Small understorey trees have greater capacity than canopy trees to adjust hydraulic traits following prolonged experimental drought in a tropical forest

Future climate change predictions for tropical forests highlight increased frequency and intensity of extreme drought events. However, it remains unclear whether large and small trees have differential strategies to tolerate drought due to the different niches they occupy. The future of tropical forests is ultimately dependent on the capacity of small trees (<10 cm in diameter) to adjust their hydraulic system to tolerate drought. To address this question, we evaluated whether the drought tolerance of neotropical small trees can adjust to experimental water stress and was different from tall trees. We measured multiple drought resistance-related hydraulic traits across nine common neotropical genera at the world’s longest-running tropical forest throughfall-exclusion experiment and compared their responses with surviving large canopy trees. Small understorey trees in both the Control and the throughfall exclusion treatment (TFE) had lower minimum stomatal conductance and maximum hydraulic leaf-specific conductivity relative to large trees of the same genera, as well as greater hydraulic safety margin (HSM), percentage loss of conductivity (PLC) and embolism resistance, demonstrating they occupy a distinct hydraulic niche. Surprisingly, in response to the drought treatment, small trees increased specific hydraulic conductivity by 56.3% and leaf:sapwood area ratio by 45.6%. The greater HSM of small understorey trees relative to large canopy trees likely enabled them to adjust other aspects of their hydraulic systems to increase hydraulic conductivity and take advantage of increases in light availability in the understorey resulting from the drought-induced mortality of canopy trees. Our results demonstrate that differences in hydraulic strategies between small understorey and large canopy trees drive hydraulic niche segregation. Small understorey trees can adjust their hydraulic systems in response to changes in water and light availability indicating natural regeneration of tropical forests following long-term drought may be possible.

Effects of Throughfall Exclusion on Photosynthetic Traits in Mature Japanese Cedar (Cryptomeria japonica (L. f.) D. Don.)

As climate change progresses, it is becoming more crucial to understand how timber species respond to increased drought frequency and severity. Photosynthetic traits in a 40-year-old clonal Japanese cedar (Cryptomeria japonica) plantation were assessed under artificial drought stress using a roof to exclude rainfall and a control with no exclusion. C. japonica is a commercial tree that is native to Japan and has high growth on mesic sites. The maximum carboxylation rate (Vcmax), maximum electron transfer rate (Jmax), and dark respiration rate (Rd) in current-year shoots in the upper canopy were determined from spring to autumn over two growing seasons. In addition, the photosynthetic rate at light saturation (Pmax), stomatal conductance (gs), and intrinsic water use efficiency (WUEi) were measured in the morning and afternoon during the same period. Leaf mass per unit area (LMA) and nitrogen concentration (N) were also measured. The values of Vcmax, Jmax, Rd, N, and LMA did not differ between the two plots. By contrast, significantly lower Pmax and gs and higher WUEi were found in the drought plot, and the reduction in Pmax was accompanied by low gs values. Midday depressions in Pmax and gs were more pronounced in the drought plot relative to the control and were related to higher WUEi. Under drought conditions, mature Japanese cedar experienced little change in photosynthetic capacity, foliar N, or LMA, but they did tend to close the stomata to regulate transpiration, thus avoiding drought-induced damage to the photosynthetic machinery and improving WUEi.

Effects of natural and experimental drought on soil fungi and biogeochemistry in an Amazon rain forest

Microbiota are essential components of the soil, driving biogeochemical cycles. Fungi affect decomposition and biotic interactions with plants across scales. Climate projections suggest that extended dry seasons may transform sensitive rain forests into savanna-like vegetation, with consequent changes in biogeochemistry. Here we compare the impacts of natural seasonality with 14 years of partial throughfall exclusion in an Amazonian rain forest, focussing on soil fungal functional diversity, extracellular soil enzyme activities (EEA) and their implications for nutrient dynamics. Large changes in fungal diversity and functional group composition occur in response to drought, with a conspicuous increase in the abundance of dark-septate fungi and a decrease in fungal pathogens. The high seasonality of EEA in the control (non droughted) and suppression of seasonality in the drought treatment, together with an increased implied nitrogen demand in the dry season induced by experimental drought, suggest that the changed soil microbiota activity may signal a pending shift in the biogeochemical functioning of the forest.

Responses of Soil Microbes to Hydrological Perturbations in Tropical Forest Soils

<p>Model projections predict that climate change impacts on the tropics will include an increased frequency of drought and precipitation cycles. Such environmental fluctuations at the soil pore-scale play an important role in shaping microbial adaptive capacity, and trait composition of a community, which feeds back on to the breakdown and formation of soil organic matter (SOM). Understanding the factors controlling the carbon balance of humid tropical forest soils remains a social imperative. Microbial feedback to SOM pools is critical. Herein, we examine the microbial response to drought perturbations across  3 different, but complementary scales. At the largest scale, we explored the impacts of drought across a 1 m precipitation gradient spanning four sites from the Caribbean coast to the interior of Panama. At each site 4, throughfall exclusion plots (10 x 10 m) were established to reduce precipitation by 50 %. In addition, 4 corresponding control plots were also constructed. At the meso-scale, we incubated intact soil cores from one of these sites (P12) under 3 different hydrological treatments (control, drought, rewetting-drying cycles) for over a 5-month period. For the field and meso-scale experiments, we evaluated changes imparted by hydrological perturbations using multi-omic approaches, and physico-chemical measurements.   In order to identify the traits involved in response to drought at the field and meso-scale, we isolated a range of bacteria to subject to stress at the scale of the single-cell and simple communities.  Cell extracts were subjected to osmotic or matric stress and the short-term physiological responses determined using non-destructive synchrotron radiation-based Fourier Transform-Infrared spectromicroscopy. Through this approach, we identified changes in metabolic allocation within different cells, in particular to the secondary metabolome of the different bacteria. Our contribution will discuss the outcomes of these multi-scale experiments.  Specifically focusing on how shifts in the microbial community and physiological changes may influence tropical soil carbon stability under future scenarios of altered drought and precipitation cycles.</p>

The effects of an experimental drought on the ecophysiology and fruiting phenology of a tropical rainforest palm

Aims Anthropogenic climate change is predicted to increase mean temperatures and rainfall seasonality. How tropical rainforest species will respond to this climate change remains uncertain. Here, we analysed the effects of a 4-year experimental throughfall exclusion (TFE) on an Australian endemic palm (Normambya normanbyi) in the Daintree rainforest of North Queensland, Australia. We aimed to understand the impact of a simulated reduction in rainfall on the species’ physiological processes and fruiting phenology. Methods We examined the fruiting phenology and ecophysiology of this locally abundant palm to determine the ecological responses of the species to drought. Soil water availability was reduced overall by ~30% under a TFE experiment, established in May 2015. We monitored monthly fruiting activity for 8 years in total (2009–2018), including 4 years prior to the onset of the TFE. In the most recent year of the study, we measured physiological parameters including photosynthetic rate, stomatal conductance and carbon stable isotopes (δ 13C, an integrated measure of water use efficiency) from young and mature leaves in both the dry and wet seasons. Important Findings We determined that the monthly fruiting activity of all palms was primarily driven by photoperiod, mean solar radiation and mean temperature. However, individuals exposed to lower soil moisture in the TFE decreased significantly in fruiting activity, photosynthetic rate and stomatal conductance. We found that these measures of physiological performance were affected by the TFE, season and the interaction of the two. Recovery of fruiting activity in the TFE palms was observed in 2018, when there was an increase in shallow soil moisture compared with previous years in the treatment. Our findings suggest that palms, such as the N. normanbyi, will be sensitive to future climate change with long-term monitoring recommended to determine population-scale impacts.

Effects of Experimental Throughfall Exclusion on Soil Respiration in a Continental Coniferous Stand, South Korea

Severe droughts and changing precipitation patterns could alter the biogeochemical properties of the soil, affecting soil carbon cycles in forest ecosystems. A throughfall exclusion (TFE) experiment was conducted in a continental climate coniferous stand in Gangwon Province, Korea, to examine the effects of excluding rainfall on total soil respiration (SR), heterotrophic soil respiration (HR), autotrophic soil respiration (AR), sapling diameter growth, and soil bacterial communities from July 2016 to October 2017. The soil water content (SWC) was significantly decreased by the exclusion of the throughfall, resulting in changes in the bacterial communities, and subsequently a decrease in HR. Although AR did not present significant differences between the control and TFE plots, the rate of sapling growth was significantly lower in the TFE plots compared with that in the control plots. An exponential function relating SR to soil temperature accounted for 0.61% and 0.82% of the variance in SR in the control and TFE plots, respectively (Q10 = 2.48 and 2.86, respectively). Furthermore, a multivariate nonlinear model based on soil temperature and SWC explained 0.89% and 0.88% of the variance in SR in the control and TFE plots, respectively. When soil temperature was high, SR showed high fluctuations due to SWC variation. However, when SWC was low, we detected relatively small fluctuations in SR due to soil temperature. The results of this study show that the activity of soil microbial and root respiration during the growing season may be lower under future drought conditions.

Mitigation of drought stress in forest stands - insights from a throughfall-exclusion and a thinning experiment

<p>Due to climate change suitable forest management measures are required to mitigate the proposed exacerbating drought events as already observed in central Europe in summer 2003, 2015, 2018, and 2019. This contribution summarizes the findings of two long-term field experiments studying different forest management measures aiming at mitigating drought in forest stands.</p><p>The first study investigated the potential of mixing tree composition for mitigating drought stress in Norway spruce (Picea abies [L.] Karst.). To this end, a five-year study with repeated summer droughts, experimentally induced via throughfall-exclusion (TE), was performed. The study objects were mature (60-80 years) old stands of Norway spruce in monoculture and mixture with European beech (Fagus sylvatica (L.). The impacts of repeated summer droughts were assessed on about 100 trees distributed on 12 plots and accessible via canopy crane (Kranzberg forest ROOF experiment in southern Germany). Predawn leaf water potentials of Norway spruce reached minima of -1.8 MPa, but were not affected by species mixture. Nevertheless, daily xylem sapflow density was increased up to 40% in mixture compared to pure strands. Likewise, stem growth, i.e. relative basal area increment, showed significantly higher drought resistance in mixture compared to monocultures.</p><p>While altering forest stand composition seems to be promising on the long-term, the conversion from monocultures is often economically not suitable for young stands in the short- to medium-term. Therefore, reducing intraspecific competition via thinning is a frequently discussed option investigated in the second, eight-year-long study. In a 26-year old Norway spruce monoculture, three thinning intensities, i.e. unchanged (control), moderate thinning (MT with reduced basal area by 43%) and heavy thinning (HT, reduced basal area by 67%) were applied, potentially mitigating drought by reducing intraspecific competition and increasing soil water availability. Indeed, in both thinning intensities the duration of drought stress for the trees (soil water content below critical value) was reduced compared to controls for up to 5-7 years following the event. However, increased radiation and higher growth rates of the individual trees accompanied by the fast establishment of a vital ground vegetation diminished the difference in stand transpiration between MT and HT within two years. Moreover, belowground competition with the understorey vegetation suppressed fine root recovery under HT compared to MT in contradiction to increased leaf area on HT on tree-level and therefore increased transpirational demand.</p><p>The presented studies suggest admixing of broadleaved beech into monocultures of Norway spruce to be a promising management measure in the long-term. In juvenile monocultures of Norway spruce frequent and intense thinning interventions while preventing the establishment of a vital understorey vegetation appears to be a promising forest measure, mitigating drought without losing sight of economic needs.</p>