Phragmites australis root secreted phytotoxin undergoes photo-degradation to execute severe phytotoxicity

Our study organism, Phragmites australis (common reed), is a unique invader in that both native and introduced lineages are found coexisting in North America. This allows one to make direct assessments of physiological differences between these different subspecies and examine how this relates to invasiveness. Recent efforts to understand plant invasive behavior show that some invasive plants secrete a phytotoxin to ward-off encroachment by neighboring plants (allelopathy) and thus provide the invaders with a competitive edge in a given habitat. Here we show that a varying climatic factor like ultraviolet (UV) light leads to photo-degradation of secreted phytotoxin (gallic acid) in P. australis rhizosphere inducing higher mortality of susceptible seedlings. The photo-degraded product of gallic acid (hereafter GA), identified as mesoxalic acid (hereafter MOA), triggered a similar cell death cascade in susceptible seedlings as observed previously with GA. Further, we detected the biological concentrations of MOA in the natural stands of exotic and native P. australis. Our studies also show that the UV degradation of GA is facilitated at an alkaline pH, suggesting that the natural habitat of P. australis may facilitate the photo-degradation of GA. The study highlights the persistence of the photo-degraded phytotoxin in the P. australis''s rhizosphere and its inhibitory effects against the native plants.Key words: ultraviolet, gallic acid, mesoxalic acid, novel weapons, invasive species, Phragmites australis     

Using wavelet analysis to examine bark microrelief

Key Message

An improved quantification of variations in bark microrelief is presented that uses wavelets on a circular domain from data acquired using the LaserBark? automated tree measurement system.

Abstract

An important metric of canopy structure, bark microrelief affects both the hydrology and biogeochemistry of forests. Increased bark microrelief leads to reduced stemflow volumes and higher concentrations of stemflow leachates and nutrient-ions. Consequently, an improved representation of bark microrelief would be useful to describe the influence of various tree species on water and solute contributions to the forest floor. Most existing methods to quantify bark microrelief are ‘global’ measures; that is, they provide a single number that represents the overall bark microrelief of the entire perimeter of the tree. To remedy this, wavelet analysis of LaserBark? automated tree measurement system data is proposed and described to quantify variations in bark microrelief around the perimeter of the tree. This measure describes the spatial differences in bark microrelief and allows representation of trees that exhibit directional variability in bark microrelief due to natural or anthropogenic effects. The results show that wavelet analysis is effective in quantifying both bark microrelief and large-scale tree asymmetry. The radial component highlights changes in the depth of bark microrelief while the tangential component relates to the distance between bark furrows in the bark cross section. Thus, wavelet analysis may be a useful tool for comparing bark structure that varies, for example, within- and between-tree species, at different stages of tree growth, and among trees grown under different environmental conditions.     

Evaporation of intercepted precipitation from fruit litter of Liquidambar styraciflua L. (sweetgum) in a clearing as a function of meteorological conditions

Interception of precipitation by fruit litter is a poorly understood component of the hydrologic cycle in forested ecosystems. Even less well understood is the effect of meteorological conditions on the evaporation of precipitation intercepted by forest litter. This study sought to examine the influence of meteorological conditions on the evaporation of intercepted precipitation by fruit litter from Liquidambar styraciflua L. (sweetgum) by deriving and calibrating a regression model to estimate evaporation from the fruit litter that may be of potential use to forest and watershed managers. Data on evaporative losses from the fruit litter used to derive and calibrate the statistical model were acquired through a larger field experiment conducted from mid November 2002 through April 2003. Results from the forward stepwise least squares multiple regression model demonstrated that evaporative losses from the fruit litter were estimated with a high degree of accuracy based on the amount of water stored, solar radiation inputs, and vapor pressure deficit (adjusted R²=0.836, F=82.28, P<0.00001). The amount of water stored in the fruit litter explained the highest proportion of variance in the regression model. Storm to storm comparisons also highlighted the importance of solar radiation and wind speed in determining evaporation from the fruit litter. The regression model potentially may be used in conjunction with a canopy interception model to predict interception losses from L. styraciflua dominated forests and plantations.     

Individual and time-varying tree-ring growth to climate sensitivity of Pinus tabuliformis Carr. and Sabina przewalskii Kom. in the eastern Qilian Mountains, China

Individual tree-ring width chronologies and mean chronologies from Pinus tabuliformis Carr. (Chinese pine) and Sabina przewalskii Kom. (Qilian juniper) tree cores were collected and analyzed from two sites in the eastern Qilian Mountains of China. The chronologies were used to analyze individual and time-varying tree-ring growth to climate sensitivity with monthly mean air temperature and total precipitation data for the period 1958–2008. Climate–growth relationships were assessed with correlation functions and their stationarity and consistency over time were measured using moving correlation analysis. Individuals’ growth–climate correlations suggested increased percentages of individuals are correlated with certain variables (e.g., current June temperature at the P. tabuliformis site; previous June, December and current May temperature and May precipitation at the S. przewalskii site). These same climatic variables also correspond to the mean chronology correlations. A decreased percentage of individuals correlated with these climatic variables indicates a reduced sensitivity of the mean chronology. Moving correlation analysis indicated a significant change over time in the sensitivity of trees to climatic variability. Our results suggested: (1) that individual tree analysis might be a worthwhile tool to improve the quality and reliability of the climate signal from tree-ring series for dendroclimatology research; and (2) time-dependent fluctuations of climate growth relationships should be taken into account when assessing the quality and reliability of reconstructed climate signals.     

Rhizobacteria Bacillus subtilis restricts foliar pathogen entry through stomata

Plants exist in a complex multitrophic environment, where they interact with and compete for resources with other plants, microbes and animals. Plants have a complex array of defense mechanisms, such as the cell wall being covered with a waxy cuticle serving as a potent physical barrier. Although some pathogenic fungi infect plants by penetrating through the cell wall, many bacterial pathogens invade plants primarily through stomata on the leaf surface. Entry of the foliar pathogen, Pseudomonas syringae pathovar tomato DC3000 (hereafter PstDC3000), into the plant corpus occurs through stomatal openings, and consequently a key plant innate immune response is the transient closure of stomata, which delays disease progression. Here, we present evidence that the root colonization of the rhizobacteria Bacillus subtilis FB17 (hereafter FB17) restricts the stomata‐mediated pathogen entry of PstDC3000 in Arabidopsis thaliana. Root binding of FB17 invokes abscisic acid (ABA) and salicylic acid (SA) signaling pathways to close light‐adapted stomata. These results emphasize the importance of rhizospheric processes and environmental conditions as an integral part of the plant innate immune system against foliar bacterial infections.     

Surface roughness affects the running speed of tropical canopy ants

Cursorial central‐place foragers like ants are expected to minimize travel costs by choosing the least resistive pathways to food resources. Tropical arboreal and semi‐arboreal ants locomote over a variety of plant surfaces, and their choice of pathways is selective. We measured the roughness of tree trunk and liana stem surfaces using laser scanning technology, and explored its consequences for running speed in various ant taxa. The average amplitude of tree trunk surface roughness differed interspecifically, and ranged from 1.4–2.2 mm among three common tree species (Anacardium excelsum, Alseis blackiana, and Dipteryx panamensis). The roughness of liana stems also varied interspecifically (among Tontelea ovalifolia, Bauhinia sp. and Paullinia sp.) and was an order of magnitude lower than tree surface roughness (mean amplitude ranged 0.09–0.19 mm). Field observations of various ant species foraging on tree trunks and liana stems, and on dowels covered with sandpaper, showed that their running speed declined with increasing amplitude of roughness. The effect of roughness on running speed was strongest for mid‐sized ants (Azteca trigona and Dolichoderus bispinosus). The accumulation rate of ants at baits did not vary with tree surface roughness, but was significantly lower on moss‐covered versus moss‐free bark. Collectively, these results indicate that the quality of plant substrates can influence the foraging patterns of arboreal ants, but likely is more important for resource discovery than for dominance on bare tree surfaces.     

Stomatal conductance and transpiration of co-occurring seedlings with varying shade tolerance

Forest ecosystems exert a large influence on the hydrologic cycle through transpiration, with the majority supplied by saplings and adult trees. However, a measurable amount is also supplied by seedlings. The contribution of seedlings is dependent upon species, which is ultimately controlled by microclimate. The objectives of this study were to (1) observe meteorological conditions of two forest microclimates; and (2) assess the intra- and interspecific stomatal conductance and transpiration responses of naturally occurring seedlings of varying shade tolerance. Naturally established seedlings in a deciduous forest understory and an adjacent clearing were monitored throughout the 2008 growing season in southeastern Pennsylvania (39°49′N, 75°43′W). The understory microclimate conditions overall had a lower degree of variability and had consistently lower mean quantum flux densities, air temperature, vapor pressure deficit, volumetric water content, and soil temperature than the clearing plot. Understory seedlings of Fagus grandifolia Ehrh. (American beech) and Prunus serotina L. (black cherry) had significantly lower mean monthly rates of water loss (p = 0.05) than clearing seedlings [P. serotina and Liriodendron tulipifera L. (yellow poplar)]. Additionally, water loss by shade-grown P. serotina seedlings was significantly lower (p = 0.05) than by sun-grown seedlings. Physiological differences, specifically shade tolerance, played an important role in determining the rates of stomatal conductance and transpiration in the seedlings. To a lesser degree, microclimate variability also influenced water loss. The results of this study validate results obtained in previous studies conducted largely under controlled conditions. Field validations are critical to developing better models and forest management strategies.