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It is #GreatSupervisor Week! I'd like to give a shout out to my supervisor, Dr. Rob Guy from @ubcforestry. He is very knowledgeable in tree physiology and also has so much integrity. My MSc and PhD experience has been greatly enhanced by his teachings and humanity. #RoleModel
I feel very fortunate to have Dr. Rob Guy from @ubcforestry as my #GreatSupervisor for both my MSc and my PhD. It has been 7 amazing years of wise support and academic mentorship from him. He is a role model to me @UBCGradSchool
Understanding how tropical forest structure and function change during the decades after logging is a key research challenge. This thesis reports functional traits, forest structure, biomass, net primary productivity (NPP) and allocation, as well as their controlling factors in an old-growth forest and a 54-year-old logged-over forest in Ghana. By analyzing root traits, I found fine-root biomass, root length, surface area, and root tissue density were higher in the logged-over forest, whereas the old-growth forest had higher specific root length and specific root area. I also found divergent exploitation strategies between the two forests; plants in the old-growth forest produced thinner roots, which increase resource uptake efficiency, while plants in the logged-over forest had thicker roots, which are associated with greater resource conservation. Through correlation analysis, I found that fine-root mass correlated positively to relative humidity, while absorbed photosynthetically active radiation and fine-root biomass were also positively correlated. Fine-root mass and soil K were also positively correlated, and fine-root necromass correlated positively with soil P. I then explored the relationships between leaf traits, taxonomic (e.g., species richness) or structural (e.g., tree diameter) variables and aboveground biomass (AGB) or coarse wood productivity (CWP) in the two forests. Leaf K related positively to tree biomass in the logged-over forest. Leaf N and P were significantly and positively related to tree productivity in the old-growth forest and logged-over forest. AGB and CWP were mostly explained by the structural variables. The shape and magnitude of the relationships between tree species richness and AGB or CWP differed between the two forests. In addition, I found that leaf area index, mean tree diameter and height were similar between the two forests, but stand density and basal area were higher in the logged-over forest than in the old-growth forest. Total biomass and annual NPP were comparable in both forests, but there was a shift in NPP allocation between wood and fine roots. I conclude that the forest structure, biomass and productivity of the logged-over forest have largely recovered, but the legacy of logging still persists, which is reflected in differences in functional traits and allocation patterns.
There is dramatic provenance level variation in tree species with geographically wide ranges. For example, in Populus trichocarpa Torr. & Gray, net photosynthesis (An) and stomatal conductance (gs) both increase with latitude of origin. This thesis reports a parallel cline in mesophyll conductance (gm) and explores its physiological basis. In addition to anatomical constraints, variation in gm should depend on chloroplast positioning, transmembrane CO₂ diffusion through aquaporins (AQPs), and biochemical facilitation of the CO₂↔HCO₃− equilibrium by carbonic anhydrase (CA), but evidence for the former has been lacking. I found that gm increases with latitude across 12 genotypes, as measured by chlorophyll fluorescence, and confirmed this pattern by the isotope discrimination method in six representative genotypes. Northern genotypes had greater CA activity. An inhibitor of CA, acetazolamide, reduced CA activity, gm, gs, chloroplast CO₂ concentration and An at normal CO₂ (400 µmol mol-¹), the latter being reversible at saturating CO₂. The relationship between CA activity and gm was similar whether the variation was inherent or inhibitor-induced. I then explored the role of chloroplast positioning in relation to gm, driven by the ratio of blue (BL) to red light supplied to leaves. Repositioning was manifested by a reversible decrease in chlorophyll content index (CCI), while actual chlorophyll content remained unchanged. Although gm was found to decrease as BL increased, and more so in northern genotypes, cytochalasin D, an inhibitor of chloroplast motility, blocked the effect of BL on CCI but not gm, suggesting that BL can mediate gm independently of repositioning. High BL reduced CA activity, consistent with a possible reduction in protein-facilitated diffusion, which might also involve AQPs. I found that the AQP inhibitor mercuric chloride reduces gm more in northern genotypes than in southern genotypes, both absolutely and proportionally, but also reduces CA activity. Although greater gm in high-latitude genotypes likely reflects contributions from several components of the liquid-phase diffusion pathway, this thesis draws particular attention to a major role for CA. Because gm is an equal or greater limitation on photosynthesis than gs, these findings may help direct crop improvement efforts to promote resource use efficiencies and yield.
Populus species or their hybrids are being increasingly used as feedstock for the growing bioenergy industry. To optimize carbon uptake efficiencies, fertilizer inputs must be minimized while, at the same time, achieving rapid growth. The complexity of nitrogen uptake and assimilation in plants and environmental heterogeneity force simplifications that limit improvements in nitrogen-use efficiency. In particular, a lack of integration limits the applicability of many traditional nitrogen-use assays to whole plant nitrogen use. Net nitrogen isotope discrimination has potential to act as a time-integrated process indicator of nitrogen-use in plants grown under steady-state conditions. The objective of this thesis was to further develop an isotope discrimination-based integrated measure of nitrogen-use that reflects whole plant and organ level nitrogen use. Observed differences in nitrogen isotope discrimination were proposed to be a function of nitrogen isotope discrimination of the assimilatory enzymes, fluxes across the root plasma membrane and translocation of inorganic nitrogen to the shoot. As a test, nitrogen supply and demand was environmentally varied in black cottonwood (Populus trichocarpa (Torr & Gray)) and genetically manipulated with knockout lines of Arabidopsis (Arabidopsis thaliana L.). Changes in isotopic composition (δ¹⁵N) were interpretable within the context of the proposed model. Furthermore, efflux/influx (E/I) across the root plasma membrane calculated from the isotope mass balance approach was positively correlated with E/I measured using an established ¹⁵N compartmental analysis approach indicating that the isotope mass balance approach produced a reliable measure of E/I. The isotope mass balance approach was then used to determine intraspecific variability in balsam poplar (Populus balsamifera L.), a species used for hybrid poplar breeding. Nitrogen use traits were calculated for 25 genotypes from five climatically dispersed provenances grown hydroponically under steady-state nitrogen conditions with either ammonium or nitrate. Genotypic variation exceeded provenance level variation in most cases and significant variation was observed in growth, nitrogen isotope composition and calculated nitrogen-use traits indicating that there is potential for breeding for nitrogen-use using balsam poplar. The isotope mass balance modelpresented here provides a new approach for probing integrated nitrogen-use traits in plants, which are often technically difficult to measure.
No abstract available.
The widespread species Populus balsamifera L. exhibits large intra-specific variation in photosynthetic rates and phenology. Northern populations have a tendency towards higher photosynthetic carbon assimilation rates (An) than trees from the south. However, because bud set occurs earlier in northern trees they accomplish far less height growth than do southern trees. Assuming that there are no physiological constraints to combining high An and long growing season, the progeny of intra-specific crosses between northern and southern populations may accomplish more growth in one growing season than their parents (i.e., heterosis). High performing F₁s could be used for a variety of agroforestry projects. Full reciprocal crosses were conducted between individuals from two northern (N) and two southern (S) populations found at the extremes of the P. balsamifera range. Representative selections of progeny and parental material were planted in a greenhouse and in the field and characters including shoot elongation rate, photosynthetic rates, stomatal conductance (gs), water-use efficiency (WUE) leaf mass per area (LMA), internal conductance (gm) in a greenhouse and phenology in the field were evaluated. Although the F₁ families did not display any evidence of heterosis, photosynthetic rates and phenology were uncorrelated in the NxS families, suggesting an uncoupling of traits. Additionally, a number of individuals which possessed a combination of high growth potential and late growth cessation (bud set) were observed and could be useful for a variety of potential deployment areas. The high An in northern populations has been partially attributed to gm, and appears to be a consequence of leaves with greater LMA, which have an increased mesophyll surface area available for carbon uptake. WUE was also correlated with LMA, suggesting that in facilitating CO₂ diffusion for carbon assimilation an increase in gm over an increase in gs provides a clear advantage in not promoting further water loss.
No abstract available.