Yatir Forest Research Articles

Disentangling Soil, Shade, and Tree Canopy Contributions to Mixed Satellite Vegetation Indices in a Sparse Dry Forest

Remote sensing (RS) for vegetation monitoring can involve mixed pixels with contributions from vegetation and background surfaces, causing biases in signals and their interpretations, especially in low-density forests. In a case study in the semi-arid Yatir forest in Israel, we observed a mismatch between satellite (Landsat 8 surface product) and tower-based (Skye sensor) multispectral data and contrasting seasonal cycles in near-infrared (NIR) reflectance. We tested the hypothesis that this mismatch was due to the different fractional contributions of the various surface components and their unique reflectance. Employing an unmanned aerial vehicle (UAV), we obtained high-resolution multispectral images over selected forest plots and estimated the fraction, reflectance, and seasonal cycle of the three main surface components (canopy, shade, and sunlit soil). We determined that the Landsat 8 data were dominated by soil signals (70%), while the tower-based data were dominated by canopy signals (95%). We then developed a procedure to resolve the canopy (i.e., tree foliage) normalized difference vegetation index (NDVI) from the mixed satellite data. The retrieved and corrected canopy-only data resolved the original mismatch and indicated that the spatial variations in Landsat 8 NDVI were due to differences in stand density, while the canopy-only NDVI was spatially uniform, providing confidence in the local flux tower measurements.

The importance of tree internal water storage under drought conditions.

Global warming and drying trends, as well as the increase in frequency and intensity of droughts, may have unprecedented impacts on various forest ecosystems. We assessed the role of internal water storage (WS) in drought resistance of mature pine trees in the semi-arid Yatir Forest. Transpiration (T), soil moisture and sap flow (SF) were measured continuously, accompanied by periodical measurements of leaf and branch water potential (Ψleaf) and water content (WC). The data were used to parameterize a tree hydraulics model to examine the impact of WS capacitance on the tree water relations. The results of the continuous measurements showed a 5-h time lag between T and SF in the dry season, which peaked in the early morning and early afternoon, respectively. A good fit between model results and observations was only obtained when the empirically estimated WS capacitance was included in the model. Without WS during the dry season, Ψleaf would drop below a threshold known to cause hydraulic failure and cessation of gas exchange in the studied tree species. Our results indicate that tree WS capacitance is a key drought resistance trait that could enhance tree survival in a drying climate, contributing up to 45% of the total daily transpiration during the dry season.

Turbulent transport of energy across a forest and a semiarid shrubland

Abstract. The role of secondary circulations has recently been studied in the context of well-defined surface heterogeneity in a semiarid ecosystem where it was found that energy balance closure over a desert–forest system and the structure of the boundary layer was impacted by advection and flux divergence. As a part of the CliFF (“Climate feedbacks and benefits of semi-arid forests”, a collaboration between KIT, Germany, and the Weizmann Institute, Israel) campaign, we studied the boundary layer dynamics and turbulent transport of energy corresponding to this effect in Yatir Forest situated in the Negev Desert in Israel. The forest surrounded by small shrubs presents a distinct feature of surface heterogeneity, allowing us to study the differences between their interactions with the atmosphere above by conducting measurements with two eddy covariance (EC) stations and two Doppler lidars. As expected, the turbulence intensity and vertical fluxes of momentum and sensible heat are found to be higher above the forest compared to the shrubland. Turbulent statistics indicative of nonlocal motions are also found to differ over the forest and shrubland and also display a strong diurnal cycle. The production of turbulent kinetic energy (TKE) over the forest is strongly mechanical, while buoyancy effects generate most of the TKE over the shrubland. Overall TKE production is much higher above the forest compared to the shrubland. The forest is also found to be more efficient in dissipating TKE. The TKE budget appears to be balanced on average both for the forest and shrubland, although the imbalance of the TKE budget, which includes the role of TKE transport, is found to be quite different in terms of diurnal cycles for the forest and shrubland. The difference in turbulent quantities and the relationships between the components of TKE budget are used to infer the characteristics of the turbulent transport of energy between the desert and the forest.

Effect of Secondary Circulations on the Surface–Atmosphere Exchange of Energy at an Isolated Semi-arid Forest

Afforestation in semi-arid regions can potentially enhance the global carbon sink by increasing the terrestrial biomass. However, the survival of planted forests under such extreme environmental conditions is not guaranteed a priori, and critically depends on the surface–atmosphere exchange of energy. We investigate the pine forest Yatir in Israel, an example of a man-made semi-arid ecosystem, by means of large-eddy simulations. We focus on the interaction between surface–atmosphere exchange and secondary circulations that couple the isolated forest to the surrounding shrubland. The large-eddy simulations feature a grid resolution that resolves the forest canopy in several layers, and are initialized by satellite data and Doppler lidar, eddy-covariance and radiosonde measurements. We perform three large-eddy simulations with different geostrophic wind speeds to investigate the influence of those wind speeds on the surface–atmosphere exchange. We reproduce the measured mean updrafts above the forest and mean downdrafts above the shrubland, which increase in strength with decreasing geostrophic wind speed. The largest updrafts emerge above the older, denser part of the forest, triggering secondary circulations. The spatial extent of these circulations does not cover the entire forest area, although we observe a reduced aerodynamic resistance in the regions of updraft. Our simulations indicate that the enhanced surface–atmosphere exchange of the Yatir forest is not sufficient to compensate for the increased net radiation, due to the lower albedo of the forest with respect to the surroundings, resulting in higher air temperatures inside the forest. However, the difference between the forest and shrubland temperatures decreases with increasing geostrophic wind speed due to reduction in the aerodynamic resistance.

Using EOF Analysis over a Large Area for Assessing the Climate Impact of Small-Scale Afforestation in a Semiarid Region

AbstractThe authors suggest an approach to analyze the effects of small-scale afforestation on the surrounding climate of a large heterogenic area. While simple statistics have difficulty identifying the effect, here a well-known eigenvector technique is used to overcome several specific challenges that result from a limited research region, complex topography, and multiple atmospheric circulation patterns. This approach is applied to investigate the influence of the isolated Yatir forest, at the north edge of Israel’s Negev Desert. It was found that this forest does influence the daily climate, primarily seen in the main pattern of the empirical orthogonal function (EOF) of temperature and humidity. The EOF explains 93% and 80%, respectively, of the total variance in the data. Although the Yatir forest is small, it is significant in regulating the climate in the nearby surroundings, as it is located in a sharp transition area toward an arid climate. The results are presented as maps of correlation and regression between the normalized principal component time series of each pattern as well as other time series of the raw data and spatially interpolated data stations. Analysis of short-term campaign measurements around the Yatir forest supports the EOF results, and shows the forest’s influence to the south, mainly during nighttime when the forest becomes cooler than its surroundings. Overall, results suggest that in areas of transition to semiarid climates, forests regulate the surrounding surface air temperature and humidity fields. Wind analysis based on a complex EOF technique reveals the pattern of the daily cycle of surface wind over the region.

Variations in the Isotopic Composition of Dissolved Inorganic Carbon in the Unsaturated Zone of a Semi-Arid Region

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ 18 O and tritium content, dissolved inorganic carbon (DIC) and its δ 13 C (‰) and Δ 14 C (‰) content, and δ 13 C (‰) and Δ 14 C (‰) in the solid sediment. The profiles of moisture and δ 18 O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr –1 ). The source of carbon in the DIC is CO 2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ 13 C (‰) profiles show clearly the chemical transition from dissolved CO 2 (δ 13 C = –22‰) to bicarbonate (δ 13 C = –14‰). At greater depth (–11.3‰), the δ 13 C becomes similar to the δ 13 C in the aquifer below (–12.5‰). The effect of secondary processes is evident in the profile of Δ 14 C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10‰ yr –1 . Precipitation of carbon from the DIC on the sediment is 1.1 mg C L sed –1 yr –1 , negligible compared to the 28 g C in 1 L sed . In the solid sediment, there is a gradient in Δ 14 C carb at the top meter. The net precipitation of 14 C from the DIC on the sediment (0.25 to 1.1‰ yr –1 ), corrected for decay, cannot be observed in the deeper sediment. The presence of 14 C in the top 1 m of the sediment is explained by two possible processes: accumulation of 14 C-tagged dust (~0.05 mm yr –1 ) and/or long-term cumulative precipitation from the DIC. DOI: 10.2458/azu_rc.57.18356

Secondary circulations at a solitary forest surrounded by semi-arid shrubland and their impact on eddy-covariance measurements

The Yatir forest in Israel is a solitary forest at the dry timberline that is surrounded by semi-arid shrubland. Due to its low albedo and its high surface roughness, the forest has a strong impact on the surface energy budget, and is supposed to induce a secondary circulation, which was assessed using eddy-covariance (EC) and Doppler lidar measurements and large-eddy simulation (LES). The buoyancy fluxes were 220–290Wm−2 higher above the forest, and the scale of the forest relative to the boundary-layer height is ideal for generating a secondary circulation, as confirmed by a LES run without background wind. However, usually a relatively high background wind (6ms−1) prevails at the site. Thus, with the Doppler lidar a persistent updraft above the forest was detected only on 5 of the 16 measurement days. Nevertheless, the secondary circulation and convective coherent structures caused low-frequency flux contributions in the mixed-layer w spectra, the surface layer u and w spectra and in the surface-layer momentum fluxes. According to the ogive functions from the tower data and a control volume approach using the LES, such low-frequency contributions with timescales >30min were a major reason the non-closure of the energy balance at the desert site. In the roughness sublayer above the forest, these large structures were broken up into smaller eddies and the energy balance was closed.

Variations in the Isotopic Composition of Dissolved Inorganic Carbon in the Unsaturated Zone of a Semi-Arid Region

A study of water and carbon isotopes was conducted in a bare plot in the unsaturated zone of the Yatir Forest in the northern Negev of Israel. Sediment cores were collected in three different seasons. Measurements include profiles of mineralogy, moisture and its δ18O and tritium content, dissolved inorganic carbon (DIC) and its δ13C (‰) and Δ14C (‰) content, and δ13C (‰) and Δ14C (‰) in the solid sediment. The profiles of moisture and δ18O in the cores show clearly the effect of evaporation. The tritium profile indicates infiltration of water (0.11 m yr−1). The source of carbon in the DIC is CO2 released by biotic activity through roots of trees and of seasonal plants, which show seasonal variations, and by decay of organic debris. The δ13C (‰) profiles show clearly the chemical transition from dissolved CO2 (δ13C = −22‰) to bicarbonate (δ13C = −14‰). At greater depth (–11.3‰), the δ13C becomes similar to the δ13C in the aquifer below (–12.5‰). The effect of secondary processes is evident in the profile of Δ14C in the DIC. It shows a clear decrease with depth due to exchange with the sediment at a rate of 10‰ yr−1. Precipitation of carbon from the DIC on the sediment is 1.1 mg C Lsed−1 yr−1, negligible compared to the 28 g C in 1 Lsed. In the solid sediment, there is a gradient in Δ14Ccarb at the top meter. The net precipitation of 14C from the DIC on the sediment (0.25 to 1.1‰ yr−1), corrected for decay, cannot be observed in the deeper sediment. The presence of 14C in the top 1 m of the sediment is explained by two possible processes: accumulation of 14C-tagged dust (∼0.05 mm yr−1) and/or long-term cumulative precipitation from the DIC.

Drought stress, growth and nonstructural carbohydrate dynamics of pine trees in a semi-arid forest.

In trees exposed to prolonged drought, both carbon uptake (C source) and growth (C sink) typically decrease. This correlation raises two important questions: (i) to what degree is tree growth limited by C availability; and (ii) is growth limited by concurrent C storage (e.g., as nonstructural carbohydrates, NSC)? To test the relationships between drought, growth and C reserves, we monitored the changes in NSC levels and constructed stem growth chronologies of mature Pinus halepensis Miller trees of three drought stress levels growing in Yatir forest, Israel, at the dry distribution limit of forests. Moderately stressed and stressed trees showed 34 and 14% of the stem growth, 71 and 31% of the sap flux density, and 79 and 66% of the final needle length of healthy trees in 2012. In spite of these large reductions in growth and sap flow, both starch and soluble sugar concentrations in the branches of these trees were similar in all trees throughout the dry season (2-4% dry mass). At the same time, the root starch concentrations of moderately stressed and stressed trees were 47 and 58% of those of healthy trees, but never <2% dry mass. Our results show that all the studied trees maintain a fairly good coordination between C supply and demand, and even during prolonged drought there is more than one way for a tree to maintain a positive C balance.

Fluxes of Fine Particles Over a Semi-Arid Pine Forest: Possible Effects of a Complex Terrain

Semi-arid forests are of growing importance due to expected ecosystem transformations following climatic changes. Dry deposition of atmospheric aerosols was measured for the first time in such an ecosystem, the Yatir forest in southern Israel. Size-segregated flux measurements for particles ranging between 0.25 μm and 0.65 μm were taken with an optical particle counter (OPC) using eddy covariance methodology. The averaged deposition velocity (Vd ) at this site was 3.8 ± 4.5 mm s−1 for 0.25–0.28 μm particles, which is in agreement with deposition velocities measured in mid and northern latitude coniferous forests, and is most heavily influenced by the atmospheric stability and turbulence conditions, and to a lesser degree by the particle size. Both downward and upward fluxes were observed. Upward fluxes were not associated with a local particle source. The flux direction correlated strongly with wind direction, suggesting topographical effects. We hypothesize that a complex terrain and a patchy fetch affec...

Fire-free natural regeneration in water limited Pinus halepensis forests: a silvicultural approach

We aimed to elucidate environmental and silvicultural factors that determine the extent of fire-free natural regeneration in east Mediterranean Pinus halepensis forests. The specific aims were to study the potential and identify bottlenecks for natural regeneration and examine the effects of overstory thinning and site preparation treatments. We integrated four experiments conducted in diverse ecoregions in Israel ranging from semiarid to subhumid. Seedling emergence and survival were traced for 2–4 years in Mishmar Ha’Emek Forest (MHF) with annual rainfall of 604 mm; Shaharia Forest (SF)—435 mm; and Yatir Forest (YF)—280 mm. Effects of thinning treatments: clearcut, 100 tree, 200 tree ha−1 and control—310 tree ha−1, were examined in MHF. Effects of site preparation treatments: soil scarification and herb clipping were examined in SF. Density of emerged seedlings varied among ecoregions and was linked to average annual rainfall. Within ecoregions, emerged seedling density was higher on north-facing slopes than on south-facing slopes and was strongly related to stand characteristics (e.g., stem basal area) and seed rain. Seedling survival rate of about 20 % was recorded in MHF while no seedling survival was observed in YF and SF during the study years. Thinning treatments reduced seed rain and emerging seedling density, but increased seedling survival and growth. Recruitment 4 years after thinning was highest in the 100 tree ha−1 treatment and lowest in the control. In SF, seedling survival was limited mainly by herbaceous vegetation and was improved by soil scarification and herb clipping. Silvicultural implications are discussed.

Transpiration and annual water balance of Aleppo pine in a semiarid region: Implications for forest management

Transpiration is a fundamental datum in understanding the ecophysiology of planted forests in dry regions and is central to the construction of an ecosystem-level water balance. The present aims were: (i) to measure daily transpiration in a dryland Pinus halepensis Mill. (Aleppo pine) forest and to examine its relationship to environmental conditions such as soil water content and evaporative demand; (ii) to determine the seasonal and annual water balances of the ecosystem; and (iii) to explore management implications in the context of a climate-change scenario of increasing aridity. The study was conducted in the Yatir forest (300treesha−1) in Israel’s semiarid northern Negev, during three consecutive years (starting 2003/4) in which rainfall (R) was 231, 334 and 224mm, the last designated a drought because of relatively long dry spells between major rain events. Tree transpiration was measured by the heat-pulse method and values were upscaled to the forest canopy level. Independent estimates were obtained for daily ecosystem-level evapotranspiration (ET), and soil and understory vegetation evapotranspiration (E). Daily canopy-level transpiration rate (T) ranged from 0.1 to 1.6mmd−1 and showed a highly dynamic and irregular pattern during the rainy season. For the two non-drought years a large part of the variation in T could be attributed to the following simple relationships. When soil water content (SWC) ⩽0.15m3m−3, the primary driver of T was SWC; regression of T on SWC yielded highly significant quadratic relationships indicating little response below SWC of approximately 0.12m3m−3, and a steep linear response above it. For SWC >0.15m3m−3, potential evapotranspiration (PET) was of paramount importance; quadratic regression of T on PET yielded highly significant relationships and explained a high proportion of the variance in T. During the wet season (215d), on average, cumulative ET (201mm) accounted for 0.76 R (R=263mm) and cumulative T (116mm; range 103–126mm) – an independently estimated component of cumulative ET – accounted for 0.45 R. Cumulative E was 70mm. On an annual basis, total evapotranspiration losses were approximately equal to R, with 58% exiting the system via the trees and 39% via soil and undergrowth vegetation. Water balance data combined with assumptions regarding tree minimum transpiration led to a first approximation of sustainable forest density. This approach indicated that the Yatir forest should be thinned to stands of 250 or 190treesha−1 in order to remain sustainable under annual rainfall regimes of 200 or 150mm, respectively.

Variations in Soil CO2 Concentrations and Isotopic Values in a Semi-Arid Region Due to Biotic and Abiotic Processes in the Unsaturated Zone

A study of CO2 in soil gas was conducted in a bare plot in the unsaturated zone (USZ) of Yatir Forest, northern Negev, Israel. In 2006, 6 tubes for sampling of soil gas were inserted into the USZ to depths of 30, 60, 90, 120, 200, and 240 cm. Profiles of soil gas in the USZ were collected from the tubes 5 times between October 2007 and September 2008. Measurements of the collected profiles of soil gas were of CO2 (ppm), δ13C (′), and Δ14C (′). At all times, the concentration of CO2 in the soil gas was higher than in the air at the surface (CO2 ≃ 400 ppm; δ13C ≃ −9′). The main source of the CO2 in soil gas is from biotic activity released through roots of trees and of seasonal plants close to the surface. In the winter, the CO2 concentrations were lowest (6000 ppm) and the δ13C was −20′. In the spring and through the summer, the CO2 concentration increased. It was estimated that the major source of CO2 is at ≃240 cm depth (δ13C ≃ −22′; CO2 ≃ 9000 ppm) or below. Above this level, the concentrations decrease and the δ13C (′) become more positive. The 14C values in the measured profile are all less than atmospheric and biotic 14C. It was deduced that biotic CO2 dissolves in porewater to form carbonic acid, which then dissolves secondary carbonate (δ13C ≃ −8′; 14C ≃ −900′) from the sediments of the USZ. With the 14C data, the subsequent release of CO2 into the soil gas was then estimated. The 14C data, supported by the 13C and CO2 data, also indicate a biotic source at the root zone, at about 90 cm depth.

Forests and Climate: A Warming Paradox

![Figure][1] Yatir forest. In their Report, Rotenberg and Yakir studied the semi-arid Yatir forest in Israel. CREDIT: THE WEIZMANN INSTITUTE E. Rotenberg and D. Yakir (“Contribution of semi-arid forests to the climate system,” Reports, 22 January, p. [451][2]) showed that forestation may not be an effective tool for climate change mitigation. They found that in a semi-arid landscape, the warming potential of a forest due to changes in the surface albedo and the longwave radiation emission far outweighs the cooling effect due to carbon sequestration. However, their analysis did not address the fact that the radiation balance of the surface is not the same as the radiation balance of the climate system. The atmosphere retains a significant portion of the longwave radiation emitted and the shortwave radiation reflected by the surface. Globally, only 10% of the surface longwave radiation escapes the atmosphere to the outer space ([ 1 ][3]). The escape fraction over Rotenberg and Yakir's site is probably higher due to low cloud cover, but not by much: The outgoing longwave radiation for a clear sky at the top of the atmosphere suggests a maximum of 20% ([ 2 ][4]). Similarly, because of atmospheric absorption and cloud reflection, the local albedo at the top of the atmosphere is lower than the surface value. By not taking into account this energy redistribution, Rotenberg and Yakir may have substantially overestimated the warming effect of forestation (and the cooling effect of desertification). A deeper issue, also related to energy redistribution, is whether it is accurate to combine the CO2 radiative forcing and the surface radiation change for the purpose of analysis. To help policy discussions, the greenhouse effects are often expressed as climate sensitivity ([ 3 ][5]), estimated at ∼0.8°C increase in the surface temperature per W m−2 increase in the radiative forcing ([ 4 ][6]). The surface exchange process does not work that way. Rotenberg and Yakir's paradoxical result—that the forest, being an efficient convector, is much cooler despite more radiation loading than the shrubland—provides a powerful argument against combining the two quantities. In humid climates, forests also cool the surface by removing its latent heat, which is released above the atmospheric boundary layer by cloud condensation. 1. [↵][7]1. K. E. Trenberth 2. et al ., Bull. Am. Meteorol. Soc. 90, 331 (2009). [OpenUrl][8] 2. [↵][9]1. S. K. Yang 2. et al ., J. Clim. 12, 477 (1999). [OpenUrl][10][CrossRef][11] 3. [↵][12]1. V. Ramanthan, 2. G. Carmichael , Nat. Geosci. 1, 221 (2008). [OpenUrl][13][CrossRef][14] 4. [↵][15]1. S. Solomon IPCC, Summary for Policymakers (SPM), in Climate Change 2007: The Physical Science Basis: Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, S. Solomon et al ., Eds. (Cambridge Univ. Press, New York, 2007). [1]: pending:yes [2]: /lookup/doi/10.1126/science.1179998 [3]: #ref-1 [4]: #ref-2 [5]: #ref-3 [6]: #ref-4 [7]: #xref-ref-1-1 View reference 1 in text [8]: {openurl}?query=rft.jtitle%253DBull.%2BAm.%2BMeteorol.%2BSoc.%26rft.volume%253D90%26rft.spage%253D331%26rft.atitle%253DBULL%2BAM%2BMETEOROL%2BSOC%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [9]: #xref-ref-2-1 View reference 2 in text [10]: {openurl}?query=rft.jtitle%253DJ.%2BClim.%26rft.volume%253D12%26rft.spage%253D477%26rft_id%253Dinfo%253Adoi%252F10.1175%252F1520-0442%25281999%2529012%253C0477%253AEOTERB%253E2.0.CO%253B2%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [11]: /lookup/external-ref?access_num=10.1175/1520-0442(1999)012 2.0.CO;2&link_type=DOI [12]: #xref-ref-3-1 View reference 3 in text [13]: {openurl}?query=rft.jtitle%253DNat.%2BGeosci.%26rft.volume%253D1%26rft.spage%253D221%26rft_id%253Dinfo%253Adoi%252F10.1038%252Fngeo156%26rft.genre%253Darticle%26rft_val_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Ajournal%26ctx_ver%253DZ39.88-2004%26url_ver%253DZ39.88-2004%26url_ctx_fmt%253Dinfo%253Aofi%252Ffmt%253Akev%253Amtx%253Actx [14]: /lookup/external-ref?access_num=10.1038/ngeo156&link_type=DOI [15]: #xref-ref-4-1 View reference 4 in text

Effects of spatial variations in soil evaporation caused by tree shading on water flux partitioning in a semi-arid pine forest

In dry environments, water availability is a major limitation to forest productivity while losses to soil evaporation ( E) are a significant component in ecosystem hydrology. We report on a 3-year study (2004–2007) in a semi-arid pine forest in Southern Israel (40-year-old Pinus halepensis; leaf area index = 1.5; mean precipitation 285 mm year −1) that estimated soil E, assessed its spatial variability and identified the factors influencing it. We used a modified and specially calibrated soil respiration chamber to directly measure E on a weekly basis at 14 permanently installed soil collars across the range of soil surface conditions. Results showed that spatial variability in E was large, with SD of ±47% between measurement sites. E fluxes measured in sun-exposed areas were on average double those in shaded areas (0.11 mm h −1 vs. 0.06 mm h −1). The spatial variability in E correlated with radiation (measured in the photosynthetically active range), which was up to 92% higher in exposed compared to shaded sites, and with soil water content, which was higher in exposed areas during the wetting season but higher in shaded areas during the drying season. The fraction of shaded forest floor area was described as a function of canopy geometry (mean tree height, crown width and stand density) and the daily variation in solar altitude. Simple simulations based on the relationship between E and the shaded fraction indicated that E/ P (precipitation) for the Yatir forest could decrease from 0.53 (undeveloped canopy of 10% cover) to 0.30 (full canopy closure). However, according to our analysis, increasing canopy cover will also increase intercepted precipitation and transpiration such that current precipitation inputs will not be able to support forest growth above a canopy cover of 65%. Combining direct measurements of environmental conditions and canopy characteristics with such simulations can provide a simple predictive and management tool to optimize tree water use in dry environments.

Afforestation of semi-arid shrubland reduces biogenic NO emission from soil

Nitric oxide (NO) plays a central role in the formation of tropospheric ozone, hydroxyl radicals, as well as nitrous and nitric acids. There are, however, large uncertainties around estimates of global NO emissions due to the paucity of data. In particular, there is little information on the rate of NO emission and its sensitivity to processes such as land use changes in dry environments. Here we report on a two-year study on the influence of afforestation on soil NO fluxes in the semi-arid afforestation system in Southern Israel (Yatir forest, mean annual precipitation ∼280 mm). Laboratory incubations were carried out under seasonally defined conditions of soil moisture and temperature using soils sampled in different seasons from the native shrubland (taken both under shrub canopy and in the inter-shrub areas), and from the adjacent ∼2800 ha, 40-year-old pine afforestation site. Combining laboratory results with field measurements of soil moisture and temperature, we up-scaled soil–atmosphere NO fluxes to the ecosystem level. The different microsites differed in their annual mean NO release rates (0.04, 0.14 and 0.03 mg m −2 d −1 for the shrubland under and between shrubs and for the forest, respectively), and exhibited high inter-seasonal variability in NO emission rates (ranging from zero up to 0.25 mg m −2 d −1 in the wet and dry-rewetting seasons, respectively), as well as in temperature responses. Up-scaling results to annual and ecosystem scales indicated that afforestation of the semi-arid shrubland could reduce soil NO emission by up to 65%.

Assessment of temporal changes in aboveground forest tree biomass using aerial photographs and allometric equations

Studies of forest biomass dynamics typically use long-term forest inventory data, available in only a few places around the world. We present a method that uses photogrammetric measurements from aerial photographs as an alternative to time-series field measurements. We used photogrammetric methods to measure tree height and crown diameter, using four aerial photographs of Yatir Forest, a semi-arid forest in southern Israel, taken between 1978 and 2003. Height and crown-diameter measurements were transformed to biomass using an allometric equation generated from 28 harvested Aleppo pine (Pinus halepensis Mill.) trees. Mean tree biomass increased from 6.37 kg in 1978 to 97.01 kg in 2003. Mean plot biomass in 2003 was 2.48 kg/m2 and aboveground primary productivity over the study period ranged between 0.14 and 0.21 kg/m2 per year. There was systematic overestimation of tree height and systematic underestimation of crown diameter, which was corrected for at all time points between 1978 and 2003. The estimated biomass was significantly related to field-measured biomass, with an R2 value of 0.78. This method may serve as an alternative to field sampling for studies of forest biomass dynamics, assuming that there is sufficient spatial and temporal coverage of the investigated area using high-quality aerial photography, and that the tree tops are distinguishable in the photographs.

The use of remote sensing and GIS for spatio-temporal analysis of the physiological state of a semi-arid forest with respect to drought years

Drought years are a very frequent phenomenon in Israel. Between the years 1994/1995 and 2001/2002, Israel experienced four (non-consecutive) years of drought. Consequently the Yatir forest, a pine forest located in the desert fringe, suffered from a notable water shortage. The aim of this research is to detect and assess seasonal/phenological changes and inter-annual changes in the forest trees with respect to the drought effect. The use of a spectral vegetation index, namely the Normalized Difference Vegetation Index (NDVI) to detect stress conditions was implemented by using eight Landsat-TM and ETM+ images. In addition, the change detection NDVI Image Differencing technique was applied for assessing seasonal and inter-annual variations in vegetation. The results indicate similarity between the photosynthetic activity and the NDVI dynamics along the growing season. Considerable NDVI decline was observed between 1995 and 2000 due to the drought events during these years, enabling assessment of the spatial and temporal effects of such a disaster. The NDVI measured from the forest trees was found to be inversely related to the age of the trees due to strong effect of soil background in the younger forest sections that are characterized by lower vegetation density. Topographic attributes such as slope orientation (aspect) were found to affect NDVI only at the year that was not under stress. Under stress conditions, it is evident that environmental factors such as soil type, parent material, and topography are not correlated with NDVI dynamics.

Genetic variation within Pinus halepensis Mill. provenances growing in different microenvironments in Israel

The aim of the study was to compare the genetic variability within F 1 -progeny populations of Aleppo pine, growing under extreme environmental condition such as drought, with the genetic variability in the parental natural populations. Two different methods were used: Protein polymorphism by means of starch gel electrophoresis of enzymes, and genomic DNA polymorphism by means of the RAPD and PCR methods. The result of isoenzyme analysis showed that the number of alleles per locus (A/L) in populations ranged from 1.3 to 1.7. Percent polymorphic loci (P%) ranged between 12 and 44% in the natural populations and between 40 and 48% in the populations growing under the stressful environment. Mean value P% increases from 31% in natural forests to 44% in the Yatir forest populations and, consequently, the mean observed heterozygosity increased from 0.096 to 0.152. RAPD analysis showed that gene diversity (h) rose from between 0.264 and 0.316 in the natural populations to between 0.406 and 0.431 in Yatir. The calculated linear regression between allozyme heterozygosity and gene diversity resulted in r = 0.690, p < 0.001, n = 5. The Yatir afforestation can be looked upon as peripheral populations under selection pressure that eliminates homozygous and favors heterozygous genomes. This forest should be treated as a biogenetic resource for seed material.