John Ogden

An introduction to plant demography with special reference to New Zealand trees

Abstract An outline of the scope and origins of plant demography emphasises the link between numerical and evolutionary approaches. The rules governing thinning in monocultures and some general features of plant competition in mixtures are described. Plant demography can be applied at several levels — that of the genetic individuals or that of the modular components of which plants are usually composed. This dual population structure puts a new emphasis on the morphological changes occurring in competing mixtures. Various approaches to the definition of strategies are discussed. Plants with short life cycles and abundant production of small seeds are contrasted with long-lived species producing fewer larger seeds. This ‘r-K’ continuum is used as a broad framework, although Grimes (1979) two dimensional strategy model may be more realistic. The concept of the ‘regeneration gap’ has been a persistent theme in forest ecology in New Zealand, and its explanation has generally been presumed to lie in past clim...

Movements, Foraging Groups, and Diurnal Migratons of the Striped Parrotfish Scarus Croicensis Bloch (Scaridae)

A total of 374 striped parrotfish (Scarus croicensis) were tagged from the reefs surroundihg Isla Pico Feo on the Caribbean coast of Panama. Many of these fish were followed individually in the field for up to 3 months. Three different behavioral categories were recognized: stationary, territorial, and foraging. Fish tend to aggregate in foraging groups in a particular pattern relative to reef structure and have a predictable set of associated species. Transfer experiments showed that striped parrotfish have strong ties to a home reef. Striped parrotfish migrate diurnally from shallow—water feeding areas to deeper nocturnal resting areas along constant pathways. Direct counts of fish on migration pathways provided information on the structure and size of the striped parrotfish population at Pico Feo. The numbers of male striped parrotfish may regulate the phenomenon of sex reversal in the scarids. See full-text article at JSTOR

Climatic Influences on the Growth of Subalpine Trees in the Colorado Front Range

We examined variations in tree growth responses to climatic variations among different tree species and habitat types in the subalpine zone of the Colorado Front Range. We constructed 25 tree ring site chronologies (11 of Picea engelmannii, 9 of Abies lasiocarpa, 4 of Pinus contorta var. tatifolia, and 1 of Pinusflexilis) from a series of subalpine habitats ranging from xeric to wet. To establish tree growth responses to climatic variation, we used correlation and response function analyses to compare variations in ring widths with monthly temperature and precipitation records. At the driest sites, growth of Picea and Abies tracked climatic variation similarly. At mesic and wet sites, however, these species differed in their responses to climatic variation. The responses of Pinus contorta, sampled over a narrower range of habitat types, differed from those of Picea and Abies but did not differ among sites. Steep environmental gradients in the subalpine zone of the Front Range accounted for most of the observed differences in growth responses to climatic variation. Even at adjacent sites that differ only slightly in topographic position, tree growth responses to climatic variation were distinct. Interspecific differences in response to climatic variations generally were less important than site differences. Intersite differences in tree growth responses to climatic variation can be used as indicators of environmental differ- ences among subalpine habitats.

Dendroecological studies in New Zealand 1. An evaluation of tree age estimates based on increment cores

Abstract Tree-ring counts from increment cores are widely used in ecological studies for determining tree ages. In New Zealand many canopy trees are slow-growing and long-lived, and have extremely narrow rings. Single rings or groups of rings may be absent on some radii. Such narrow and absent rings cause difficulties in ring counting and necessitate careful sample preparation. The errors associated with age estimates derived from cores which do not reach the chronological centre of the tree (partial cores) are discussed. Four partial core lengths and three methods of estimation were used on cross-sections of known age from Agathis australis, Libocedrus bidwillu, Nothofagus solandri, and Prumnopitys taxifolia. It is concluded that mean errors may be less than ± 10% where the core length represents 90% of the geometric radius (half the measured diameter) increase with shorter cores. However, much greater errors may apply to individual estimates, up to ± 78% in one case. Moreover, the direction of the error...

14C calibration in the southern hemisphere and the date of the last taupo eruption : Evidence from tree-ring sequences

Tree rings from a section of Prumnopitys taxifolia (matai) covering the period AD 1335-1745 have been radiocarbon dated and used to generate a (super 14) C calibration curve for southern hemisphere wood. Comparison of this curve with calibration data for northern hemisphere wood does not show a systematic difference between (super 14) C ages measured in the northern and southern hemispheres. A floating chronology covering 270 yr and terminating at the last Taupo (New Zealand) eruption, derived from a sequence of 10-yr samples of tree rings from Phyllocladus trichomanoides (celery pine, or tanekaha), is also consistent with the absence of a systematic north-south difference, and together with the matai data, fixes the date of the Taupo eruption at AD 232+ or -15.

Dendrochronological studies and the deternination of tree ages in the Australian tropics

In temperate regions the ages of trees can be measured by counting the annual rings revealed on an increment core. The central principle of dendro- chronology - crossdating - simply ensures greater accuracy. In the tropics and sub-tropics growth periodicity may not be clearly limited to a particular season, so that it is necessary first to ascertain whether any anatomical structures delimit annual periods. This problem is discussed with particular reference to the Austra- lian tropics. The climate and forest vegetation of tropical Australia are described. Studies on Eucalyptus and Callitris in open woodlands in the monsoon climates of northern Australia indicate that trunk diameter growth is confined to the wet season. The resulting growth bands, although predominantly annual, are difficult to count, but the trees are not long-lived. However, trees 1000 years old, and possibly considerably older, do occur in the tropical rain forests of eastern Australia. A variety of techniques for measuring age is discussed and it is con- cluded that in the case of Araucaria, and probably some other conifers, the growth bands are approximately annual. Discrepancies between radiocarbon dates and ages derived by other techniques do occur however, and indicate a need for further research. Dendrochronological studies on Pisonia grandis are described in detail to illustrate how ring-width measurements and correlation with climatic factors can sometimes be used to verify the annual nature of the growth rings.

Environmental change during the last glacial maximum (c. 25 000-c. 16 500 years BP) at Mt Richmond, Auckland Isthmus, New Zealand

Abstract A 2 m section at the base of Mt Richmond contains a palynological record of the last glacial maximum (LGM) (c. 25 000‐c. 16 500 14C years BP) vegetation of the Auckland Isthmus. Three silicic tephra layers derived from the Taupo Volcanic Centre (Okaia Tephra c. 23 500 14C years BP, Kawakawa Tephra c. 22 500 14C years BP) and the Okataina Volcanic Centre (Okareka Tephra c. 18 000 14C years BP), both centres lying within the Taupo Volcanic Zone, provide the basis of the chronology supported by radiocarbon dates. The pollen diagram is divided into two pollen zones separated by the deposition of a locally derived basaltic ash. From c. 25 000–23 000 14C years BP the site was initially a eutrophic lake fringed by Leptospermum and Typha, which altered to a Cyperaceae/Leptospermum‐dominated swamp. The regional vegetation at the time was beech‐dominated forest; canopy conifers were present but formed a minor part of the local forest. From c. 23 000–16 500 14C years BP regional forest was further restricted to local patches in extensive shrubland/grassland. Temperatures may have been depressed by more than 4–5°C. The eruption of a local volcano dammed the swamp outlet resulting in a return to lacustrine conditions. Local volcanism may have accelerated vegetation change already under way as a result of climate change to cooler conditions. This record provides a view of LGM vegetation of the Auckland Isthmus and a template for earlier cold stages of the Quaternary.

Population dynamics of the emergent conifer Agathis australis (D. Don) Lindl. (kauri) in New Zealand I. Population structures and tree growth rates in mature stands

Abstract Twenty five plots of mature kauri Agathis australis (D. Don) Lindl., covering the range of the species in northern New Zealand, were sampled for density, basal area, and species composition using a modified point-centered quarter technique. Two increment cores were taken from at least ten trees at most sites, and used to estimate tree ages and growth rates. The density of kauri stems ≥ 10 cm d.b.h. ranged from 17 to 416 ha-1, and the basal area from 23 to 127 m2 ha-1 in the 25 stands. Diameter distributions ranged from highly skewed and unimodal to flat and multimodal, with all size classes represented in most plots. Combined frequency distributions suggest that two or three kauri generations (cohorts) may be present on many sites. There is only a weak relationship between age and diameter; individuals in the same 10 cm diameter class may vary in age by 300 years, and the largest individual on the site is often not the oldest. Mean annual diameter increments range from 0.15 to 0.46 cm yr-1 on dif...

Dynamics of forests with Araucariaceae in the western Pacific

Several species of Araucaria and Agathis (Araucariaceae) occur as canopy emergents in rain forests of the western pacific region, often representing major components of total stand biomass. New data from permanent forest plots (and other published work) for three species (Araucaria hunsteinii from New Guinea, A. laubenfelsii from New Caledonia, and Agathis australis from New Zealand) are used to test the validity of the temporal stand replacement model proposed by Ogden (1985) and Ogden and Stewart (1995) to explain the structural and compositional properties of New Zealand rain forests containing the conifer Agathis australis. Here we propose the model as a general one which explains the stand dynamics of rain forests with Araucariaceae across a range of sites and species in the western Pacific. Forest stands representing putative stages in the model were examined for changes through time in species recruitment, growth and survivorship, and stand richness, density and basal area. Support for the model was found on the basis of: 1. Evidence for a phase of massive conifer recruitment following landscape-scale disturbances (e.g. by fire at the Huapai site, New Zealand for Agathis australis); 2. Increasing species richness of angiosperm trees in the pole stage of forest stand development (i.e. as the initial cohort of conifers reach tree size; > 10 cm DBH); 3. A high turnover rate for angiosperms ( > 100 yr) in the pole stage, but similar turnover rates for both components (50-100 yr) as forests enter the mature to senescent phase for the initial conifer cohort; 4. Very low rates of recruitment for conifers within mature stands, and projected forest compositions which show increasing dominance by angiosperm tree species; 5. A low probability of conifer recruitment in large canopy gaps created by conifer tree falls during the initial cohort senescent phase, which could produce a second generation low density stand in the absence of landscape scale disturbance; 6. Evidence that each of the three species examined required open canopy conditions (canopy openness > 10%) for successful recruitment. The evidence presented here supports the temporal stand replacement model, but more long-term supporting data are needed, especially for the phase immediately following landscape level disturbance.

A 28 000–7600 cal yr BP pollen record of vegetation and climate change from Pukaki Crater, northern New Zealand

A pollen record of vegetation and climate change representing the interval ca. 28 000–7600 cal yr BP was obtained from a 52.5-m sediment core taken from a volcanic explosion crater at present sea level in Auckland, New Zealand (36°59′S). This provides one of the few continuous terrestrial records covering Marine Isotope Stage 2 and part of Stage 1 from the mid-latitudes of the Southern Hemisphere. Chronology of the record is underpinned by eight distal Taupo Volcanic Zone rhyolitic tephra of known age augmented by thirteen AMS radiocarbon dates. The Last Glacial Maximum cooling extended from at least c. 28 000 cal yr BP to ca. 18 000 cal yr BP when vegetation apparently consisted of patches of beech-dominated forest embedded within shrubland and grassland. Forest patches were probably larger or more widespread than those in central North Island to the south but less extensive than further to the north, in Northland. Climate warming commenced ca. 18 000 cal yr BP, marked by the replacement of beech by conifers and angiosperms, and by the expansion of forest at the expense of shrubland and grassland. No clear Late Glacial climatic reversal is evident at this site. Early Holocene forest is characterised by the replacement of the dominant forest tree Prumnopitys taxifolia by Dacrydium cupressinum. Following breach of the crater rim ca. 7600 cal yr BP by rising sea level the maar was rapidly infilled with marine sediments over a period of ca. 1000 years.