Alex Mosseler

Old-growth red spruce forests as reservoirs of genetic diversity and reproductive fitness

Abstract. Old-growth forests are assumed to be potential reservoirs of genetic diversity for the dominant tree species, yet there is little empirical evidence for this assumption. Our aim was to characterize the relationship of stand traits, such as age, height and stem diameter, with the genetic and reproductive status of old-growth and older second-growth stands of red spruce (Picea rubens Sarg.) in eastern Canada. We found strong relationships between height growth (a fitness trait) and measures of genetic diversity based on allozyme analyses in red spruce. The negative relationship between height and the proportion of rare alleles suggests that high proportions of these rare alleles may be deleterious to growth performance. Latent genetic potential, however, showed a significant and positive relationship with height. Stand age was not correlated to height, but was correlated to seedling progeny height. In late-successional species such as red spruce, age and size (e.g., height and stem diameter) relationships may be strongly influenced by local stand disturbance dynamics that determine availability of light, growing space, moisture and nutrients. In larger and older stands, age appeared to provide a good surrogate measure or indicator for genetic diversity and progeny height growth. However, in smaller and more isolated populations, these age and fitness relationships may be strongly influenced by the effects of inbreeding and genetic drift. Therefore, older populations or old-growth forests may represent superior seed sources, but only if they are also of sufficient size and structure (e.g., stem density and spatial family structure) to avoid the effects of inbreeding and genetic drift. Thus, larger and older forests appear to have an important evolutionary role as reservoirs of both genetic diversity and reproductive fitness. Given the rapid environmental changes anticipated (as a result of climate change, increasing population isolation through fragmentation, or following the introduction of exotic pests and diseases) these older populations of trees may have a valuable function in maintaining the adaptive potential of tree species.

Reproductive and genetic characteristics of rare, disjunct pitch pine populations at the northern limits of its range in Canada

Pitch pine, Pinus rigida Mill., is a rare species in Canada, existing as a disjunct population in the St. Lawrence River Valley in eastern Ontario and two northern outlier stands in southern Quebec along Canadas southern border with the United States. Reproductive and genetic characteristics of these small, scattered stands were investigated to develop a foundation for management and restoration in the event of range expansion northwards under anticipated climate warming. Seed yields and seed quality appear to be comparable to other eastern conifers, and to pitch pine at the center of its geographic range. For seed and seedling growth traits, most of the variation was attributable to differences among trees within stands and, to a lesser extent, among stands within a population; whereas the population effect was non-significant. For reproductive traits, such as numbers of filled and empty seeds per cone, reproductive efficiency, and inbreeding estimates, high levels of variation (ranging from 26% to 33%) were found among stands, suggesting that stand structural features, such as stand size and tree density within stands, play an important role in pollination environment and overall reproductive success. Estimates of genetic diversity at 32 allozyme gene loci indicate that these small, isolated stands have maintained relatively high levels of genetic diversity compared with populations at the center of its geographic range, and also relative to other widely dispersed eastern conifers. The relatively high levels of viable seed production and genetic diversity in native pitch pine populations indicate that native Canadian populations may be suitable seed sources for species restoration and range expansion in Canada.

Comparative nutrient economy, stable isotopes, and related adaptive traits in Picea rubens, Picea mariana, and their hybrids

Nutrient- and water economy-related traits in plants have significant implications for growth and fitness. We explored, examined, and compared nutrient concentrations, use efficiencies, assimilation, and informative isotopic elements in a seedling provenance experiment, and in seedling and mature tree controlled-cross hybrid experiments of red spruce (RS) (Picea rubens Sarg.) and black spruce (BS) (P. mariana (Mill.) BSP). Provenance experiment results showed RS had consistently lower carbon (C) and nitrogen (N) concentrations, and N assimilation ratio (NAR), but higher N-use efficiency (NUE), C:N ratio, water-use efficiency (WUE), and needle calcium (Ca) and magnesium (Mg) concentrations than BS. The hybrid seedling experiment showed similar results and additive inheritance for needle N, C:N ratio, NAR, Ca, and Mg, evident by a near-linear progression from one species to the other. Within both species, seedling height showed a negative relationship with needle N and a positive relationship with NUE. However, across hybrid indices, seedling height showed a positive relationship with needle N and a negative relationship with NUE. Also across hybrid indices, seedling height showed a negative relationship to Ca and C:N, and a positive relationship with NAR and 13C discrimination (without hybrid 25). Mature tree hybrid experiment results were similar to those of the seedling experiment, but with a dampening of differences caused by low nutrient availability and possibly age effects. The similarity was not true for 13C discrimination as mature tree height showed a strong negative relationship to 13C discrimination, indicating that BS had greater WUE. The reversal is most probably caused by the large difference in water availability.

Accelerating the selection process for Populus and Salix clones using short-term photosynthetic acclimation responses under greenhouse conditions

ABSTRACT Leaf photosynthetic characteristics could be determinant factors to identify the most productive clones of hybrid poplars (Populus spp.) and willows (Salix spp.). Photosynthetic acclimation of hybrid poplars and willows was studied under greenhouse conditions. Seven Populus and five Salix clones were grown for 3 months at three spacings [20 × 20, 35 × 35, and 60 × 60 cm] and two nitrogen (N) levels (20 and 200 µg g-1). There were no significant spacing effects on leafless aboveground biomass per tree (AGBT) and height. Clonal acclimation to higher density was associated to increases in leaf area index (LAI) by 347% and specific leaf area (SLA) by 13% despite decreased leaf N content per unit leaf area (Narea) by 31%. There were no changes in net CO2 assimilation rate (A) and photosynthetic N-use efficiency in the ambient light condition (PNUEamb) within different spacings. The N addition alleviated competition effects by maximizing leaf area (LA) and SLA. Compared with less productive clones, more productive clones had 28% greater SLA, greater LA and AGBT per unit of increase in Narea over all treatments. The increased development of LA and SLA under high planting density is a key indicator of more productive clones.