Quantifying and modeling processes involved in the global carbon cycle is
important to evaluate the temporal and spatial variability of these processes and
understand the effect of this variability on future response to changing climate and
land use patterns. Biomass accumulation and Net Primary Productivity (NPP) are
large components of ecosystem carbon exchange with the atmosphere and thus are the
focus of many modeling efforts. When scaling estimates of NPP temporally from days
to years and spatially from square meters to landscapes and regions the spatial
coherence of these processes through time must be taken into account. Spatial
coherence is the degree to which pairs of sites across space are synchronous (i.e.,
correlated) through time with respect to a given process or variable. In this thesis I
determined the spatial coherence of a major component of NPP, tree bole productivity
(NPP[subscript B]), and examine how it influences scaling and our ability to predict NPP and
forecast change of this flux.
In Chapter 2 I developed and tested a method modeling radial tree increment
growth from sub-sampled trees and estimating annual site-level biomass accumulation
that allows quantification of the uncertainty in these estimates. Results demonstrated
that a simple model using the mean and standard deviation of growth increments
underestimated bole biomass increment in all three age classes examined by 1% at the
largest sample sizes and up to 15% at the smallest sample sizes. The long term average
NPP[subscript B] and inter-annual variability were also underestimated by as much as 10% and
22%, respectively. Stratification of trees by size in sampling and modeling methods
increased accuracy and precision of estimates markedly. The precision of both models
was sufficient to detect patterns of inter-annual variability. To estimate bole biomass
accumulation with acceptable levels of accuracy and precision our results suggest
sampling at least 64 trees per site, although one site required a sample size of more
than 100 trees.
In Chapter 3 I compared year to year variability of NPP for tree boles (NPP[subscript B])
for two adjacent small watersheds (second-growth and old-growth) in the western
Cascades of Oregon using the methods developed in Chapter 2. Spatial coherence of
NPP[subscript B] within and between watersheds was assessed using multivariate analysis
techniques. NPP[subscript B] was found to be less coherent between watersheds than within
watersheds, indicating decreased spatial coherence with differences in age class and increased spatial scale. However, a larger degree of spatial coherence existed within
the old-growth watershed compared to the second-growth watershed, which may be a
result of the smaller degree of variation in environmental characteristics in the former
watershed. Within a watershed, potential annual direct incident radiation and heat load
were more strongly associated with the variation of NPP[subscript B] than climate. Climatic
factors correlated with the temporal variation of annual NPP[subscript B] varied between the two
watersheds. Results suggest that inter-annual variability and spatial coherence of forest
productivity is a result of both internal (e.g., environment and stand dynamics) and
external (climate) factors. An unexpected conclusion was that spatial coherence was
not consistent and changed through time. Therefore, the coherence of sites over time is
not a simple relationship. Instead the patterns of spatial coherence exhibit complex
behaviors that have implications for scaling estimates of productivity. This result also
indicates that a correlation coefficient alone may not capture the complexity of change
through time across space.
In Chapter 4 I estimated year to year variation of NPP[subscript B] for eleven sites of
varying age, elevation, moisture, and species composition in the Western Cascades of
Oregon. Spatial coherence of tree growth within sites and NPP[subscript B] between sites was
assessed using Pearson's product-moment correlation coefficient (r). Results suggest
that spatial coherence is highly variable between sites (r=-O.18 to 0.92). The second-growth
sites exhibited the greatest temporal variability of annual NPP[subscript B] due to the large
accumulation of biomass during stand initiation, but old-growth sites exhibited the
greatest variation of coherence of NPP[subscript B] between sites. In some years all sites behaved
similarly, but for other years some sites were synchronous while others were not. As
growth of individual trees and NPP[subscript B] at the site scale increased, inter-annual variability
of those variables increased. Climate in part affected annual NPP[subscript B], but intrinsic factors
and spatial proximity also affected the coherence between sites in this landscape. / Graduation date: 2006
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28799 |
| Date | 05 December 2005 |
| Creators | Woolley, Travis J. |
| Contributors | Harmon, Mark E., O'Connell, Kari E. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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