Spatial and temporal variation of sources of water across multiple tropical rainforest trees

Abstract

Tropical forest water use is important for maintaining ecosystem function, tree productivity, growth, survival and nutrient cycling. However, explaining such use is complex in the field. Water stable isotope tracing of plant water use can shed light on such plant water sources but to date, species numbers tested at any given site across the globe have been minimal. Additionally, past tree water source studies were mostly based on methods using single isotopes of either (δD or δ 18 O) at a coarse spatiotemporal resolution. This thesis used a combination of a dual stable isotope approach together with sap flux to understand species specific water utilization strategies of co-occurring tropical rainforest species at a fine spatiotemporal scale.A systematic review of global research on woody plant water sources was undertaken to evaluate the spatiotemporal dynamics of research on woody species water sources and to assess the research priorities in the study of woody species water sources (Chapter 2). Most studies were from the USA with various other countries having between one and four studies only and mostly focused on the Pinaceae family. The review indicates there is a clear variation in woody plant water sources in the forest due to season, climate, leaf phenology and method of measurement.Majority of the tree species obtained water from soil, followed by groundwater. Most of the research focus has been on identifying plant water sources using a single isotope. Much less focus was given to the nexus between water source and tree growth, drought, water use efficiency, agroforestry, groundwater interaction and many other topics.A further systematic review was undertaken on research related to tree water use (Chapter 3).This review found a clear bias in research focus relating to geographic area and species group selection. Most of the studies (33.33%) were undertaken in Central America. Only Fabaceae, Myrtaceae, Dipterocarpaceae and Anacardiaceae families were given priority. Tree size and seed mass was positively correlated with water use. In contrast, wood density showed a negative relationship with tree water use. Season is highly significant in explaining the variation of tree water use as was leaf phenology. Tropical trees' water use significantly increases in dry season.There is no significant difference in water use between native and exotic species.A comparison of δD and δ 18 O isotopes between soil water and xylem water was used to investigate niche segregation among wet tropical rainforest tree species at a fine spatial scale (Chapter 4). Tropical forest water use is critical for tree productivity, growth, survival and nutrient iii cycling, but describing such uses is difficult in the field. Stable isotope tracing of plant water use can illuminate plant water sources but to date, the number of species tested at any given site has been minimal. Here, 46 tropical hardwood tree species (49 individual trees) in a 0.32 ha plot with uniform soils were sampled. Soil water was characterized at 6 depths at 0.2 m ...

Variability in tree water uptake determined with stable water isotopes in an African tropical montane forest

“…Predominant shallow water use was also observed in other studies for several evergreen species in seasonally dry tropical forests on fine textured soils (Goldsmith et al, 2012; Liu et al, 2010) and in a tropical forest with sandy soils (Sohel, 2018). However, uptake of deeper soil water occurs occasionally in seasonal tropical climates as well (Liu et al, 2010 reported >60 cm; Meinzer et al, 1999 reported >100 cm; Schwendenmann et al, 2015 reported >30 cm; Stahl et al, 2013 reported 100–120 cm).…”

“…For instance, tree species or vegetation cover can affect soil water isotopic composition (Schwendenmann et al, 2015; Sprenger, Tetzlaff, & Soulsby, 2017) and roots or animal burrows can lead to preferential flow paths. Compared to a lateral soil water isotopic range of 10.7‰ δ 18 O in a 1 ha temperate forest (Goldsmith et al, 2018), our lateral range of up to 6‰ δ 18 O was lower, but twice as high as in seemingly homogeneous soils in an Australian tropical forest (Sohel, 2018). Unlike Goldsmith et al (2018) and Sohel (2018), who only sampled once, we sampled four times.…”

“…However, the dual isotope space plot (Figure 3) suggested that the xylem δ 2 H isotopic values were frequently more depleted than any of the expected water sources. This behaviour has frequently been observed in other studies, but underlying reasons are yet to be resolved (Barbeta et al, 2019; Bertrand et al, 2014; Bowling, Schulze, & Hall, 2017; Brooks, Barnard, Coulombe, & McDonnell, 2010; Evaristo, McDonnell, Scholl, Bruijnzeel, & Chun, 2016; Goldsmith et al, 2012; Muñoz‐Villers et al, 2018; Sohel, 2018). Possible explanations comprise hydrogen fractionation during root water uptake or within woody tree tissues (Barbeta et al, 2019; Vargas, Schaffer, Yuhong, & Sternberg, 2017; Zhao et al, 2016), or the concept of ecohydrological separation (i.e.…”

“…Goldsmith et al (2018) also report no interspecific water uptake patterns in a tropical montane cloud forest with roughly comparable climate and soils to our study area in the Mau Forest. This contradicts the clear species‐specific differences in other (seasonally dry) tropical (Hasselquist, Allen, & Santiago, 2010; Liu et al, 2010; Schwendenmann et al, 2015; Sohel, 2018; Stratton, Goldstein, & Meinzer, 2000), arid (Huang & Zhang, 2015), and temperate climates (Barbeta et al, 2019; Brinkmann, Eugster, Buchmann, & Kahmen, 2019). No general spatial patterns in uptake depth between quadrants and short‐term temporal patterns in preferred tree water uptake depth could be observed.…”

An Assessment of Woody Plant Water Source Studies from across the Globe: What Do We Know after 30 Years of Research and Where Do We Go from Here?

“…An increased study of woody plant water sources will help in identifying complementarities among tree species, which can then inform the design and management of mixed-species plantations. Even though we considered article until 2014, there are some researches [56][57][58][59][60][61] published after 2014. In most cases, those research themes matched with our proposed research themes.…”