Common Riparian Tree Species Research Articles

Functional traits of riparian trees in the lower Fitzroy River, Western Australia

AbstractRiparian trees are critically important for maintaining the ecological function of freshwater ecosystems. Globally, anthropogenic changes to water regimes are impacting the health and distribution of riparian trees. Understanding the physiological constraints on the distribution of riparian tree species in relation to the water regime is essential for informed water resource management that seeks to limit impacts on riparian trees. To fill an identified knowledge gap for the Fitzroy River, a significant river in the wet–dry tropics of north‐western Australia, we used a trait‐based approach to characterise nine common riparian tree species in relation to their distribution along a hydrological gradient. We assessed key functional traits related to drought and flood flows. Leaf mass per unit area (LMA), leaf dry matter content (LDMC), foliar carbon content (% C) and the ratio of carbon to nitrogen (C:N) are broadly related to plant productivity and durability and may reflect resistance to fluvial stress. Traits related to water availability were stem specific density (SSD), mean xylem vessel diameter and xylem vessel density, as well as foliar δ13C which is related to water use efficiency, and leaf osmotic potential at full hydration (π100) as a measure of drought tolerance. We found that π100, δ13C and SSD values reflected species hydrological habitat preferences, with higher δ13C values and lower π100 for species distributed in the drier floodplain habitats, compared with species constrained to the riverbank. Low SSD values for species close to the riverbank may be indicative of aerenchyma tissue in response to flooding. Differences in leaf trait values were primarily attributed to differences between evergreen Myrtaceous and deciduous non‐Myrtaceous species, rather than hydrological habitat preferences. LMA was greatest for the Myrtaceous study species, with deciduous non‐Myrtaceous species associated with wetter habitats having the lowest values. The physiology of riparian trees in northern Australia is not well described for the majority of species. Our study provides insight into plant functional strategies in response to both flooding and drought and is a critical step in understanding plant responses to future water‐take scenarios to support evidence‐based decision making.

Extreme climate events can slow down litter breakdown in streams

Extreme temperatures have increased in intensity, duration and frequency in the last century, with potential consequences on key ecological processes such as organic matter breakdown. Many stream ecosystems are fueled by the breakdown of terrestrial leaf litter, which is exposed to atmospheric conditions for certain periods of time before entering the stream. Thus, extreme warming or freezing events may affect the litter physicochemical structure, which could translate into altered breakdown within the stream. The above prediction was tested by exposing litter of common riparian tree species in southern Chile to freezing (−20 oC; dry or wet litter) or heating (40 oC) and comparing breakdown with control litter exposed to room temperature (20 oC), separating the effects of different breakdown agents (i.e., leaching, microorganisms and detritivores). The greatest effects were found in wet litter subjected to freezing; this treatment significantly increased leaching in the short term (48 h) and slowed down breakdown in the long term (30 days), mostly due to the inhibition of microbial breakdown. Heating also retarded microbial breakdown, but the effect was smaller. Our results suggest that short-term extreme temperatures—particularly cold ones—have the potential to slow down litter breakdown in streams, which will most likely impact global biogeochemical cycles where streams play a key role.

Leaf litter decomposition in Guinean savannah streams

ABSTRACTLow-order streams of Upper Guinea receive substantial leaf litter inputs and are characterized by intermittent flow. The ecosystem functioning of such streams flowing in savannah, a common tropical vegetation and the dominant biome in West Africa, remains largely overlooked. We hypothesized litter decomposition in such streams to be (1) fast due to the expectedly high nutrient content of tropical litter, and (2) mostly driven by microorganisms due to the widely revealed paucity of tropical macroinvertebrate consumers, and even exacerbated in these systems by water temporariness. Decomposition rates of 2 common riparian tree species, Alchornea cordifolia and Pterocarpus santalinoides, in 2 streams ranged from 0.0287 to 0.0649 d−1. The process was clearly governed by microbial decomposers, as shown by the exceptionally low mass loss discrepancy between fine and coarse mesh bags, a lack of shredders, and high leaf-associated fungal biomass (measured as ergosterol: up to 0.92 and 0.37 mg g−1 litter of ash-free dry mass for Alchornea and Pterocarpus, respectively), together with moderate conidial production rate. In such savannah streams, the absence of leaf-shredding invertebrates seems to be compensated by a high microbial activity, providing an ultimate case for the reported preponderance of microbial decomposers over invertebrate decomposers at low latitudes. In contrast with temperate temporary streams, the processing of leaf litter from riparian trees, characterized by high nitrogen and phosphorus content, is sufficiently fast to be unaffected by flow intermittency.

Effects of plant species on stream bacterial communities via leachate from leaf litter

Leaf litter provides an important resource to forested stream ecosystems. During leaf fall a significant amount of dissolved organic carbon (DOC) enters streams as leaf leachate. We compared the effects of plant species and leaf leachate bioavailability on the composition of stream bacterial communities and rates of DOC decomposition. We used four common riparian tree species that varied in foliar chemistry, leachate optical properties, and litter decomposition rate. We used laboratory microcosms from two streams and amended with a standard concentration of DOC derived from leaf leachate of the four tree species. After 24 h, we measured rates of DOC biodegradation and determined the composition of the bacterial communities via bar-coded pyrosequencing of the 16S rRNA gene. The composition, diversity, and abundance of the bacterial community differed significantly among plant species from both streams. The phylogenetic distance of the different bacterial communities correlated with species-specific leachate optical properties and rates of DOC biodegradation. Highest rates of DOC decomposition were associated with high tannin and lignin leaf types. Results demonstrate that riparian plant species strongly influences stream bacterial communities via their leachate suggesting that alterations to the presence or abundance of riparian plant taxa may influence these communities and associated ecosystem processes.

Spatial–structural properties of woody riparian vegetation with a view to reconfiguration under hydrodynamic loading

This paper investigates the structural properties of four common riparian tree species and their reconfiguration under hydrodynamic loading in a towing tank in foliated and defoliated conditions. 3D tree models were generated by digitizing twenty 0.8–3.3m tall specimens at branch level. Branch diameters and lengths were measured in order to calculate the one-sided stem area and stem volume over the plant height. The novelty of the investigations originated from the characterization of the reconfiguration which was achieved by combining the contracted width, the deflected height, and the underwater projected area in order to determine the porosity at different velocities. The results showed that the basal diameter could be used to predict the entire total one-sided stem area, although this method was not capable of reproducing the observed non-linear vertical distributions. The flow-induced width contraction contributed significantly to the reduction of the rectangular cross-sectional area occupied by the plant. The porosity of the foliated trees increased at the lower velocities, and then decreased at the higher velocities. Overall, detailed spatial–structural analyses of woody vegetation provided valuable information about plant behaviour under load, and thus are helpful for improving the determination of the physically based parameters of complex vegetative elements which is highly relevant for environmental modelling in order to fill the gap of knowledge concerning the hydrodynamic and aerodynamic flow around trees.

Water-borne hyphomycetes in tree canopies of Kaiga (Western Ghats), India

The canopy samples such as trapped leaf litter, trapped sediment (during summer), stemflow and throughfall (during monsoon) from five common riparian tree species (<em>Artocarpus heterophyllus, Cassia fistula, Ficus recemosa, Syzygium caryophyllatum</em> and <em>Xylia xylocarpa</em>) in Kaiga forest stand of the Western Ghats of southwest India were evaluated for the occurrence of water-borne hyphomycetes. Partially decomposed trapped leaf litter was incubated in bubble chambers followed by filtration to assess conidial output. Sediments accumulated in tree holes or junction of branches were shaken with sterile leaf disks in distilled water followed by incubation of leaf disks in bubble chamber and filtration to find out colonized fungi. Stemflow and throughfall samples were filtered directly to collect free conidia. From five canopy niches, a total of 29 water-borne hyphomycetes were recovered. The species richness was higher in stemflow and throughfall than trapped leaf litter and sediments (14-16 vs. 6-10 species). Although sediments of <em>Syzygium caryophyllatum</em> were acidic (5.1), the conidial output was higher than other tree species. Stemflow and throughfall of <em>Xylea xylocarpa</em> even though alkaline (8.5-8.7) showed higher species richness (6-12 species) as well as conidial load than rest of the tree species. <em>Flagellospora curvula</em> and <em>Triscelophorus acuminatus</em> were common in trapped leaf litter and sediments respectively, while conidia of <em>Anguillospora crassa</em> and A. longissima were frequent in stemflow and throughfall. Diversity of water-borne hyphomycetes was highest in throughfall of <em>Xylea xylocarpa</em> followed by throughfall of <em>Ficus recemosa</em>. Our study reconfirms the occurrence and survival of diverse water-borne hyphomycetes in different niches of riparian tree canopies of the Western Ghats during wet and dry regimes and predicts their possible role in canopy as saprophytes, endophytes and alternation of life cycle between canopy and aquatic habitats.

Phosphorus content in detritus controls life‐history traits of a detritivore

SummaryEcological stoichiometry, that is, the study of mass balance of multiple key chemical elements during ecological interactions, has recently been generalized to most ecosystems. Yet, effects of stoichiometric constraints on the life‐history traits of detritivores, one of the most important trophic level through its importance in nutrient cycling, are still poorly documented.Forested headwater streams constitute representative detritus‐based ecosystems, relying on leaf litter inputs as the main energy and nutrient source for their functioning. Leaf litter is quickly colonized by aquatic hyphomycetes, which are known to improve the quality of detritus, enabling nutrient and energy transfer to higher trophic levels.In this study, we manipulated leaf litter nutrient content of two common riparian tree species, differing in their initial biochemical composition, by short‐term phosphorus addition to leaf litter precolonized by aquatic hyphomycetes. Then, we tested the impact of leaf litter phosphorus content on some key life‐history traits, homeostasis and growth, of a common detritivorous invertebrate,Gammarus fossarum(Crustacea Amphipoda).G. fossarumfed individually for 5 weeks with P‐enriched leaf litter showed higher survival and growth rates than organisms fed with leaf litter colonized by fungi without P addition. Phosphorus content of resources did not alterG. fossarumelemental composition, suggesting that these detritivores are able to maintain their elemental homeostasis.This study sheds new light on the importance of phosphorus content of detritus for detritivores and paves the way for further studies aimed at transferring ecological stoichiometry concepts to detritus‐based ecosystems.

Water sources in riparian trees of the southern Appalachian foothills, USA: A preliminary study with stable isotope analysis

The aim of this study was to determine the sources of water uptake for two common riparian tree species found in the southeastern United States, Acer negundo and Betula nigra. The study site was located within a riparian zone typical of this region and those of the temperate USA. Water sources were determined by analyzing signatures of stable isotopes found naturally in water, 2 H and 18 O. Samples from surface water, groundwater, and soil, plus woody tissue from mature individuals of each species, were taken once each month during the 2011 summer growth season. Both species relied strongly on an unidentified ground water source, although A. negundo also showed a strong correlation with deep soil moisture (r 2 = 0.972). Sampling limitations did not permit an accurate determination of the fractional contribution of each source to plant water, limiting the strength of the results. The evidence collected leads to conclusions comparable to those of studies that have been conducted in arid parts of North America, corroborating that streamside species may rely heavily on groundwater sources, not surface streamwater. Further studies are needed in this region to confirm the evidence reported here to establish a baseline for vegetation in these systems.

Does leaf chemistry differentially affect breakdown in tropical vs temperate streams? Importance of standardized analytical techniques to measure leaf chemistry

Comparisons of the effects of leaf litter chemistry on leaf breakdown rates in tropical vs temperate streams are hindered by incompatibility among studies and across sites of analytical methods used to measure leaf chemistry. We used standardized analytical techniques to measure chemistry and breakdown rate of leaves from common riparian tree species at 2 sites, 1 tropical and 1 temperate, where a relatively large amount of information is available on litter chemistry and breakdown rates in streams (La Selva Biological Station, Costa Rica, and Coweeta Hydrologic Laboratory, North Carolina, USA). We selected 8 common riparian tree species from La Selva and 7 from Coweeta that spanned the range of chemistries of leaf litter naturally entering streams at each site. We predicted that concentrations of secondary compounds would be higher in the tropical species than in the temperate species and that high concentrations of condensed tannins would decrease breakdown rates in both sites. Contrary to our predictions, mean concentration of condensed tannins was significantly greater (2.6×, p < 0.001) for species at Coweeta than for species at La Selva. Concentration of condensed tannins was negatively correlated with breakdown rate among Coweeta species (r = −0.80), not among La Selva species, and negatively correlated when the 2 sites were combined (r = −0.53). Concentrations of structural compounds were strongly correlated with breakdown rate at both sites (Coweeta species, lignin r = −0.94, cellulose r = −0.77; La Selva species, cellulose r = −0.78, C r = −0.73). The chemistries of 8 riparian species from La Selva and 7 riparian species from Coweeta were not as different as expected. Our results underline the importance of standardized analytical techniques when making cross-site comparisons of leaf chemistry.

The Understorey Light Regime and Patterns of Tree Seedlings in Tropical Riparian Forest Patches

The understorey light regime and its influence on forest tree seedling distribution was examined in isolated riparian forest patches where large numbers of co-existing species have been found. Understorey light intensity was highest at the forest edge, approaching 13% of full sunlight, but rapidly decreased towards the forest interior. By 7- 12 m, light levels had stabilized at <2.0%, comparable to light levels in the interior of continuous tropical forest. Total seedling densities were not correlated with variations in understorey light intensity, suggesting that rates of germination and establishment were similar throughout the forest. However, examination of seedling light sensitivity among six common riparian tree species revealed differenc- es in distribution: five species had higher seedling abundance in the darker forest interior or at the brighter forest-patch edge while one species was indifferent to light variation. These results suggest that the riparian forest light regime has sufficient variation to support several regenera- tion strategies, despite the small patch sizes.