Precipitation mediates sap flux sensitivity to evaporative demand in the neotropics.

Transpiration in humid tropical forests modulates the global water cycle and is a key driver of climate regulation. Yet, our understanding of how tropical trees regulate sap flux in response to climate variability remains elusive. With a progressively warming climate, atmospheric evaporative demand [i.

A mesic maximum in biological water use demarcates biome sensitivity to aridity shifts.

Biome function is largely governed by how efficiently available resources can be used and yet for water, the ratio of direct biological resource use (transpiration, E ) to total supply (annual precipitation, P) at ecosystem scales remains poorly characterized. Here, we synthesize field, remote sensing and ecohydrological modelling estimates to show that the biological water use fraction (E /P) reaches a maximum under mesic conditions; that is, when evaporative demand (potential evapotranspiration, E ) slightly exceeds supplied precipitation. We estimate that this mesic maximum in E /P occurs at an aridity index (defined as E /P) between 1.

Leaf surface traits and water storage retention affect photosynthetic responses to leaf surface wetness among wet tropical forest and semiarid savanna plants.

While it is reasonable to predict that photosynthetic rates are inhibited while leaves are wet, leaf gas exchange measurements during wet conditions are challenging to obtain due to equipment limitations and the complexity of canopy-atmosphere interactions in forested environments. Thus, the objective of this study was to evaluate responses of seven tropical and three semiarid savanna plant species to simulated leaf wetness and test the hypotheses that (i) leaf wetness reduces photosynthetic rates (Anet), (ii) leaf traits explain different responses among species and (iii) leaves from wet environments are better adapted for wet leaf conditions than those from drier environments. The two sites were a tropical rainforest in northern Costa Rica with ~4200 mm annual rainfall and a savanna in central Texas with ~1100 mm.

Tree mortality from an exceptional drought spanning mesic to semiarid ecoregions.

Significant areas of the southern USA periodically experience intense drought that can lead to episodic tree mortality events. Because drought tolerance varies among species and size of trees, such events can alter the structure and function of terrestrial ecosystem in ways that are difficult to detect with local data sets or solely with remote-sensing platforms. We investigated a widespread tree mortality event that resulted from the worst 1-year drought on record for the state of Texas, USA.

Nocturnal transpiration in riparian Tamarix thickets authenticated by sap flux, eddy covariance and leaf gas exchange measurements.

Tamarix chinensis Lour., which is common throughout the southwestern USA, is a phreatophytic riparian tree capable of high water use. We investigated temporal congruence between daily total evapotranspiration (E) estimated from stem sap flux (J(s)) measurements (E(sf)) and eddy covariance (E(cv)), both seasonally and immediately following rain events, and used measurements of leaf-level gas exchange, stem water content and diurnal changes in leaf water potential to track drivers of transpiration.

Structural and compositional controls on transpiration in 40- and 450-year-old riparian forests in western Oregon, USA.

Large areas of forests in the Pacific Northwest are being transformed to younger forests, yet little is known about the impact this may have on hydrological cycles. Previous work suggests that old trees use less water per unit leaf area or sapwood area than young mature trees of the same species in similar environments. Do old forests, therefore, use less water than young mature forests in similar environments, or are there other structural or compositional components in the forests that compensate for tree-level differences? We investigated the impacts of tree age, species composition and sapwood basal area on stand-level transpiration in adjacent watersheds at the H.

Epiphyte host preferences and host traits: mechanisms for species-specific interactions.

We investigated species-specific relationships among two species of vascular epiphytes and ten host tree species in a coastal plain forest in the southeastern United States. The epiphytes Tillandsia usneoides and Polypodium polypodioides were highly associated with particular host species in the field, but host traits that favored colonization were inadequate to fully explain the epiphyte-host associations for either epiphyte. Field transplant experiments that bypassed epiphyte colonization demonstrated that the growth of epiphytes was significantly higher on host tree species that naturally bore high epiphyte loads than on host species with few or no epiphytes.