► Leaf area index (LAI) in the mature semi-arid forest's predominantly
Pinus halepensis plantation was measured intensively during the years 2001 to 2006 by a number of non-contact optical devices. ► ...The measurements showed a gradual increase in LAI from ∼1 to ∼2 during these years. ► The LAI measurements at the start of each season were used to constrain phenology-based estimates of annual LAI development that predicts intra-seasonal LAI variation in the order of 10% of total LAI. ► The mean clumpiness index, 0.61, is considered representative for the specific environment.
Effective leaf area index (LAI
e) in the semi-arid
Pinus halepensis plantation, located between arid and semi-arid climatic zones at the edge of the Negev and Judean deserts, was measured bi-annually during four years (2001–2004) and more intensively (monthly) during the following two years (2004–2006) by a number of non-contact optical devices. The measurements showed a gradual increase in LAI
e from ∼1 (±0.25) to ∼1.8 (±0.11) during these years. All instruments, when used properly, gave similar results that were also comparable with actual leaf area index measured by litter collection and destructive sampling and allometric estimates. Because of the constraint of clear sky conditions, which limited the use of the fisheye type sensors to times of twilight, the fisheye techniques were less useful. The tracing radiation and architecture of canopies system, which includes specific treatment of two levels of clumpiness of the sparse forest stand, was used successfully for the intensive monitoring. The mean clumpiness index, 0.61, is considered representative for the specific environment. Finally, the LAI
e measurements at the start of each season were used to constrain phenology-based estimates of annual LAI
e development, resulting in a continuous course of LAI
e in the forest over the five-year period. Intra-seasonal LAI
e variation in the order of 10% of total LAI
e predicted by the model was also observed in the intensive TRAC measurements, giving confidence in the TRAC system and indicating its sensitivity and applicability in woodlands even with low LAI
e values. The results can be important for forest management decision support as well as for use in evaluation of remote sensing techniques for forests at the lowest range of LAI
e values.
Atmospheric CO2 enrichment may stimulate plant growth directly through (1) enhanced photosynthesis or indirectly, through (2) reduced plant water consumption and hence slower soil moisture depletion, ...or the combination of both. Herein we describe gas exchange, plant biomass and species responses of five native or semi-native temperate and Mediterranean grasslands and three semi-arid systems to CO2 enrichment, with an emphasis on water relations. Increasing CO2 led to decreased leaf conductance for water vapor, improved plant water status, altered seasonal evapotranspiration dynamics, and in most cases, periodic increases in soil water content. The extent, timing and duration of these responses varied among ecosystems, species and years. Across the grasslands of the Kansas tallgrass prairie, Colorado shortgrass steppe and Swiss calcareous grassland, increases in aboveground biomass from CO2 enrichment were relatively greater in dry years. In contrast, CO2-induced aboveground biomass increases in the Texas C3/C4 grassland and the New Zealand pasture seemed little or only marginally influenced by yearly variation in soil water, while plant growth in the Mojave Desert was stimulated by CO2 in a relatively wet year. Mediterranean grasslands sometimes failed to respond to CO2-related increased late-season water, whereas semiarid Negev grassland assemblages profited. Vegetative and reproductive responses to CO2 were highly varied among species and ecosystems, and did not generally follow any predictable pattern in regard to functional groups. Results suggest that the indirect effects of CO2 on plant and soil water relations may contribute substantially to experimentally induced CO2-effects, and also reflect local humidity conditions. For landscape scale predictions, this analysis calls for a clear distinction between biomass responses due to direct CO2 effects on photosynthesis and those indirect CO2 effects via soil moisture as documented here.
The large boreal carbon (C) stocks in Alaska are vulnerable to losses from disturbance, such as clearcut logging and deforestation for agricultural development. Here we investigated impacts of ...logging in uplands and agricultural deforestation in lowlands on C and nitrogen (N) stocks in Interior Alaska, using chronosequences, and synthesized results from other studies in the boreal region. Two years after logging, ecosystem C stocks in upland forests were reduced by 11 kg m⁻²(46% of the original ecosystem C stock), mainly as a consequence of stem removal. Soil C and N stocks increased over the first few years after logging, but returned to pre-harvest levels during the following decades to century. Studies across the boreal region showed that mean initial C loss was four times greater, but long-term C cycling was similar in logged as compared to burned forests. Agricultural development in Alaskan lowlands permanently reduced ecosystem C stocks, reaching losses of 11 kg m⁻²(34% of the ecosystem C stock) on non-permafrost soils after several decades and 31 kg m⁻²(69%) on permafrost soils over 6 years. These C losses are much more rapid than the 5–6 kg m⁻²over 500 years that models project to be lost by warming or warming-plus-wildfire in lowland boreal forests. If economic incentives and climate warming augment boreal land-use change in lowlands because of improved agricultural opportunities and performance, this could magnify warming-induced C loss and amplify climate warming. These impacts can be reduced by conserving permafrost-dominated sites for C storage and focusing agriculture on permafrost-free sites.
Soil respiration (SR) constitutes the largest flux of CO(2) from terrestrial ecosystems to the atmosphere. However, there still exist considerable uncertainties as to its actual magnitude, as well as ...its spatial and interannual variability. Based on a reanalysis and synthesis of 80 site-years for 57 forests, plantations, savannas, shrublands and grasslands from boreal to tropical climates we present evidence that total annual SR is closely related to SR at mean annual soil temperature (SR(MAT)), irrespective of the type of ecosystem and biome. This is theoretically expected for non water-limited ecosystems within most of the globally occurring range of annual temperature variability and sensitivity (Q(10)). We further show that for seasonally dry sites where annual precipitation (P) is lower than potential evapotranspiration (PET), annual SR can be predicted from wet season SR(MAT) corrected for a factor related to P/PET. Our finding indicates that it can be sufficient to measure SR(MAT) for obtaining a well constrained estimate of its annual total. This should substantially increase our capacity for assessing the spatial distribution of soil CO(2) emissions across ecosystems, landscapes and regions, and thereby contribute to improving the spatial resolution of a major component of the global carbon cycle.
Nitrogen (N) and water availability are important factors affecting ecosystem productivity that can be influenced by land-use change. We hypothesized that the observed increase in carbon (C) ...sequestration associated with afforestation of semi-arid sparse shrubland must also be associated with an increase in N input. We tested this hypothesis by reconstructing the ecosystem N budget of two ecosystems, a semi-arid shrubland and a nearby planted pine forest, using measurements augmented with literature-based estimates. Our findings demonstrate that, contrary to our hypothesis, massive sequestration by the pine forest could be accounted for without a change in the net N budget (i. e., neither elevated N inputs nor reduced N losses). However, in comparison to the shrubland, the forest showed an almost tripling in aboveground N use efficiency (NUE; 235 vs. 83 kg dry mass kg⁻1 N) and a doubling in ecosystem level C/N ratio (16 vs. 8, for the forest and shrubland, respectively). Nitrogen cycling slowed in the forest compared to the shrubland: net N mineralization rates in soils decreased by approximately 50%, decomposition rates decreased by approximately 20%, and NOₓ loss decreased by approximately 64%. These adjustments in N cycling provide a possible basis for increased NUE and subsequent sequestration without net change in the overall N budget, which should be addressed in future investigations.
Rising atmospheric CO2 concentrations may lead to increased water availability because the water use efficiency of photosynthesis (WUE) increases with CO2 in most plant species. This should allow the ...extension of afforestation activities into drier regions. Using eddy flux, physiological and inventory measurements we provide the first quantitative information on such potential from a 35‐year old afforestation system of Aleppo pine (Pinus halepensis Mill.) at the edge of the Negev desert. This 2800 ha arid‐land forest contains 6.5 ± 1.2 kg C m−2, and continues to accumulate 0.13–0.24 kg C m−2 yr−1. The CO2 uptake is highest during the winter, out of phase with most northern hemispheric forest activity. This seasonal offset offers low latitude forests ∼10 ppm higher CO2 concentrations than that available to higher latitude forests during the productive season, in addition to the 30% increase in mean atmospheric CO2 concentrations since the 1850s. Expanding afforestation efforts into drier regions may be significant for C sequestration and associated benefits (restoration of degraded land, reducing runoff, erosion and soil compaction, improving wildlife) because of the large spatial scale of the regions potentially involved (ca. 2 × 109 ha of global shrub‐land and C4 grassland). Quantitative information on forest activities under dry conditions may also become relevant to regions predicted to undergo increasing aridity.
The ratio of CO2 efflux to O2 influx (ARQ, apparent respiratory quotient) in tree stems is expected to be 1.0 for carbohydrates, the main substrate supporting stem respiration. In previous studies of ...stem fluxes, ARQ values below 1.0 were observed and hypothesized to indicate retention of respired carbon within the stem. Here, we demonstrate that stem ARQ < 1.0 values are common across 85 tropical, temperate, and Mediterranean forest trees from nine different species. Mean ARQ values per species per site ranged from 0.39 to 0.78, with an overall mean of 0.59. Assuming that O2 uptake provides a measure of in situ stem respiration (due to the low solubility of O2), the overall mean indicates that on average 41 % of CO2 respired in stems is not emitted from the local stem surface. The instantaneous ARQ did not vary with sap flow. ARQ values of incubated stem cores were similar to those measured in stem chambers on intact trees. We therefore conclude that dissolution of CO2 in the xylem sap and transport away from the site of respiration cannot explain the low ARQ values. We suggest refixation of respired CO2 in biosynthesis reactions as possible mechanism for low ARQ values.
Climate warming is most pronounced at high latitudes, which could result in the intensification of the extensively cultivated areas in the boreal zone and could further enhance rates of forest ...clearing in the coming decades. Using paired forest‐field sampling and a chronosequence approach, we investigated the effect of conversion of boreal forest to agriculture on carbon (C) and nitrogen (N) dynamics in interior Alaska. Chronosequences showed large soil C losses during the first two decades following deforestation, with mean C stocks in agricultural soils being 44% or 8.3 kg m−2 lower than C stocks in original forest soils. This suggests that soil C losses from land‐use change in the boreal region may be greater than those in other biomes. Analyses of changes in stable C isotopes and in quality of soil organic matter showed that organic C was lost from soils by combustion of cleared forest material, decomposition of organic matter and possibly erosion. Chronosequences indicated an increase in C storage during later decades after forest clearing, with 60‐year‐old grassland showing net ecosystem C gain of 2.1 kg m−2 over the original forest. This increase in C stock resulted probably from a combination of large C inputs from belowground biomass and low C losses due to a small original forest soil C stock and low tillage frequency. Reductions in soil N stocks caused by land‐use change were smaller than reductions in C stocks (34% or 0.31 kg m−2), resulting in lower C/N ratios in field compared with forest mineral soils, despite the occasional incorporation of high‐C forest‐floor material into field soils. Carbon mineralization per unit of mineralized N was considerably higher in forests than in fields, which could indicate that decomposition rates are more sensitive in forest soils than in field soils to inorganic N addition (e.g. by increased N deposition from the atmosphere). If forest conversion to agriculture becomes more widespread in the boreal region, the resulting C losses (51% or 11.2 kg m−2 at the ecosystem level in this study) will induce a positive feedback to climatic warming and additional land‐use change. However, by selecting relatively C‐poor soils and by implementing management practices that preserve C, losses of C from soils can be reduced.
Field‐chamber measurements of soil respiration from 17 different forest and shrubland sites in Europe and North America were summarized and analyzed with the goal to develop a model describing ...seasonal, interannual and spatial variability of soil respiration as affected by water availability, temperature, and site properties. The analysis was performed at a daily and at a monthly time step. With the daily time step, the relative soil water content in the upper soil layer expressed as a fraction of field capacity was a good predictor of soil respiration at all sites. Among the site variables tested, those related to site productivity (e.g., leaf area index) correlated significantly with soil respiration, while carbon pool variables like standing biomass or the litter and soil carbon stocks did not show a clear relationship with soil respiration. Furthermore, it was evidenced that the effect of precipitation on soil respiration stretched beyond its direct effect via soil moisture. A general statistical nonlinear regression model was developed to describe soil respiration as dependent on soil temperature, soil water content, and site‐specific maximum leaf area index. The model explained nearly two thirds of the temporal and intersite variability of soil respiration with a mean absolute error of 0.82 μmol m−2 s−1. The parameterized model exhibits the following principal properties: (1) At a relative amount of upper‐layer soil water of 16% of field capacity, half‐maximal soil respiration rates are reached. (2) The apparent temperature sensitivity of soil respiration measured as Q10 varies between 1 and 5 depending on soil temperature and water content. (3) Soil respiration under reference moisture and temperature conditions is linearly related to maximum site leaf area index. At a monthly timescale, we employed the approach by Raich et al. 2002 that used monthly precipitation and air temperature to globally predict soil respiration (T&P model). While this model was able to explain some of the month‐to‐month variability of soil respiration, it failed to capture the intersite variability, regardless of whether the original or a new optimized model parameterization was used. In both cases, the residuals were strongly related to maximum site leaf area index. Thus, for a monthly timescale, we developed a simple T&P&LAI model that includes leaf area index as an additional predictor of soil respiration. This extended but still simple model performed nearly as well as the more detailed time step model and explained 50% of the overall and 65% of the site‐to‐site variability. Consequently, better estimates of globally distributed soil respiration should be obtained with the new model driven by satellite estimates of leaf area index. Before application at the continental or global scale, this approach should be further tested in boreal, cold‐temperate, and tropical biomes as well as for non‐woody vegetation.
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