Browsing by Author "Cramer, Michael"

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    Mechanisms determining the coexistence of open- and closed-canopy biomes
    (2018) Power, Simon C; Cramer, Michael; Bond, William; Verboom, Tony
    Open- (e.g. grassland, savanna, shrubland) and closed-canopy (e.g. forest) biomes frequently coexist in the same landscape, where open environments tend to be fire-prone with higher light, but lower nutrient and water availability than closed environments. Environmental heterogeneity could select for divergent floristic assemblages and adaptive traits, from which emergent differences in resource availability and fire incidence contribute to excluding species from the alternate habitat. In this thesis, I investigated whether the coexistence of open–closed canopy biomes, such as forest and fynbos in the Cape Floristic Region, is contingent on environmental heterogeneity coupled with contrasting species traits. Given the heterogeneity in multiple environmental properties between open- and closed-canopy biomes, I hypothesized that boundaries between open- and closed-canopy biomes will display greater floristic turnover compared to boundaries between structurally similar biomes (e.g. open- and opencanopy biomes). To explore this, genus- and family-level turnover were correlated with climate, fire, leaf area index (LAI: proxy for understorey light) and soil properties across biome boundaries in South Africa. Both genus- and family-level turnovers were highest across open–closed boundaries and most strongly predicted by increased differences in LAI, suggesting that contrasting light regimes provide significant adaptive challenges for plants. The potential effect of contrasting light regimes is highlighted by the absence of open-canopy species from forest understoreys, where low, dynamic light could limit the ability of plants to acquire sufficient carbon. This apparent shade intolerance led to the hypothesis that open-canopy species lack the traits to maintain a positive carbon balance under low and dynamic light. To test this, leaf traits and photosynthetic response to continuous or dynamic light were compared between forest and fynbos species grown under three light treatments. Fynbos species experienced high mortality under shade treatments, produced leaves that were thicker, up to 1000 times smaller, had lower photosynthetic rates (0.8 versus 3.4mol m-2 s -1 ) under continuous low light (400 mol m-2 s -1 ) and lower light-use efficiency during dynamic light sequences than forest species. These differences imply that shade intolerance in fynbos species is associated with traits that are inefficient at harvesting light and require relatively continuous high intensity light for carbon assimilation. Moreover, these inefficiencies would make it difficult to support the carbon intensive traits (e.g. cluster roots, lignotubers, sclerophyllous leaves) that facilitate fire survival and nutrient acquisition/conservation in open habitats. In contrast, forest species are able to colonize open habitats during the long-term absence of fire, implying that they are able to tolerate high light and low nutrient conditions. Given that plants frequently cope with contrasting conditions through the expression of phenotypic plasticity, it was hypothesized that closed-canopy species possess greater plasticity than open-canopy species. To assess this, the response of leaf traits and foliar nutrition to changes in LAI and soil nutrition were compared between forest and fynbos species in the field. Leaf size and specific leaf area in forest species correlated positively with LAI and soil nutrition, whereas fynbos species response was weak, suggesting that forest species are more plastic. This plasticity may be realised by the variable light conditions forest species experience through their canopy and the occupation of higher nutrient soils, which alleviate belowground constraints. By comparison, the occupation of low nutrient soils by fynbos may inhibit plasticity given the selection of inflexible, conservative leaves. Consequently, I propose that the coexistence of open- and closed-canopy biomes arises from the steep turnover in selective regimes, which together with the contrasting adaptive traits and degrees of phenotypic plasticity they require, act together to competitively exclude species from the alternate habitat.
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    Sub-Antarctic plant-soil interactions in a changing world: plant N acquisition and growth under warming on Marion Island
    (2023) Pallett, Nita; Cramer, Michael
    It is widely accepted that in cold ecosystems, strong abiotic (e.g., temperature) controls over soil decomposition processes result in N-limited plant productivity and soils replete in organic N (oN, e.g., amino acids) but not inorganic N (iN, i.e., NH4 + and NO3 - ). Recent advances in our understanding of plant N use have shown that cold ecosystem plants meet the bulk of their N requirements through direct oN acquisition. Climate warming in cold ecosystems under global change is expected to alleviate the temperature limitations to soil decomposition, effectively increasing N-availability through iN release. This has resulted in predictions for stronger indirect than direct effects on plant productivity. Additionally, increasing iN fractions may affect coldecosystem plant nutrition by altering the N-form predominantly acquired from oN to iN. Investigations into plant oN acquisition and the effects of soil warming on plant productivity are largely restricted to northern high latitudes, leaving southern cold ecosystems such as the subAntarctic underrepresented. Sub-Antarctic soils are typically replete in oN but not iN, however, plant oN uptake has not been accounted for in terrestrial N-budgets. Furthermore, the islands are experiencing high rates of climate change including increasing temperatures. This thesis examines sub-Antarctic plant-soil interactions, investigating whether sub-Antarctic grasses acquire oN and how soil warming affects plant growth and nutrition. Potted and field experiments were run with four common grasses (Polypogon magellanicus, Poa cookii, Agrostis stolonifera, and Poa annua) from sub-Antarctic Marion Island (MI, -46.9, 37.8). I hypothesised that sub-Antarctic grasses acquire oN which affects plant growth relative to iN and that soil warming influences plant growth directly and indirectly through stimulating microbial iN mineralisation and plant nutrition by altering bioavailable oN and iN soil fractions. Grasses supplied with either 15N-enriched oN (glycine) or iN (NO3 - ) provided evidence for oN acquisition in hydroponics. Experimental oN and iN provision (in hydroponics) resulted in higher relative growth rates (RGR) on iN compared to oN, but species-specific differences in biomass allocation under the different N-forms and [N]. Grasses supplied with 15N-glycine in situ resulted in significant 15N enrichment, although intact oN acquisition in situ cannot be determined with the use of only 15N-labelled glycine. Considering the high [oN] in MI soils, this evidence suggests that oN represents an important N-resource for sub-Antarctic vegetation. A five-month warming experiment (MI ambient summer temperatures +3°C) resulted in limited biomass increases, which was only significant (p < 0.05) for P. annua (by 42%), and soil NH4 + increased by 12%. A fertilisation (NPK) treatment resulted in substantially higher plant biomass increases than warming (449% relative to 24%, respectively), suggesting that warming-induced N-release should not be assumed to strongly stimulate plant biomass. Soil warming did not influence plant acquisition of oN or iN. A soil incubation experiment (42 d; 5°C control, +3°C and +6°C warming) showed no effect of warming on soil iN, oN, or PO4 3- , although iN increased and PO4 3- decreased with increasing total organic C (TOC). Microbial biomass (C) increased with soil TOC but was not affected by warming. While microbial P increased with TOC and the +6°C warming treatment, microbial N was unaffected by warming and did not change with TOC. These results highlight the importance of investigating prevailing assumptions on soil-plant processes in cold ecosystems, including outdated assumptions that subAntarctic grasses only acquire iN, and that warming-induced N-release strongly stimulates plant productivity. The evidence presented here suggests that early work on MI underestimated total plant-available N, thus the sub-Antarctic terrestrial N-budget requires re-evaluation. Plant biomass and microbial mineralisation only showed limited or non-significant responses to soil warming, challenging the predictions for large, widespread effects of soil warming on N-release in cold ecosystems. The large differences in plant biomass under NPK relative to warming suggest that plant responses to increased N are limited if other nutrients, e.g., P, do not increase concomitantly. Despite strong temperature controls on cold-ecosystem soil decomposition rates, the stimulatory effects of short-term warming may be curtailed by a combination of interactive plant-soil processes.
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    Testing alternative stable state theory at Afromontane and Milkwood forest-fynbos ecotones
    (2023) Fouche, Bianke; Cramer, Michael; Hoffman Michael
    Alternative stable state (ASS) and ‘sharpening switch' theories provide a conceptual framework within which to understand abrupt ecotones between two (or more) contrasting ecosystem configurations. ASS theory is more suited for explaining ecosystems that exist within the same abiotic conditions. In the Greater Cape Region, forest-fynbos ecotones are often juxtaposed in landscapes where fire is the most important disturbance that maintains open canopy fire-prone fynbos and closed- canopy forests in fire refugia. Forest and fynbos ecotones are, however, additionally maintained by biotic feedbacks that result in edaphic differences between the contrasting ecosystems. Anthropogenic modifications to disturbance regimes alter the stability of forests and fynbos and may shift forest and fynbos into a degraded species-poor state. Conservation practitioners are concerned over perceived loss of forest habitat, and tree planting initiatives are common in parts of the Western Cape of South Africa. I hypothesised that diverse (e.g., Afromontane-and Milkwood-forests) forest and fynbos (e.g., different fynbos types) assemblages are ASS on the same nutrient-poor geology, and that biotic feedbacks with the soil result in edaphic discontinuities across the forest-fynbos ecotone. A sub- hypothesis is that anthropogenically altered disturbance regimes (e.g., timber harvesting, herbivory, fire management) have the potential to keep forest and fynbos in a transient state, so that a regime shift may be more likely. Soil samples (n=5 per vegetation type) were taken, and vegetation plot surveys conducted (n=3 per vegetation type) at seven study sites in Grootbos Private Nature Reserve (GPNR) and Platbos Forest Reserve (PFR) in South Africa. These data were collected in areas identified as forests, transitional vegetation, reforested areas (from conservation management), and the surrounding fynbos matrix. Forests were floristically distinct from fynbos at all study sites (sharing max. = 5% spp. at one site). Most forests had similar soil texture to fynbos however, one site associated with an Afromontane Forest had a much higher silt: sand ratio (ca. 1: 0.9) than the fynbos matrix (ca. 1: 1.6), indicating differences in pedogenic processes in the forest compared to the surrounding fynbos. Forest soils had higher soil nutrient content (N, P, K, Mg, Ca, Fe, and loss-on-ignition) than fynbos soils. The edaphic nutrient differences across ecotones were not due to geological variability, indicating that edaphic differences are largely due to differences in nutrient cycling and vegetation-soil feedback mechanisms. The soil nutrient status of transitional vegetation was variable and not always intermediate between forest and fynbos. Historical imagery revealed that a complex history of disturbance has negatively affected all of the Southern Coastal/Milkwood Forests in the past, but that the forests have recovered in the absence of continued disturbances. Tree cover for most Southern Coastal/Milkwood forests has increased since 1938 (ca. 65% tree cover increase at one of the forests). Afromontane Forests have, however, not changed in extent or tree density since 1938. These findings add to the evidence that soil nutrient differences are sufficient to prevent a regime shift between forest and fynbos. The Southern Coastal/Milkwood Forests can be considered a fire-derived ASS, where edaphic differences reinforce the stability of forest and fynbos boundaries. Afromontane Forest- fynbos ecotones are perhaps better explained by a ‘sharpening switch' model than ASS. Most existing transitional vegetation should not be considered a degraded vegetation state since transitional vegetation is a natural spatial and temporal gradation between fynbos and forest and also a natural buffer for forests against fire. Keywords: alternative stable state, regime shift, bistability, multi-stability, hysteresis, tipping point, resilience, vulnerability, edaphic properties