Douglas W. Larson

Light Attenuation by Limestone Rock and Its Constraint on the Depth Distribution of Endolithic Algae and Cyanobacteria

To understand how light attenuation in limestone rock constrains the distribution and abundance of endolithic photoautotrophic organisms, we examined light‐level profiles in 25 rock samples containing endolithic algae and cyanobacteria. We collected samples from three representative cliff sites along the Niagara Escarpment, Canada, using methods that allowed us to make comparisons among as well as within sites. Photosynthetically active radiation profiles were generated by manufacturing cavities in the bottom of thick slabs of surface rock and enclosing radiation sensors in the cavities. The thickness of the flat layer of limestone between the sensor and a light source was then abraded in steps of ca. 0.5 mm. An exponential curve was fitted to the measurements for each sample to allow statistical comparisons of the light levels at different depths. We examined the correlations between physical characteristics of the rock and light attenuation and between light attenuation and the maximum depth, relative biomass, and taxonomic richness of endolithic algae and cyanobacteria. The depth at which 0.01% of available light remained varied from 2.1 to 4.5 mm at different sites, but there was large within‐site variability on intermediate and small spatial scales. In contrast, the maximum depth of photoautotrophic endoliths was homogeneous within sites but different among sites (1.1–3.5 mm). Although the endoliths all have access to light, there was no significant correlation between the limit of their depth penetration and threshold quantum flux densities, indicating that the mechanisms controlling distribution and abundance of endolithic photoautotrophs are as complicated as those found for aquatic algae and cyanobacteria.

Vegetation, environmental characteristics and ideas on the maintenance of alvars on the Bruce Peninsula, Canada.

Despite international recognition that alvar habitats are important reservoirs of biodiversity, they remain little studied in North America. In this paper, the results are re- ported on an investigation of alvars in the central portion of their known distribution on this continent. 210 plots were distributed among seven sites and were quantitatively sampled for vascular plants, lichens, bryophytes and a suite of environ- mental variables. Detrended and Canonical Correspondence Analyses and other methods were used to investigate differ- ences among alvars, within alvars and between alvars and adjacent forested habitats. The plant communities and envi- ronmental conditions were highly similar among sites in the study region, yet very different from surrounding habitats. There were abrupt changes in vegetation and environmental conditions from alvar to forest, without the presence of transi- tion zones in the vegetation or environmental gradients as the forest was approached. The environmental factors associated with the change from alvar to forest and with variation within alvar habitat were examined. Some alvars in the study were found to contain stunted, slow-growing trees reaching ages of 524 yr. These same sites appear to have remained unburned for several centuries, while other sites likely burned 90 yr ago. The plant communities were very similar between the alvars that lacked a major, biomass-removing disturbance in centuries and alvars that had experienced catastrophic fire relatively recently. Maintenance of the plant communities and open nature of alvars appears site-specific rather than habitat specific.

Rooting Characteristics of Trees in Rock: A Study of Thuja occidentalis on Cliff Faces

Sixty-two slow-growing Thuja occidentalis (eastern white cedar), 6-28 yr in age and 11-52 cm in size, were excavated from a vertical limestone cliff. Their root extension, distribution, biomass, and belowground allocation were investigated, as was the degree to which these rooting characteristics were influenced by substrate factors such as rock fracturing and the presence of soil pockets and ledges. Alson examined was the correlation between such microsite characteristics and individual tree growth rates. The results showed that the majority of trees grew in rock without soil or in very small soil pockets. Rooting was shallow, penetrating solid rock to an average of only 9 cm (maximum 30 cm). Roots were found almost exclusively in rock fissures of the softer, more weathered rock layers, penetrating harder layers only via crevices. Root competition was largely absent when trees grew in rock. The prescence of absence of soil had no effect on aboveground or belowground biomass, root: Shoot ratio, or growth rate but significantly affected the pattern of root deployment. Root: shoot ratios were within the normal range for temperature-zone trees (average 0.48). All microsite factors were poor predictors of individual plant growth rate. The likely availability of both water and nutrients near the rock surface may explain the lack of deep root penetration and a minimal need for soil. Plasticity of root deployment without a loss of uptake efficiency may be one of the characteristics that enable T. occidentalis to persist on cliff faces.

Experimental separation of resource quantity from temporal variability: seedling responses to water pulses

We tested the hypothesis that higher temporal variability in water supply will promote higher species richness of germinating and surviving seedlings using assemblages of 70 species of herbaceous plants from limestone pavement habitats. In a two-factor greenhouse experiment, doubling the total volume of water added led to greater germination (measured as number of germinated seeds and species) and establishment (survival and biomass) but the effects of temporal variability depended on the response variable considered. Low pulse frequencies of water addition with total volume added held constant resulted in greater temporal variability in soil moisture concentration that in turn promoted higher density and richness of germinated seedlings. Low pulse frequencies caused an eight-fold greater mortality in the low total volume treatment and biomass production to decline by one-third in the high total volume treatment. The effects of increasing temporal variability in water supply during recruitment stages can thus be opposite on different components of plant fitness and may also depend on total resource quantity. While greater species richness in more temporally variable soil moisture conditions was attributable to sampling effects rather than species-specific responses to the water treatments, species relative abundances did vary significantly with temporal variability. Changes in the amplitude or frequency of resource fluctuations may alter recruitment patterns, and could have severe and relatively rapid effects on community structure in unproductive ecosystems.

ABUNDANCE, BIOMASS, AND PRODUCTIVITY OF ENDOLITHIC AND EPILITHIC LOWER PLANTS ON THE TEMPERATE-ZONE CLIFFS OF THE NIAGARA ESCARPMENT, CANADA

To investigate the cover, frequency, biomass, and productivity of endolithic and epilithic lower plants on temperate-zone cliff faces, rock samples were randomly collected from the Niagara Escarpment in southern Ontario, Canada. Chlorophyll was extracted using dimethyl sulfoxide (DMSO), and CO2 gas exchange was measured in the laboratory using infrared gas analysis. Epilithic surface cover averaged 26% for cyanobacteria, 3% for green algae, and 20% for lichens. Endoliths were present below 6% of the surface area, with chasmoendoliths about twice as common as cryptoendoliths. Fungi were by far the most common organisms. The cliffs contained 73.0 mg chl a and 19.8 mg chl b per square meter; 15% and 23% of these, respectively, were in the endolithic zone. Biomass of primary producers (algae, cyanobacteria, and lichen photobionts) was 1.5-73 g dry mass m-2 total, and

Environmental relations of the bryophytic and vascular components of a talus slope plant community

0.3-14\ {\rm g}\ {\rm m}^{-2}

Environmental stress and Umbilicaria lichens: the effect of high temperature pretreatments

in the endolithic zone alone. Photosynthetic carbon uptake was 0.0021 mg

The role of seed rain in determining the assembly of a cliff community

{\rm CO}_{2}\ {\rm cm}^{-2}\ {\rm h}^{-1}

Cambial Dieback and Partial Shoot Mortality in Cliff-Face Thuja occidentalis: Evidence for Sectored Radial Architecture

at 700 μ mol m-2 s-1 photosynthetically active radiation (PAR). At

Dominance As An Overlooked Measure of Invader Success

{\rm PAR}=130\ \mu {\rm mol}\ {\rm m}^{-2}\ {\rm s}^{-1}