Higher Education for High Risk Students, April 1968

https://egrove.olemiss.edu/civ_pubs/1084/thumbnail.jp

Disturbance is required for CO2-dependent promotion of woody plant growth in grasslands

The relative effects of disturbance (here defined as bare soil), competition for edaphic resources, thermal interference and elevated [CO 2] on growth of tree seedlings in grasslands were studied under field conditions. Snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings were grown in open-top chambers flushed with either ambient or elevated [CO 2] from March 2004 to January 2005 (autumn to summer). These seedlings were planted into three treatments (i.e. bare soil, soil covered with straw or soil supporting a sward of live pasture grass) to separate effects of grass on seedling growth into those due to competition with grass for soil resources or to alteration of the thermal environment caused by a grassy surface (Ball et al. 2002). After the first major autumn frost, seedlings growing in competition with grass lost 59% of their canopy area, whereas those growing in bare soil or straw suffered negligible damage. These results reveal the complexity of competitive inhibition of plant growth in which ineffective competition for resources such as soil water enhances the vulnerability of the plant to abiotic stress, in this case frost. Tree seedlings growing in bare soil and straw commenced growth earlier in spring than those growing in competition with grass, where soil moisture was consistently lowest. Under ambient [CO 2], growth was greater in bare soil than in straw, consistent with thermal interference, but these differences disappeared under elevated [CO 2]. Elevated [CO2] significantly increased biomass accumulation for seedlings growing in bare soil and straw treatments, but not in grass. Thus, elevated [CO2] alleviated apparent thermal interference of seedling growth in spring but did not overcome adverse effects on seedling growth of either competitive reduction in soil resources or competitive enhancement of environmental stress. Nevertheless, elevated [CO2] could promote invasion of grasslands due to enhancement of woody plant growth in bare soil created by disturbances

Effect of reduced light during autumn and winter on snow gum seedling establishment

The present study tested whether eucalypt seedlings benefit from a reduction in irradiance when growth is limited by low temperatures. Growth of snow gum (Eucalyptus pauciflora Sieb. ex Spreng.) seedlings was studied for five consecutive seasons in a treeless pasture at 1000 m elevation where frost is common. Seedlings were planted at the beginning of autumn on north and south sides of vertical panels transmitting 50 % incident sunlight. This planting arrangement allowed assessment of excess irradiance on growth of seedlings subject to similar minimum temperatures. The panels remained in place for the first two seasons of the study and at the beginning of spring were removed to determine under common conditions the influence of different irradiance regimes during autumn and winter. Over winter, shaded seedlings were less photoinhibited, had higher photosynthetic CO2 assimilation rates, lost less leaf area and maintained higher leaf area ratio than unshaded seedlings. These differences were consistent with greater growth in shaded seedlings by the end of winter. The benefit of reduced irradiance during winter however did not translate into continued superior growth after the shade shelters were removed. During spring, seedlings which were previously shaded suffered more frost damage and had reduced photosynthetic rates so that overall, seedlings that were formerly shaded accumulated less biomass than seedlings exposed to full sunlight for the entire experiment. These results show that in the short term, shading at the coldest time of the year may explain why nurse plants can improve regeneration. However in the longer term if seedlings are subsequently exposed to full sunlight, the prior benefit of reduced light may not continue. Finally, recommendations are provided for tree planting in cold regions

Hydroacoustics as a tool to examine the effects of Marine Protected Areas and habitat type on marine fish communities

Abstract Hydroacoustic technologies are widely used in fisheries research but few studies have used them to examine the effects of Marine Protected Areas (MPAs). We evaluate the efficacy of hydroacoustics to examine the effects of closure to fishing and habitat type on fish populations in the Cabo Pulmo National Park (CPNP), Mexico, and compare these methods to Underwater Visual Censuses (UVC). Fish density, biomass and size were all significantly higher inside the CPNP (299%, 144% and 52% respectively) than outside in non-MPA control areas. These values were much higher when only accounting for the reefs within the CPNP (4715%, 6970% and 97% respectively) highlighting the importance of both habitat complexity and protection from fishing for fish populations. Acoustic estimates of fish biomass over reef-specific sites did not differ significantly from those estimated using UVC data, although acoustic densities were less due to higher numbers of small fish recorded by UVC. There is thus considerable merit in nesting UVC surveys, also providing species information, within hydroacoustic surveys. This study is a valuable starting point in demonstrating the utility of hydroacoustics to assess the effects of coastal MPAs on fish populations, something that has been underutilised in MPA design, formation and management