Arctic tundra has been a net sink for carbon dioxide during historic and recent geological times1-4, and large amounts of carbon are stored in the soils of northern ecosystems. Many regions of the Arctic are warmer now than they have been in the past5-10, and this warming may cause the soil to change from a carbon dioxide sink to a source by lowering the water table11,12, thereby accelerating the rate of soil decomposition (CO2 source)3,13-15 so that this dominates over photosynthesis (CO2 sink). Here we present data indicating that the tundra on the North Slope of Alaska has indeed become a source of carbon dioxide to the atmosphere. This change coincides with recent warming in the Arctic, whether this is due to increases in greenhouse gas concentrations in the atmosphere or to some other cause. Our results suggest that tundra ecosystems may exert a positive feedback on atmospheric carbon dioxide and greenhouse warming.
Giant sequoia [Sequoiadendron giganteum (Lindl.) Buchholz] groves are found entirely within the Sierra Nevada mixed-conifer type. Several of its companion tree species, mainly ponderosa pine (Pinus ponderosa Dougl. ex Laws.) and Jeffrey pine (P. jeffreyi Grev. & Balf.), show foliar injury after exposure to present levels of ozone in the southern Sierra Nevada. Observations at nine giant sequoia groves in Sequoia National Park from August 1983 through September 1986 showed that surviving seedlings in 1986 averaged 18 percent of the original number. These observations did not provide evidence of a causal relationship between ozone exposure and seedling mortality. Field fumigation studies with container-grown seedlings at Giant Forest during the summers of 1987 and 1988 tested morphological and physiological responses of seedlings exposed to charcoal-filtered air, I X, and 1.5X the ambient ozone concentration for the entire summer season. In both 1987 and 1988, very slight levels of visible ozone injury to cotyledonary and primary leaves were observed in fumigation chambers and at sites in natural groves at 1X ambient ozone concentrations; however, at 1.5X ambient in chambers the symptoms of foliar injury were extensive. The end-of-season harvests of seedlings exposed to 1.5X ambient ozone showed no significant reductions of root, shoot, or total plant weights in 1987 or 1988. Gas exchange measurements made during the 1988 experiment found that ozone exposure of seedlings and rooted cuttings to 1.5X ambient ozone over the duration of a growing season increased the light compensation point, lowered CO, exchange rate at light saturation, and increased dark respiration compared to controls. A 2-month branch-chamber fumigation of large giant sequoia saplings (120 years old) with charcoal-filtered air and ozone at IX, 2X, and 3X ambient ozone concentrations did not yield visible injury or any detectable changes in photosynthetic rates.
There has been much debate about the effect of increased atmospheric CO2 concentrations on plant net primary production1,3 and on net ecosystem CO2 flux3–10. Apparently conflicting experimental findings could be the result of differences in genetic potential11–15 and resource availability16–20, different experimental conditions21–24 and the fact that many studies have focused on individual components of the system2,21,25–27 rather than the whole ecosystem. Here we present results of an in situ experiment on the response of an intact native ecosystem to elevated CO2. An undisturbed patch of tussock tundra at Toolik Lake, Alaska, was enclosed in greenhouses in which the CO2 level, moisture and temperature could be controlled28, and was subjected to ambient (340 p.p.m.) and elevated (680 p.p.m.) levels of CO2 and temperature (+4 °C). Air humidity, precipitation and soil water table were maintained at ambient control levels. For a doubled CO2 level alone, complete homeostasis of the CO2 flux was re-established within three years, whereas the regions exposed to a combination of higher temperatures and doubled CO2 showed persistent fertilization effect on net ecosystem carbon sequestration over this time. This difference may be due to enhanced sink activity from the direct effects of higher temperatures on growth16,29–33 and to indirect effects from enhanced nutrient supply caused by increased mineralization10,11,19,27,34. These results indicate that the responses of native ecosystems to elevated CO2 may not always be positive, and are unlikely to be straightforward. Clearly, CO2 fertilization effects must always be considered in the context of genetic limitation, resource availability and other such factors.
We examined the physiological response of foliage in the upper third of the canopy of 125-year-old giant sequoia (Sequoiadendron giganteum Buchholz.) trees to a 61-day exposure to 0.25x, 1x, 2x or 3x ambient ozone concentration. Four branch exposure chambers, one per ozone treatment, were installed on 1-m long secondary branches of each tree at a height of 34 m. No visible symptoms of foliar ozone damage were apparent throughout the 61-day exposure period and none of the ozone treatments affected branch growth. Despite the similarity in ozone concentrations in the branch chambers within a treatment, the trees exhibited different physiological responses to increasing ozone uptake. Differences in diurnal and seasonal patterns of gs among the trees led to a 2-fold greater ozone uptake in tree No. 2 compared with trees Nos. 1 and 3. Tree No. 3 had significantly higher CER and gs at 0.25x ambient ozone than trees Nos. 1 and 2, and gs and CER of tree No. 3 declined with increasing ozone uptake. The y-intercept of the regression for dark respiration versus ozone uptake was significantly lower for tree No. 2 than for trees Nos. 1 and 3. In the 0.25x and 1x ozone treatments, the chlorophyll concentration of current-year foliage of trees Nos. 1 and 2 was significantly higher than that of current-year foliage of tree No. 3. Chlorophyll concentration of current-year foliage on tree No. 1 did not decline with increasing ozone uptake. In all trees, total needle water potential decreased with increasing ozone uptake, but turgor was constant. Although tree No. 2 had the greatest ozone uptake, gs was highest and foliar chlorophyll concentration was lowest in tree No. 3 in the 0.25x and 1x ambient atmospheric ozone treatments.
Ponderosa pines (Pinus ponderosa Dougl. ex. Laws) 21 to 60 yr old were used to assess the relative importance of environmental stressors (O3, drought) versus an enhancer (N deposition) on foliar retention, components of aboveground growth, and whole tree biomass allocation. Sites were chosen across a well-described gradient in ozone exposure (40 to 80 ppb per h, 24 h basis, 6 month growing season) and nitrogen deposition (5 to 40 kg ha−1 yr−1) in the San Bernardino Mountains east of Los Angeles, California. A high level of chlorotic mottle indicated high 0, injury at sites closest to the pollution source, despite potential for the mitigating effects of N deposition. At the least polluted site, foliar biomass was evenly distributed across three of the five needle-age classes retained. At the most polluted site, 95% of the foliar biomass was found in the current year’s growth. High N deposition and O3 exposure combined to shift biomass allocation in pine to that of a deciduous tree with one overwintering needle age class. Based on whole tree harvests, root biomass was lowest at sites with the highest pollution exposure, confirming previous chamber exposure and field studies. Aboveground growth responses in the high-pollution sites were opposite to those expected for O3 injury. Needle and lateral branch elongation growth, and measures of wood production increased with increasing proximity to the pollution source. An enhancement of these growth attributes suggested that N deposition dominated the ponderosa pine response despite high O3 exposure.
Plant physiological models are generally parameterized from many different sources of data, including chamber experiments and plantations, from seedlings to mature trees. We obtained a comprehensive data set for a natural stand of ponderosa pine (Pinus ponderosa Laws.) and used these data to parameterize the physiologically based model, TREGRO. Representative trees of each of five tree age classes were selected based on population means of morphological, physiological, and nearest neighbor attributes. Differences in key physiological attributes (gas exchange, needle chemistry, elongation growth, needle retention) among the tree age classes were tested. Whole-tree biomass and allocation were determined for seedlings, saplings, and pole-sized trees. Seasonal maxima and minima of gas exchange were similar across all tree age classes. Seasonal minima and a shift to more efficient water use were reached one month earlier in seedlings than in older trees because of decreased soil water availability in the rooting zone of the seedlings. However, carbon isotopic discrimination of needle cellulose indicated increased water-use efficiency with increasing tree age. Seedlings had the lowest needle and branch elongation biomass growth. The amount of needle elongation growth was highest for mature trees and amount of branch elongation growth was highest for saplings. Seedlings had the highest biomass allocation to roots, saplings had the highest allocation to foliage, and pole-sized trees had the highest allocation to woody tissues. Seedlings differed significantly from pole-sized and older trees in most of the physiological traits tested. Predicted changes in biomass with tree age, simulated with the model TREGRO, closely matched those of trees in a natural stand to 30 years of age.
Seasonal patterns of carbohydrate concentration in coarse and fine roots, stem or bole, and foliage of ponderosa pine (Pinus ponderosa Laws) were described across five treeage classes from seedlings to mature trees at an atmospherically clean site. Relative to all other tree-age classes, seedlings exhibited greater tissue carbohydrate concentration in stems and foliage, and greater shifts in the time at which maximum and minimum carbohydrate concentration occurred. To determine the effect of environmental stressors on tissue carbohydrate concentration, two tree-age classes (40-year-old and mature) were compared at three sites along a well-established, long-term O3 and N deposition gradient in the San Bernardino Mountains, California. Maximum carbohydrate concentration of 1-year-old needles declined with increasing pollution exposure in both tree-age classes. Maximum fine root monosaccharide concentration was depressed for both 40-year-old and mature trees at the most polluted site. Maximum coarse and fine root starch concentrations were significantly depressed at the most polluted site in mature trees. Maximum bole carbohydrate concentration of 40-year-old trees was greater for the two most polluted sites relative to the cleanest site: the bole appeared to be a storage organ at sites where high O3 and high N deposition decreased root biomass.
The suggestion has been made that most wildland fire operations personnel base their expectations of how a fire will behave largely on experience and, to a lesser extent, on guides to predicting fire behavior (Burrows 1984). Experienced judgment is certainly needed in any assessment of wildland fire potential but it does have its limitations. The same can be said for mathematical models and computerized decision-support systems. Case history knowledge will prove a useful complement to fire behavior modeling and experienced judgment when it comes to appraising potential fire behavior (Alexander and others 2013b). Weighing each type of input in predicting wildland fire behavior is vital and yet is as much an art s a science.