How important is groundwater availability and stream perenniality to riparian and floodplain tree growth?

Riparian vegetation is important for stream functioning and as a major landscape feature. For many riparian plants, shallow groundwater is an important source of water, particularly in areas where rainfall is low, either annually or seasonally, and when extended dry conditions prevail for all or part of the year. The nature of tree water relationships is highly complex. Therefore, we used multiple lines of evidence to determine the water sources used by the dominant tree species Eucalyptus camaldulensis (river red gum), growing in riparian and floodplain areas with varying depth to groundwater and stream perenniality. Dendrometer bands were used to measure diel, seasonal, and annual patterns of tree water use and growth. Water stable isotopes (δ2H and δ18O) in plant xylem, soil water, and groundwater were measured to determine spatial and temporal patterns in plant water source use. Our results indicated riparian trees located on relatively shallow groundwater had greater growth rates, larger diel responses in stem diameter, and were less reactive to extended dry periods, than trees in areas of deep groundwater. These results were supported by isotope analysis that suggested all trees used groundwater when soil water stores were depleted at the end of the dry season, and this was most pronounced for trees with shallow groundwater. Trees may experience more frequent periods of water deficit stress and undergo reduced productivity in scenarios where water table accessibility is reduced, such as drawdown from groundwater pumping activities or periods of reduced rainfall recharge. The ability of trees to adapt to changing groundwater conditions may depend on the speed of change, the local hydrologic and soil conditions as well as the species involved. Our results suggest that E. camaldulesis growing at our study site is capable of utilizing groundwater even to depths \u3e10 m, and stream perenniality is likely to be a useful indicator of riparian tree use of groundwater

Smaller, faster stomata: scaling of stomatal size, rate of response, and stomatal conductance

Maximum and minimum stomatal conductance, as well as stomatal size and rate of response, are known to vary widely across plant species, but the functional relationship between these static and dynamic stomatal properties is unknown. The objective of this study was to test three hypotheses: (i) operating stomatal conductance under standard conditions (gop) correlates with minimum stomatal conductance prior to morning light [gmin(dawn)]; (ii) stomatal size (S) is negatively correlated with gop and the maximum rate of stomatal opening in response to light, (dg/dt)max; and (iii) gop correlates negatively with instantaneous water-use efficiency (WUE) despite positive correlations with maximum rate of carboxylation (Vc max) and light-saturated rate of electron transport (J max). Using five closely related species of the genus Banksia, the above variables were measured, and it was found that all three hypotheses were supported by the results. Overall, this indicates that leaves built for higher rates of gas exchange have smaller stomata and faster dynamic characteristics. With the aid of a stomatal control model, it is demonstrated that higher g op can potentially expose plants to larger tissue water potential gradients, and that faster stomatal response times can help offset this risk

Rapid root elongation by phreatophyte seedlings does not imply tolerance of water table decline

Key message Despite high rates of root elongation during phreatophyte establishment once connection to groundwater has occurred and leaf area develops, seedlings demonstrate limited capacity for root elongation in response to groundwater decline. Abstract In a water-limited environment, rapid root elongation immediately after germination can be critical for a plant to reach deeper water sources such as a water table to avoid water deficit stress. However, once plants have accessed a water table, their continued survival may depend on their ability to adapt their root distribution to changes in the depth to a water table. In glasshouse experiments using two Banksia species with contrasting water requirements, we investigated (1) the rate of root elongation by young seedlings in the presence of a shallow water table, and (2) whole plant response to rapid water table decline using older seedlings that had established root contact with a water table. The results of the first experiment agree with the hypothesis that the facultative phreatophyte, B. attenuata, has a faster rate of root elongation than the obligate phreatophyte, B. littoralis. These differences are likely related to the contrasting habitat preferences of the two species. Older seedlings in the second experiment demonstrated a water-saving response to a declining water table, rapidly closing stomata to limit water loss. Additionally, roots did not elongate to follow the water table and plants were quickly disconnected from the saturated zone. For the two phreatophytic Banksia species, the capacity for rapid growth by young seedlings did not translate to an ability for established seedlings to adapt their root distribution to survive rapid water table decline

The ecology of conifer persistence in tropical rainforests: Podocarpus neriifolius in northern Thailand

A range of hypotheses seek to explain why conifers are infrequent in tropical rainforests. Here, we explore how the conifer, Podocarpus neriifolius, persists at low density in tropical lower montane rainforests of northern Thailand. Recruitment, growth and survival of seedlings and small saplings was monitored for 3.5 years in plots near and away from putative parent trees, and for large saplings and trees in a 16 ha forest plot over 8 years. Seeds and strobili were collected in litterfall traps for 2 years, and pollen rain extracted from forest floor moss samples. Demographic rates and ecophysiological traits of saplings were compared with co-occurring angiosperms Castanopsis acuminatissima and Calophyllum polyanthum. Survivorship was higher in the conifer for all size classes \u3e 30 cm ht (0.97–0.99 y−1 vs. 0.92–0.98). Growth rate was similar for small saplings (9.3–12.6 cm y−1 ht), higher in P. neriifolius for large saplings (0.21 cm y−1 dbh vs. 0.11–0.12), but lower for trees (0.10 cm y−1 vs. 0.16–0.45). Canopy openness above saplings was similar amongst species (3.2–4.2%), whilst leaf N was lower (1.33% vs. 1.43–1.49%), and mass leaf per area (1.96 vs. 1.88) and δ13C higher (− 33.4 ‰ vs. – 34 to 34.9‰) for P. neriifolius. Pollen rain varied between plots, as did strobilus and seed fall between both plots and years, with highest pollen density, strobilus and seed fall close to parent trees, as were areas with highest seedling density and recruitment. High survivorship, high recruitment near parent trees and more widespread recruitment in mast years, offsets slower growth to ensure population persistence of P. neriifolius in angiosperm-dominated rainforest. Low stand density and patchy distribution is best explained by pollen limitation on seed production, with strong recruitment only where male and female trees occur in close proximity, and in mast seed years

Spatio-temporal patterns of evapotranspiration from groundwater-dependent vegetation

Understanding hydrological processes in water-limited systems requires consideration of temporal and spatial vegetation water use patterns at the landscape scale. We used data derived from the MODerate Resolution Imaging Spectroradiometer (MODIS) satellite instrument and interpolated climate data covering a ten-year period to contrast the spatio-temporal patterns of actual evapotranspiration (AET) from known phreatophytic and non-phreatophytic vegetation overlying a large superficial aquifer. We assessed shallow to deeper groundwater habitats and compared AET responses to seasonal and inter-annual variation in precipitation. Overall, vegetation in shallow groundwater habitats had higher AET rates during the growth season (spring and summer) than vegetation growing in deeper groundwater habitats, suggesting that the former was not physiologically constrained by water deficit. Vegetation in areas of consistently high (ground-)water availability maintained higher AET, reaching a peak of 95 mm in mid-summer. In contrast, plantation maritime pines had the highest AET rates at deep groundwater habitats. Inter-annual variability in AET correlated with rainfall and AET rates peaked two months after the majority of effective rainfall had fallen. During low rainfall years, maximum AET peaked one month earlier relative to higher rainfall years. The results of this study suggest that remote sensing of AET can give a conditional indication of where groundwater is important in supporting vegetation and can be a valuable tool in identifying management focus areas where vegetation is variably sensitive to water deficit. Copyright © 2016 John Wiley & Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd

Can hydraulically redistributed water assist surrounding seedlings during summer drought?

Plant interaction studies provide a good understanding of the roles of key species, which can assist restoration of natural ecosystems. Among the interactions, facilitation and competition are known to affect ecosystem structure and function. We investigated whether a deep-rooted species could positively affect surrounding seedlings through hydraulic redistribution during dry months. We conducted two experiments in which seedlings from two species were growing together or isolated from source plants (field experiment) and where plants were isolated from source plants that were connected to or separated from a water table (glasshouse experiment). Survival, growth, water relations and soil water content were measured. We also applied δ2H enriched water adjacent to, or into, the roots of source plants to track water movement between plants. Soil water content was higher in shallow layers where source plants could interact with seedlings (field) and when accessing water tables (glasshouse). Seedlings from all treatments had an increase in leaf δ2H. Seedlings of Banksia attenuata that were isolated from source plants had the highest survival, growth and stomatal conductance rates. Seedlings of Gompholobium tomentosum presented higher stomatal conductance rates when growing with source plants than when isolated from them during the first months, but this relationship reversed towards the end of summer. These results suggest that source plants and seedlings competed, but the influence of facilitation and competition might change during the year, at least for the shallow-rooted species. Therefore, competition for water and/or other limiting factors must be considered when planning ecological restoration in such area