Steven K. Rice

Variation in carbon isotope discrimination within and among Sphagnum species in a temperate wetland

Abstract Field samples of bryophytes are highly variable in carbon isotope discrimination values (Δ, a measure of 13CO2 uptake relative to 12CO2), but it is unknown what affects Δ under field conditions, or how variation in Δ relates to bryophyte performance. This study employed field and greenhouse common garden studies to evaluate the influence of microsite, seasonal, and genetic variation on Δ in peatmosses. Three species of Sphagnum that occupy hollow (S. recurvum), carpet (S. palustre), and hummock (S. tenerum) habitats were sampled for relative growth rates (RGR), C:N ratio, and Δ throughout a growing season. Values of Δ ranged from 19.0 to 27.1‰. This variation was unrelated to species (P=0.61). However, Δ varied seasonally (P<0.001), with lower discrimination in the spring (mean 22.5‰), followed by summer (23.8‰) and winter (24.7‰). There was also significant microsite variation (P=0.015) which disappeared when plants were grown in a common garden. In both spring and summer, microsite variation in Δ was inversely related to RGR (P<0.001), but unrelated to C:N ratios (P>0.08). These results suggest that environmental, not genetic, variation at microsites affects Δ in non-vascular plants. However, environmental control of Δ is unlike that in vascular plants where water limitation lowers chloroplastic demand and increases resistance to carbon uptake. In non-vascular plants, water limitation lowers chloroplastic demand and decreases resistance to carbon uptake. These processes have additive effects and generate high spatial and seasonal variability in Δ.

Functional significance of variation in bryophyte canopy structure

In most bryophytes, the thickness of boundary layers (i.e., unstirred layers) that surrounds plant surfaces governs rates of water loss. Architectural features of canopies that influence boundary layer thickness affect the water balance of bryophytes. Using field samples (9.3 cm diameter cushions) from 12 populations (11 species) of mosses and liverworts, we evaluated the relationship between canopy structure and boundary layer properties. Canopy structure was characterized using a contact surface probe to measure canopy depth along perpendicular transects at spatial scales ranging from 0.8 to 30 mm on 186 points per sample. Semivariance in depth measurements at different spatial scales was used to estimate three architectural properties: surface roughness (L(r)), the scale of roughness elements (S(r)), and fine-scale surface texture, the latter characterized by the fractal dimension (D) of the canopy profile. Boundary layer properties were assessed by evaporation of ethanol from samples in a wind-tunnel at wind speeds from 0.6 to 4.2 m/s and applied to characterize mass transfer using principles of dynamic similarity (i.e., using dimensionless representations of conductance and flow). In addition, particle image velocimetry (PIV) was used to visualize and quantify flow over two species. All cushions exhibited the characteristics of turbulent as opposed to laminar boundary layers, and conductance increased with surface roughness. Bryophyte canopies with higher L(r) had greater conductances at all wind speeds. Particle image velocimetry analysis verified that roughness elements interacted with flow and caused turbulent eddies to enter canopies, enhancing evaporation. All three morphological features were significantly associated with evaporation. When L(r), S(r), and D were incorporated with a flow parameter into a conductance model using multiple linear regression, the model accounted for 91% of the variation in mass transfer.

Do bryophyte shoot systems function like vascular plant leaves or canopies? Functional trait relationships in Sphagnum mosses (Sphagnaceae)

Vascular plant leaf traits that influence photosynthetic function form the basis of mechanistic models of carbon exchange. Given their unique tissue organization, bryophytes may not express similar patterns. We investigated relationships among tissue, shoot, and canopy traits, and their associations with photosynthetic characteristics in 10 Sphagnum species. Trait relationships were organized around a primary dimension accounting for 43% of variation in 12 traits. There was no significant relationship between nitrogen content of shoot systems and maximum photosynthesis expressed on mass (A(mass)) or area (A(area)) bases due to nitrogen sequestration and storage within the canopy interior. This pattern differs from the distribution of nitrogen in vascular plant canopies. Thus, nitrogen and its relationship to carbon uptake in Sphagnum shoots does not conform to patterns of either vascular plant leaves or canopies. Species that concentrate biomass and nitrogen in the capitulum have enhanced rates of A(mass) and A(area). Consequently, A(area) was positively associated with N(area) of the capitulum only. Overall, water content and carotenoid concentration were the strongest predictors of both A(mass) and A(area) and these were expressed as inverse relationships. The relationships of plant traits in Sphagnum defines a principal trade-off between species that tolerate environmental stress and those that maximize carbon assimilation.

Vegetation establishment in post-fire Adenostoma chaparral in relation to fine-scale pattern in fire intensity and soil nutrients

At fine spatial scales (0.1–10m), chaparral communities have been shown to be strongly influenced by canopy-gap patterns, leading to periodicities in vegetation at 4–5 m spatial scales. Fine-scale variations in fire behavior and post-fire erosion can lead to changes in the patterning of viable seeds and nutrients and may alter the spatial patterning of post-fire chaparral communities. This study deals with the relationship among fire behavior, post-fire nutrient availabilities and vegetation patterns in a 1-yr old, post-fire Adenostoma fasciculatum chaparral community in the Sierra Nevada Mountains, California, USA. Variations in mineral soil exchangeable cations (Ca, Mg) and extractable phosphorus (P04-P) were correlated with ash distribution. Cations and measures of ammonium and nitrate were also correlated with fire intensity, measured by the diameter of the smallest remaining A. fasciculatum twigs following fire. Fire intensity was correlated with the pattern of post-fire vegetation establishment based on first axis DC A scores. However, ash PO4-P was more highly correlated with sample DCA scores, local species richness and total cover (p < 0.01), suggesting that small-scale variations in PO4-P which correlate with ash distributions may be important in structuring this community. Two- and three-term local variance analysis revealed a maximum of pattern intensity in DCA first axis scores at 4–5 m intervals that likely corresponds to pre-fire canopy-gap patterns. However, total cover showed pattern at spatial scales of 8–10 m, and was correlated at this scale with patterns of ash distribution and fire intensity. Microtopographic patterns also occur at similar spatial scales. Microtopographic patterns appear important in determining post-fire plant nutrient and water distributions and, thereby, patterns of plant establishment. Thus, the scale and intensity of post-fire vegetation pattern may differ considerably from pre-fire conditions.

Photosynthesis in Bryophytes and Early Land Plants

http://lisboa.academia.edu/RicardoCruzde Carvalho/Talks/36958/Desiccation_in_the_aquatic_ bryophyte_Fontinalis_antipyretica . 17 Aug 2011 Cruz de Carvalho R, Branquinho C, da Marques Silva J (2008) Oxygen evolution and chlorophyll fl uorescence under extreme desiccation in the aquatic bryophyte Fontinalis antipyretica . Photosynth Energy Sun 24:1425–1430 Cruz de Carvalho R, Branquinho C, da Marques Silva J (2011) Physiological consequences of desiccation in the aquatic bryophyte Fontinalis antipyretica . Planta

Relationships among shoot tissue, canopy and photosynthetic characteristics in the feathermoss Pleurozium schreberi

Abstract In bryophytes, rates of net photosynthesis vary among populations. How this variation is shaped by shoot biochemical or structural traits has not been established, yet would be essential to develop useful models of forest floor function in the boreal zone. The objectives of this study were to characterize functional trait relationships in the widespread feathermoss Pleurozium schreberi, to develop an empirical model to predict net photosynthesis in this species, and to compare the performance of a surface model that incorporates the shoot systems average properties with a canopy model that accounts for the vertical distribution of light and shoot area. Maximal rates of net photosynthesis (Amax) and dark respiration (Rd) were measured (n = 25) using gas exchange at optimal water contents. Shoot system concentrations of chlorophyll (a+b), carotenoids and nitrogen were measured in addition to the water content, surface roughness (Lr), canopy height, and the vertical distribution of shoot area and light within the canopy. The light extinction coefficient and transmission parameters were estimated from the latter. Amax ranged from 2.20 to 7.78 µmol CO2 m−2 s−1 with a mean value of 4.97. A linear multiple regression model using shoot area index (SAI) and chlorophyll concentration explained 55% of variation in Amax, and no other factors associated significantly with the residuals. The Monsi-Saeki canopy model was also fit to the data, which explained only 33% of variation in Amax. Residuals were related to Lr, and the full model improved to explain 53% of the variation. Given senescence and acclimation of shoots within the canopy, a more refined model will be necessary to add predictive power to the canopy model. Unlike vascular plants, the canopy models are not likely to be improved by considering the allocation of nitrogen because it does not associate with photosynthetic characteristics as it does in vascular plant leaves.

Best Practices for Measuring Photosynthesis at Multiple Scales

Studies of bryophyte photosynthetic performance have generally adapted techniques developed for use in vascular plants and relied on underlying vascular plant functional models as guides. Within this context, bryophytes present intellectual and methodological challenges, but also opportunities relative to their vascular plant counterparts. For example, although the leaf is clearly a functional unit for vascular plants, the comparable bryophyte structure may or may not serve a similar purpose. Instead, shoot systems and their organization into canopies are often employed as the functional equivalent. Unfortunately, due to issues of scale and alternative functional demands on bryophyte shoots like external transport and nutrient uptake, neither the methodologies nor the underlying models that lead to an integrated understanding of photosynthesis in vascular plants apply well to bryophytes. This chapter will consider the appropriate functional units for studies of bryophyte photosynthesis and relate it to the growth form and life form literature. Methods to characterize photosynthetic “leaf” area, water content, and canopy structure will be evaluated relative to their use in characterizing rates of photosynthesis. In addition, various methods are used to study photosynthetic function and these will be considered in light of their appropriate spatial and temporal domains.

Structural and Functional Analyses of Bryophyte Canopies

Although not often discrete, the canopy (i.e., the organization of branches, shoot systems and their extent) remains the most definable and useful unit of function in bryophytes. Chambers used for gas exchange provide an integrated summary of canopy photosynthetic function. However, other techniques can provide more information on spatial variation in physiological process in both the horizontal and vertical planes. Three examples of such studies are presented here. First, variation in photosystem II (PSII) function has been evaluated, along a canopy surface, using an imaging chlorophyll fluorometer. We evaluated the quantum yield of PSII, ϕPSII, and calculated the relative rate of photosynthetic electron transport (RETR) on 7 cm diameter samples of ten Sphagnum species during drying. Spatial variation in RETR increased both during drying as well as in high light, which led to different relationships between mean RETR and its variation—across light gradients, the relationship was positive, but negative when RETR was reduced by tissue desiccation. Studies of photosynthetic function using chlorophyll fluorescence measurements need to match their sampling protocols to account for this difference. Further, combining a laser scanning approach that provides three-dimensional information on canopy structure with functional imaging allows assessment of function in three dimensions (3D) within the canopy. This is illustrated using a thermal imaging camera to measure temperature distribution within Pleurozium schreberi canopies under still conditions and with wind. This imaging system resolved 9 °C temperature differences within the canopy and localized shoot temperature relative to canopy height. Finally, computational canopy (i.e., virtual) models have been developed for bryophyte canopies, particularly ones with simple branching structure. A model of this type is shown here for the liverwort Bazzania trilobata and a light model implemented using a ray tracing algorithm. Output from this model followed the attenuation of light predicted by the Lambert-Beer Law and such a technique can be used to evaluate how branching architecture and density affect the dynamics of light capture in bryophytes. New approaches based on novel imaging technologies are in rapid development and present opportunities to further our understanding of function within bryophyte canopies.

What Can We Learn From Bryophyte Photosynthesis

Bryophytes have been evolving in terrestrial and aquatic environments longer than any other group of land plants, surviving and thriving through an incredible range of climatic and environmental variation. Several of the bryophyte growth forms we find today closely resemble those found in ancient fossils whereas many of the other early land plant forms lack modern representatives. What is it about bryophyte growth form and physiology that has allowed them to persist through time and radiate into every terrestrial ecosystem, even dominating some of them? What can we learn from modern bryophytes to address this question and to predict how plants will respond to future environmental change? In this chapter, we briefly examine these questions as a preview to the volume as a whole.

Opportunities in Bryophyte Photosynthesis Research

The synthesis of important concepts in bryophyte and early land plant photosynthesis provided by the authors in this volume compiles a foundation of knowledge for new and experienced scientists interested in the field. In this prospective chapter, we highlight some areas where additional research is needed.