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• Mediterranean‐type ecosystems contain 20% of all vascular plant diversity on Earth and have been identified as being particularly threatened by future increases in drought. Of particular concern is the Cape Floral Region of South Africa, a global biodiversity hotspot, yet there are limited experimental data to validate predicted impacts on the flora. In a field rainout experiment, we tested whether rooting depth and degree of isohydry or anisohydry could aid in the functional classification of drought responses across diverse growth forms. • We imposed a 6‐month summer drought, for 2 yr, in a mountain fynbos shrubland. We monitored a suite of parameters, from physiological traits to morphological outcomes, in seven species comprising the three dominant growth forms (deep‐rooted proteoid shrubs, shallow‐rooted ericoid shrubs and graminoid restioids). • There was considerable variation in drought response both between and within the growth forms. The shallow‐rooted, anisohydric ericoid shrubs all suffered considerable reductions in growth and flowering and increased mortality. By contrast, the shallow‐rooted, isohydric restioids and deep‐rooted, isohydric proteoid shrubs were largely unaffected by the drought. • Rooting depth and degree of iso/anisohydry allow a first‐order functional classification of drought response pathways in this flora. Consideration of additional traits would further refine this approach.

Stable isotope analysis is a powerful tool for assessing plant carbon and water relations and their impact on biogeochemical processes at different scales. Our process-based understanding of stable isotope signals, as well as technological developments, has progressed significantly, opening new frontiers in ecological and interdisciplinary research. This has promoted the broad utilisation of carbon, oxygen and hydrogen isotope applications to gain insight into plant carbon and water cycling and their interaction with the atmosphere and pedosphere. Here, we highlight specific areas of recent progress and new research challenges in plant carbon and water relations, using selected examples covering scales from the leaf to the regional scale. Further, we discuss strengths and limitations of recent technological developments and approaches and highlight new opportunities arising from unprecedented temporal and spatial resolution of stable isotope measurements.

Fine root (<2 mm) cycling rates are important for understanding plant ecology and carbon fluxes in forests, but they are difficult to determine and remain uncertain. This paper synthesizes minirhizotron and isotopic data and a root model and concludes that (1) fine roots have a spectrum of turnover times ranging from months to many years and (2) the mean age of live root biomass (A) and the mean age of roots when they die (i.e., their turnover time (τ)) are not equal. We estimated A and τ of fine roots in three forests using the root model Radix. For short‐lived roots, we constrained τ with existing minirhizotron data; for long‐lived roots, we used new radiocarbon measurements of roots sampled by diameter size class and root branch order. Long‐lived root pools had site mean τ of 8–13 y and 5–9 y when sampled by diameter and branch order, respectively. Mean turnover times across sites were in general not significantly different as a function of branch‐order, size class, or depth. Our modeling results indicate that ∼20% of fine root biomass has turnover times of about a year, and ∼80% has decadal turnover times. This partitioning is reflected in our predicted mean ages of ∼9 y and turnover times of ∼3 y. We estimate that fine root mortality contributes between 38 and 104 g C m−2 y−1 to soil in these forests. These estimates are 20 to 80% of previous estimates in these and similar forests, in part because we explicitly account for the large portion of fine‐root biomass with decadal cycling rates. Our work shows that both fast and slow cycling roots must be modeled jointly to account for the heterogeneous nature of fine‐root dynamics.

Tropical dry forests (TDFs) undergo a substantial dry season in which plant species must endure several months of drought. Although TDFs support a diverse array of plant growth forms, it is not clear how they vary in mechanisms for coping with seasonal drought. We measured organic tissue stable isotopic composition of carbon (δ13C) and nitrogen (δ15N) across six plant growth forms including epiphytes, terrestrial succulents, trees, shrubs, herbs, and vines, and oxygen (δ18O) of four growth forms, to distinguish among patterns of resource acquisition and evaluate mechanisms for surviving annual drought in a lowland tropical dry forest in Yucatan, Mexico. Terrestrial succulent and epiphyte δ13C was around -14‰, indicating photosynthesis through the Crassulacean acid metabolism pathway, and along with one C4 herb were distinct from mean values of all other growth forms, which were between -26 and -29‰ indicating C3 photosynthesis. Mean tissue δ15N across epiphytes was -4.95‰ and was significantly lower than all other growth forms, which had values around +3‰. Tissue N concentration varied significantly among growth forms with epiphytes and terrestrial succulents having significantly lower values of about 1% compared to trees, shrubs, herbs and vines, which were around 3%. Tissue C concentration was highest in trees, shrubs and vines, intermediate in herbs and epiphytes and lowest in terrestrial succulents. δ18O did not vary among growth forms. Overall, our results suggest several water-saving aspects of resource acquisition, including the absolute occurrence of CAM photosynthesis in terrestrial succulents and epiphytes, high concentrations of leaf N in some species, which may facilitate CO2 drawdown by photosynthetic enzymes for a given stomatal conductance, and potentially diverse N sources ranging from atmospheric N in epiphytes with extremely depleted δ15N values, and a large range of δ15N values among trees, many of which are legumes and dry season deciduous.

Fog has been viewed as an important source of moisture in many coastal ecosystems, yet its importance for the plants which inhabit these ecosystems is virtually unknown. Here, I report the results of a 3-year investigation of fog inputs and the use of fog water by plants inhabiting the heavily fog inundated coastal redwood (Sequoia sempervirens) forests of northern California. During the study period, 34%, on average, of the annual hydrologic input was from fog drip off the redwood trees themselves (interception input). When trees were absent, the average annual input from fog was only 17%, demonstrating that the trees significantly influence the magnitude of fog water input to the ecosystem. Stable hydrogen and oxygen isotope analyses of water from fog, rain, soil water, and xylem water extracted from the dominant plant species were used to characterize the water sources used by the plants. An isotopic mixing model was employed to then quantify how much fog water each plant used each month during the 3-year study. In summer, when fog was most frequent, ∼19% of the water within S. sempervirens, and ∼66% of the water within the understory plants came from fog after it had dripped from tree foliage into the soil; for S. sempervirens, this fog water input comprised 13-45% of its annual transpiration. For all plants, there was a significant reliance on fog as a water source, especially in summer when rainfall was absent. Dependence on fog as a moisture source was highest in the year when rainfall was lowest but fog inputs normal. Interestingly, during the mild El Niño year of 1993, when the ratio of rainfall to fog water input was significantly higher and fog inputs were lower, both the proportion and coefficient of variation in how much fog water was used by plants increased. An explanation for this is that while fog inputs were lower than normal in this El Niño year, they came at a time when plant demand for water was highest (summer). Therefore, proportional use of fog water by plants increased. The results presented suggest that fog, as a meteorological factor, plays an important role in the water relations of the plants and in the hydrology of the forest. These results demonstrate the importance of understanding the impacts of climatic factors and their oscillations on the biota. The results have important implications for ecologists, hydrologists, and forest managers interested in fog-inundated ecosystems and the plants which inhabit them.

The economically important rootstock species Poncirus trifoliata is resistant to most isolates of Citrus tristeza virus (CTV), but not to members of the CTV resistance-breaking (RB) strain presently found in New Zealand. In this study, five known and suspected RB isolates were separated from field mixtures, and their genomes were sequenced in full. It was found that the RB isolates are members of a single phylogenetically distinct clade with an average of 90.3% genomic nucleotide sequence identity to the closest extant isolate, T36. These isolates also show evidence of multiple recombination events throughout their evolutionary history, with T36, T30 and VT-like isolates, and with each other. Finally, the genomic sequences of these isolates show that several genes contain unique polymorphisms that may or may not be involved in overcoming resistance. These data will aid in the understanding of host-virus interactions, and the mechanism of resistance in P. trifoliata.

Recent studies suggest that savanna trees in semi-arid areas can increase understorey plant production. We hypothesized that one of the mechanisms that explains the facilitation between trees and grasses in East African savannas is hydraulic lift (HL). HL in large Acacia tortilis trees was studied during the first 3 months of the dry season during a relatively wet year (1998) and a very dry year (2000). In 1998, we found distinct diel fluctuation in soil water potential ($\\psi _{\\text{s}}$), with increasing values during the night and decreasing again the following day. These fluctuations in$\\psi _{\\text{s}}$are consistent with other observations of HL and in A. tortilis were found up to 10 m from the tree. In 2000, during a severe drought,$\\psi _{\\text{s}}$measurements indicated that HL was largely absent. The finding that HL occurred in wetter years and not in drier years was supported by data obtained on the δ18O values in soil, rain and groundwater. The δ18O of water extracted from the xylem water of grasses indicated that when they grew near trees they had values similar to those of groundwater. This could be because they either (1) use water from deeper soil layers or (2) use hydraulically lifted water provided by the tree; this was not seen in the same grass species growing outside tree canopies. While our data indicate that HL indeed occurs under Acacia trees, it is also true that$\\psi _{\\text{s}}$was consistently lower under trees when compared to outside tree canopies. We believe that this is because tree-grass mixtures take up more water from the upper soil layers than is exuded by the tree each night. This limits the beneficial effect of HL for understorey grasses and suggests that in savannas both facilitation via HL and competition are active processes. The importance of each process may depend upon how wet or dry that particular site or year is.

Watersheds of the Catskill Mountains, New York have marked differences in nitrogen (N) dynamics among dominant tree species stands. Our objectives were to study how tree species vary in N uptake to better understand the basis for the observed variation in these forested watersheds. We conducted a ¹⁵N tracer greenhouse study to determine \\[NH_4^ - \\] and \\[NH_3^ - \\] uptake of American beech (Fagus grandifolia Ehrh.), eastern hemlock (Tsuga Canadensis L.), red oak (Quercus rubra L.) and sugar maple (Acer saccharum Marsh.) seedlings. Seedlings and their native soil were collected in November 1997, over-wintered and allowed to break dormancy in spring 1998. Half of the seedlings of each tree species received ¹⁵NH₄-NO₃ to examine \\[NH_4^ - \\] uptake and the other half received NH₄-¹⁵NO₃ to examine \\[NH_3^ - \\] uptake. Plants were harvested 4 days following ¹⁵N addition. Tree species varied in their preference for \\[NH_4^ - \\] and \\[NH_3^ - \\]. Sugar maple and eastern hemlock seedlings took up more \\[NH_4^ - \\] than \\[NH_3^ - \\] per unit plant biomass, while beech was the only species to take up more \\[NH_3^ - \\] than \\[NH_4^ - \\]. Red oak took up more NH^ than NO^~ into roots, stems and leaves, but the difference between the two forms of N was not statistically significant. These results demonstrate that tree species of the Catskill Mountains vary in their capacity to take up \\[NH_4^ - \\] and \\[NH_3^ - \\]. Coupled with stand-level studies of N dynamics, this variation can help explain some of the patterns of forested watershed N retention and loss in the Catskill Mountains shown in our field investigations (Templer, 2001; Templer et al., in press Ecosystems).

Two Citrus tristeza virus (CTV) isolates from New Zealand that display distinct phenotypes were isolated, examined and sequenced in full. The first isolate, NZ-M16, is largely asymptomatic and non-transmissible by the aphid vector Toxoptera citricida, while the second, NZ-B18, is highly transmissible and induces very severe symptoms on C. sinensis and C. aurantii. Phylogenetic analysis of the genome sequences showed that both isolates were approximately 90-93% similar to the VT and T318 isolates but possessed only 89% identity to one another. Based on sequence identity, both isolates are VT subtypes, with NZ-M16 being T3-like, while NZ-B18 is a member of a novel subtype with B165 from India.

A natural abundance hydrogen stable isotope technique was used to study seasonal changes in source water utilization and water movement in the xylem of dimorphic root systems and stem bases of several woody shrubs or trees in mediterranean-type ecosystems of south Western Australia. Samples collected from the native tree Banksia prionotes over 18 months indicated that shallow lateral roots and deeply penetrating tap (sinker) roots obtained water of different origins over the course of a winter-wet/summer-dry annual cycle. During the wet season lateral roots acquired water mostly by uptake of recent precipitation (rain water) contained within the upper soil layers, and tap roots derived water from the underlying water table. The shoot obtained a mixture of these two water sources. As the dry season approached dependence on recent rain water decreased while that on ground water increased. In high summer, shallow lateral roots remained well-hydrated and shoots well supplied with ground water taken up by the tap root. This enabled plants to continue transpiration and carbon assimilation and thus complete their seasonal extension growth during the long (4-6 month) dry season. Parallel studies of other native species and two plantation-grown species of Eucalyptus all demonstrated behavior similar to that of B. prionotes. For B. prionotes, there was a strong negative correlation between the percentage of water in the stem base of a plant which was derived from the tap root (ground water) and the amount of precipitation which fell at the site. These data suggested that during the dry season plants derive the majority of the water they use from deeper sources while in the wet season most of the water they use is derived from shallower sources supplied by lateral roots in the upper soil layers. The data collected in this study support the notion that the dimorphic rooting habit can be advantageous for large woody species of floristically-rich, open, woodlands and heath-lands where the acquisition of seasonally limited water is at a premium.