Environment Institute

Permanent URI for this community

https://hdl.handle.net/2440/65485

Browse

Browsing Environment Institute by Advisors "Facelli, Jose Maria"

Now showing 1 - 3 of 3
  • No Thumbnail Available
    ItemOpen Access
    Broad crowned trees and the hydraulic limitation hypothesis.
    (2011) Escoto-Rodriguez, Martin; Watling, Jennifer Robyn; Facelli, Jose Maria; School of Earth and Environmental Sciences
    The hydraulic limitation hypothesis (HLH) provides a physiological explanation of what limits height in trees. It states that resistance to water flow increases with pathway length, causing water potential to decrease and, as a consequence, the premature closing of stomata thus limiting photosynthesis and growth. The existence of broad crowned trees, however, appears to present a challenge to the HLH as vertical growth is more limited than that of longer horizontal shoots. This suggests that pathway length may not be the main factor leading to height limitation, because water is travelling a longer distance in the horizontal stems than in the vertical ones. In this thesis I investigated the HLH and factors influencing tree shape and height in Acacia papyrocarpa Benth, a broad crowned tree from south-eastern Australia. Mature, isolated A. papyrocarpa trees from two different sites were found to have asymmetric crowns with a non-random, northerly orientation. This orientation could not be explained by wind direction, or loss of branches due to mistletoe infection. The most likely explanation is that the northerly orientation maximises light interception during the Southern Hemisphere winter. At two sites with contrasting water availability, trees were taller at the more mesic site whereas phyllode δ¹³C at the top of the canopy was similar in trees from both sites. These results are in agreement with a water limiting mechanism. However, in trees with longer horizontal pathways than vertical ones, phyllode δ¹³C of the longest horizontal stems was lower than that at the top of the tallest vertical stems. Thus, longer path length did not result in more conservative water use as has been argued for the HLH. Because there were no differences in light environment or in hydraulic conductivity between branches sampled at the two canopy positions, the difference in phyllode δ¹³C suggests that the effects of gravity on water transport could be more important than pathway resistances. Following these results, I had planned to quantify some effects of gravity on water status in small trees, however, preliminary measurements of xylem pressure potentials in fully hydrated leaves showed a large variability that overcame the intra-canopy differences that gravity would be predicted to generate. In attempting to account for this variability I measured balance pressure (BP) on fully hydrated, non-transpiring detached leaves from 4 different species. BP in such leaves should be close to 0 kPa, however, it ranged from 3 kPa to 200 kPa or higher, despite a calculated measurement error of only 2 kPa. The variability in BP could not be solely accounted for by differences in species, hydration time, plant water status, light history, or leaf position on the plant. Leaf area and LMA, however, did explain up to 61% of BP variability in some species. The negative non-linear relationships between these leaf characteristics and BP suggest that leaf growth was causing part of the disequilibrium. In order to reduce confounding factors during pressure chamber measurements, leaves need to be selected carefully to avoid the large variability that may be associated with leaf growth.
  • No Thumbnail Available
    ItemOpen Access
    Cassytha pubescens: germination biology and interactions with native and introduced hosts.
    (2010) Tsang, Hong Tai (Steven); Facelli, Jose Maria; Watling, Jennifer Robyn; Prider, Jane Noeleen; School of Earth and Environmental Sciences
    The native hemiparasitic vine Cassytha pubescens infects and often kills the invasive weeds Cytisus scoparius and Ulex europaeus in the Mount Lofty Ranges, South Australia. This leads to the consideration of whether this parasite is a suitable biological control agent for these weeds. The aims of this study were to investigate germination characteristics of the parasite, the direct effects of the parasite on both invasive and native hosts, and the indirect effects of the parasite on interactions between native and invasive hosts. Seed dormancy and germination of C. pubescens were examined. Imbibition tests revealed that the seeds are enclosed in a water impermeable seed coat, which produces physical dormancy. Germination experiments showed that heat and scarification broke the physical dormancy, but the germination rate of heated seeds was over three times higher than that of scarified seeds. Thus this parasite may have evolved to share similar fire-related germination cues as some of its native hosts. The direct impact of C. pubescens on growth of Acacia myrtifolia (a native legume) and Cytisus scoparius (an invasive legume) was investigated in a pot experiment. None of the parasites on infected A. myrtifolia survived, so none of the A. myrtifolia was successfully infected with the parasite. In contrast, C. pubescens successfully infected C. scoparius. Host biomass accumulation was reduced by 21%, relative to uninfected plants. Photosystem II efficiencies were reduced but only on the infected branches. The total nitrogen content of infected plants plus parasite was the same as that of uninfected plants, and there was no impact of infection on nodulation by Rhizobium. Thus, it is likely that the removal of nitrogen by the parasite, reduces the supply to the host, and this limits the biomass accumulation of C. scoparius. The differences in resistance to the parasite by native and invasive hosts were studied. A pot experiment was conducted using [superscript]32P to examine the uptake of nutrients by the parasite from either C. scoparius or A. myrtifolia. In this experiment, C. pubescens was able to successfully attach to A. myrtifolia hosts. The parasite absorbed no [superscript]32P when attached to the native host, but did take up [superscript]32P from the invasive host. This suggests A. myrtifolia resists the formation of functional haustoria by the parasite, while the invasive host does not. It is likely that this resistance of the native host to the native parasite may have evolved through long-term coexistence, whereas the invasive host has had only a short-term association with the parasite. To investigate if the differences in host resistance to C. pubescens lead to changes in competitive outcomes between hosts, plants of the invasive weed Ulex europaeus were grown together with either A. myrtifolia, or a native non-legume, Leptospermum myrsinoides, and either with or without C. pubescens. There was no effect on either biomass accumulation of hosts or on the intensity of competition between hosts. However, as these are perennial species, it is possible that the experiment was too short to detect any effects. Long-term experiments and field monitoring may be required to resolve these competitive interactions. These results provide an important insight into the germination ecology of C. pubescens, and the nature of its impact on both native and invasive hosts. Unlike the morphologically similar holoparasites of the genus Cuscuta, C. pubescens does not seems to act as a carbon sink, thus had little effect on symbiotic nitrogen fixation. This suggests a different carbon-nitrogen economy model form the one proposed for the morphologically similar holoparasites, Cuscuta spp. The study also detected differences in resistance of hosts to the parasite; however, this appeared to have no effect on host competition in a short-term pot experiment.
  • No Thumbnail Available
    ItemOpen Access
    The reproductive ecology of two terrestrial orchids, Caladenia Rigida and Caladenia tentaculata.
    (2010) Faast, Renate; Facelli, Jose Maria; Austin, Andrew Donald; School of Earth and Environmental Sciences : Ecology and Evolutionary Biology
    The reproductive outcome of plants is often determined by a multitude of interacting factors operating at both the plant level and the population level. For many plants, fruit production and the subsequent release of seeds are paramount for the persistence of the species. Understanding the processes that influence variation within and among populations is therefore crucial for the successful long-term management of threatened plants. While abiotic factors such as resource availability and environmental conditions can influence seed production directly through their effects on plant growth, biological interactions such as those between plants and pollinators or herbivores can be equally important. The relative intensity and direction of such interactions are often determined by the nature of the plants themselves, or by characteristics of the plant population or the habitat in which it occurs. This thesis examines the processes that influence spatio-temporal variation in the reproductive success of two terrestrial orchids, Caladenia rigida and Caladenia tentaculata. The study was carried out over three years (2005 – 2007), in several populations located in the Mount Lofty region of South Australia. A detailed investigation of the pollination strategy employed by C. rigida revealed that this species is a generalist, being pollinated by a suite of food-seeking insects, possibly attracted by the presence of small amounts of nectar. Successful pollination and seed release for C. rigida was highly variable across space and time. Furthermore, both measures were consistently higher than for the sexually deceptive species, C. tentaculata, leading to the suggestion that the highly specialised pollination syndrome of the latter species may place it at a reproductive disadvantage. Pollination success of C. rigida was influenced by the height of flowers, but not by the local density of conspecifics. Small populations of C. rigida did not produce capsules when environmental conditions were stressful, suggesting that resource availability may indirectly restrict reproductive success by limiting the availability of pollinators. Poor seed quality in some populations may also be attributed to reduced population size. Both orchid species were subject to intense levels of vertebrate florivory and capsule predation, leading to significant reductions in seed output. A herbivore exclusion experiment was carried out to help elucidate the size and type of herbivores, and video surveillance identified birds as a predominant florivore in some populations. The intensity of florivory varied within and among populations, as well as among years, in response to several factors including flower height, the local density of conspecifics, concealment amongst neighbourhood vegetation and proximity to the habitat edge. Spatio-temporal variation in seed release was thus the net outcome of processes acting on both mutualistic and antagonistic interactions. This work provides valuable baseline data of factors that influence the reproductive ecology and, hence, population dynamics of Caladenia species. Implications for the conservation and management of threatened populations are discussed, with respect to both short-term and long-term goals. The thesis is presented as a series of five manuscripts. Two of these have been published, and the remaining three have been prepared for submission as publications.