Palynology and the Ecology of the New Zealand Conifers

Abstract

The New Zealand conifers (20 species of trees and shrubs in the Araucariaceae, Podocarpaceae, and Cupressaceae) are often regarded as ancient Gondwanan elements, but mostly originated much later. Often thought of as tall trees of humid, warm forests, they are present throughout in alpine shrublands, tree lines, bogs, swamps, and in dry, frost-prone regions. The tall conifers rarely form purely coniferous forest and mostly occur as an emergent stratum above evergreen angiosperm trees. During Maori settlement in the thirteenth century, fire-sensitive trees succumbed rapidly, most of the drier forests being lost. As these were also the more conifer-rich forests, ecological research has been skewed toward conifer dynamics of forests wetter and cooler than the pre-human norm. Conifers are well represented in the pollen record and we here we review their late Quaternary history in the light of what is known about their current ecology with the intention of countering this bias. During glacial episodes, all trees were scarce south of c. 40 • S, and extensive conifer-dominant forest was confined to the northern third of the North Island. Drought-and cold-resistant Halocarpus bidwillii and Phyllocladus alpinus formed widespread scrub in the south. During the deglacial, beginning 18,000 years ago, tall conifers underwent explosive spread to dominate the forest biomass throughout. Conifer dominance lessened in favor of angiosperms in the wetter western lowland forests over the Holocene but the dryland eastern forests persisted largely unchanged until settlement. Mid to late Holocene climate change favored the more rapidly growing Nothofagaceae which replaced the previous conifer-angiosperm low forest or shrubland in tree line ecotones and montane areas. The key to this dynamic conifer history appears to be their bimodal ability to withstand stress, and dominate on poor soils and in cool, dry regions but, in wetter, warmer locations, to slowly grow thorough competing broadleaves to occupy an exposed, emergent stratum where their inherent stress resistance ensures little effective angiosperm competition.

Early tropical crop production in marginal subtropical and temperate Polynesia

“…Although this pollen can be dispersed long distances by wind, in these proportions, it was likely locally abundant on Ahuahu. Prehuman forests of similar composition occurred on other offshore islands in northern New Zealand (37–39), with the pollen abundance of other conifers too low to detect local presence (40). Some invertebrate remains were recovered from prehuman sediments on Ahuahu at Waitetoke, and mostly comprised oribatid mites, curculionid weevils, and carabid beetles.…”

Late Quaternary Climate Variability and Change from Aotearoa New Zealand Speleothems: Progress in Age Modelling, Oxygen Isotope Master Record Construction and Proxy-Model Comparisons

“…LGM isotopic signatures for some New Zealand speleothems show anomalously negative δ 18 O and positive δ 13 C values. These signatures align with widespread grass/herb expansion and tall tree/thermophilous plant reduction [78,79] at a time when offshore sea surface temperatures were as much as 5 • C cooler than the Holocene [82]. There is also evidence of near-maximally expanded glaciers on the North and South Islands [83,84] during the LGM, which required a combination of both colder and drier conditions to achieve geometries that best match moraine limits [85].…”

“…The isotopic signatures evaluated in previous New Zealand speleothem studies [47,48] have been compared with Late Quaternary vegetation and marine climate records [72][73][74][75][76][77][78][79][80]. Contemporary ecology studies that support Late Quaternary pollen interpretations establish an intimate link between New Zealand plant ecosystems and multiple climate variables [81].…”

New Zealand forest dynamics: a review of past and present vegetation responses to disturbance, and development of conceptual forest models

“…The use of chronosequences has received criticism where observations are used to infer mechanisms or erroneous temporal linkages are assumed, yet when appropriately applied they can allow valuable insight into vegetation dynamics and soil development over time (Johnson & Miyanishi 2008;Walker et al 2010). Finally, although palaeoecology has much to contribute to the understanding of forest dynamics, the opportunity for increased integration of palaeoecology with forest ecology and conservation science remains (Willis et al 2007;Lindbladh et al 2013;Wilmshurst et al 2015;McGlone et al 2017).…”