Paula I. Campanello

Hydraulic differences along the water transport system of South American Nothofagus species: do leaves protect the stem functionality?

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.

Water storage dynamics in the main stem of subtropical tree species differing in wood density, growth rate and life history traits

Wood biophysical properties and the dynamics of water storage discharge and refilling were studied in the trunk of canopy tree species with diverse life history and functional traits in subtropical forests of northeast Argentina. Multiple techniques assessing capacitance and storage capacity were used simultaneously to improve our understanding of the functional significance of internal water sources in trunks of large trees. Sapwood capacitances of 10 tree species were characterized using pressure-volume relationships of sapwood samples obtained from the trunk. Frequency domain reflectometry was used to continuously monitor the volumetric water content in the main stems. Simultaneous sap flow measurements on branches and at the base of the tree trunk, as well as diurnal variations in trunk contraction and expansion, were used as additional measures of stem water storage use and refilling dynamics. All evidence indicates that tree trunk internal water storage contributes from 6 to 28% of the daily water budget of large trees depending on the species. The contribution of stored water in stems of trees to total daily transpiration was greater for deciduous species, which exhibited higher capacitance and lower sapwood density. A linear relationship across species was observed between wood density and growth rates with the higher wood density species (mostly evergreen) associated with lower growth rates and the lower wood density species (mostly deciduous) associated with higher growth rates. The large sapwood capacitance in deciduous species may help to avoid catastrophic embolism in xylem conduits. This may be a low-cost adaptation to avoid water deficits during peak water use at midday and under temporary drought periods and will contribute to higher growth rates in deciduous tree species compared with evergreen ones. Large capacitance appears to have a central role in the rapid growth patterns of deciduous species facilitating rapid canopy access as these species are less shade tolerant than evergreen species.

Functional relationships between leaf hydraulics and leaf economic traits in response to nutrient addition in subtropical tree species

Leaves can be both a hydraulic bottleneck and a safety valve against hydraulic catastrophic dysfunctions, and thus changes in traits related to water movement in leaves and associated costs may be critical for the success of plant growth. A 4-year fertilization experiment with nitrogen (N) and phosphorus (P) addition was done in a semideciduous Atlantic forest in northeastern Argentina. Saplings of five dominant canopy species were grown in similar gaps inside the forests (five control and five N + P addition plots). Leaf lifespan (LL), leaf mass per unit area (LMA), leaf and stem vulnerability to cavitation, leaf hydraulic conductance (K(leaf_area) and K(leaf_mass)) and leaf turgor loss point (TLP) were measured in the five species and in both treatments. Leaf lifespan tended to decrease with the addition of fertilizers, and LMA was significantly higher in plants with nutrient addition compared with individuals in control plots. The vulnerability to cavitation of leaves (P50(leaf)) either increased or decreased with the nutrient treatment depending on the species, but the average P50(leaf) did not change with nutrient addition. The P50(leaf) decreased linearly with increasing LMA and LL across species and treatments. These trade-offs have an important functional significance because more expensive (higher LMA) and less vulnerable leaves (lower P50(leaf)) are retained for a longer period of time. Osmotic potentials at TLP and at full turgor became more negative with decreasing P50(leaf) regardless of nutrient treatment. The K(leaf) on a mass basis was negatively correlated with LMA and LL, indicating that there is a carbon cost associated with increased water transport that is compensated by a longer LL. The vulnerability to cavitation of stems and leaves were similar, particularly in fertilized plants. Leaves in the species studied may not function as safety valves at low water potentials to protect the hydraulic pathway from water stress-induced cavitation. The lack of rainfall seasonality in the subtropical forest studied probably does not act as a selective pressure to enhance hydraulic segmentation between leaves and stems.

Removal of nutrient limitations in forest gaps enhances growth rate and resistance to cavitation in subtropical canopy tree species differing in shade tolerance

A 4-year fertilization experiment with nitrogen (N) and phosphorus (P) was carried out in natural gaps of a subtropical forest in northeastern Argentina. Saplings of six dominant canopy species differing in shade tolerance were grown in five control and five N + P fertilized gaps. Hydraulic architectural traits such as wood density, the leaf area to sapwood area ratio (LA : SA), vulnerability to cavitation (P50) and specific and leaf-specific hydraulic conductivity were measured, as well as the relative growth rate, specific leaf area (SLA) and percentage of leaf damage by insect herbivores. Plant growth rates and resistance to drought-induced embolisms increased when nutrient limitations were removed. On average, the P50 of control plants was -1.1 MPa, while the P50 of fertilized plants was -1.6 MPa. Wood density and LA : SA decreased with N + P additions. A trade-off between vulnerability to cavitation and efficiency of water transport was not observed. The relative growth rate was positively related to the total leaf surface area per plant and negatively related to LA : SA, while P50 was positively related to SLA across species and treatments. Plants with higher growth rates and higher total leaf area in fertilized plots were able to avoid hydraulic dysfunction by becoming less vulnerable to cavitation (more negative P50). Two high-light-requiring species exhibited relatively low growth rates due to heavy herbivore damage. Contrary to expectations, shade-tolerant plants with relatively high resistance to hydraulic dysfunction and reduced herbivory damage were able to grow faster. These results suggest that during the initial phase of sapling establishment in gaps, species that were less vulnerable to cavitation and exhibited reduced herbivory damage had faster realized growth rates than less shade-tolerant species with higher potential growth rates. Finally, functional relationships between hydraulic traits and growth rate across species and treatments were maintained regardless of soil nutrient status.

Facing Shortage or Excessive Light: How Tropical and Subtropical Trees Adjust Their Photosynthetic Behavior and Life History Traits to a Dynamic Forest Environment

Light is critical for plant establishment, growth, and survival in wet tropical forests. The objective of this chapter is to analyze paradigms of photosynthetic performance and life history traits of tropical forest trees to contrasting light environments across the forest floor, gaps and upper canopy. Physiological and morphological plasticity as well as genetically fixed adaptive traits are analyzed, including leaf optical properties and photoprotection from high irradiance. Photosynthetic adaptations to contrasting light environments of closely related species are discussed. This approach has the advantage among comparative studies of adaptations across species in that genetic relationships among species are known. Species-specific variations in maximum photosynthetic rates, which reflect the degree of adaptation to growth irradiance, are shown to be gradual, suggesting that classification into two distinct functional groups in terms of light requirements is somewhat arbitrary. Trees growing in gaps or in the upper canopy rely strongly on biochemical mechanisms to dissipate excess energy and to avoid damage to the light reaction centers and photosystems. Consistent with their high photosynthetic capacity, light demanding species are capable of plastic changes in hydraulic architecture , such as increases in hydraulic conductivity under high irradiance, which makes them more competitive in open habitats.

Carbon Economy of Subtropical Forests

Compared to tropical and temperate forests, subtropical forests have received little attention in physiological and ecological studies until now, and the contribution of this ecosystem type to the global carbon cycle has not been fully assessed. In this chapter we discuss results on the carbon balance of subtropical forests at different spatial and temporal scales, analyze the potential limitation of seasonal low temperatures and water deficits on physiological processes of subtropical trees, and characterize the uniqueness of subtropical forest ecosystems in terms of carbon economy. Results from multiple techniques and scales were included in the carbon balance assessment. The largest two regions with subtropical forests are located in Asia and South America. The net ecosystem carbon gain of subtropical forests in these two regions, which have annual precipitations larger than 800 mm, is probably neither strongly limited by soil water availability nor by seasonal low temperatures. Relatively low evapotranspiration in the winter/dry season and high soil water-holding capacity help maintain good water availability for trees in most subtropical forests. High solar radiation, light penetration and low ecosystem respiration in winter may compensate for the negative effects of low temperatures on gross photosynthesis . Therefore, subtropical forests in many areas can assimilate carbon in excess of respiration throughout the year and they are, probably, among the largest terrestrial carbon sinks across terrestrial ecosystems worldwide. In addition, because leaf and ecosystem respiration respond to temperature changes to a larger extent compared to ecosystem carbon assimilation, a negative relationship between net ecosystem carbon gain and mean annual temperature was found in Asian subtropical and tropical forests. This relationship suggests that global warming may weaken the carbon sink strength of these forest ecosystems. These results indicate the important contribution of subtropical forests to the global carbon cycle and the potentially negative response of these forests to global warming. We hope this information will promote additional physiological and ecological research and conservation in subtropical forests.

Hydraulic architecture and photoinhibition influence spatial distribution of the arborescent palm Euterpe edulis in subtropical forests.

Physiological characteristics of saplings can be considered one of the most basic constraints on species distribution. The shade-tolerant arborescent palm Euterpe edulis Mart. is endemic to the Atlantic Forest of Argentina, Brazil and Paraguay. At a local scale, saplings of this species growing in native forests are absent in gaps. We tested the hypothesis whether sensitivity to photoinhibition or hydraulic architecture constrains the distribution of E. edulis saplings in sun-exposed forest environments. Using shade houses and field studies, we evaluated growth, survival, hydraulic traits and the susceptibility of Photosystem II to photoinhibition in E. edulis saplings under different growth irradiances. Survival rates in exposed sites in the field were very low (a median of 7%). All saplings exhibited photoinhibition when exposed to high radiation levels, but acclimation to a high radiation environment increased the rate of recovery. Petiole hydraulic conductivity was similar across treatments regardless of whether it was expressed per petiole cross-sectional area or per leaf area. At the plant level, investment in conductive tissues relative to leaf area (Huber values) increased with increasing irradiance. Under high irradiance conditions, plants experienced leaf water potentials close to the turgor-loss point, and leaf hydraulic conductance decreased by 79% relative to its maximum value. Euterpe edulis saplings were able to adjust their photosynthetic traits to different irradiance conditions, whereas hydraulic characteristics at the leaf level did not change across irradiance treatments. Our results indicate that uncoupling between water demand and supply to leaves apparently associated with high resistances to water flow at leaf insertion points, in addition to small stems with low water storage capacity, weak stomatal control and high vulnerability of leaves to hydraulic dysfunction, are the main ecophysiological constraints that prevent the growth and survival of E. edulis saplings in gaps in the native forest where native lianas and bamboos show aggressive growth.

Leaf phenology and water-use patterns of canopy trees in Northern Argentinean subtropical forests

Tree physiological processes are affected not only by environmental conditions, but also by phenological leaf stages. During foliar expansion, rapid changes occur, such as the activation of metabolic processes that encompass a hydraulic link between xylem and phloem pathways at a whole-tree level. Daily and seasonal changes in stem diameter may reveal different temporal dynamics of water use and recharge in tree reservoirs. Foliar phenological patterns were studied in relation to stem dimensional changes in 10 canopy tree species with different phenological patterns (three deciduous, three brevideciduous and four evergreen species). Additionally, we assessed (i) daily sap flow fluctuations in branch and main stem, (ii) diurnal changes in sapwood volumetric water content and (iii) stem radius variations during leafless, expanding and mature leaves periods in three of the 10 tree species (two deciduous and one brevideciduous). During the leaf expansion phase, the diameter of trees decreased in all 10 species, with a larger impact on deciduous and brevideciduous species. For the subset of deciduous and brevideciduous species, the movement of long-distance water transport occurred first near the branches and then in the main stem during the leafless stage. Changes in stored water use and a decrease in the volumetric water content and the radius of the main stem during this period suggest that there is a contribution of water from internal stem reservoirs toward the construction of new leaves.

Carbon Allocation and Water Relations of Lianas Versus Trees

Despite lianas being fundamental components of tropical and subtropical forest ecosystems throughout the world, the physiological characteristics of this growth form are not well known. Different behaviors at the seedling stage were until recently largely unnoticed. In one extreme of a continuum of adaptive traits, freestanding liana seedlings invest a large proportion of biomass in self-support tissue while on the other extreme support-seeker seedlings invest more resources in rapid elongation of slender stems with an efficient hydraulic conductive system. Adult lianas often have lower wood density and higher specific leaf area than trees and have most of their leaves deployed at the top of the canopy, experiencing high irradiance and transpirational demands, which requires effective regulation of water loss to avoid desiccation. Recent studies show that lianas have faster stomatal responses to increasing vapor pressure deficit (VPD) and exhibit stronger partial stomatal closure compared to trees. Strong stomatal control and efficient water transport help lianas maintain leaf water potential (Ψleaf) within a safe hydraulic range to avoid xylem dysfunction despite their low stem water storage capacity, which is achieved at a minimum cost in terms of carbon assimilation. Liana colonization of tree crowns can significantly reduce tree growth and transpiration with consequences for carbon and water economy at individual tree and ecosystem levels.

Geographical, Taxonomical and Ecological Aspects of Lianas in Subtropical Forests of Argentina

Lianas are more diverse and typically more abundant in tropical than temperate forests, with subtropical forests being intermediate. In this chapter, we analyze geographical, taxonomical and ecological patterns of lianas in subtropical forests of Northern Argentina, including Mountain Forests (MF), Atlantic Forests (AF); and Dry and Humid Chaco Forests (DCh and HCh, respectively). A total of 184 woody species of climbing plants were recognized in all four subtropical forests, with 35 species exclusive to MF, 38 exclusive to AF, while DCh and HCh had 2 and 8 exclusive species, respectively. In MF most liana species belonged to Sapindaceae and Bignoniaceae (16 % each), followed by Malpighiaceae (11 %) and Apocynaceae (10 %). In AF most liana species belonged to Bignoniaceae (21 %) followed by Apocynaceae (12 %), Fabaceae (11 %), Malpighiaceae (11 %) and Sapindaceae (10 %). Considering all liana species together, the most common climbing mechanisms included tendrils and twiners. The highest liana density was observed in the semideciduous Atlantic Forest, followed by the deciduous Humid Chaco Forest and the semideciduous Montane Forest. The semideciduous Atlantic Forest has also relatively high liana species richness as compared to other subtropical forests, followed by semideciduous MF. Besides geographical location and forest disturbances, little is known about how lianas respond to other environmental factors that drive patterns of liana density and diversity in these subtropical forests.