John S. King

Simple additive effects are rare: a quantitative review of plant biomass and soil process responses to combined manipulations of CO2 and temperature

In recent years, increased awareness of the potential interactions between rising atmospheric CO2 concentrations ([ CO2 ]) and temperature has illustrated the importance of multifactorial ecosystem manipulation experiments for validating Earth System models. To address the urgent need for increased understanding of responses in multifactorial experiments, this article synthesizes how ecosystem productivity and soil processes respond to combined warming and [ CO2 ] manipulation, and compares it with those obtained in single factor [ CO2 ] and temperature manipulation experiments. Across all combined elevated [ CO2 ] and warming experiments, biomass production and soil respiration were typically enhanced. Responses to the combined treatment were more similar to those in the [ CO2 ]-only treatment than to those in the warming-only treatment. In contrast to warming-only experiments, both the combined and the [ CO2 ]-only treatments elicited larger stimulation of fine root biomass than of aboveground biomass, consistently stimulated soil respiration, and decreased foliar nitrogen (N) concentration. Nonetheless, mineral N availability declined less in the combined treatment than in the [ CO2 ]-only treatment, possibly due to the warming-induced acceleration of decomposition, implying that progressive nitrogen limitation (PNL) may not occur as commonly as anticipated from single factor [ CO2 ] treatment studies. Responses of total plant biomass, especially of aboveground biomass, revealed antagonistic interactions between elevated [ CO2 ] and warming, i.e. the response to the combined treatment was usually less-than-additive. This implies that productivity projections might be overestimated when models are parameterized based on single factor responses. Our results highlight the need for more (and especially more long-term) multifactor manipulation experiments. Because single factor CO2 responses often dominated over warming responses in the combined treatments, our results also suggest that projected responses to future global warming in Earth System models should not be parameterized using single factor warming experiments.

Reduction of soil carbon formation by tropospheric ozone under increased carbon dioxide levels

In the Northern Hemisphere, ozone levels in the troposphere have increased by 35 per cent over the past century, with detrimental impacts on forest and agricultural productivity, even when forest productivity has been stimulated by increased carbon dioxide levels. In addition to reducing productivity, increased tropospheric ozone levels could alter terrestrial carbon cycling by lowering the quantity and quality of carbon inputs to soils. However, the influence of elevated ozone levels on soil carbon formation and decomposition are unknown. Here we examine the effects of elevated ozone levels on the formation rates of total and decay-resistant acid-insoluble soil carbon under conditions of elevated carbon dioxide levels in experimental aspen (Populus tremuloides) stands and mixed aspen–birch (Betula papyrifera) stands. With ambient concentrations of ozone and carbon dioxide both raised by 50 per cent, we find that the formation rates of total and acid-insoluble soil carbon are reduced by 50 per cent relative to the amounts entering the soil when the forests were exposed to increased carbon dioxide alone. Our results suggest that, in a world with elevated atmospheric carbon dioxide concentrations, global-scale reductions in plant productivity due to elevated ozone levels will also lower soil carbon formation rates significantly.

Soil respiration, root biomass, and root turnover following long‐term exposure of northern forests to elevated atmospheric CO2 and tropospheric O3

The Rhinelander free-air CO(2) enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO(2)) and elevated tropospheric ozone (+O(3)). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen (Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. Soil respiration was greatest in the +CO(2) and +CO(2) +O(3) treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m(-2)) were positively affected by +O(3). After 10 yr of exposure, +CO(2) +O(3)-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO(2) on belowground net primary productivity (NPP) may not be offset by negative effects of O(3). For the aspen community, fine-root biomass is actually stimulated by +O(3), and especially +CO(2) +O(3).

Clonal variation in above- and below-ground growth responses of Populus tremuloides Michaux: Influence of soil warming and nutrient availability

Trembling aspen (Populus tremuloides Michx.) is the most widely distributed tree species in North America making it important to terrestrial carbon and nutrient cycles. Due to anthropogenic climate change high latitude temperatures are expected to increase, making it necessary to assess the feedback between above- and below-ground carbon pools to increased temperature at sites of both high and low N-availability. We grew four clones of aspen at two levels of soil temperature and two levels of soil N-availability for 98 days and quantified photosynthesis, growth, biomass allocation, and root length production and mortality. High soil temperature increased rates of photosynthesis (65%), resulting in greater whole-plant growth (37%) through increases in roots, stems, and foliage; however these increases generally occurred only in soil of high N-availability. Root:shoot biomass allocation varied between clones but was unaffected by the soil temperature or N-availability treatments. Root length production and mortality increased at elevated soil temperature, but this response was modified by soil N-availability. At high soil temperature, soil N-availability had little effect on root dynamics, while at low soil temperature, high soil N-availability increased both the production and mortality (turnover) of roots. We conclude that trembling aspen has the potential for substantially greater growth and root turnover under conditions of warmer soil at sites of both high and low N-availability, but that allometric patterns of growth are under strong genetic, rather than environmental control.

Hydraulic redistribution of soil water by roots affects whole‐stand evapotranspiration and net ecosystem carbon exchange

*Hydraulic redistribution (HR) of water via roots from moist to drier portions of the soil occurs in many ecosystems, potentially influencing both water use and carbon assimilation. *By measuring soil water content, sap flow and eddy covariance, we investigated the temporal variability of HR in a loblolly pine (Pinus taeda) plantation during months of normal and below-normal precipitation, and examined its effects on tree transpiration, ecosystem water use and carbon exchange. *The occurrence of HR was explained by courses of reverse flow through roots. As the drought progressed, HR maintained soil moisture above 0.15 cm(3) cm(-3) and increased transpiration by 30-50%. HR accounted for 15-25% of measured total site water depletion seasonally, peaking at 1.05 mm d(-1). The understory species depended on water redistributed by the deep-rooted overstory pine trees for their early summer water supply. Modeling carbon flux showed that in the absence of HR, gross ecosystem productivity and net ecosystem exchange could be reduced by 750 and 400 g C m(-2) yr(-1), respectively. *Hydraulic redistribution mitigated the effects of soil drying on understory and stand evapotranspiration and had important implications for net primary productivity by maintaining this whole ecosystem as a carbon sink.

Decoupling the influence of leaf and root hydraulic conductances on stomatal conductance and its sensitivity to vapour pressure deficit as soil dries in a drained loblolly pine plantation

The study examined the relationships between whole tree hydraulic conductance (K(tree)) and the conductance in roots (K(root)) and leaves (K(leaf)) in loblolly pine trees. In addition, the role of seasonal variations in K(root) and K(leaf) in mediating stomatal control of transpiration and its response to vapour pressure deficit (D) as soil-dried was studied. Compared to trunk and branches, roots and leaves had the highest loss of conductivity and contributed to more than 75% of the total tree hydraulic resistance. Drought altered the partitioning of the resistance between roots and leaves. As soil moisture dropped below 50%, relative extractable water (REW), K(root) declined faster than K(leaf). Although K(tree) depended on soil moisture, its dynamics was tempered by the elongation of current-year needles that significantly increased K(leaf) when REW was below 50%. After accounting for the effect of D on g(s), the seasonal decline in K(tree) caused a 35% decrease in g(s) and in its sensitivity to D, responses that were mainly driven by K(leaf) under high REW and by K(root) under low REW. We conclude that not only water stress but also leaf phenology affects the coordination between K(tree) and g(s) and the acclimation of trees to changing environmental conditions.

Wound-Induced Terpene Synthase Gene Expression in Sitka Spruce That Exhibit Resistance or Susceptibility to Attack by the White Pine Weevil

We analyzed the expression pattern of various terpene synthase (TPS) genes in response to a wounding injury applied to the apical leader of Sitka spruce (Picea sitchensis Bong. Carr.) genotypes known to be resistant (R) or susceptible (S) to white pine weevil (Pissodes strobi Peck.) attack. The purpose was to test if differences in constitutive or wound-induced TPS expression can be associated with established weevil resistance. All wounding treatments were conducted on 9-year-old R and S trees growing under natural field conditions within the range of variation for weevil R and S genotypes. Representative cDNAs of the monoterpene synthase (mono-TPS), sesquiterpene synthase (sesqui-TPS), and diterpene synthase (di-TPS) classes were isolated from Sitka spruce to assess TPS transcript levels. Based on amino acid sequence similarity, the cDNAs resemble Norway spruce (Picea abies) (−)-linalool synthase (mono-TPS; PsTPS-Linl) and levopimaradiene/abietadiene synthase (di-TPS; PsTPS-LASl), and grand fir (Abies grandis) δ-selinene synthase (sesqui-TPS; PsTPS-Sell). One other mono-TPS was functionally identified as (−)-limonene synthase (PsTPS-Lim). No significant difference in constitutive expression levels for these TPSs was detected between R and S trees. However, over a postwounding period of 16 d, only R trees exhibited significant transcript accumulation for the mono- and sesqui-TPS tested. Both R and S trees exhibited a significant accumulation of PsTPS-LASl transcripts. An assessment of traumatic resin duct formation in wounded leaders showed that both R and S trees responded by forming traumatic resin ducts; however, the magnitude of this response was significantly greater in R trees. Collectively, our data imply that the induced resinosis response is an important aspect of defense in weevil R Sitka spruce trees growing under natural conditions.

Effects of Soil Temperature on Nutrient Uptake

More than a decade ago, Bowen (1991) underscored the curious fact that soil temperature had received relatively little attention from scientists, even though it is of fundamental importance to plant productivity. With the realization that soil temperatures may rise as a part of anthropogenic global change, there is renewed interest in studying the mechanisms and effects of terrestrial ecosystem responses to changes in soil temperature (Billings et al. 1982; Peterjohn et al. 1994; Rosenzweig and Hillel 2000). However, this area of investigation is still much underrepresented. In any consideration of soil temperature, we must be cognizant that it continually interacts with biotic and abiotic components of the soil–plant system in both space and time (Pregitzer et al. 2000). For example, not only does soil temperature influence rate constants of chemical reactions, water content, and nutrient transport in soil, but it simultaneously affects plant physiological aspects of ion uptake, root growth, and the composition and function of soil microbial communities. In fact, virtually all processes occurring in soil, from the weathering of primary minerals to plant nutrition and storage of organic carbon, are strongly influenced by soil temperature (Fig. 10.1; also Chap. 2, this Vol.). In particular, acquisition of mineral nutrients by plants is governed by myriad chemical, physical, and biological processes that are all sensitive to temperature. Soil temperature therefore exerts a major influence on primary productivity in terrestrial ecosystems around the globe. In this chapter, we address how plant nutrient uptake is influenced by changes in soil temperature. There have been several excellent treatments of soil temperature effects on root growth and related processes (Gliński and Lipiec 1990; Bowen 1991; McMichael and Burke 1996, 1998; Pregitzer et al. 2000). Our contribution is a synthesis of information from a wide range of sources to provide insight on controls and global patterns of soil temperature, and how it affects the biotic and abiotic control points of nutrient uptake. Recent work has demonstrated the capacity of some plants to utilize organic

Tracing Changes in Ecosystem Function under Elevated Carbon Dioxide Conditions

Abstract Responses of ecosystems to elevated levels of atmospheric carbon dioxide (CO2) remain a critical uncertainty in global change research. Two key unknown factors are the fate of carbon newly incorporated by photosynthesis into various pools within the ecosystem and the extent to which elevated CO2 is transferred to and sequestered in pools with long turnover times. The CO2 used for enrichment in many experiments incorporates a dual isotopic tracer, in the sense that ratios of both the stable carbon-13 (13C) and the radioactive carbon-14 (14C) isotopes with respect to carbon-12 are different from the corresponding ratios in atmospheric CO2. Here we review techniques for using 13C and 14C abundances to follow the fate of newly fixed carbon and to further our understanding of the turnover times of ecosystem carbon pools. We also discuss the application of nitrogen, oxygen, and hydrogen isotope analyses for tracing changes in the linkages between carbon, nitrogen, and water cycles under conditions of elevated CO2.

The Response of Belowground Carbon Allocation in Forests to Global Change

Belowground carbon allocation (BCA) in forests regulates soil organic matter formation and influences biotic and abiotic properties of soil such as bulk density, cation exchange capacity, and water holding capacity. On a global scale, the total quantity of carbon allocated belowground by terrestrial plants is enormous, exceeding by an order of magnitude the quantity of carbon emitted to the atmosphere through combustion of fossil fuels. Despite the importance of BCA to the functioning of plant and soil communities, as well as the global carbon budget, controls on BCA are relatively poorly understood. Consequently, our ability to predict how BCA will respond to changes in atmospheric greenhouse gases, climate, nutrient deposition, and plant community composition remains rudimentary. In this synthesis, we examine BCA from three perspectives: coarse-root standing stock, belowground net primary production (BNPP), and total belowground carbon allocation (TBCA). For each, we examine methodologies and methodological constraints, as well as constraints of terminology. We then examine available data for any predictable variation in BCA due to changes in species composition, mean annual temperature, or elevated CO2 in existing Free Air CO2 Exposure (FACE) experiments. Finally, we discuss what we feel are important future directions for belowground carbon allocation research, with a focus on global change issues.