N Swarts

Terrestrial orchid conservation in the age of extinction.

BACKGROUND Conservation through reserves alone is now considered unlikely to achieve protection of plant species necessary to mitigate direct losses of habitat and the pervasive impact of global climate change. Assisted translocation/migration represent new challenges in the face of climate change; species, particularly orchids, will need artificial assistance to migrate from hostile environments, across ecological barriers (alienated lands such as farmlands and built infrastructure) to new climatically buffered sites. The technology and science to underpin assisted migration concepts are in their infancy for plants in general, and orchids, with their high degree of rarity, represent a particularly challenging group for which these principles need to be developed. It is likely that orchids, more than any other plant family, will be in the front-line of species to suffer large-scale extinction events as a result of climate change. SCOPE The South West Australian Floristic Region (SWAFR) is the only global biodiversity hotspot in Australia and represents an ideal test-bed for development of orchid conservation principles. Orchids comprise 6 % of all threatened vascular plants in the SWAFR, with 76 out of the 407 species known for the region having a high level of conservation risk. The situation in the SWAFR is a portent of the global crisis in terrestrial orchid conservation, and it is a region where innovative conservation solutions will be required if the impending wave of extinction is to be averted. Major threatening processes are varied, and include land clearance, salinity, burning, weed encroachment, disease and pests. This is compounded by highly specialized pollinators (locally endemic native invertebrates) and, in the most threatened groups such as hammer orchids (Drakaea) and spider orchids (Caladenia), high levels of mycorrhizal specialization. Management and development of effective conservation strategies for SWAFR orchids require a wide range of integrated scientific approaches to mitigate impacts that directly influence ecological traits critical for survival. CONCLUSIONS In response to threats to orchid species, integrated conservation approaches have been adopted (including ex situ and translocation principles) in the SWAFR with the result that a significant, multidisciplinary approach is under development to facilitate conservation of some of the most threatened taxa and build expertise to carry out assisted migration to new sites. Here the past two decades of orchid conservation research in the SWAFR and the role of research-based approaches for managing effective orchid conservation in a global biodiversity hotspot are reviewed.

Ecological specialization in mycorrhizal symbiosis leads to rarity in an endangered orchid

Terrestrial orchid germination, growth and development are closely linked to the establishment and maintenance of a relationship with a mycorrhizal fungus. Mycorrhizal dependency and specificity varies considerably between orchid taxa but the degree to which this underpins rarity in orchids is unknown. In the context of examining orchid rarity, large scale in vitro and in situ germination trials complemented by DNA sequencing were used to investigate ecological specialization in the mycorrhizal interaction of the rare terrestrial orchid Caladenia huegelii. Common and widespread sympatric orchid congeners were used for comparative purposes. Germination trials revealed an absolute requirement for mycorrhisation with compatibility barriers to germination limiting C. huegelii to a highly specific and range limited, efficacious mycorrhizal fungus. DNA sequencing confirmed fidelity between orchid and fungus across the distribution range of C. huegelii and at key life history stages within its life cycle. It was also revealed that common congeners could swap or share fungal partners including the fungus associated with the rare orchid but not vice versa. Data from this study provides evidence for orchid rarity as a cause and consequence of high mycorrhizal specialization. This interaction must be taken into account in efforts to mitigate the significant extinction risk for this species from anthropogenically induced habitat change and illustrates the importance of understanding fungal specificity in orchid ecology and conservation.

Perspectives on orchid conservation in botanic gardens

Orchids, one of the largest families of flowering plants, face an uncertain future through overexploitation, habitat loss and impacts of climate change. With their intricate abiotic and biotic dependencies, orchids typify the plight of global plant resources and, thus, provide ideal model species for ecological tracking and focussing conservation programs. Botanic gardens worldwide have traditionally been major centres of excellence in orchid horticulture, research and conservation as orchids generate wide public and educational appeal. Here, we highlight the role of botanic gardens in areas key to orchid conservation. With pristine habitats under threat globally, the challenge for orchid conservation programs will ultimately depend upon developing ecological restoration technologies, whereby orchids are reinstated into sustainably restored habitats.

Mycorrhizal preference promotes habitat invasion by a native Australian orchid: Microtis media

BACKGROUND AND AIMS Mycorrhizal specialization has been shown to limit recruitment capacity in orchids, but an increasing number of orchids are being documented as invasive or weed-like. The reasons for this proliferation were examined by investigating mycorrhizal fungi and edaphic correlates of Microtis media, an Australian terrestrial orchid that is an aggressive ecosystem and horticultural weed. METHODS Molecular identification of fungi cultivated from M. media pelotons, symbiotic in vitro M. media seed germination assays, ex situ fungal baiting of M. media and co-occurring orchid taxa (Caladenia arenicola, Pterostylis sanguinea and Diuris magnifica) and soil physical and chemical analyses were undertaken. KEY RESULTS It was found that: (1) M. media associates with a broad taxonomic spectrum of mycobionts including Piriformospora indica, Sebacina vermifera, Tulasnella calospora and Ceratobasidium sp.; (2) germination efficacy of mycorrhizal isolates was greater for fungi isolated from plants in disturbed than in natural habitats; (3) a higher percentage of M. media seeds germinate than D. magnifica, P. sanguinea or C. arenicola seeds when incubated with soil from M. media roots; and (4) M. media-mycorrhizal fungal associations show an unusual breadth of habitat tolerance, especially for soil phosphorus (P) fertility. CONCLUSIONS The findings in M. media support the idea that invasive terrestrial orchids may associate with a diversity of fungi that are widespread and common, enhance seed germination in the host plant but not co-occurring orchid species and tolerate a range of habitats. These traits may provide the weedy orchid with a competitive advantage over co-occurring orchid species. If so, invasive orchids are likely to become more broadly distributed and increasingly colonize novel habitats.

Variation in nutrient-acquisition patterns by mycorrhizal fungi of rare and common orchids explains diversification in a global biodiversity hotspot

BACKGROUND AND AIMS Many terrestrial orchids have an obligate requirement for mycorrhizal associations to provide nutritional support from germination to establishment. This study will investigate the ability of orchid mycorrhizal fungi (OMF) to utilize a variety of nutrient sources in the nutrient-impoverished (low organic) soils of the Southwest Australian Floristic Region (SWAFR) in order to effectively compete, survive and sustain the orchid host. METHODS Mycorrhizal fungi representing key OMF genera were isolated from three common and widespread species: Pterostylis recurva, Caladenia flava and Diuris corymbosa, and one rare and restricted species: Drakaea elastica. The accessibility of specific nutrients was assessed by comparing growth including dry biomass of OMF in vitro on basal CN MMN liquid media. KEY RESULTS Each of the OMF accessed and effectively utilized a wide variety of nutrient compounds, including carbon (C) sources, inorganic and organic nitrogen (N) and inorganic and organic phosphorus (P). The nutrient compounds utilized varied between the genera of OMF, most notably sources of N. CONCLUSIONS These results suggest that OMF can differentiate between niches (micro-niche specialization) in a constrained, highly resource-limited environment such as the SWAFR. Phosphorus is the most limited macronutrient in SWAFR soils and the ability to access phytate by OMF indicates a characterizing functional capacity of OMF from the SWAFR. Furthermore, compared with OMF isolated from the rare D. elastica, OMF associating with the common P. recurva produced far greater biomass over a wider variety of nutritional sources. This suggests a broader tolerance for habitat variation providing more opportunities for the common orchid for recruitment and establishment at a site.

Propagation and reintroduction of Caladenia

Many Caladenia species have been reduced to extremely small and/or fragmented populations, and reintroduction/translocation into natural or rehabilitated habitats, by using ex situ propagated plants or via direct seeding, represents an important adjunct in conservation planning. However, Caladenia species are some of the most difficult terrestrial orchid taxa to propagate, in part because of the specificity of the mycorrhizal associations and the need to provide growing conditions that suit both the mycorrhizal fungi and Caladenia plants. The present paper reviews recent advances in Caladenia propagation and reintroduction methods, including in vitro seed germination, transferral from in vitro to nursery environments, ex vitro symbiotic germination (germination in inoculated nursery media), nursery cultivation, the use of nurse plants and reintroduction of Caladenia into natural habitats by using seed, dormant tubers or growing plants. Techniques discussed in the present paper increase the options for future Caladenia conservation programs, especially for those species currently on the brink of extinction.

Ex situ conservation and cryopreservation of orchid germplasm

Premise of research. Orchids are among the most enigmatic of plant species. Yet the Orchidaceae comprises more species at risk of extinction than any other plant family. The collection and storage of orchid germplasm—principally seeds and associated mycorrhizal fungi but also protocorm-like bodies using encapsulation and vitrification techniques—allows for secure ex situ conservation. This article reviews the approaches and techniques used for the ex situ conservation of orchid germplasm, with a focus on seed banking and the use of cryopreservation techniques to improve the longevity of germplasm. Pivotal results. It is increasingly apparent that cryopreservation—the storage of germplasm at ultra-low temperatures (e.g., in liquid nitrogen)—is required for the long-term and low-maintenance conservation of all types of orchid germplasm. For orchid seeds, desiccation tolerance is common, but longevity in storage is poor. Cryopreservation of orchid seeds shows promise, but some complexities in low-temperature storage behavior still require explanation and resolution. The application of more advanced cryopreservation techniques, including encapsulation-dehydration and vitrification, is becoming increasingly common. These techniques provide for the simultaneous storage of orchid propagules with their compatible fungus, while for seeds, vitrification techniques show potential for improving tolerance to the stresses of cryopreservation. Conclusions. A renewed focus on describing the low-temperature storage physiology of orchid seeds to more precisely define the relationship between seed water content, storage temperature, and seed survival is required, as is perhaps the wider adoption of the use of cryoprotectants for seeds. This research, coupled with the development of improved methods of seed viability testing, will support the growing work of germplasm banks to protect orchid biodiversity in the face of habitat loss and potential species extinction.

Benchmarking nitrous oxide emissions in deciduous tree cropping systems

Nitrous oxide (N2O) emissions contribute 6% of the global warming effect and are derived from the activity of soil-based microorganisms involved in nitrification and denitrification processes. There is a paucity of greenhouse gas emissions data for Australia’s horticulture industry. In this study we investigated N2O flux from two deciduous fruit tree crops, apples and cherries, in two predominant growing regions in eastern Australia, the Huon Valley in southern Tasmania (Lucaston – apples and Lower Longley – cherries), and high altitude northern New South Wales (Orange – apples and Young – cherries). Estimated from manual chamber measurements over a 12-month period, average daily emissions were very low ranging from 0.78gN2O-Nha–1day–1 in the apple orchard at Lucaston to 1.86gN2O-Nha–1day–1 in the cherry orchard in Lower Longley. Daily emissions were up to 50% higher in summer (maximum 5.27gN2O-Nha–1day–1 at Lower Longley) than winter (maximum 2.47gN2O-Nha–1day–1 at Young) across the four trial orchards. N2O emissions were ~40% greater in the inter-row than the tree line for each orchard. Daily flux rates were used as a loss estimate for annual emissions, which ranged from 298gN2O-Nha–1year–1 at Lucaston to 736gN2O-Nha–1year–1 at Lower Longley. Emissions were poorly correlated with soil temperature, volumetric water content, water filled porosity, gravimetric water content and matric potential – with inconsistent patterns between sites, within the tree line and inter-row and between seasons. Stepwise linear regression models for the Lucaston site accounted for less than 10% of the variance in N2O emissions, for which soil temperature was the strongest predictor. N2O emissions in deciduous tree crops were among the lowest recorded for Australian agriculture, most likely due to low rates of N fertiliser, cool temperate growing conditions and highly efficient drip irrigation systems. We recommend that optimising nutrient use efficiency with improved drainage and a reduction in soil compaction in the inter-row will facilitate further mitigation of N2O emissions.