Browsing by Author "Oren, Ram"

Quantifying carbon fluxes and pools of forest ecosystems is an active research area in global climate study, particularly in the currently and projected increasing atmospheric carbon dioxide concentration environment. Forest carbon dynamics are closely linked to the water cycle through plant stomata which are regulated by environmental conditions associated with atmospheric and soil humidity, air temperature and light. Thus, it is imperative to study both carbon and water fluxes of a forest ecosystem to be able to assess the impact of environmental changes, including those resulting from climate change, on global carbon and hydrologic cycles. However, challenges hampering such global study lie in the spatial heterogeneity of and the temporal variability of fluxes in forests around the globe. Moreover, continuous, long-term monitoring and measurements of fluxes are not feasible at global forest scale. Therefore, the need to quantify carbon and water fluxes and to identify key variables controlling them at multiple stands and time scales is growing. Such analyses will benefit the upscaling of stand-level observations to large- or global-scale modelling approaches.

I performed a series of studies investigating carbon and water fluxes in pine forests of various site characteristics, conditions and latitudinal locations. The common techniques used in these studies largely involved sap flux sensors to measure tree-level water flow which is scaled up to stand-level transpiration and a process-based model which calculates canopy light absorption and carbon assimilation constrained by the sap-flux beased canopy stomatal conductance (called Canopy Conductance Constrained Carbon Assimilation or 4C-A model). I collected and analyzed sap flux data from pine forests of two major species: Pinus taeda in temperate (36 °N) and Pinus sylvestris in boreal (64 °N) climatic zones. These forests were of different stage-related canopy leaf area and some were under treatments for elevated atmospheric CO2 concentration or fertilization.

I found that (Chapter 2) the 17-year long free-air CO2 enrichment (FACE) had little effect on canopy transpiration of a mixed forest with the dominant P. taeda and other broadleaved species as the understory in North Carolina, USA (Duke FACE). The result was due to the compensation of elevated [CO2]-induced increase of canopy leaf area for the reduction of mean canopy stomatal conductance. My next theoretical study (Chapter 3), comparing P. taeda (native at 36 °N in North Carolina), P. sylvestris (native at 64 °N in norther Sweden) and Pinus contorta (native at 58 °N in British Columbia, Canada) canopies, revealed that the interaction between crown architecture and solar elevation associated with site latitude of pine canopies affected the distribution and total amount of canopy light absorption and potentially photosynthesis such that the latitudinally prescribed needle organization of a pine canopy is optimal for light interception and survival in its native location. Then, I quantified and analyzed water fluxes in four pine forests: one composed of P. taeda in North Carolina and three containing P. sylvestris in northern Sweden (Chapter 4). The latter forests consisted of various stage-related canopy leaf area and nutrient status. Combining my estimates with other published results from forests of various types and latitudinal locations, I derived an approach to estimate daily canopy transpiration during the growing season based on a few environmental variables including atmospheric and soil humidity and canopy leaf area. Moreover, based on a water budget analysis, I discovered that the intra-annual variation of precipitation in a forest has a small effect on evapotranspiration and primarily affecting outflow; however, variation of precipitation across latitudes proportionally influences anuual evapotranspiration and outflow. Furthermore, the hydrologic analyses implied the `disequilibrium' of forest water cycling during the growing season when forests may use less and more water in dry and wet regions, respectively, than the incoming precipitation. Nevertherless, at annual timescale, most forests became in `equilibrium' by using similar proportion of incoming precipitation. Finally, (Chapter 5) I estimated and analyzed the temporal and spatial variabilities of carbon fluxes of the same four forests measured in Chapter 4 using the 4C-A computational approach and analyzed their resource-use efficiencies. I concluded that, based on my results and others as available, despite the differences in species clumping and latitudes which influence growing season length and solar elevation, the gross primary productivity can be conservatively linearly related to the canopy light absorption. However, based on previous findings from a global study, different allocation of the acquired carbon to the above- and belowground is regulated by soil nutrient status.

Overall, the findings in this dissertation offer new insights into the impacts of environmental changes on carbon and water dynamics in forests across multiple sites and temporal scales which will be useful for larger-scale analyses such as those pertaining to global climate projection.

Uptake and storage of carbon by forest ecosystems continues to be a major research topic needed for the quantification of global budgets in an increasing atmospheric carbon dioxide environment. However, there are considerable challenges in quantifying carbon budgets of forest across a wide range of spatial and temporal scales. Although general trends in the components of carbon budgets emerge when analyzed over large spatial or temporal scales, these relationships tend to weaken, or even reverse, at smaller spatial (e.g. stand level) and temporal scales. On the other hand, continuous measuring and monitoring is not a feasible or sensible approach for the range of global forests. There is growing need to identify the key variables that drive variability in these localized budgets at multiple time scales. These results will assist in upscaling stand-level observations into large-scale modeling approaches.

Forest carbon dynamics are closely-coupled with the hydrologic cycle, so an approach that attempts to bridge these dynamics must incorporate water availability and use. Water is necessary for trees to transport nutrients, maintain cellular function, and regulate stomatal conductance; however, water is also related to other biological processes, including microbial decomposition of soil carbon, and physiologically-important abiotic factors, such as atmospheric vapor pressure deficit. Thus, much of the key to understanding the variability in forest carbon cycles is identifying the sensitivity of the processes of the carbon cycle to water availability.

Therefore, my research takes the following approach: I begin by using sap flux sensors to measure tree-level transpiration over a four-year period and combine these values with other estimates of stand-level evaporation to generate an accurate estimate of total evapotranspiration, partitioned by component and tree species (Chapter 2). To assess the sensitivity of the water fluxes in the forest, I next establish a complete hydrologic budget for the forest stand over four years, including one severe and one mild drought (Chapter 3). I then focus on the flux of carbon from the soil and its variability over space and time. Using automated, high-frequency measurements of soil CO₂ flux over a 10-year period and including 3 forest stands, I assess inter- and intra-stand variability as well as inter- and intra-annual variability in soil flux in relation to climatic factors and stand characteristics representing productivity (Chapter 4). In order to assess how soil CO₂ flux may change over longer periods of time within the context of global change, I analyze how enrichment of [CO₂] independent of and combined with soil nitrogen availability alter the balance of carbon in a stand (Chapter 5). Finally, building off these previous chapters, I examine the relationship between carbon uptake, allocation, and turnover in a mixed-species forest experiencing interannual variability in water availability (Chapter 6).

I conclude that (Chapter 2) sap flux sensors can successfully be used to estimate tree- to stand-level transpiration if one accounts for both nocturnal water movement through the tree stem and spatial variability of species composition and demography within a stand. (Chapter 3) Despite reductions in transpiration by some species during water-limited (i.e. drought) periods, the magnitude and duration of these reductions results in annual water use that is similar to a non-drought year. The consequence of this invariability in transpiration and evapotranspiration for the hydrologic cycle is that changes in annual precipitation translate directly to changes in water supplied to rivers and streams. (Chapter 4) Diurnal to seasonal variability in soil CO2 flux is driven by temperature, whereas interannual variability is most-strongly influenced by soil moisture. Furthermore, spatial variability of soil CO₂ flux is directly related to forest productivity, and by proxy, leaf production, across biomes and, to a lesser extent, across stands within a region. However, within-stand variability may be inversely related to leaf production as a result of differential allocation of carbon between aboveground and belowground uses based on local resource availability. (Chapter 5) Although elevated atmospheric [CO₂] enhances productivity, it may only result in a small increase in the flux of CO₂ from soils. Instead, nitrogen availability explains much of the variability within a forest stand, regardless of [CO₂], with increasing nitrogen availability resulting in lower allocation of carbon belowground and greater aboveground productivity. (Chapter 6) Interannual variability in water availability can affect gross primary productivity in mature forests but these effects may primarily affect the following growing season. The proportionate changes in gross primary productivity appears to show greater reductions with previous year's soil moisture than net primary productivity, leading to increased carbon use efficiency following drought. Variability in leaf biomass in this relatively stable, mature stand appears to drive the interannual variability in photosynthesis as well as the demand for carbon used for biomass production and metabolic activity.

This dissertation examined the response to termination of CO2 enrichment of a forest ecosystem exposed to long-term elevated atmospheric CO2 condition, and aimed at investigating responses and their underlying mechanisms of two important factors of carbon cycle in the ecosystem, stomatal conductance and soil respiration. Because the contribution of understory vegetation to the entire ecosystem grew with time, we first investigated the effect of elevated CO2 on understory vegetation. Potential growth enhancing effect of elevated CO2 were not observed, and light seemed to be a limiting factor. Secondly, we examined the importance of aerodynamic conductance to determine canopy conductance, and found that its effect can be negligible. Responses of stomatal conductance and soil respiration were assessed using Bayesian state space model. In two years after the termination of CO2 enrichment, stomatal conductance in formerly elevated CO2 returned to ambient level, while soil respiration became smaller than ambient level and did not recovered to ambient in two years.

This dissertation examines issues concerning sap flux scaled estimates of the canopy-averaged transpiration rate of trees per unit leaf area (E_L) and stomatal conductance (G_S), as well as their implications in the water and carbon balance of individuals and stands, with the final goal of an integrated assessment of 11 years of such data from two species (Pinus taeda and Liquidambar styraciflua) at the Duke Free Air Carbon dioxide Enrichment (Duke FACE) facility. These issues include (1) the effects of allometric relationships and xylem characteristics on the gas phase transport of water from leaves and the hydraulic supply of it, (2) consideration of the hydraulic capacitance in the inference of stomatal behavior from sap flux data and (3) the dynamic modeling of stomatal conductance to environmental drivers using Bayesian techniques. It is shown that a) for resolution of sap flux in conifers at the scale of minutes under dynamic conditions, time constants for both stomatal responses and hydraulic capacitance of sapwood must be considered, (b) nighttime conductance can lead to large errors in rates of sap flux measured under some conditions, (c) variation in allometry between P. taeda individuals can lead to different rates of transpiration and carbon assimilation per unit leaf area and that (d) hydraulic time constants for the stems of mature P. taeda at Duke FACE trees varied by the stem length considered and were on the order of 30-45 minutes for a 10-m segment. An analysis incorporating all these elements leads to the conclusions that (e) both elevated CO₂ (eCO₂) and fertilization (FR) resulted in proportionally larger reductions in the E_L and G_S of P. taeda as soil moisture decreased with (f) eCO₂ having little to no effect in months of high soil moisture and (g) FR leading to ~14% reduction of GS under high soil moisture in absence of eCO₂, while (h) both eCO₂ and FR led to reduced E_L and G_S of L. styraciflua across soil moisture conditions.

An increasing number of experimental studies attempt to maximize biomass production of trees in plantations by removing nutrient and water limitations. The results from these studies begin to inform operational managers. We investigated a Populus trichocarpa Torr. x P. deltoides Bartr. & Marsh plantation with a combined irrigation and nutrient supply system designed to optimize biomass production. Sap flux density was measured continuously over four of the six growing season months, supplemented with periodic measurements of leaf gas exchange and water potential. Measurements of tree diameter and height were used to estimate leaf area and biomass production using allometric relations. Sap flux was converted to canopy conductance, and analyzed based on an empirical model to isolate the effects of water limitation. Actual and soil water-unlimited potential CO2 uptakes were estimated using a Canopy Conductance Constrained Carbon Assimilation (4C-A) scheme, which couples actual or potential canopy conductance with vertical gradients of light distribution, leaf-level conductance, maximum Rubisco capacity (Vcmax) and maximum electron transport (Jmax). Net primary production (NPP) was ~0.43 of gross primary production (GPP); when estimated for individual trees, this ratio was independent of tree size. Based on the same ratio, we found that current irrigation reduced growth by ~18 % compare to growth with no water limitation. To achieve this maximum growth, however, would require 70% more water for transpiration, and would reduce water use efficiency by 27 %, from 1.57 to 1.15 g stem wood C kg-1 water. Given the economic and social values of water, plantation managers appear to have optimized water use.

The purpose of this study was to investigate hydraulic limitations associated with long, tall stems. Longer stems may lead to decreased water delivery to leaves due to increased friction against the walls of the conduit, and taller stems lead to decreased water delivery due to increased hydrostatic pressure at greater heights. Leaves that receive less water must close stomata to avoid cavitation, and thus limit uptake and growth rates. Although both of these limitations have been demonstrated to lead to reduced growth in trees, there is evidence that some vines can compensate. I hypothesize that despite growing long stems, kudzu is able to maintain high rates of gas exchange and photosynthesis regardless of leaf position. This is accomplished through hydraulic compensations that manifest in structural changes of the conducting tissue along the pathway.

I observed only a slight (11%) decline in stomatal conductance as expected, but was unable to predict the small decline with height based on structural and hydraulic changes to the vine. Across treatment groups, leaf area increases with path length, so equal water delivery to each leaf would result in less water per unit leaf area to distal leaves, suggesting kudzu does not compensate by adjusting the sapwood to leaf area ratio. To compensate by increasing the driving force, leaf water potential would need to fall below the wilting point. To compensate by decreasing hydraulic resistance, kudzu stems would need to be two orders of magnitude more conductive than measured. Since the total amount of water storage in kudzu based on volume is too small to maintain water supply for 20 minutes, leaves cannot rely on capacitance to compensate for slow delivery through the stem. I am unable to describe the mechanism that allows kudzu to maintain high gas exchange and growth rates in distal leaves.