The hare, the tortoise and the crocodile: the ecology of angiosperm dominance, conifer persistence and fern filtering

Abstract

Summary1 Angiosperm trees often dominate forests growing in resource-rich habitats, whereas conifers are generally restricted to less productive habitats. It has been suggested that conifers may be displaced by angiosperms except where competition is less intense, because conifer seedlings are inherently slow growing, and are outpaced by faster-growing angiosperm species. Here we investigate whether competition with ferns and deeply shading trees also contributes to a failure of conifers to regenerate in resource-rich habitats. 2 We examined how changes in soil nutrient availability and drainage affected vegetation along the retrogressive stages of a soil chronosequence in southern New Zealand. Vegetation composition shifted from angiosperm-tree dominance on 'recent' alluvial terraces (< 24 ky), via coniferous-tree dominance on older marine terraces (79-121 ky), to coniferous-shrub dominance on the oldest marine terraces (291 ky). Soil drainage deteriorated along the sequence, and N : P leaves and N : P soil indicate increasing Plimitation. Conifer species appear to be adapted to persistence on infertile and poorly drained soils. 3 The floor of the relatively fertile alluvial forests was deeply shaded ( ∼ 1% light transmission) by dense groves of tree-ferns and ground-ferns, and by large-leaved subcanopy trees. Few seedlings of any type were found on the forest floor, even in tree-fall gaps, and establishment was restricted to rotting logs and tree-fern trunks. Angiosperms were particularly successful at colonizing these raised surfaces. 4 Less shade was cast by the conifer-dominated forests on infertile marine terraces ( ∼ 5% light transmission), which lacked tall ferns. There were many opportunities for conifer establishment, with high seedling densities recorded on the forest floor and on logs. By contrast, angiosperm seedlings were mainly restricted to logs. 5 Our results suggest that several mechanisms act in concert to reduce regeneration opportunities for conifers in productive habitats. In particular, we suggest that tall ferns and deep shade are responsible for a restriction of regeneration opportunities in relatively productive forests in New Zealand, diminishing the opportunities for conifers to escape the competitive effects of fast-growing angiosperms. Thus 'crocodiles' may alter the outcome of the race between 'hares' and 'tortoises'.

Efecto de la tala de Podocarpus glomeratus (Podocarpaceae) sobre la estructura de un bosque de neblina en los Andes (Cochabamba, Bolivia)

“…Los cambios ocasionados se relacionan con la formación de claros de dosel y la perdida progresiva de estratos de dosel de P. glomeratus (estrato superior) y W. microphylla (estrato medio), ocasionando mayores niveles de iluminación a medida que avanza el disturbio. La perdida de estratos de dosel diversificados conllevan a formas doseles monoespecíficos, que en muchos otros estudios ha sido el principal factor para mejorar la regeneración de especies pioneras (principalmente coníferas) (Pinazo et al 2003, Vaca 2003, Soto y Figueroa 2008; por otra parte, bosques maduros compuestos por doseles multiestrato presentan poca regeneración de especies pioneras (Donoso y Nyland 2005, Coomes et al 2005). …”

“…En el hemisferio sur se observaron muchos bosques maduros con especies de coníferas que tienen poca regeneración natural debajo de sus doseles, deduciendo que son intolerantes a la sombra debido a que la mayoría mejoran su regeneración cuando reciben mejor iluminación por disturbios naturales ó antropogénicos (Wardlh 1963, Bergin 2000, Coomes et al 2005, Soto y Figueroa 2008. Las respuestas a estos disturbios por especies de la familia Podocarpaceae muestran diferentes estrategias de regeneración, las mismas se explican a través de tres hipótesis: a) la regeneración por "claros" indica que éstas especies mejoran su regeneración cuando se forman claros por la caída de individuos o grupos de árboles maduros dentro del bosque debido a disturbios naturales u otros (p.e.…”

Influência Das Condições Ambientais No Crescimento De Dois Grupos Arbóreos Em Floresta Com Araucária

“…Para Coomes et al (2005), as coníferas normalmente são restritas aos sítios menos produtivos, adaptadas aos solos pouco férteis ou mal drenados, os quais limitam o desenvolvimento das espécies folhosas (DUTRA; STRANZ, 2003). Essas características foram observadas na UE I, sítio com muitas espécies higrófilas e com baixa profundidade, que, embora não tenha diferido estatisticamente das demais UEs, manteve as menores médias para as alturas.…”

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

“…However, there is surprisingly sparse coverage of the dynamics of such communities in the literature. One notable exception is the demonstration by Coomes et al (2005) that dense tree ferns and ground ferns can affect forest composition by influencing the regeneration success of other taxa, resulting in an advantage for angiosperm canopy tree species over their coniferous counterparts. In addition, recent work by Brock et al (2018) demonstrates that, in northern NZ forests, C. medullarisdominated early successions are associated with trajectories towards podocarp-broadleaved forests, while those dominated by Kunzea robusta are associated with trajectories towards Agathis australis forest; although there may also be potential effects of edaphic conditions and initiating disturbance that are difficult to disentangle.…”