Dynamics of transpiration, sap flow and use of stored water in tropical forest canopy trees

In large trees, the daily onset of transpiration causes water to be withdrawn from internal storage compartments, resulting in lags between changes in transpiration and sap flow at the base of the tree. We measured time courses of sap flow, hydraulic resistance, plant water potential and stomatal resistance in co-occurring tropical forest canopy trees with trunk diameters ranging from 0.34-0.98 m, to determine how total daily water use and daily reliance on stored water scaled with size. We also examined the effects of scale and tree hydraulic properties on apparent time constants for changes in transpiration and water flow in response to fluctuating environmental variables. Time constants for water movement were estimated from whole-tree hydraulic resistance (R) and capacitance (C) using an electric circuit analogy, and from rates of change in water movement through intact trees. Total daily water use and reliance on stored water were strongly correlated with trunk diameter, independent of species. Although total daily withdrawal of water from internal storage increased with tree size, its relative contribution to the daily water budget (approximately 10%) remained constant. Net withdrawal of water from storage ceased when upper branch water potential corresponded to the sapwood water potential (Psi(sw)) at which further withdrawal of water from sapwood would have caused Psi(sw) to decline precipitously. Stomatal coordination of vapor and liquid phase resistances played a key role in limiting stored water use to a nearly constant fraction of total daily water use. Time constants for changes in transpiration, estimated as the product of whole- tree R and C, were similar among individuals (~0.53 h), indicating that R and C co-varied with tree size in an inverse manner. Similarly, time constants estimated from rates of change in crown and basal sap flux were nearly identical among individuals and therefore independent of tree size and species.     

Direct seeding of late-successional trees to restore tropical montane forest

Natural regeneration of large-seeded, late-successional trees in fragmented tropical landscapes can be strongly limited by a lack of seed dispersal resulting in the need for more intensive restoration approaches, such as enrichment planting, to include these species in future forests. Direct seeding may be an alternative low-cost approach to planting nursery-raised tree seedlings, but there is minimal information on its efficacy or when in the successional process this technique will be most successful. We tested directly seeding five native tree species into habitats representing passive and active restoration approaches: (1) recently abandoned pasture; (2) naturally establishing, young secondary forests; and (3) young, mixed-species (fast-growing N-fixers and commercially valuable species) tree plantations established to facilitate montane forest recovery in southern Costa Rica. We monitored germination, survival, growth, and above- and below-ground biomass over a 2-year period. Germination in pastures, secondary forests, and tree plantations was similar (∼43%). Seedling survival after one and two years was significantly higher under tree plantations (91% year 1, 75% year 2) compared to secondary forests (76, 44%) or pastures (74, 41%). Moreover, seedlings had greater total biomass and lower root:shoot ratios in the plantations, suggesting higher nutrient availability in that treatment. Costs for direct seeding were 10- to 30-fold less per 100 seedlings after 2-year compared to nursery-raised seedlings planted at the same sites; however, there are important trade-offs to the two restoration approaches. Planting nursery-raised seedling is a more effective but higher cost approach for rapidly establishing canopy cover and restoring large areas whereas direct seeding is a more efficient way to enrich an existing system. We particularly recommend using direct seeding as a complimentary measure to the more intensive restoration approach of planting fast-growing and N-fixing trees.     

Species association in tropical montane rain forest at two successional stages in Diaoluo Mountain, Hainan

Species association is one of the basic concepts in community succession. There are different viewpoints on how species interaction changes with the progress of succession. In order to assess these relationships, we examined species associations in the tropical montane rain forest at early and late successional stages in Diaoluo Mountain, Hainan Island. Based on data from a 2 × 2 contingency table of species presence or absence, statistical methods including analysis of species association and X ² tests were applied. The results show that: 1) an overall positive association was present among tree species in the communities during the two successional stages and were statistically significant at the late stage. The number of species pairs with positive and negative associations decreased throughout the process of succession, while the number with null associations was greatly increased. The same trend existed among the dominant and companion species. The results indicate that the communities are developing towards a stable stage where the woody species coexist in harmony. 2) In the early-established and later invading species, all positive associations were not significant. Compared with positive and null associations, fewer negative associations were found. This implies that these species are inclined to coexist independently through portioning of resources. 3) Among the later invading species, positive associations were significant and no negative associations were found which suggest that these species have similar adaptive ability in the habitat and occupied overlapping niches in the community. __________ Translated from Biodiversity Science, 2007, 15(3): 257–263 [译自: 生物多样性]     

Impact of soil drought on sap flow and water status of evergreen trees in a tropical monsoon forest in northern Thailand

Hill evergreen forest is the dominant vegetation type in northern Thailand. In this region, there is higher atmospheric evaporative demand and lower soil moisture during the 5- to 7-month dry season than in the rainy season under influences from Asian monsoons. In an earlier study we revealed that canopy-scale transpiration is actively maintained even during the latter part of the dry season in hill evergreen forest. However, the impact of soil drought on tree water use was not investigated. To clarify the ecohydrological processes at this site, we used individual tree-scale measurements during a 2-year period to base our examination of whether limited water use in individual trees is caused by soil drought in the latter part of the dry season. Sap flow and water potential measurements were conducted in four evergreen trees, two large emergent trees 29.8 and 25.4 m high, and two smaller understory trees 4.8 and 1.4 m high.The amount of rainfall preceding the late dry season of 2004 was significantly less than that preceding the late dry season of 2003. Although a distinct decrease in sap-flow velocities in individual trees due to soil water stress was not found in the late dry season of 2003, it did become comparatively apparent in the late dry season of 2004; ranging from 10 to 40% for a given atmospheric evaporative demand. Furthermore, the reductions in sap-flow velocities and predawn stem-water potential were most significant in the smallest tree. The recovery of sap-flow velocities and water potential in the smallest tree after irrigation confirmed that the reductions in sap-flow velocity and predawn stem-water potential in the smallest tree were caused by soil drought. These results suggest that shallower roots could be reason for the significant decrease in water use in the smallest trees. The deeper roots of larger trees could be the reason for the reduced impact of soil drought on water use in larger trees, and canopy-scale transpiration might be maintained by larger trees, even in an unusually severe drought. These possibilities provide a new insight for management of evergreen forests under Asian monsoon influences.     

Foliar temperature tolerance of temperate and tropical evergreen rain forest trees of Australia

Australian rain forests extend from tropical climates in the north to temperate climates in the south, providing an opportunity to investigate physiological responses to temperature of both temperate and tropical species within the same forest type. Eight, rain forest canopy tree species were selected to cover the 33 degrees latitudinal range of rain forests in eastern Australia. Temperature tolerance was measured in 6-year-old plants grown in a common environment, by exposing leaves to a series of high temperatures during late summer and a series of freezing temperatures during midwinter. Damage was evaluated based on chlorophyll fluorescence measurements made 2 h after exposure and by visual assessment of leaf damage made a week after exposure. Leaves of the tropical species were more heat tolerant and less frost tolerant than leaves of the temperate species, which is consistent with their climate distributions. In contrast, the temperature tolerance of the photosynthetic apparatus was unrelated to climate in a species' native habitat. However, the tropical species underwent significant photoinhibition during winter. All species maintained the integrity of the photosynthetic apparatus and avoided tissue damage over a similar span of temperatures (about 60 degrees C), reflecting the similar annual temperature ranges in Australia's temperate and tropical rain forests. Chlorophyll fluorescence measurements and visual assessment of leaf damage provided different estimates of the absolute and relative temperature tolerances of the species, thus emphasizing the importance of a direct assessment of tissue damage for determining a species' temperature tolerance.     

Growth,survival, carbon rates for some dry tropical forest trees used in enrichment planting in the Cerro Blanco protected forest on the Ecuadorian coast

Deforestation principally to establish cattle pastures has created large expanses of land dominated by exotic grass prior to the establishment of the Cerro Blanco Protected Forest in Ecuador. A dry forest restoration program was carried out from 2008 to 2017. Tree planting sites were cleared in parallel lines through abandoned pasture and secondary vegetation using manual labor. Native tree species were produced in a nursery on site and used in yearly plantings during the rainy season. A total of 637 hectares were planted at an average stocking density of 1,000 trees per hectare. Growth data was collected from trees planted in nine sites from 2008 to 2017. Mortality rates were determined from a sample of 400 trees of twenty-two species. Survival of all species was greater than 50%. Ten species had survival rates greater than 90%. Vitex gigantea, with high survival rates, also had significantly greater growth rates than the other species (p < 0.0001) and showed the greatest yearly accumulation of total carbon (2.07 Kg C yr ^?1 stem^?1). Other species with high growth rates were Cordia alliodora and Centrolobium ochroxylum. This restoration program demonstrates that the dry forest restoration with timely maintenance and protection from fire and grazing is possible with intensive tree planting of native species.     

Stomatal control and hydraulic conductance, with special reference to tall trees

A better understanding of the mechanistic basis of stomatal control is necessary to understand why modes of stomatal response differ among individual trees, and to improve the theoretical foundation for predictive models and manipulative experiments. Current understanding of the mechanistic basis of stomatal control is reviewed here and discussed in relation to the plant hydraulic system. Analysis focused on: (1) the relative role of hydraulic conductance in the vicinity of the stomatal apparatus versus whole-plant hydraulic conductance; (2) the influence of guard cell inflation characteristics and the mechanical interaction between guard cells and epidermal cells; and (3) the system requirements for moderate versus dramatic reductions in stomatal conductance with increasing evaporation potential. Special consideration was given to the potential effect of changes in hydraulic properties as trees grow taller. Stomatal control of leaf gas exchange is coupled to the entire plant hydraulic system and the basis of this coupling is the interdependence of guard cell water potential and transpiration rate. This hydraulic feedback loop is always present, but its dynamic properties may be altered by growth or cavitation-induced changes in hydraulic conductance, and may vary with genetically related differences in hydraulic conductances. Mechanistic models should include this feedback loop. Plants vary in their ability to control transpiration rate sufficiently to maintain constant leaf water potential. Limited control may be achieved through the hydraulic feedback loop alone, but for tighter control, an additional element linking transpiration rate to guard cell osmotic pressure may be needed.     

Soil physical properties and preferential flow pathways in tropical rain forest, Bukit Tarek, Peninsular Malaysia

Soil physical properties and water movement within soil were investigated using dyes in a tropical rain forest, the Bukit Tarek Experimental Watershed of Peninsular Malaysia. The saturated hydraulic conductivity (K _s) decreased with increasing soil depth. TheK _s values were higher than those reported for other tropical soils. The geometric means of theK _s values ranged from 4.69×10⁻³ (80 cm) to 4.07×10⁻² cm s⁻¹ (10cm). This suggests saturation overland flow may not be dominant but that subsurface flow must play an important role in stormflow generation. The shapes of the soil moisture characteristic curves resembled those of forest soils which have large changes in volumetric water content at pressure heads <30 cmH₂O. The relatively high conductivities were due to the presence of a porous zone of decomposed root channels which existed continuously in vertical direction. Besides decayed roots, living roots also encourage preferential flow in vertical and lateral (downslope) directions. Termite activities may also form water flow pathways in tropical regions. These detailed results help us analyze water flow within the soil in tropical rain forests.     

Conservation of forest biodiversity and ecosystem properties in a pastoral landscape of the Ecuadorian Andes

High Andean cloud forests are home to a diversity of unique wildlife and are important providers of ecosystem services to people in the Andean regions. The extent of these cloud forests has been widely reduced through conversion to pasture for livestock, which threatens the forests’ ability to support biodiversity and provide ecosystem services. This paper explores whether impacts on woody plant biodiversity and four ecosystem properties (woody plant species richness, juvenile timber tree abundance, soil organic matter content and soil moisture) from converting forest to pasture can be mitigated if some woody forest vegetation is maintained within pastures. Woody vegetation in pastures was found to conserve those woody plant species that are more tolerant to exposure and grazing, but conservation of the high montane cloud forest community required areas of forest from which livestock were restricted. The sampled sites clustered according to woody plant species cover; these clusters represented a gradient from pasture with patches of shrubs to mature forest. Clusters differed in both woody plant species richness and number of juvenile timber trees whereas soil organic matter and soil moisture were observed to be similar among all clusters. This suggests that the different habitats may have some equivalent ecosystem properties. We conclude that the presence of woody vegetation in pastures may reduce some of the impacts of converting forest to pasture, but should not be considered a substitute for protecting large areas of forest, which are essential for maintaining woody plant species diversity in high Andean cloud forest.     

Comparative seed ecophysiology of wild and cultivated Carica papaya trees from a tropical rain forest region in Mexico

To ascertain the effects of centuries of cultivation practices on seed behavior and dormancy mechanisms, we compared seed size and germination characteristics of wild and cultivated (domesticated) populations of Carica papaya L. Germination experiments were carried out under various conditions of temperature, light, seed soaking and gibberellic acid treatments. Wild papaya seeds showed responses to treatment that are characteristic of seeds of many rain forest pioneer trees. Seeds were small and light sensitive, whereas cultivated papaya seeds were 33% larger and their light responses as well as other physiological traits indicated that cultivation had resulted in a lessening in the importance of specific environmental conditions for dormancy breaking and germination.     

Soil-plant hydrology of indigenous and exotic trees in an Ethiopian montane forest

Fast-growing exotic trees are widely planted in the tropics to counteract deforestation; however, their patterns of water use could be detrimental to overall ecosystem productivity through their impact on ecosystem water budget. In a comparative field study on seasonal soil-plant water dynamics of two exotic species (Cupressus lusitanica Mill. and Eucalyptus globulus Labill.) and the indigenous Podocarpus falcatus (Thunb.) Mirb. in south Ethiopia, we combined a 2.5-year record for climate and soil water availability, natural-abundance oxygen isotope ratios (delta(18)O) of soil and xylem water, destructive root sampling and transpiration measurements. Soil was generally driest under C. lusitanica with its dense canopy and shallow root system, particularly following a relatively low-rainfall wet season, with the wettest soil under E. globulus. Wet season transpiration of C. lusitanica was twice that of the other species. In the dry season, P. falcatus and C. lusitanica reduced transpiration by a factor of six and two, respectively, whereas E. globulus showed a fivefold increase. In all species, there was a shift in water uptake to deeper soil layers as the dry season progressed, accompanied by relocation of live fine root biomass (LFR) of C. lusitanica and P. falcatus to deeper layers. Under P. falcatus, variability in soil matric potential, narrow delta(18)O depth gradients and high LFR indicated fast water redistribution. Subsoil water uptake was important only for E. globulus, which had low topsoil LFR and tap roots exploiting deep water. Although P. falcatus appeared better adapted to varying soil water availability than the exotic species, both conifers decreased growth substantially during dry weather. Growth of E. globulus was largely independent of topsoil water content, giving it the potential to cause substantial dry-season groundwater depletion.     

Elevational change in woody tissue CO2 efflux in a tropical mountain rain forest in southern Ecuador

Much uncertainty exists about the magnitude of woody tissue respiration and its environmental control in highly diverse tropical moist forests. In a tropical mountain rain forest in southern Ecuador, we measured the apparent diurnal gas exchange of stems and coarse roots (diameter 1-4 cm) of trees from representative families along an elevational transect with plots at 1050, 1890 and 3050 m a.s.l. Mean air temperatures were 20.8, 17.2 and 10.6 degrees C, respectively. Stem and root CO(2) efflux of 13 to 21 trees per stand from dominant families were investigated with an open gas exchange system while stand microclimate was continuously monitored. Substantial variation in respiratory activity among and within species was found at all sites. Mean daily CO(2) release rates from stems declined 6.6-fold from 1.38 micromol m(-2) s(-1) at 1050 m to 0.21 micromol m(-2) s(-1) at 3050 m. Mean daily CO(2) release from coarse roots decreased from 0.35 to 0.20 micromol m(-2) s(-1) with altitude, but the differences were not significant. There was, thus, a remarkable shift from a high ratio of stem to coarse root respiration rates at the lowest elevation to an apparent equivalence of stem and coarse root CO(2) efflux rates at the highest elevation. We conclude that stem respiration, but not root respiration, greatly decreases with elevation in this transect, coinciding with a substantial decrease in relative stem diameter increment and a large increase in fine and coarse root biomass production with elevation.     

Estimating water use of sclerophyllous species under East-Mediterranean climate: III. Tabor oak forest sap flow distribution and transpiration

The annual course of daily transpiration and the hydrological balance of a Tabor oak forest were determined. The study was done in a representative forest within the natural geographical range of the species in the lower Galilee region of Israel. The climate is sub-humid with a rainless dry season from May to October. A partial water balance of a 0.1 ha area supporting an average of 14 trees was calculated from: (a) soil water content (SWC) measured by a Neutron Probe at depths of from 0.2 to 8 m, and (b) daylight transpiration rate measured with sap flow sensors by the heat pulse technique.Soil–bedrock complex water content (%) in the first 2 m of the profile fluctuated strongly between 5 and 20% depending on the season. The water content increased with depth from about 10% at 2.0 m depth to more than 20% at 5.0 m depth. For depths exceeding 5.5 m seasonal fluctuations in water content were negligible and water content ranged from 30 to 35%. After a dry winter, water content generally decreased within the main root zone down to about 2.0 m depth. Monthly changes in water content (mm) were greatest at depths of 0.35–1.0 m. Only minor changes in the soil–bedrock complex water content were recorded at greater depths. After a very rainy winter (2002/2003), decreases in soil–bedrock complex water content in the upper 2 m were much larger than after a dry winter. Fluctuations of soil–bedrock complex water content in deeper regions were larger in the wetter year, probably the result of drainage.Sap velocity was measured at six depths in the sapwood, from 4 to 44 mm, at 8 mm intervals. Sap velocity declined with depth, hence, sap flux density too.Based on sap velocity measurements performed during 4 years, the annual average daily transpiration (T) was 0.796 mm/day. This sums up to 239 mm during ∼300 days of leaf carriage, i.e. 41.3% of the 578 mm average annual rainfall for the area in the last 50 years. In a relatively dry year (rainfall of 432.7 mm) total water withdrawal from the 8 m soil–bedrock profile was 81% of the annual rainfall; of this amount 69% were transpired by the oak trees (239.0 mm), or 55% of the annual rainfall. In a relatively wet year (annual rainfall 801.4 mm) total water withdrawal was 67%; of this amount 45% would be transpired by the oak trees, or 30% of the annual rainfall.     

Interspecific variation of photosynthesis and leaf characteristics in canopy trees of five species of Dipterocarpaceae in a tropical rain forest

Photosynthetic rate, nitrogen concentration and morphological properties of canopy leaves were studied in 18 trees, comprising five dipterocarp species, in a tropical rain forest in Sarawak, Malaysia. Photosynthetic rate at light saturation (Pmax) differed significantly across species, varying from 7 to 18 micro mol m(-2) s(-1). Leaf nitrogen concentration and morphological properties, such as leaf blade and palisade layer thickness, leaf mass per area (LMA) and surface area of mesophyll cells per unit leaf area (Ames/A), also varied significantly across species. Among the relationships with leaf characteristics, Pmax had the strongest correlation with leaf mesophyll parameters, such as palisade cell layer thickness (r2 = 0.76, P < 0.001) and Ames/A (r2 = 0.73, P < 0.001). Leaf nitrogen concentration and Pmax per unit area also had a significant but weaker correlation (r2 = 0.46, P < 0.01), whereas Pmax had no correlation, or only weakly significant correlations, with leaf blade thickness and LMA. Shorea beccariana Burck, which had the highest P(max) of the species studied, also had the thickest palisade layer, with up to five or more layers. We conclude that interspecific variation in photosynthetic capacity in tropical rain forest canopies is influenced more by leaf mesophyll structure than by leaf thickness, LMA or leaf nitrogen concentration.     

Abundance, growth and mortality of very large trees in neotropical lowland rain forest

Very large trees, arbitrarily defined as those over 70 cm diameter above buttresses, account for a major portion of the above-ground biomass in neotropical rain forests. Owing to the scarcity of individuals of a given species and the difficulty of accurate measurement, there are few species-level data on the growth, mortality, and abundance of species that regularly reach emergent status. We report such data for very large individuals from old-growth tropical wet forest at the La Selva Biological Station in the Atlantic lowlands of the Republic of Costa Rica. The landscape-scale abundance of all species reaching over 70 cm diameter was assessed using 515 0.01-ha quadrats located at grid points in a 500 ha area of old-growth forest. In the total sample of 2301 stems 10 cm or more in diameter, very large individuals accounted for 2% of the stems, 23% of the basal area, and 27% of the estimated above-ground biomass. Growth and survival for five species that regularly attain emergent status were measured in a 150 ha area within the 500 ha plot. Survival of 282 very large individuals of the five species was measured over 6 years. The mean annual mortality rate of the total sample was only 0.6% year⁻¹. Mean annual diameter growth increments varied from 1.9 to 5.2 mm year⁻¹ among species, and were negatively correlated with diameter in four of the five species. For a sample of 193 individuals measured over 7 years, growth almost exactly equalled losses in basal area and biomass due to mortality. Because all of these species are regularly recruiting new trees into the over 70 cm diameter class, the amount of biomass in the large-individual size class is increasing over the 150 ha old-growth study area. Historic disturbance and/or current climatic change are hypothesized to account for the increase. We identify lack of standard diameter measurement criteria, and small and potentially unrepresentative plot locations as two problems in assessing the role of very large trees in other neotropical forests. Future studies should sample larger areas; this will increase the generality of the conclusions and will make possible a species-level comparison of the ecology of very large tropical trees.     

Evaluation of transpiration in a Douglas-fir stand by means of sap flow measurements

Transpiration of a Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) stand was evaluated by sap flow measurements during a 4-month period. Between-tree variation in sap flow depended on crown class. On a sunny day, total transpiration was 1.6, 8.0 and 22.0 liters day(-1) for suppressed, codominant and dominant trees, respectively. Transpiration estimated by sap flow fell below potential evapotranspiration when available soil water decreased below 30% of its maximum value. Sap flow measurements gave transpiration values similar to those obtained by the water balance method.     

Rainfall interception and fog precipitation in a tropical montane cloud forest of Guatemala

Tropical montane cloud forest hydrology is complex because of the presence of epiphytic life-forms that increase canopy surfaces and fog persistency. Fog precipitation is a hydrological input common to cloud forests, and forms when fog droplets are intercepted by the canopy and fall to the forest floor. Interception and fog precipitation was determined for a 2100 m site and a 2550 m site in a first-order tributary of the Sierra de las Minas Biosphere Reserve, Guatemala by calculating the difference between throughfall and gross precipitation for a 44-week period. Both sites were situated within closed-canopy cloud forests. The 2100 m site was on the windward slope of Montaña de Miranda near the lower boundary of the cloud forest and the 2550 m site was at the summit. Fog precipitation was found during periods in which throughfall exceeds gross precipitation. Fog precipitation was greater at 2550 m than at 2100 m. Data collected by precipitation and throughfall gauges demonstrate the existence of seasonal fog precipitation with the greatest fog precipitation occurring in the dry season (November–April). Fog precipitation contributes approximately 1 mm per day to the hydrological budget of the cloud forest at 2550 m during the dry season, and 0.5 mm per day during the rainy season (May–October).     

Partition of nocturnal sap flow in Acacia mangium and its implication for estimating the whole-tree transpiration

We analyzed the partition of nocturnal sap flow into refilling of internal water storage and transpiration in Acacia mangium. Sap flow of trees was monitored continuously with Granier’s sensors for estimating the whole-tree transpiration. Possible night transpiration and stomatal conductance at the leaf level in the canopy were measured with a LI-6400 photosynthesis measuring system. For nocturnal leaf transpiration and stomatal conductance were weak, nocturnal sap flow of mature A. mangium trees was mainly associated with water recharge in the trunk. No significant change in night water recharge of the trunk was found at both seasonal and inter-annual scales. Morphological features of trees including diameter at the breast height (DBH), tree height, and canopy size could explain variances of night water recharge. Furthermore, although the contribution of nocturnal sap flow to the total transpiration varied among seasons and DBH classes, the error caused by night water recharge on wholetree transpiration was negligible. __________ Translated from Journal of Plant Ecology (Chinese Version), 2007, 31 (5): 777–786 [译自: 植物生态学报]