The combined effects of a long-term experimental drought and an extreme drought on the use of plant-water sources in a Mediterranean forest.

Vegetation in water-limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species-specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long-term experimental drought (12 years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long-term experimental drought shifted water uptake toward deeper (10-35 cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought-affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.

On the vapour trail of an atmospheric imprint in insects.

Terrestrial arthropods, at constant risk from desiccation, are highly sensitive to atmospheric temperature and humidity. A physiological marker of these abiotic conditions could highlight phenotypic adaptations, indicate niche partitioning, and predict responses to climate change for a group representing three-quarters of the Earth's animal species. We show that the (18)O composition of insect haemolymph is such a measure, providing a dynamic and quantitatively predictable signal for respiratory gas exchange and inputs from atmospheric humidity. Using American cockroaches (Periplaneta americana) under defined experimental conditions, we show that insects respiring at low humidity demonstrate the expected enrichment in the (18)O composition of haemolymph because of evaporation. At high humidity, however, diffusional influx of atmospheric water vapour into the animal forces haemolymph to become depleted in (18)O. Additionally, using cockroaches sampled from natural habitats, we show that the haemolymph (18)O signature is transferred to the organic material of the insect's exoskeleton. Insect cuticle, therefore, exhibits the mean atmospheric conditions surrounding the animals prior to moulting. This discovery will help to define the climatic tolerances of species and their habitat preferences, and offers a means of quantifying the balance between niche partitioning and 'neutral' processes in shaping complex tropical forest communities.

Leaf water δ18O reflects water vapour exchange and uptake by C3 and CAM epiphytic bromeliads in Panama.

The distributions of CAM and C3 epiphytic bromeliads across an altitudinal gradient in western Panama were identified from carbon isotope (δ13C) signals, and epiphyte water balance was investigated via oxygen isotopes (δ18O) across wet and dry seasons. There were significant seasonal differences in leaf water (δ18Olw), precipitation, stored 'tank' water and water vapour. Values of δ18Olw were evaporatively enriched at low altitude in the dry season for the C3 epiphytes, associated with low relative humidity (RH) during the day. Crassulacean acid metabolism (CAM) δ18Olw values were relatively depleted, consistent with water vapour uptake during gas exchange under high RH at night. At high altitude, cloudforest locations, C3 δ18Olw also reflected water vapour uptake by day. A mesocosm experiment with Tillandsia fasciculata (CAM) and Werauhia sanguinolenta (C3) was combined with simulations using a non-steady-state oxygen isotope leaf water model. For both C3 and CAM bromeliads, δ18Olw became progressively depleted under saturating water vapour by day and night, although evaporative enrichment was restored in the C3 W. sanguinolenta under low humidity by day. Source water in the overlapping leaf base 'tank' was also modified by evaporative δ18O exchanges. The results demonstrate how stable isotopes in leaf water provide insights for atmospheric water vapour exchanges for both C3 and CAM systems.

Oxygen isotope composition of CAM and C3 Clusia species: non-steady-state dynamics control leaf water 18O enrichment in succulent leaves.

Leaf gas exchange and leaf water (18)O enrichment (Delta(18)O(L)) were measured in three Clusia species under field conditions during dry and wet seasons and in Miconia argentea during the dry season in the Republic of Panama. During the dry season, all three Clusia species used crassulacean acid metabolism (CAM); during the wet season Clusia pratensis operated in the C(3) mode, while Clusia uvitana and Clusia rosea used CAM. Large departures from isotopic steady state were observed in daytime Delta(18)O(L) of the Clusia species, especially during the dry season. In contrast, daytime Delta(18)O(L) was near isotopic steady state in the C(3) tree M. argentea. Across the full data set, non-steady-state predictions explained 49% of variation in observed Delta(18)O(L), whereas steady-state predictions explained only 14%. During the wet season, when Delta(18)O(L) could be compared with Clusia individuals operating in both C(3) and CAM modes, steady-state and non-steady-state models gave contrasting predictions with respect to interspecific variation in daytime Delta(18)O(L). The observed Delta(18)O(L) pattern matched that predicted for the non-steady state. The results provided a clear example of how non-steady-state control of leaf water (18)O dynamics can shift the slope of the relationship between transpiration rate and daytime Delta(18)O(L) from negative to positive.

Water vapour isotopic exchange by epiphytic bromeliads in tropical dry forests reflects niche differentiation and climatic signals.

The 18O signals in leaf water (delta18O(lw)) and organic material were dominated by atmospheric water vapour 18O signals (delta18O(vap)) in tank and atmospheric life forms of epiphytic bromeliads with crassulacean acid metabolism (CAM), from a seasonally dry forest in Mexico. Under field conditions, the mean delta18O(lw) for all species was constant during the course of the day and systematically increased from wet to dry seasons (from 0 to +6 per thousand), when relative water content (RWC) diminished from 70 to 30%. In the greenhouse, progressive enrichment from base to leaf tip was observed at low night-time humidity; under high humidity, the leaf tip equilibrated faster with delta18O(vap) than the other leaf sections. Laboratory manipulations using an isotopically depleted water source showed that delta18O(vap) was more rapidly incorporated than liquid water. Our data were consistent with a Craig-Gordon (C-G) model as modified by Helliker and Griffiths predicting that the influx and exchange of delta18O(vap) control delta18O(lw) in certain epiphytic life forms, despite progressive tissue water loss. We use delta18O(lw) signals to define water-use strategies for the coexisting species which are consistent with habitat preference under natural conditions and life form. Bulk organic matter (delta18O(org)) is used to predict the deltaO18(vap) signal at the time of leaf expansion.

Fungal diversity and plant disease in mangrove forests: salt excretion as a possible defense mechanism.

Salt excretion in leaves of some mangrove species may serve as an important defense against fungal attack, reducing the vulnerability of typically high-density, monospecific forest stands to severe disease pressure. In field surveys of a Caribbean mangrove forest in Panama, Avicennia germinans suffered much less damage from foliar diseases than did Laguncularia racemosa or Rhizophora mangle. Similarly, Avicennia leaves supported the least superficial fungal growth, endophytic colonization, and diversity, followed by Laguncularia and Rhizophora. Host specificity of leaf-colonizing fungi was greater than expected at random. We hypothesize that the different salt tolerance mechanisms in the three mangrove species may differentially regulate fungal colonization. The mangroves differ in their salt tolerance mechanisms such that Avicennia (which excretes salt through leaf glands) has the highest salinity of residual rain water on leaves, Laguncularia (which accumulates salt in the leaves) has the greatest bulk salt concentration, and Rhizophora (which excludes salt at the roots) has little salt associated with leaves. The high salt concentrations associated with leaves of Avicennia and Laguncularia, but not the low salinity of Rhizophora, were sufficient to inhibit the germination of many fungi associated with mangrove forests.

Dense taxonomic EST sampling and its applications for molecular systematics of the Coleoptera (beetles).

Expressed sequence tag (EST) sequences can provide a wealth of data for phylogenetic and genomic studies, but the utility of these resources is restricted by poor taxonomic sampling. Here, we use small EST libraries (<1,000 clones) to generate phylogenetic markers across a broad sample of insects, focusing on the species-rich Coleoptera (beetles). We sequenced over 23,000 ESTs from 34 taxa, which produced 8,728 unique sequences after clustering nonredundant sequences. Between taxa, the sequences could be grouped into 731 gene clusters, with the largest corresponding to mitochondrial DNA transcripts and gene families chymotrypsin, actin, troponin, and tubulin. While levels of paralogy were high in most gene clusters, several midsized clusters including many ribosomal protein (RP) genes appeared to be free of expressed paralogs. To evaluate the utility of EST data for molecular systematics, we curated available transcripts for 66 RP genes from representatives of the major groups of Coleoptera. Using supertree and supermatrix approaches for phylogenetic analysis, the results were consistent with the emerging phylogenetic conclusions about basal relationships in Coleoptera. Numerous small EST libraries from a taxonomically densely sampled lineage can provide a core set of genes that together act as a scaffold in phylogenetic reconstruction, comparative genomics, and studies of gene evolution.