From trees to stands: production ecology, growth dominance and carbon partitioning
Date
2021
Authors
Fernandez Tschieder, Ezequiel, author
Binkley, Daniel E., advisor
Ryan, Michael M., committee member
Hobbs, Nicholas T., committee member
Bauerle, William L., committee member
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Abstract
Growth of a stand is the sum of the growth of individual trees, and it can be distributed among trees proportional to their size or a group of trees may produce a disproportional share of the stand's growth. Large trees within a stand usually have higher growth rates than smaller trees. The production ecology of trees shows that this is the result of large trees' greater resource acquisition, and greater efficiency of wood production per unit of resource used. However, the fact that large trees grow faster than small trees does not necessarily imply that these trees produce a disproportional share of the stand growth. The distribution of a stand's growth among trees is influenced by how trees compete for resources (symmetric or asymmetric competition) and by the efficiency with which trees used those resources to grow. This dissertation had two main questions: (1) how growth distribution relates to patterns of competition and patterns of resource use efficiency with tree size (Chapter I, II and III), and (2) why large trees have greater resource use efficiency for wood production than small trees within a stand (Chapter IV). In the first chapter, I proposed a specific connection between production ecology of trees and growth dominance patterns. Growth dominance is a measure of how the growth of a stand is distributed among trees. It can be negative or positive whether small or large trees account for a greater proportion of stand growth than its contribution to stand biomass, or null if all trees contribute a similar proportion to the growth and biomass of a stand (Figure 1). Specifically, positive growth dominance should relate to asymmetric competition for resources and (or) to increasing resource use efficiency with tree size in a stand. Null growth dominance should result from symmetric competition for resources and similar resource use efficiency among trees in a stand. Reverse growth dominance should arise from symmetric competition for resources and (or) from a decreasing resource use efficiency with tree size in a stand. In the second chapter, I used a Pinus ponderosa stand undergoing strong negative growth dominance (growth dominance negative = −0.22) to test the corresponding pattern proposed in Chapter I. Dominant trees were 5-times larger than suppressed trees but captured a less-than-proportional amount of light relative to their size compared with suppressed trees (90.4 vs. 20.9 GJ year-1 tree-1) and light use efficiency declined with tree size. Suppressed trees were twice as efficient as dominant trees (0.11 vs. 0.05 kg[wood] GJ [PAR]-1). In the third chapter, I studied the relationship between growth dominance and production ecology across species including conifer and broadleaf. Both light competition and patterns of resource use efficiency with tree size explained a large portion of the variation in the distribution of growth across tree sizes. Growth dominance increased with the asymmetry of competition for light (i.e., growth dominance increased as larger trees increased their share of light interception) and as light use efficiency increased with tree size. In the fourth chapter, I analyzed the pattern of water use efficiency across trees in eucalyptus experimental plots. I hypothesized that differences in water use efficiency related to changes in carbon partitioning between trees. Specifically, dominant trees should partition less photosynthate belowground than smaller trees, resulting in greater wood growth per unit of resource used. I combined tree transpiration and integrated crown water use efficiency to estimate tree-scale gross primary production, and belowground fluxes were estimated by subtracting aboveground production and respiration from gross primary production. Dominant trees produced 2.3-times more wood per unit of water transpired (0.87 vs. 0.38 gC LH2O-1), fixed 1.1-more carbon per unit of water transpired (3.4 vs. 3 gC LH2O-1) and partitioned 2.2-times more carbon to wood production than suppressed trees (0.26 vs 0.12). Belowground partitioning decreased with tree size; however, the uncertainty in transpiration measurements showed that this pattern might be the result of the underestimation of gross primary production in dominant trees. Overall, this study indicated that growth distribution (growth dominance) and production ecology patterns were related, but in variable ways. Stands with asymmetric distributions of growth are likely to have greater asymmetries in resource interception and resource use efficiency among trees. Variation in resource use efficiency related to both photosynthetic efficiency of trees and carbon partitioning to wood. However, the evidence supporting lower belowground carbon partitioning by dominant trees needs to be corroborated with future tests.
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Subject
trees
growth rate
stands
growth distribution
patterns
carbon partitioning
wood production