Abstract
Dry periods are predicted to become more frequent and severe in the future in some parts of the tropics, including Amazonia, potentially causing reduced productivity, higher tree mortality and increased emissions of stored carbon. Using a long-term (12 year) through-fall exclusion (TFE) experiment in the tropics, we test the hypothesis that trees produce leaves adapted to cope with higher levels of water stress, by examining the following leaf characteristics: area, thickness, leaf mass per area, vein density, stomatal density, the thickness of palisade mesophyll, spongy mesophyll and both of the epidermal layers, internal cavity volume and the average cell sizes of the palisade and spongy mesophyll. We also test whether differences in leaf anatomy are consistent with observed differential drought-induced mortality responses among taxa, and look for relationships between leaf anatomy, and leaf water relations and gas exchange parameters. Our data show that trees do not produce leaves that are more xeromorphic in response to 12 years of soil moisture deficit. However, the drought treatment did result in increases in the thickness of the adaxial epidermis (TFE: 20.5 ± 1.5 µm, control: 16.7 ± 1.0 µm) and the internal cavity volume (TFE: 2.43 ± 0.50 mm3 cm−2, control: 1.77 ± 0.30 mm3 cm−2). No consistent differences were detected between drought-resistant and drought-sensitive taxa, although interactions occurred between drought-sensitivity status and drought treatment for the palisade mesophyll thickness (P = 0.034) and the cavity volume of the leaves (P = 0.025). The limited response to water deficit probably reflects a tight co-ordination between leaf morphology, water relations and photosynthetic properties. This suggests that there is little plasticity in these aspects of plant anatomy in these taxa, and that phenotypic plasticity in leaf traits may not facilitate the acclimation of Amazonian trees to the predicted future reductions in dry season water availability.
Highlights
A key issue in the prediction of future climate change is understanding how forests, significant stores of carbon (Pan et al 2011, Grace et al 2014), will respond to current and future changes in temperature and water availability (Bonan 2008)
This study uses a long-term (>12 years) through-fall exclusion (TFE) experiment in the lowland Amazon Rainforest to address the following questions: (i) do trees respond to long-term imposed soil moisture deficit through changes in leaf structure or anatomy? and (ii) do differences in anatomy, or anatomical plasticity, explain contrasts in drought sensitivity among taxa? We examine any further associations between leaf anatomy, water relations and gas exchange traits and drought using multivariate analyses
cavity volume (CV) and palisade thickness showed significant interactions between treatment and sensitivity status (P = 0.025 and 0.034, respectively), whereby the treatment effect was stronger amongst the resistant compared to the sensitive genera (Figure 3)
Summary
A key issue in the prediction of future climate change is understanding how forests, significant stores of carbon (Pan et al 2011, Grace et al 2014), will respond to current and future changes in temperature and water availability (Bonan 2008). Tree mortality has reportedly increased in response to episodic severe drought (Breshears et al 2005, Allen et al 2010), including in the tropics 2005, Phillips et al 2009, Brienen et al 2015), and understanding the physiology underlying drought-induced mortality is essential for estimating forest sensitivity to drought (Christensen et al 2013, Allen et al 2015, Meir et al 2015b). 2013, Fu et al 2013, Reichstein et al 2013, Boisier et al 2015) These changed conditions will exert a selection pressure affecting the generation of trees, but the persistence of the current generation depends on their capacity for acclimation or resilience in the face of climate change. Investigating the capacity of trees to cope with drought in tropical forests is of paramount importance in estimating the magnitude of biosphere–atmosphere feedbacks
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