Delphis F. Levia

Variability of throughfall volume and solute inputs in wooded ecosystems

Throughfall is a critical component of the hydrological and biogeochemical cycles of wooded ecosystems with a characteristically large degree of temporal and spatial variability. The highly variable nature of throughfall is of importance to scientists and natural resource managers concerned with the effects of water and solute inputs to the subcanopy, including understory vegetation, soil moisture, soil solution chemistry, and the fate of atmospheric dryfall. The purpose of this study is to critically review and evaluate the present state of knowledge pertaining to the temporal and spatial variability of throughfall volume and solute inputs in wooded ecosystems. The authors are optimistic that this review will facilitate the advancement of science by exposing gaps in our current understanding and mitigating the duplication of unwarranted research efforts. Several key areas where current knowledge is weak are: (1) the effect of meteorological conditions on the variability of throughfall volume; a data gap exists concerning the effects of precipitation type (eg, rain, snow, snow-to-rain, rain-to-snow), incident rain drop size, intensity, duration, wind speed and direction, and wind run on the throughfall variability; (2) the effect of meteorological conditions on the variability of throughfall solute inputs; (3) the role of canopy structure on precipitation partitioning into throughfall and stemflow and the variability of throughfall volume and solute inputs; (4) effects of epiphytes on the spatial variation of throughfall volume and solute inputs; (5) the physics and fluid dynamics of water flow over vegetative surfaces and its impact on throughfall yield and chemical enrichment; and (6) intraspecific variation of throughfall water and solute inputs. Future research projects undertaken with the specific aim of addressing the deficiencies identified will improve our understanding of interactions among the biosphere-atmosphere-lithosphere and promote better stewardship of forest and water resources.

Soil moisture: A central and unifying theme in physical geography

Soil moisture is a critical component of the earth system and plays an integrative role among the various subfields of physical geography. This paper highlights not just how soil moisture affects atmospheric, geomorphic, hydrologic, and biologic processes but that it lies at the intersection of these areas of scientific inquiry. Soil moisture impacts earth surface processes in such a way that it creates an obvious synergistic relationship among the various subfields of physical geography. The dispersive and cohesive properties of soil moisture also make it an important variable in regional and microclimatic analyses, landscape denudation and change through weathering, runoff generation and partitioning, mass wasting, and sediment transport. Thus, this paper serves as a call to use research in soil moisture as an integrative and unifying theme in physical geography.

Throughfall and Stemflow in Wooded Ecosystems

Incident precipitation is routed to the subcanopy by throughfall and stemflow. Throughfall is defined as the precipitation that passes directly through a canopy or is initially intercepted by aboveground vegetative surfaces and subsequently drips from the canopy, whereas stemflow is the precipitation that drains from outlying leaves and branches and is channeled to the bole (or stem) of plants. Throughfall and stemflow inputs constitute the majority of incident precipitation (Table 21.1) and are of critical importance to wooded ecosystems, ranging from 70 to 90% of incoming precipitation in most cases with the remainder lost to interception (Levia and Frost 2003). The inputs of throughfall and stemflow are highly variable over space and through time with consequent “hot spots” and “hot moments” of water and solute inputs from the canopy to the subcanopy (Stout and McMahon 1961; Levia 2003; Zimmermann et al. 2007). There are marked differences in the routing of intercepted water to the forest floor via throughfall (Keim and Skaugset 2004) and stemflow (Herwitz 1987) in terms of flowpaths and residence times along vegetative surfaces, which result in notable differences in solute concentrations and mass fluxes of canopy leachates (Levia and Frost 2003; Zimmermann et al. 2007). Recent work has documented the demonstrable effects of throughfall and stemflow to the hydrology and biogeochemistry of hillslopes (Keim et al. 2006a; Liang et al. 2009). Liang et al. (2009), for example, have reported that stemflow has led to a root-induced bypass flow infiltration process on hillslopes, a coupled mechanism termed “double-funneling” (Johnson and Lehmann 2006) discussed further in Chap. 24. Stemflow also has been documented to contribute to the enrichment of soils beneath shrubs in semiarid climates, leading to a “fertile island” effect (Whitford et al. 1997), whereas the spatial distribution of fine roots was observed to mirror throughfall inputs (Ford and Deans 1978).

A review of stemflow generation dynamics and stemflow‐environment interactions in forests and shrublands

Many geoscientists now recognize stemflow as an important phenomenon which can exert considerable effects on the hydrology, biogeochemistry, and ecology of wooded ecosystems and shrublands. Despite the explosive growth of stemflow research, until this review there has been no comprehensive attempt to summarize and synthesize this literature since 2003. Topical areas of substantive new knowledge in stemflow research include the following: (1) the interrelationships among stemflow and meteorological conditions, especially within individual rain events; (2) the dynamic interplay between stemflow and canopy structure; (3) stemflow and the cycling of solutes and transport of particulate matter; (4) stemflow and its interactions with canopy fungi and corticolous lichens; and (5) stemflow-soil interactions. Each of these five topical areas of substantive new stemflow research is summarized and synthesized, with areas of future research opportunities discussed. In addition, we have reviewed the parameters which can be used to describe stemflow and critically evaluate their utility for different purposes. This review makes a call for scientists studying stemflow to utilize common metrics in an effort to increase the cross-site comparability of stemflow studies. Capitalizing on the insights of prior research, exciting research opportunities await hydrologists, biogeoscientists, and forest ecologists who will conduct studies to deepen our knowledge of stemflow which will enable a better and more accurate framing of stemflow in the larger context of watershed hydrology and biogeochemistry.

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.

Canopy Structure in Relation to Hydrological and Biogeochemical Fluxes

The structure of forest canopies is highly heterogeneous at multiple scales. Leaves, twigs, and stems are not organized uniformly in space. For example, some plants have highly clustered leaves (e.g., conifers) while others are less clustered (e.g., Kira et al. 1969). Forest canopies contain gaps, but the size and distribution of these gaps is highly variable and forest/disturbance-dependent (e.g., Yavitt et al. 1995; Asner et al. 2004). Leaf area and woody biomass are not evenly distributed along the vertical axis, with some forests having a larger proportion of the leaf area closer to the forest floor, whilst other forests have most of their foliage near the top (e.g., Parker et al. 2004b). How these elements are organized and connected in space can have profound influences on ecosystem process such as hydrological and biogeochemical fluxes.

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.

An Automated Instrument for the Measurement of Bark Microrelief

Bark microrelief is of importance to the physiological ecology of forested ecosystems because it has been documented to influence the distribution of corticolous lichens, stemflow generation, and forest biogeochemical cycles. Hitherto no instrument existed to characterize the inherent variability of bark microrelief with high spatial resolution. Our newly designed prototype instrument consists of a hinged ring, laser rangefinder, and motor linked to a standard laptop. The prototype produces trunk cross sections at a 0.33° horizontal resolution and detects bark-ridge-to-furrow heights at < 1 resolution. The prototype was validated by comparing measurements of bark microrelief between the instrument and digital calipers. The mean absolute error of the prototype as a percentage of the measured average microrelief was 1.0%, with a mean absolute error of 0.83 mm. Our bark microrelief prototype instrument can supply critical requisite information of bark microstructure that can be used by researchers to interpret the distribution of lichens and bryophytes on tree surfaces, relate stemflow yield and chemistry to bark microrelief, and provide detailed measurements of the changes of bark microrelief with stem dehydration. In short, the prototype instrument can be used to gain a more holistic understanding of the physiological ecology of forest ecosystems.

Farmland conversion and residential development in North Central Massachusetts

The conversion of farmland for residential development is a serious environmental concern which results in land degradation. The factors responsible for farmland conversion in Leominster, Massachusetts, were found using simultaneous discriminant analysis. The statistically significant factors determining farmland conversion were relief, distance from the city center and route 190, and farm size. These factors were employed to develop a model for farmland preservation. This model should be used by planning offices to identify the farms likely to be converted to non-farm activities to control urban sprawl.

A Comparison of Three Canopy Interception Models for a Leafless Mixed Deciduous Forest Stand in the Eastern United States

Abstract Canopy interception of incident precipitation is a critical component of the forest water balance during each of the four seasons. Models have been developed to predict precipitation interception from standard meteorological variables because of acknowledged difficulty in extrapolating direct measurements of interception loss from forest to forest. No known study has compared and validated canopy interception models for a leafless deciduous forest stand in the eastern United States. Interception measurements from an experimental plot in a leafless deciduous forest in northeastern Maryland (39°42′N, 75°50′W) for 11 rainstorms in winter and early spring 2004/05 were compared to predictions from three models. The Mulder model maintains a moist canopy between storms. The Gash model requires few input variables and is formulated for a sparse canopy. The WiMo model optimizes the canopy storage capacity for the maximum wind speed during each storm. All models showed marked underestimates and overestimat...