Michael G. Ryan

Hydraulic Limits to Tree Height and Tree Growth

Examines what determines the height to which a tree will grow in a particular region and climate. The relationship between maximum tree height and the speed at which the tree grew when young; Mechanisms for growth including the respiration hypothesis, the nutrient limitation hypothesis, the maturation hypothesis and the hydraulic limitation hypothesis; Details about each hypothesis; Evidence for hydraulic limitation; Conclusions.

Effects of Climate Change on Plant Respiration

Plant respiration is a large, environmentally sensitive component of the ecosystem carbon balance, and net ecosystem carbon flux will change as the balance between photosynthesis and respiration changes. Partitioning respiration into the functional components of construction, maintenance, and ion uptake will aid the estimation of plant respiration for ecosystems. Maintenance respiration is the component most sensitive to changes in temperature, CO2 , protein concentration and turnover, water stress, and atmospheric pollutants. For a wide variety of plant tissues, maintenance respiration, corrected for temperature, appears to be linearly related to Kjeldahl nitrogen content of live tissue. Total and maintenance respiration may decline under CO2 enrichment, but the mechanism, independence from changes in protein content, and acclimation are unknown. Response of respiration to temperature can be modelled as a Q10 relationship, if corrections for bias arising from daily and annual temperature amplitude are applied. Occurrence and control of the cyanide-resistant respiratory pathway and acclimation of respiration rates to different climates are poorly understood, but may substantially affect the reliability of model estimates of plant respiration.

Age-related decline in forest productivity : Pattern and process

Publisher Summary This chapter reviews the evidence for the pattern of growth decline with age and discusses the evidence for the mechanisms that may be responsible. It begins with an overview of the proposed mechanisms. The chapter also presents a framework for understanding the changes in stand productivity with age, because many of the proposed mechanisms are linked and affect carbon allocation. The available information on the importance of various mechanisms behind growth decline, in the context of the stand carbon cycle is presented. The common patterns of a decline in stand leaf area and leaf photosynthetic capacity suggest a new model of carbon balance with stand development. In this model, photosynthesis and above-ground dry-matter production increase with canopy development. After the forest reaches a maximum leaf area, photosynthesis and dry-matter production decline as leaf area, photosynthetic capacity, and photosynthesis also decline. The model assumes that allocation to respiration and below ground to roots and symbionts is a constant fraction of assimilation over the life of a forest stand.

Reconciling carbon-cycle concepts, terminology, and methods

Recent projections of climatic change have focused a great deal of scientific and public attention on patterns of carbon (C) cycling as well as its controls, particularly the factors that determine whether an ecosystem is a net source or sink of atmospheric carbon dioxide (CO2). Net ecosystem production (NEP), a central concept in C-cycling research, has been used by scientists to represent two different concepts. We propose that NEP be restricted to just one of its two original definitions—the imbalance between gross primary production (GPP) and ecosystem respiration (ER). We further propose that a new term—net ecosystem carbon balance (NECB)—be applied to the net rate of C accumulation in (or loss from [negative sign]) ecosystems. Net ecosystem carbon balance differs from NEP when C fluxes other than C fixation and respiration occur, or when inorganic C enters or leaves in dissolved form. These fluxes include the leaching loss or lateral transfer of C from the ecosystem; the emission of volatile organic C, methane, and carbon monoxide; and the release of soot and CO2 from fire. Carbon fluxes in addition to NEP are particularly important determinants of NECB over long time scales. However, even over short time scales, they are important in ecosystems such as streams, estuaries, wetlands, and cities. Recent technological advances have led to a diversity of approaches to the measurement of C fluxes at different temporal and spatial scales. These approaches frequently capture different components of NEP or NECB and can therefore be compared across scales only by carefully specifying the fluxes included in the measurements. By explicitly identifying the fluxes that comprise NECB and other components of the C cycle, such as net ecosystem exchange (NEE) and net biome production (NBP), we can provide a less ambiguous framework for understanding and communicating recent changes in the global C cycle.

The Boreal Ecosystem–Atmosphere Study (BOREAS): An Overview and Early Results from the 1994 Field Year

Abstract The Boreal Ecosystem Atmosphere Study (BOREAS) is large-scale international field experiment that has the goal of improving our understanding of the exchanges of radiative energy, heat water, CO2, and trace gases between the boreal forest and the lower atmosphere. An important objective of BORES is collect the data needed to improve computer simulation models of the processes controlling these exchanges so that scientists can anticipate the effects of global change. From August 1993 through September 1994, a continuous set of monitoring measurements—meteorology, hydrology, and satellite remote sensing—were gathered over the 1000 × 1000 km BOREAS study region that covers most of Saskatchewan and Manitoba, Canada. This monitoring program was punctuated by six campaigns that saw the deployment of some 300 scientists and aircrew into the field, supported by 11 research aircraft. The participants were drawn primarily from U.S. and Canadian agencies and universities, although there were also important ...

BOREAS in 1997: Experiment overview, scientific results, and future directions

The goal of the Boreal Ecosystem-Atmosphere Study (BOREAS) is to improve our understanding of the interactions between the boreal forest biome and the atmosphere in order to clarify their roles in global change. This overview paper describes the science background and motivations for BOREAS and the experimental design and operations of the BOREAS 1994 and BOREAS 1996 field years. The findings of the 83 papers in this journal special issue are reviewed. In section 7, important scientific results of the project to date are summarized and future research directions are identified.

Annual carbon cost of autotrophic respiration in boreal forest ecosystems in relation to species and climate

Autotrophic respiration (Ra) in forest ecosystems can be >50% of the carbon fixed in photosynthesis and may regulate productivity and carbon storage in forest ecosystems, because Ra increases with temperature. We estimated annual Ra from chamber measurements in aspen, black spruce, and jack pine forests in Canada for 1994. Mean foliage respiration at 10°C for expanded leaves was 0.21–0.95 μmol m−2 (leaf surface) s−1 for all species and differed little from May to September. Wood respiration at 15°C (0.2–1 μmol m−2 (stem surface) s−1 for all species) was strongly seasonal, with high rates in midsummer that coincided with wood growth. Fine root respiration at 10°C was 2.5–7.7 μmol kg−1 s−1 for all species and declined throughout the growing season for the conifers. Annual costs of Ra for foliage, wood, and roots (overstory and understory) were 490, 610, and 450 g C m−2 (ground) yr−1 for aspen, black spruce, and jack pine (old) in northern Manitoba and 600, 480, and 310 g C m−2 yr−1 for aspen, black spruce, and jack pine (old) in central Saskatchewan. Carbon use efficiency (CUE), the ratio of net production to production plus Ra, averaged 0.44, 0.34, and 0.39 for aspen, black spruce, and jack pine (old) for all tissues and 0.61, 0.36, and 0.44 for aboveground tissues. Differences in CUE between the northern and the southern sites were small for all species, and CUE did not vary with stand biomass. Species differences in CUE suggest that models assuming a constant CUE across species may poorly estimate production and carbon balance for any given site.

Continued warming could transform Greater Yellowstone fire regimes by mid-21st century

Climate change is likely to alter wildfire regimes, but the magnitude and timing of potential climate-driven changes in regional fire regimes are not well understood. We considered how the occurrence, size, and spatial location of large fires might respond to climate projections in the Greater Yellowstone ecosystem (GYE) (Wyoming), a large wildland ecosystem dominated by conifer forests and characterized by infrequent, high-severity fire. We developed a suite of statistical models that related monthly climate data (1972–1999) to the occurrence and size of fires >200 ha in the northern Rocky Mountains; these models were cross-validated and then used with downscaled (∼12 km × 12 km) climate projections from three global climate models to predict fire occurrence and area burned in the GYE through 2099. All models predicted substantial increases in fire by midcentury, with fire rotation (the time to burn an area equal to the landscape area) reduced to <30 y from the historical 100–300 y for most of the GYE. Years without large fires were common historically but are expected to become rare as annual area burned and the frequency of regionally synchronous fires increase. Our findings suggest a shift to novel fire–climate–vegetation relationships in Greater Yellowstone by midcentury because fire frequency and extent would be inconsistent with persistence of the current suite of conifer species. The predicted new fire regime would transform the flora, fauna, and ecosystem processes in this landscape and may indicate similar changes for other subalpine forests.

AN EXPERIMENTAL TEST OF THE CAUSES OF FOREST GROWTH DECLINE WITH STAND AGE

The decline in aboveground wood production after canopy closure in even-aged forest stands is a common pattern in forests, but clear evidence for the mechanism causing the decline is lacking. The problem is fundamental to forest biology, commercial forestry (the decline sets the rotation age), and to carbon storage in forests. We tested three hypotheses about mechanisms causing the decline in wood growth by quantifying the complete carbon budget of developing stands for over six years (a full rotation) in replicated plantations of Eucalyptus saligna near Pepeekeo, Hawaii. Our first hypothesis was that gross primary production (GPP) does not decline with stand age, and that the decline in wood growth results from a shifft in partitioning from wood production to respiration (as tree biomass accumulates), total belowground carbon allocation (as a result of declining soil nutrient supply), or some combination of these or other sinks. An alternative hypothesis was that GPP declines with stand age and that the decline in aboveground wood production is proportional to the decline in GPP. A decline in GPP could be driven be reduced canopy leaf area and photosynthetic capacity resulting from increasing nutrient limitation, increased abrasion between tree canopies, lower turgor pressure to drive foliar expansion, or hydraulic limitation of water flux as tree height increases. A final hypothesis was a combination of the first two: GPP declines, but the decline in wood production is disproportionately larger because partitioning shifts as well. We measured the entire annual carbon budget (aboveground production and respiration, total belowground carbon allocation [TBCA], and GPP) from 0.5 years after seedling planting through 6 1/2 years (when trees were ~25m tall). The replicated plots included two densities of trees (1111 trees/ha and 10 000 trees/ha) to vary the ratio of canopy leaf mass to wood mass in the individual trees, and three fertilization regimes (minimal, intensive, and minimal followed by intensive after three years) to assess the role of nutrition in shaping the decline in GPP and aboveground wood production. The forest closed its canopy in 1-2 years, with peak aboveground wood production, coinciding with canopy closure, of 1.2-1.8 kg C.m-2yr-1. Aboveground wood production declined from 1.4 kg C.m-2yr-1 at age 2 to 0.60 kg C.m-2yr-1 at age 6. Hypothesis 1 failed: GPP declined from 5.0 kg C.m-2yr-1 at age 2 to 3.2 kg C.m-2yr-1 at age 6. Aboveground woody respiration declined from 0.66 kg C.m-2yr-1 at age 2 to 0.22 kg C.m-2yr-1 at age 6 and TBCA declined from 1.9 kg C.m-2yr-1 at age 2 to 1.4 kg C.m-2yr-1 at age 6. Our data supported hypothesis 3: the decline in aboveground wood production (42% of peak) was proportionally greater than the decline in canopy photosynthesis (64% of peak). The fraction of GPP partitioned to belowground allocation and foliar respiration increased with stand age and contributed to the decline in aboveground wood production. The decline in GPP was not caused by nutrient limitation, a decline in leaf area or in photsynthetic capacity, or (from a related study on the same site) by hydraulic limitation. Nutrition did interact with the decline in GPP and aboveground wood production, because treatments with high nutritient availablity declined more slowly than did our control treatment, which was fertilized only during stand establishment.

MAINTENANCE RESPIRATION AND STAND DEVELOPMENT IN A SUBALPINE LODGEPOLE PINE FOREST

We examined a chronosequence of subalpine lodgepole pine stands to test the hypothesis that low net primary production in older forest stands is caused by higher maintenance respiration costs of woody tissues. We predicted that respiration of woody tissues (particularly stem sapwood) would be greater in older stands and that the higher maintenance costs would account for observed low wood production. For a unit of ground surface, the carbon flux involved in wood production and associated construction respi- ration was 210 gm-2-yr-l in a 40-yr-old stand, but declined to 46 g.m-2.yr-l in a 245- yr-old stand. However, maintenance respiration of woody tissues in stems and branches consumed only 61 g.m-2.yr-l in the 40-yr-old stand and 79 g-m-2 yr-l in the 245-yr-old stand. The slight, nonsignificant increase in maintenance respiration of woody tissues could not explain the dramatic decline in aboveground wood production in the old-growth stand.