Hydraulic traits were studied for six Nothofagus species from South America (Argentina and Chile), and for three of these species two populations were studied. The main goal was to determine if properties of the water conductive pathway in stems and leaves are functionally coordinated and to assess if leaves are more vulnerable to cavitation than stems, consistent with the theory of hydraulic segmentation along the vascular system of trees in ecosystems subject to seasonal drought. Vulnerability to cavitation, hydraulic conductivity of stems and leaves, leaf water potential, wood density and leaf water relations were examined. Large variations in vulnerability to cavitation of stems and leaves were observed across populations and species, but leaves were consistently more vulnerable than stems. Water potential at 50% loss of maximum hydraulic efficiency (P(50)) ranged from -0.94 to -2.44 MPa in leaves and from -2.6 to -5.3 MPa in stems across species and populations. Populations in the driest sites had sapwood and leaves more vulnerable to cavitation than those grown in the wettest sites. Stronger diurnal down-regulation in leaf hydraulic conductance compared with stem hydraulic conductivity apparently has the function to slow down potential water loss in stems and protect stem hydraulics from cavitation. Species-specific differences in wood density and leaf hydraulic conductance (K(Leaf)) were observed. Both traits were functionally related: species with higher wood density had lower K(Leaf). Other stem and leaf hydraulic traits were functionally coordinated, resulting in Nothofagus species with an efficient delivery of water to the leaves. The integrity of the more expensive woody portion of the water transport pathway can thus be maintained at the expense of the replaceable portion (leaves) of the stem-leaf continuum under prolonged drought. Compensatory adjustments between hydraulic traits may help to decrease the rate of embolism formation in the trees more vulnerable to cavitation.

The effect of the pathogen Phytophthora austrocedrae on tree physiology of Austrocedrus chilensis in Patagonia was studied in a 4‐week study. In the first week, stem‐inoculated saplings showed a significant decrease in photosynthesis (A) without alteration of stomatal conductance (gs) or stem‐specific hydraulic conductivity (Ks). From the second week on, progressive decreases in A, gs and Ks were observed, concomitantly with development of significant stem lesions. Water use efficiency (WUE) increased in the second week and declined progressively from the third week. Hyphae and resinous materials were observed in tracheids and rays below lesions. Necrosis of parenchyma ray cells and blockage of tracheids torus were observed. Healthy xylem showed no resinous materials or tracheid blockage, but abundant starch in rays, which was absent in altered xylem. The culture filtrate (CF) of the pathogen was shown to induce changes in extracellular pH and conductivity, and increased necrosis in tissues of leaves and stem challenged with CF in vitro. Similar results were obtained in leaf tissues of the inoculated saplings in vivo. CF injection into xylem of saplings induced a decline in A and disturbance of leaf tissue integrity, without altering gs, WUE or Ks. The decrease of A correlated with changes in tissue integrity. A possible mechanism of A. chilensis decline induced by P. austrocedrae is discussed.

Phytophthora austrocedri is a straminipilous (heterokonta) organism that causes mortality of Austrocedrus chilensis, an endemic Cupressaceae from the Patagonian Andes forest in temperate South America. This soil pathogen colonizes and kills the roots and extends up to the stem causing necrosis of cambium, phloem and xylem ray parenchyma. An anatomical study of affected tissues was conducted in order to better understand the process of pathogen colonization and tree response. It was found that tracheids of the xylem of affected trees showed large numbers of trabeculae, both rod- and plate-shaped. The occurrence of these structures was clearly associated with the necrotic lesion area, since the trabeculae were rare in healthy tissues above the necrotic lesion. Trabeculae occurred in a variety of arrangements: solitary or in long files, single, double or triple. Our results could indicate that trabeculae proliferation in tracheids of A. chilensis trees is induced by the stress generated by the P. austrocedri invasion. Whether this is triggered by a nonspecific stress response or in direct response to the pathogen remains to be tested.

Phytophthora austrocedri is a straminipilous (heterokonta) organism that causes mortality of Austrocedrus chilensis, an endemic Cupressaceae from the Patagonian Andes forest in temperate South America. This soil pathogen colonizes and kills the roots and extends up to the stem causing necrosis of cambium, phloem and xylem ray parenchyma. An anatomical study of affected tissues was conducted in order to better understand the process of pathogen colonization and tree response. It was found that tracheids of the xylem of affected trees showed large numbers of trabeculae, both rod- and plate-shaped. The occurrence of these structures was clearly associated with the necrotic lesion area, since the trabeculae were rare in healthy tissues above the necrotic lesion. Trabeculae occurred in a variety of arrangements: solitary or in long files, single, double or triple. Our results could indicate that trabeculae proliferation in tracheids of A. chilensis trees is induced by the stress generated by the P. austrocedri invasion. Whether this is triggered by a nonspecific stress response or in direct response to the pathogen remains to be tested.

Fungi are the main decomposers of lignocellulose in temperate forests, and are classified as either white- or brown-rot, based on the ability to degrade lignin along with cellulose and hemicellulose. In this work, decomposition of Nothofagus pumilio wood by different wood-inhabiting fungal species was investigated through in vitro assays. Sapwood and heartwood blocks were individually exposed to 11 fungal species; mass loss was determined after 75, 135, and 195 days of exposure, comparatively analyzing the fungal ability to colonize and degrade this lignocellulosic substrate corresponding to both parts of the wood. Transverse section slices of the blocks were cut and separately stained with two types of dyes, Congo red and phloroglucinol, that are specifically associated with cellulose and lignin, respectively. Most of the species showed a different performance in sapwood and heartwood. Rhizochaete brunnea, Aurantiporus albidus and Phanerochaete velutina produced the greatest mass losses in sapwood. The latter two and Laetiporus portentosus produced the highest mass losses in heartwood, whereas Rh. brunnea was among the worst decomposers of this substrate. White rotters generally showed a higher ability to degrade the sapwood and brown rotters the heartwood. The fungal species that produced greater mass losses in heartwood than in sapwood grow on heartwood of living trees. Among white-rot fungi, two modes of action were identified: a) localized degradation, with zones of advanced decay in a less deteriorated matrix, and b) homogeneous degradation, with an even decay. Our results showed that many species have different performances in different substrates, reinforcing the importance of analyzing sapwood and heartwood decomposition separately, usually not done in this kind of studies.