Michael E. Day

The adaptive physiology of Metasequoia to eocene high-latitude environments

Publisher Summary This chapter explores the physiological basis for Metasequoias success in the Eocene high-Arctic and the ecophysiological attributes that imparted adaptive value to trees inhabiting this unique environment (Eocene high-Arctic). The environment that has been advanced for Eocene high-latitude forests provided a unique combination of light, temperature, and water regimes that offers an unparalleled set of adaptive challenges to plant species. The temperate continuous light (CL) regime offered both substantial physiological challenges and the potential benefit of high growth rates for species able to overcome those physiological hurdles. Among the unusual aspects of Metasequoia is a collection of characteristics that do not fit contemporary models of shade-adaptation or sun-adaptation. Metasequoia possesses an aggregation of characteristics not generally associated with either adaptive strategy, but competitively adaptive at high latitudes. While the photosynthetic rates of Metasequoia under light intensities typical of the growing season at middle latitudes are clearly much lower than those of sun-adapted conifers, its resource allocation to photosynthetic systems (leaf-level) is close to optimum for the moderate light intensities of the Arctic lowland forests, or cloudier lower palaeolatitudes. Metasequoia could rapidly produce canopies with extensive leaf area, not only efficiently capturing incident light for photosynthesis, but minimizing the transmitted light that would be available for competitors. Production of low-density stem wood permitted Metasequoia rapidly to overtop potential competitors that were establishing concurrently. The importance of photorespiration and alternate pathways for photoenergetics in Metasequoia and other species growing under CL regimes are yet to be explored.

Is early life cycle success a determinant of the abundance of red spruce and balsam fir

Red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea (L.) Mill.) are sympatric in much of the Acadian Forest, but their relative regeneration success during the changing climates of the H...

Separating the effects of tree size and meristem maturation on shoot development of grafted scions of red spruce (Picea rubens Sarg.)

In order to separate the effects of size and meristem maturation on age-related changes in shoot growth behaviour, a reciprocal grafting experiment was conducted involving juvenile (J), mid-age (MA) and old-growth (OG) red spruce (Picea rubens Sarg.) trees as both scion donors and rootstock. The effects of rootstock and scion age on vegetative growth, foliar morphology and reproductive development were assessed over 7 years after grafting. Vegetative growth potential declines with meristem maturation, but the high growth potential of J meristems on J rootstock cannot be expressed when J scions are grafted on MA and OG rootstock. Branch density decreases with meristem maturation. The tendency for high J branch density is expressed across all rootstock ages, but is minimally expressed on MA rootstock where elongation of terminal shoots is significantly greater than on OG rootstock. Both needle width and leaf mass area increase with meristem maturation and increasing tree size. Reproductive competence is mainly a function of meristem maturity, and rootstock had little effect on strobilus development, although the more fecund rootstocks did promote some flowering on J scions. Developmental decline in height growth does not appear to be a function of decreased meristem vigour, but reduced terminal long shoot elongation and decreased apical control in OG trees will reduce height growth.

Regulation of Ontogeny in Temperate Conifers

Through their life stages, long-lived forest trees must adapt to challenges resulting from vast changes in size and external environment. Trees accomplish this by producing new tissues and growth habits adapted to specific life stages by pluripotent meristems. The morphology and physiology of these new tissues are determined by complex interactions of the external and internal environments of the tree, and gene expression both within meristems and during differentiation of cells produced by meristems. The trajectories of various age-related changes are both inter-related and independent at various scales, and, for the majority of species, are not purely physio-mechanical responses to increased tree height. Understanding the relationships between tree developmental state, tree size and its environment requires both a whole-tree approach integrated through time to encompass life stage strategies, and a molecular approach to understand the cues, transduction pathways and epigenetic mechanisms that regulate whole-life ontogeny.

Regulation of foliar plasticity in conifers: developmental and environmental factors.

Foliar plasticity is widespread among woody plants, and historically most observations have been made in angiosperms. This review concentrates on examples from gymnosperms, particularly the Pinaceae, since there have been a number of recent studies on coniferous forest species. Foliar plasticity can be defined as variation in the morphology and physiological function of foliage produced over time and space within a single individual. For example, leaves produced in the sun have a higher leaf mass area (LMA) than leaves produced in the shade, and differ in their photosynthetic capacities. Sun and shade leaves are examples of heterophylly, or variation in leaf morphology in response to environmental variation in the immediate space surrounding the apical meristem producing the leaf primordia. The LMA of coniferous foliage also tends to increase with age, independently of the external environment, and this variation, called heteroblasty, appears to be a result of maturation of the apical meristem, which occurs over time. The regulation of variation due to heterophylly and heteroblasty appears to be very different. Heterophylly in response to light appears to vary linearly with available light. More massive sun foliage produced by a given apical meristem may be a response to available Photosynthetically Active Radiation (PAR) and its effects on net photosynthesis, or may be a photomorphogenetic response to the ratio of red to far red light. In contrast, heteroblastic variation can occur independently of available light, and may be the result of the developmental age of the apical meristem. It appears to vary curvilinearly with age, with the most rapid change occurring during the earliest life stages of the plant. Heteroblastic variation appears to be less plastic than heterophyllic, since grafted scions from mature or juvenile donors retain LMA characteristics of donor whether grafted onto juvenile or mature rootstocks.

Seedling ontogeny and environmental plasticity in two co-occurring shade-tolerant conifers and implications for environment–population interactions

PREMISE OF THE STUDY Seedling success is determined by evolved strategies of intrinsic genetic programming and plasticity that are regulated by extrinsic pathways. We tested the relative importance of these mechanisms in red spruce (Picea rubens Sarg.) and balsam fir (Abies balsamea Lin.), which share understory regeneration niches in northeastern North America. Although its reproductive effort is adequate, spruce has decreased in abundance, in relation to fir, in seedling and sapling populations, even in forests that have a predominance of spruce in the overstory. METHODS To understand the factors that regulate this phenomenon and their implications for tree populations, we compared intrinsic and plastic regulation of first- and second-year seedlings under steady understory irradiance levels and in response to increases in light environment. KEY RESULTS Both species exhibited interactions of ontogenetic patterns and plasticity in first- and second-year seedlings. Physiologically, spruce had higher photosynthetic capacity, allocation to photoprotective xanthophylls, and greater plasticity in response to light treatments. Although both species demonstrated an inability to plastically increase photosynthetic capacity in the short term, spruce benefited from greater allocation to foliage under increased irradiance. Fir showed a conservative strategy in root-shoot allocation that may better equip seedlings to withstand drought adaptations and attributes associated with greater shade tolerance. CONCLUSIONS These attributes likely contribute to the relative success of fir seedlings in the current climate. By contrast, they indicate that spruce would be a superior competitor in cooler, moister climates, which suggests that future forest composition will be largely determined by an interaction of disturbance and moisture regimes.