Comparative Cryptogam Ecology: A Review of Bryophyte and Lichen Traits that Drive Biogeochemistry

Epiphyte biomass on branches and trunks was estimated for 42 individual felled trees, distributed among three Pseudotsuga menziesii-Tsuga heterophylla stands aged 95, 145, and 400+ years, in the western Cascade Range of Oregon and Washington, then extrapolated to the whole stands by regression techniques. Epiphytes were sorted into four groups defined by ecological roles rather than taxonomy: cyanolichens, alectorioid lichens, other lichens, and bryophytes. In general the spatial sequence of dominance of these four groups, from upper canopy to forest jloor, was: "other" lichens, alectorioid lichens, cyanolichens, and bryophytes. The zones of these functional groups of epiphytes apparently migrate upward in forests through time. For example the Hypogymnia and Platismatia that dominate throughout canopies in young forests are found primarily in the upper canopies of old forests. Similarly, bryophytes enter a stand near the forest jloor and gradually expand their dominance upwards. Epiphyte biomass was greatest in the old-growth stand, with about 2.6 t/ha. In the two younger stands total epiphyte biomass was about 1 t/ha. The old-growth stand differed from the younger stands in having over 1 t/ha of cyanolichens, while this group was essentially absent from the younger stands. As a synthesis of these and previous results, a similar gradient hypothesis is proposed: epiphyte species are ordered similarly on three distinct spatial and temporal gradients: 1) vertical differences in species composition within a given stand, 2) species compositional differences among stands differing in moisture regime but of the same age, and 3) changes in species composition through time in a given stand.

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Gradients in Epiphyte Biomass in Three Pseudotsuga-Tsuga Forests of Different Ages in Western Oregon and Washington

The influence of environmental characteristics on life history attributes of Antarctic terrestrial biota

Considerable debate has accompanied efforts to integrate the selective impacts of environmental stresses into models of life-history evolution. This study was designed to determine if different environmental stresses have consistent phenotypic effects on life-history characters and whether selection under different stresses leads to consistent evolutionary responses. We created lineages of a wild mustard (Sinapis arvensis) that were selected for three generations under five stress regimes (high boron, high salt, low light, low water, or low nutrients) or under near-optimal conditions (control). Full-sibling families from the six selection histories were divided among the same six experimental treatments. In that test generation, lifetime plant fecundity and six phenotypic traits were measured for each plant. Throughout this greenhouse study, plants were grown individually and stresses were applied from the early seedling stage through senescence. Although all stresses consistently reduced lifetime fecundity and most size- and growth-related traits, different stresses had contrasting effects on flowering time. On average, stress delayed flowering compared to favorable conditions, although plants experiencing low nutrient stress flowered earliest and those experiencing low light flowered latest. Contrary to expectations of Grime's triangle model of life-history evolution, this ruderal species does not respond phenotypically to poor environments by flowering earlier. Most stresses enhanced the evolutionary potential of the study population. Compared with near-optimal conditions, stresses tended to increase the opportunity for selection as well as phenotypic variance, although both of these quantities were reduced in some stresses. Rather than favoring traits characteristic of stress tolerance, such as slow growth and delayed reproduction, phenotypic selection favored stress-avoidance traits: earlier flowering in all five stress regimes and faster seedling height growth in three stresses. Phenotypic correlations reinforced direct selection on these traits under stress, leading to predicted phenotypic change under stress, but no significant selection in the control environment. As a result of these factors, selection under stress resulted in an evolutionary shift toward earlier flowering. Environmental stresses may drive populations of ruderal plant species like S. arvensis toward a stress-avoidance strategy, rather than toward stress tolerance. Further studies will be needed to determine when selection in stressful environments leads to these alternative life-history strategies.

Evolution in stressful environments. I. Phenotypic variability, phenotypic selection, and response to selection in five distinct environmental stresses.

Bryophytes are a successful group of non-flowering land plants that mostly avoid competition with tracheophytes. Many bryophytes grow on substrata that are impenetrable to tracheophyte roots and tolerate frequent desiccation and poor conditions. Most species exhibit clonal or colonial life-forms. These rather than the individual shoots or thalli are the ecologically functional units. Approximately eleven distinct life-forms can be recognized although the schemes of no two authors agree. Bryophyte life-forms can be interpreted as recurring arrangements of the photosynthetic tissues that minimize evaporative water loss and maximize primary production. However, other factors such as prevention of photoinhibition and scavenging of cloudwater may also be important. Many species show plasticity of life-form according to environmental conditions. Aerodynamically competent colonies are probably fashioned passively because protruding individuals are more prone to desiccation than those remaining within the laminar boundary layer. Alterations of density and shoot and leaf orientation appear to be regulated by phytochrome-mediated growth responses to shading. The different life-forms can be arranged in sequences reflecting water availability and light intensity in different habitats but fewer categories are represented on soils than on hard substrata because of negative interactions (stress and disturbance) with tracheophytes. Despite the strong correlation of life-forms with moisture and light conditions, there are limitations to their use as a framework in ecological studies of bryophytes. Different categories of life-form have been loosely applied and an improved system based on objective criteria is required. Also, life-forms have not evolved independently of other attributes of life-strategy. Colonial life-forms confer a competitive advantage in pre-empting space in the patchy habitats occupied by bryophytes; moreover some types, like 'dendroids', may be specialists at capturing space occupied by other species. Life-forms may sometimes also be interpreted, especially in the more highly productive species like Brachythecium rutabulum. as adaptations for foraging. Internal movement of nutrients from favourably. placed ramets may be beneficial to the competitive fitness of the whole colony. Life-forms and breeding systems in bryophytes are unlikely to have evolved independently as dioicous taxa tend to form unisexual colonies with low chances of fertilization. It is concluded that life-form is a useful concept because of the exceptionally high dependence of bryophytes on transient external water supplies; however, major advances in understanding will depend upon investigation of the relationships of life-form to other attributes of bryophyte life-strategy.

E. R. Rincon

Papers

Bryophytes and plant strategy theory