Contrasting stem water uptake and storage dynamics of water-saver and water-spender species during drought and recovery.

Drought is projected to occur more frequently and intensely in the coming decades, and the extent to which it will affect forest functioning will depend on species-specific responses to water stress. Aiming to understand the hydraulic traits and water dynamics behind water-saver and water-spender strategies in response to drought and recovery, we conducted a pot experiment with two species with contrasting physiological strategies, Scots pine (Pinus sylvestris L.) and Portuguese oak (Quercus faginea L.). We applied two cycles of soil drying and recovery and irrigated with isotopically different water to track fast changes in soil and stem water pools, while continuously measuring physiological status and xylem water content from twigs. Our results provide evidence for a tight link between the leaf-level response and the water uptake and storage patterns in the stem. The water-saver strategy of pines prevented stem dehydration by rapidly closing stomata which limited their water uptake during the early stages of drought and recovery. Conversely, oaks showed a less conservative strategy, maintaining transpiration and physiological activity under dry soil conditions, and consequently becoming more dehydrated at the stem level. We interpreted this dehydration as the release of water from elastic storage tissues as no major loss of hydraulic conductance occurred for this species. After soil rewetting, pines recovered pre-drought leaf water potential rapidly, but it took longer to replace the water from conductive tissues (slower labeling speed). In contrast, water-spender oaks were able to quickly replace xylem water during recovery (fast labeling speed), but it took longer to refill stem storage tissues, and hence to recover pre-drought leaf water potential. These different patterns in sap flow rates, speed and duration of the labeling reflected a combination of water-use and storage traits, linked to the leaf-level strategies in response to drought and recovery.

Home gardens' agrobiodiversity and owners' knowledge of their ecological, economic and socio-cultural multifunctionality: a case study in the lowlands of Tabasco, México.

BACKGROUND: Home gardens (HGs) are hotspots of in situ agrobiodiversity conservation. We conducted a case study in Tabasco, México, on HG owners' knowledge of HG ecological, economical and socio-cultural multifunctionality and how it relates to agrobiodiversity as measured by species richness and diversity. The term multifunctionality knowledge refers to owners' knowledge on how HGs contribute to ecological processes, family economy, as well as human relations and local culture. We hypothesized a positive correlation between owners' multifunctionality knowledge and their HGs' agrobiodiversity. METHODS: We inventoried all perennial species in 20 HGs, determined observed species richness, calculated Shannon diversity indexes and analysed species composition using non-metric multidimensional scaling (NMDS). Based on literature, semi-structured interviews and a dialogue of knowledge with HG owners, we catalogued the locally recognized functions in the ecological, economic and socio-cultural dimensions. We determined the score of knowledge on each function in the three dimensions on explicit scales based on the interviews and observed management. We determined Spearman rs correlations of HGs' observed species richness, Shannon diversity index (H) and of HGs' scores on NMDS-axis and multifunctionality knowledge scores. We dialogued on the results and implications for agrobiodiversity conservation at workshops of HG owners, researchers and local organizations. RESULTS: HG agrobiodiversity and owners' multifunctionality knowledge in the study area showed large variation. Average richness was 59.6 perennial species, varying from 21 to 107 species, and total observed richness was 280 species. A total of 38 functions was distinguished, with 14, 12 and 12 functions in the ecological, economic and socio-cultural dimensions. Total multifunctionality knowledge scores varied from 64.1 to 106.6, with an average of 87.2. Socio-cultural functionality knowledge scores were the highest, followed by scores in the ecological and economic dimensions. Species richness and Shannon H were significantly correlated with ecological functionality knowledge (rs = 0.68 and P < 0.001 in both cases), and species richness was also correlated with economic functionality knowledge (rs = 0.47, P = 0.03). Species composition scores on the first and second axes of NMDS was significantly correlated with knowledge of ecological multifunctionality, with rs = 0.49 resp-0.49 and P = 0.03 in both cases. Other functionality knowledge scores showed no correlation with NMDS scores. Dialogue in workshops confirmed the interwovenness of multifunctionality knowledge and agrobiodiversity. CONCLUSION: The rich agrobiodiversity of home gardens cherished by rural families in Tabasco relates with the knowledge about HG functionality in the ecological and economic dimensions. Also, species composition relates with ecological functionality knowledge. The socio-cultural functionality knowledge, which includes many elements beyond the individual HG, is not correlated with agrobiodiversity, but had the highest scores. Our results show that multifunctionality knowledge provides many opportunities for the participative conception and planning of policies and actions necessary to conserve agrobiodiversity.

Coupled plant traits adapted to wetting/drying cycles of substrates co-define niche multidimensionality.

Theories attempting to explain species coexistence in plant communities have argued in favour of species' capacities to occupy a multidimensional niche with spatial, temporal and biotic axes. We used the concept of hydrological niche segregation to learn how ecological niches are structured both spatially and temporally and whether small scale humidity gradients between adjacent niches are the main factor explaining water partitioning among tree species in a highly water-limited semiarid forest ecosystem. By combining geophysical methods, isotopic ecology, plant ecophysiology and anatomical measurements, we show how coexisting pine and oak species share, use and temporally switch between diverse spatially distinct niches by employing a set of functionally coupled plant traits in response to changing environmental signals. We identified four geospatial niches that turned into nine, when considering the temporal dynamics of the wetting/drying cycles in the substrate and the particular plant species adaptations to garner, transfer, store and use water. Under water scarcity, pine and oak exhibited water use segregation from different niches, yet under maximum drought when oak trees crossed physiological thresholds, niche overlap occurred. The identification of niches and mechanistic understanding of when and how species use them will help unify theories of plant coexistence and competition.

Ecohydrogeochemical functioning of coastal freshwater herbaceous wetlands in the Protected Natural Area, Ciénaga del Fuerte (American tropics): Spatiotemporal behaviour.

Coastal zones are characterized by the interactions between continents and oceans and, therefore, between fresh and salt surface and groundwater. The wetlands of coastal zones represent transitional ecosystems that are affected by these conditions, although little is known about the hydrogeochemistry of wetlands, especially coastal wetlands. In the present study, the hydrogeochemical characterization of coastal freshwater herbaceous wetlands in the Ciénaga del Fuerte Protected Natural Area in Veracruz, Mexico, in the American tropics was carried out per plant community. Four herbaceous wetlands (alligator flag, saw grass, cattail, and floodplain pasture) were monitored to understand the origin of the water feeding these ecosystems, the hydrogeochemical composition of groundwater, and the relationship between the groundwater and ecology of these ecosystems during dry and rainy seasons. The results indicate that Ciénaga del Fuerte is located in a regional discharge area and receives local recharge, so it is fed by both regional and local flows. The chemical composition varied temporally and spatially, creating unique conditions that determined the habitat occupied by the hydrophytic vegetation. The spatiotemporal behaviour of groundwater is one factor that, along with the hydroperiod, determines wetland dynamics and affects wetland biota (ecohydrogeochemistry). Generalist plant communities established in zones of local recharge, whereas other more specialized and/or plastic communities inhabited zones receiving regional flows with greater ion concentrations. This information forms the basis for establishing an appropriate scale (municipal, state, or larger regions) for the sustainable management of goods and services provided by the wetlands.

Geoecohydrological mechanisms couple soil and leaf water dynamics and facilitate species coexistence in shallow soils of a tropical semiarid mixed forest.

Trees growing on shallow rocky soils must have exceptional adaptations when underlying weathered bedrock has no deep fractures for water storage. Under semiarid conditions, hydrology of shallow soils is expected to decouple from plant hydrology, as soils dry out as a result of rapid evaporation and competition for water increases between coexisting tree species. Gas exchange and plant-water relations were monitored for 15 months for Pinus cembroides and Quercus potosina tree species in a tropical semiarid forest growing on c. 20-cm-deep soils over impermeable volcanic bedrock. Soil and leaf water potential maintained a relatively constant offset throughout the year in spite of high intra-annual fluctuations reaching up to 5 MPa. Thus, hydrology of shallow soils did not decouple from hydrology of trees even in the driest period. A combination of redistribution mechanisms of water stored in weathered bedrock and hypodermic flow accessible to oak provided the source of water supply to shallow soils, where most of the actively growing roots occurred. This study demonstrates a unique geoecohydrological mechanism that maintains a tightly coupled hydrology between shallow rocky soils and trees, as well as species coexistence in this mixed forest, where oak facilitates water access to pine.