentials (Ψ_L) relative to late-seral trees (−1.01 ± 0.14 and −0.54 ± 0.07 MPa, respectively). Although Ψ_L did not differ between evergreen and drought-deciduous trees, results from stable isotope analyses indicated different strategies to overcome seasonal water limitations. Differences were especially pronounced in the early-seral stage where evergreen trees had significantly lower xylem water δ¹⁸O values relative to drought-deciduous trees (−2.6 ± 0.5 and 0.3 ± 0.6‰, respectively), indicating evergreen species used deeper sources of water. In contrast, drought-deciduous trees showed greater enrichment of foliar ¹⁸O (∆¹⁸O_l) and ¹³C, suggesting lower stomatal conductance and greater water-use efficiency. Thus, the rapid development of deep roots appears to be an important strategy enabling evergreen species to overcome seasonal water limitation, whereas, in addition to losing a portion of their leaves, drought-deciduous trees minimize water loss from remaining leaves during the dry season.

Carbon stable isotopic composition of soluble sugars in Tillandsia epiphytes varies in response to shifts in habitat

We studied C stable isotopic composition (δ¹³C) of bulk leaf tissue and extracted sugars of four epiphytic Tillandsia species to investigate flexibility in the use of crassulacean acid metabolism (CAM) and C₃ photosynthetic pathways. Plants growing in two seasonally dry tropical forest reserves in Mexico that differ in annual precipitation were measured during wet and dry seasons, and among secondary, mature, and wetland forest types within each site. Dry season sugars were more enriched in ¹³C than wet season sugars, but there was no seasonal difference in bulk tissues. Bulk tissue δ¹³C differed by species and by forest type, with values from open-canopied wetlands more enriched in ¹³C than mature or secondary forest types. The shifts within forest habitat were related to temporal and spatial changes in vapor pressure deficits (VPD). Modeling results estimate a possible 4% increase in the proportional contribution of the C₃ pathway during the wet season, emphasizing that any seasonal or habitat-mediated variation in photosynthetic pathway appears to be quite moderate and within the range of isotopic effects caused by variation in stomatal conductance during assimilation through the C₃ pathway and environmental variation in VPD. C isotopic analysis of sugars together with bulk leaf tissue offers a useful approach for incorporating short- and long-term measurements of C isotope discrimination during photosynthesis.

Fog interception by Sequoia sempervirens (D. Don) crowns decouples physiology from soil water deficit

Although crown wetting events can increase plant water status, leaf wetting is thought to negatively affect plant carbon balance by depressing photosynthesis and growth. We investigated the influence of crown fog interception on the water and carbon relations of juvenile and mature Sequoia sempervirens trees. Field observations of mature trees indicated that fog interception increased leaf water potential above that of leaves sheltered from fog. Furthermore, observed increases in leaf water potential exceeded the maximum water potential predicted if soil water was the only available water source. Because field observations were limited to two mature trees, we conducted a greenhouse experiment to investigate how fog interception influences plant water status and photosynthesis. Pre-dawn and midday branchlet water potential, leaf gas exchange and chlorophyll fluorescence were measured on S. sempervirens saplings exposed to increasing soil water deficit, with and without overnight canopy fog interception. Sapling fog interception increased leaf water potential and photosynthesis above the control and soil water deficit treatments despite similar dark-acclimated leaf chlorophyll fluorescence. The field observations and greenhouse experiment show that fog interception represents an overlooked flux into the soil-plant-atmosphere continuum that temporarily, but significantly, decouples leaf-level water and carbon relations from soil water availability.

Crassulacean acid metabolism and epiphytism linked to adaptive radiations in the Orchidaceae

Species of the large family Orchidaceae display a spectacular array of adaptations and rapid speciations that are linked to several innovative features, including specialized pollination syndromes, colonization of epiphytic habitats, and the presence of Crassulacean acid metabolism (CAM), a water-conserving photosynthetic pathway. To better understand the role of CAM and epiphytism in the evolutionary expansion of tropical orchids, we sampled leaf carbon isotopic composition of 1,103 species native to Panama and Costa Rica, performed character state reconstruction and phylogenetic trait analysis of CAM and epiphytism, and related strong CAM, present in 10% of species surveyed, to climatic variables and the evolution of epiphytism in tropical regions. Altitude was the most important predictor of photosynthetic pathway when all environmental variables were taken into account, with CAM being most prevalent at low altitudes. By creating integrated orchid trees to reconstruct ancestral character states, we found that C3 photosynthesis is the ancestral state and that CAM has evolved at least 10 independent times with several reversals. A large CAM radiation event within the Epidendroideae, the most species-rich epiphytic clade of any known plant group, is linked to a Tertiary species radiation that originated 65 million years ago. Our study shows that parallel evolution of CAM is present among subfamilies of orchids, and correlated divergence between photosynthetic pathways and epiphytism can be explained by the prevalence of CAM in low-elevation epiphytes and rapid speciation of high-elevation epiphytes in the Neotropics, contributing to the astounding diversity in the Orchidaceae.

Why are non-photosynthetic tissues generally 13C enriched compared with leaves in C3 plants? Review and synthesis of current hypotheses

Non-photosynthetic, or heterotrophic, tissues in C₃ plants tend to be enriched in ¹³C compared with the leaves that supply them with photosynthate. This isotopic pattern has been observed for woody stems, roots, seeds and fruits, emerging leaves, and parasitic plants incapable of net CO₂ fixation. Unlike in C₃ plants, roots of herbaceous C₄ plants are generally not ¹³C-enriched compared with leaves. We review six hypotheses aimed at explaining this isotopic pattern in C₃ plants: (1) variation in biochemical composition of heterotrophic tissues compared with leaves; (2) seasonal separation of growth of leaves and heterotrophic tissues, with corresponding variation in photosynthetic discrimination against ¹³C; (3) differential use of day v. night sucrose between leaves and sink tissues, with day sucrose being relatively ¹³C-depleted and night sucrose ¹³C-enriched; (4) isotopic fractionation during dark respiration; (5) carbon fixation by PEP carboxylase; and (6) developmental variation in photosynthetic discrimination against ¹³C during leaf expansion. Although hypotheses (1) and (2) may contribute to the general pattern, they cannot explain all observations. Some evidence exists in support of hypotheses (3) through to (6), although for hypothesis (6) it is largely circumstantial. Hypothesis (3) provides a promising avenue for future research. Direct tests of these hypotheses should be carried out to provide insight into the mechanisms causing within-plant variation in carbon isotope composition.

[Growth, survival and herbivory of seedlings in Brosimum alicastrum (Moraceae), a species from the Neotropical undergrowth]

Growth responses, survival, and herbivory, on seedlings of Brosimum alicastrum were studied in a neotropical Mexican forest. We selected 122 seedlings and divided them into three groups assigned to defoliation treatments: control or 0 (n=21), 50 (n=51) and 90% (n=50). Every 4 months during two years we measured seedling growth (in terms of relative growth rate in biomass, leaf area growth, produced leaves and height growth) and survival. In addition, we evaluated every 12 months pathogen damage and insect herbivory using a 2 mm(-2) grid. Separately, we estimated mammal herbivory in 3-month old seedlings that were selected within a plot of 500 m x 10 m (N=1095). Pathogen damage and insect herbivory were evaluated within the same plot in 113 seedlings. We found that 50% defoliated seedlings showed compensatory responses in all growth parameters. Relative growth rate and height growth also had a compensatory response in seedlings at 90% defoliation. Relative growth rate and leaf area growth gradually decreased with time although height growth seedling showed an opposite pattern. Leaves produced were not affected by time. Estimated seedling survival probability increased with defoliation to a maximum of 97%, decreasing at 24 month to 37%. Mammal herbivory was more frequent and severe than herbivory caused by pathogens and insects. In some cases, mammal herbivory produced total defoliation. Compensatory growth in leaf area growth, produced leaves and height growth seedling suggest a synergic compensatory mechanism expressed in a whole-plant growth biomass (relative growth rate). Compensation and survival results suggest trade-offs at the leaf level, such as leaf area growth and produced leaves versus chemical defenses, respectively.

A comparison of sap flow measurements and potometry in two tropical lowland tree species with contrasting wood properties

We evaluated the performance of the Heat Dissipation Technique (HDT) to measure sap flow in whole trees by comparison with potometric water uptake. Two tropical lowland species, Ochroma lagopus (balsa), a pioneer species with light wood and Hyeronima alchorneoides (pilón), a late-successional species with hard wood were examined. Diurnal courses of sap flow measured with the HDT showed good agreement with potometry. At the low sap flow rates (below 1 Kg h(-1)) occurring during nocturnal recharge HDT consistently underestimated sap flow rates. This resulted in the failure of the current version of the HDT to measure nocturnal water uptake, an important component of the water budget of at least one of the two species examined.

Nighttime transpiration in woody plants from contrasting ecosystems

It is commonly assumed that transpiration does not occur at night because leaf stomata are closed in the dark. We tested this assumption across a diversity of ecosystems and woody plant species by various methods to explore the circumstances when this assumption is false. Our primary goals were: (1) to evaluate the nature and magnitude of nighttime transpiration, E(n), or stomatal conductance, g(n); and (2) to seek potential generalizations about where and when it occurs. Sap-flow, porometry and stable isotope tracer measurements were made on 18 tree and eight shrub species from seven ecosystem types. Coupled with environmental data, our findings revealed that most of these species transpired at night. For some species and circumstances, nighttime leaf water loss constituted a significant fraction of total daily water use. Our evidence shows that E(n) or g(n) can occur in all but one shrub species across the systems we investigated. However, under conditions of high nighttime evaporative demand or low soil water availability, stomata were closed and E(n) or g(n) approached zero in eleven tree and seven shrub species. When soil water was available, E(n) or g(n) was measurable in these same species demonstrating plasticity for E(n) or g(n). We detected E(n) or g(n) in both trees and shrubs, and values were highest in plants from sites with higher soil water contents and in plants from ecosystems that were less prone to atmospheric or soil water deficits. Irrespective of plant or ecosystem type, many species showed E(n) or g(n) when soil water deficits were slight or non-existent, or immediately after rainfall events that followed a period of soil water deficit. The strongest relationship was between E(n) or g(n) and warm, low humidity and (or) windy (> 0.8 m s(-1)) nights when the vapor pressure deficit remained high (> 0.2 kPa in wet sites, > 0.7 kPa in dry sites). Why E(n) or g(n) occurs likely varies with species and ecosystem type; however, our data support four plausible explanations: (1) it may facilitate carbon fixation earlier in the day because stomata are already open; (2) it may enhance nutrient supply to distal parts of the crown when these nutrients are most available (in wet soils) and transport is rapid; (3) it may allow for the delivery of dissolved O(2) via the parenchyma to woody tissue sinks; or (4) it may occur simply because of leaky cuticles in older leaves or when stomata cannot close fully because of obstructions from stomatal (waxy) plugs, leaf endophytes or asymmetrical guard cells (all non-adaptive reasons). We discuss the methodological, ecophysiological, and theoretical implications of the occurrence of E(n) or g(n) for investigations at a variety of scales.

Distribution of crassulacean acid metabolism in orchids of Panama: evidence of selection for weak and strong modes

Crassulacean acid metabolism (CAM) is one of three metabolic pathways found in vascular plants for the assimilation of carbon dioxide. In this study, we investigate the occurrence of CAM photosynthesis in 200 native orchid species from Panama and 14 non-native species by carbon isotopic composition (δ¹³C) and compare these values with nocturnal acid accumulation measured by titration in 173 species. Foliar δ¹³C showed a bimodal distribution with the majority of species exhibiting values of approximately -28‰ (typically associated with the C₃ pathway), or -15‰ (strong CAM). Although thick leaves were related to δ¹³C values in the CAM range, some thin-leaved orchids were capable of CAM photosynthesis, as demonstrated by acid titration. We also found species with C₃ isotopic values and significant acid accumulation at night. Of 128 species with δ¹³C more negative than -22‰, 42 species showed nocturnal acid accumulation per unit fresh mass characteristic of weakly expressed CAM. These data suggest that among CAM orchids, there may be preferential selection for species to exhibit strong CAM or weak CAM, rather than intermediate metabolism.