Under drought conditions, even tropical rainforests might turn from carbon sinks to sources, and tree species composition might be altered by increased mortality. We monitored stem diameter variations of 40 tree individuals with stem diameters above 10 cm belonging to eleven different tree genera and three tree life forms with high-resolution dendrometers from July 2007 to November 2010 and additionally March 2015 to December 2017 in a tropical mountain rainforest in South Ecuador, a biodiversity hotspot with more than 300 different tree species belonging to different functional types. Although our study area receives around 2200 mm of annual rainfall, dry spells occur regularly during so-called “Veranillo del Niño” (VdN) periods in October-November. In climate change scenarios, droughts are expected with higher frequency and intensity as today. We selected dry intervals with a minimum of four consecutive days to examine how different tree species respond to drought stress, raising the question if some species are better adapted to a possible higher frequency and increasing duration of dry periods. We analyzed the averaged species-specific stem shrinkage rates and recovery times during and after dry periods. The two deciduous broadleaved species Cedrela montana and Handroanthus chrysanthus showed the biggest stem shrinkage of up to 2 mm after 10 consecutive dry days. A comparison of daily circumference changes over 600 consecutive days revealed different drought responses between the families concerning the percentage of days with stem shrinkage/increment, ranging from 27.5 to 72.5% for Graffenrieda emarginata to 45–55% for Podocarpus oleifolius under same climate conditions. Moreover, we found great difference of recovery times after longer-lasting (i.e., eight to ten days) VdN drought events between the two evergreen broadleaved species Vismia cavanillesiana and Tapirira guianensis. While Vismia replenished to pre-VdN stem circumference after only 5 days, Tapirira needed 52 days on average to restore its circumference. Hence, a higher frequency of droughts might increase inter-species competition and species-specific mortality and might finally alter the species composition of the ecosystem.
<p>Ecosystems play an important role in controlling the exchange of energy, water and carbon between the land surface and the atmosphere, which contributes to the regulation of the climate through biogeochemical process. Changes in vegetation or biomass impacts the microclimatological conditions of the landscapes with feedbacks to the heat and water budgets. Knowledge about the dynamics and driving factors of the exchange processes contributes to our understanding of the land surface &#8211; atmosphere interactions as drivers of the Earth&#8217;s surface energy budget.</p> <p>In the Tumbesian mountain dry forest (MDF) in the Laipuna reserve on the western escarpment of the Andes mountains in South Ecuador two eddy-covariance measurement stations have been installed over natural forest and an anthropogenic replacement system to observe atmospheric water and carbon fluxes. The MDF is characterized by a distinct seasonality, which can be divided into a dry (May - December) and wet (November - May) season following the inter-hemispherical shift of the ITCZ. Mean monthly precipitation totals ranges between 50 and 400mm with an annual amount of 650mm, while the temperature varies between 21 &#8211; 26&#176;C. The forest ecosystem is dominated by deciduous trees and hold a clear annual cycle in the water budget and carbon sequestration. In the scope of global climate change such water limited landscapes are strongly vulnerable to climatic stress situations which lead to changes in the phenological cycles in the ecosystem associated with feedbacks to the water and carbon cycle. The aim is thus, to investigate the energy, water and carbon dynamics along a land use gradient in order to estimate the impact of deforestation on net-ecosystem exchange and evapotranspiration in the MDF region. The study shows first results of microclimatological conditions, such as radiative fluxes, moisture and soil conditions of both sites as well as water and carbon fluxes.</p>
This study addresses transpiration in a tropical evergreen mountain forest in the Ecuadorian Andes from the leaf to the stand level, with emphasis on nocturnal plant-water relations. The stand level: Evapotranspiration (ET) measured over 12 months with the Eddy-Covariance (ECov) technique proved as the major share (79%) of water received from precipitation. Irrespective of the humid climate, the vegetation transpired day and night. On average, 15.3% of the total daily ET were due to nocturnal transpiration. Short spells of drought increased daily ET, mainly by enhanced nighttime transpiration. Following leaf transpiration rather than air temperature and atmospheric water vapor deficit, ET showed its maximum already in the morning hours. The tree level: Due to the humid climate, the total water consumption of trees was generally low. Nevertheless, xylem sap flux measurements separated the investigated tree species into a group showing relatively high and another one with low sap flux rates. The leaf level: Transpiration rates of Tapirira guianensis, a member of the high-flux-rate group, were more than twice those of Ocotea aciphylla, a representative of the group showing low sap flux rates. Representatives of the Tapirira group operated at a relatively high leaf water potential but with a considerable diurnal amplitude, while the leaves of the Ocotea group showed low water potential and small diurnal fluctuations. Overall, the Tapirira group performed anisohydrically and the Ocotea group isohydrically. Grouping of the tree species by their water relations complied with the extents of the diurnal stem circumference fluctuations. Nighttime transpiration and hydrological type: In contrast to the isohydrically performing trees of the Ocotea group, the anisohydric trees showed considerable water vapour pressure deficit (VPD)-dependent nocturnal transpiration. Therefore, we conclude that nighttime ET at the forest level is mainly sourced by the tree species with anisohydric performance.
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