Abstract
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.
Highlights
Global change, especially climate change, affects forests worldwide, with adverse effects on biodiversity and ecological services like carbon sequestration
Due to the constraints of the mentioned approaches we focus in the present study on analyzing daily stem diameter responses to drought between different tropical tree species and families of different plant functional types regarding their resilience and ability to cope with dry spells of varying length
Evergreen broadleaved and coniferous species showed a lower rate of stem diameter shrinkage in relation to dry interval length than the deciduous broadleaved species H. chrysanthus and C. montana, with average circumference losses of 1.876 and 2.190 mm after nine dry days, respectively (Figure 1)
Summary
Especially climate change, affects forests worldwide, with adverse effects on biodiversity and ecological services like carbon sequestration. Drought as one of the most frequent climatic extremes on a global scale (Allen et al, 2009, 2015) and the accompanied increase in atmospheric vapor pressure deficit is, due to its partly devastating impact on forests, still under intense debate (Sass-Klaassen et al, 2016). There is still limited knowledge regarding the seasonal occurrence of positive or negative radial diameter or circumference variations of tree stems, especially at hourly or daily scale (De Swaef et al, 2015). The possibilities of the interpretation of dendrometer readings considering plant physiology are not fully exploited yet (Zweifel, 2016). These deficiencies are in particular valid for the tropics. To directly monitor the impact of drought on a forest and tree species specific response, it is necessary to have a close look at the individual tree level
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