Drought, axe and goats. More variable and synchronized growth forecasts worsening dieback in Moroccan Atlas cedar forests.

Understanding how climate warming and land-use changes determine the vulnerability of forests to drought is critical. However, we still lack: (i) robust quantifications of long-term growth changes during aridification processes, (ii) links between growth decline, changes in forest cover, stand structure and soil conditions, and (iii) forecasts of growth variability to projected climate warming. We investigated tree-ring records over the past 400-700 years, quantified changes in grazing area and forest cover during the 20th century, sampled current stand structure, and analyzed soil organic carbon δ13C and total nitrogen δ15N of Atlas cedar (Cedrus atlantica (Endl.) Manetti ex Carrière) Moroccan forests to characterize their dieback. Atlas cedar forests experienced massive dieback after the 1970s, particularly in the xeric High Atlas region. Forest cover increased in the less xeric regions (Middle Atlas and Rif) by almost 20%, while it decreased about 18% in the High Atlas, where soil δ13C and δ15N showed evidences of grazing. Growth declined and became more variable in response to recent droughts. The relative growth reduction (54%) was higher in the Middle Atlas than elsewhere (Rif, 32%; High Atlas, 36%). Growth synchrony between forests located within the Middle and High Atlas regions increased after the 1970s. Simulations based on a worst-case emission scenario and rapid warming forecast a stronger limitation of growth by low soil moisture in all regions, but particularly in the Middle Atlas and after the mid-21st century. Climate warming is expected to strengthen growth synchronization preceding dieback of conifer forests in xeric regions. The likelihood of similar dieback episodes is further exacerbated by historical degradation of these forests.

The cost of stress resistance: construction and maintenance costs of leaves and roots in two populations of Quercus ilex.

We tested whether growth and maintenance costs of plant organs vary with environmental stress. Quercus ilex L. seedlings from acorns collected from natural populations in the northern Iberian Peninsula and in a lower elevation and putatively less stressful habitat in the southern Iberian Peninsula were grown in pots under the same conditions. Growth and maintenance respiration were measured by CO(2) exchange. Young leaves from 5-month-old seedlings of both populations had similar mean specific leaf areas, nitrogen and carbon concentrations and specific growth rates, and almost identical growth costs (1.26 g glucose g(-1)). Leaf maintenance cost was higher in northern than in the southern population (27.3 versus 22.4 mg glucose g(-1) day(-1), P < 0.01). In both populations, leaf maintenance cost decreased by 90% as leaves aged, but even in mature leaves, the maintenance cost was higher in the northern population than in the southern population (3.38 versus 2.53 mg glucose g(-1) day(-1), P < 0.01). The growth costs of fine roots < 1 mm in diameter were similar in the two populations (1.20 g glucose g(-1)), whereas fine root maintenance cost was higher in the northern population than in the southern population (9.86 versus 7.45 mg glucose g(-1) day(-1); P < 0.05). The results suggest that the cost of organ maintenance is related to the severity of environmental stress in the native habitat. Because the observed differences in both leaves and roots were constitutive, the two populations may be considered ecotypes.

A study of the leachate toxicity of metal-containing solid wastes using Daphnia magna.

This paper examines the ecotoxicity of different actual inorganic wastes containing some potentially toxic metals commonly found in many industrial residues and the reduction in ecotoxicity obtained when the wastes are submitted to a solidification/stabilization process. The ecotoxicity was assessed as defined in the Spanish regulations on hazardous wastes, by means of the Daphnia magna Straus acute toxicity test applied to the toxicity characteristic leaching procedure (TCLP) waste leachate. On the basis of the EC50 values of the individual metals and the effect of the pH of the aqueous medium on D. magna, we have explored a method for estimating the toxicity of the TCLP waste leachate by calculating its EC50 value in the D. magna test. Although the method is only valid in cases in which a clear cause for the toxicity exists, the procedure described allows us to more completely explain the observed effects on the daphnids. The ecotoxicity of the residues after stabilization is less than that of the untreated waste, generally as a consequence of the reduction in metal leachability achieved by the immobilization treatment. The stabilization process was analyzed in terms of both the chemical and the ecotoxicological parameters, and the influence of leachate pH on the objectives of waste treatment is discussed.