Effects of different representations of stomatal conductance response to humidity across the African continent under warmer CO2-enriched climate conditions

General circulation models (GCMs) forecast higher global vapor pressure deficit (VPD) but unchanged global relative humidity (RH) in future climates. A literature survey revealed that 50% of Earth system models (ESMs) and land surface models (LSMs) embedded within GCMs employ RH as an atmospheric aridity index when describing stomatal conductance (gs), whereas the remaining 50% employ VPD. The consequences of using RH or VPD in gs models for water cycling and vegetation productivity in future climates on large spatial and temporal scales remain to be explored. Process-based global dynamic vegetation model runs, changes in the hydrological cycle, and concomitant vegetation productivity for the 21st century projected climate were conducted by altering only gs responses to VPD or RH and not changing any other formulations. In the simulations of the African continent under a 21st century warming trend, both stomatal functions of VPD and RH resulted in similar gross geographic patterns in primary production (GPP). However, continental total GPP was larger for the VPD response than that for the RH response. Transpiration rates were lower, resulting in a 13% increase in water-use efficiency for the VPD response compared with its RH counterpart

Dynamic and synergistic influences of air temperature and rainfall on general flowering in a Bornean lowland tropical forest

Supra‐annually synchronized flowering events occurring in tropical forests in Southeast Asia, known as general flowering (GF), are “spectacular and mysterious” forest events. Recently, studies that combined novel molecular techniques and model‐based theoretical approaches suggested that cool temperature and drought synergistically drove GF. Although these advanced our understanding of GF, it is still difficult to know whether the individual‐based molecular measurements and model‐based mathematical representations reasonably well capture the complex and dynamic GF processes at the community level. In the present study, we collected a 17‐year set of community‐wide phenology data from Lambir Hills National Park in Borneo, Malaysia, and analyzed it using a model‐free approach, empirical dynamic modeling (EDM), which does not rely on specific assumptions about the underlying mechanisms, to overcome and complement the previous limitations. We found that GF in the region is driven synergistically, not independently, by cool air temperature and drought, which is consistent with the previous studies. More importantly, our model‐free approach showed for the first time that effects of cumulative meteorological variables on GF changed over time. The time‐varying influences of meteorological variables on GF imply that the relationship between GF and meteorological variables might be influenced by other factors such as plant/soil nutrient resource dynamics. Our study provides a novel insight about the mechanism underlying the spectacular tropical forest event GF, and future studies integrating advanced mathematical/statistical frameworks, long‐term and large spatial scale ecosystem monitoring and molecular phenology data are promising for achieving better understanding and forecasting of GF events in Southeast Asia

Persistence and memory time scales in root-zone soil moisture dynamics

The memory timescale that characterizes root-zone soil moisture remains the dominant measure in seasonal forecasts of land-climate interactions. This memory is a quasi-deterministic timescale associated with the losses (e.g. evapotranspiration) from the soil column and is often interpreted as persistence in soil moisture states. Persistence, however, represents a distribution of time periods where soil moisture resides above or below some prescribed threshold, and is therefore inherently probabilistic. Using multiple soil moisture datasets collected at high resolution (sub-hourly) across different biomes and climates, this paper explores the differences, underlying dynamics, and relative importance of memory and persistence timescales in root-zone soil moisture. A first-order Markov process, commonly used to interpret soil moisture fluctuations derived from climate simulations, is also used as a reference model. Persistence durations of soil moisture below the plant water-stress level (chosen as the threshold), and the temporal spectrum of up- and down-crossings of this threshold, are compared to the memory timescale and spectrum of the full time series, respectively. The results indicate that despite the differences between meteorological drivers, the spectrum of threshold-crossings is similar across sites, and follows a unique relation with that of the full soil moisture series. The distribution of persistence times exhibits an approximate stretched exponential type and reflects a likelihood of exceeding the memory at all sites. However, the rainfall counterpart of these distributions shows that persistence of dry atmospheric periods is less likely at sites with long soil moisture memory. The cluster exponent, a measure of the density of threshold crossings in a time frame, reveals that the clustering tendency in rainfall events (on-off switches) does not translate directly to clustering in soil moisture. This is particularly the case in climates where rainfall and evapotranspiration are out of phase, resulting in less ordered (more independent) persistence in soil moisture than in rainfall

Effects of vertical air temperature distribution within forest canopies on photosynthesis and transpiration

This study examined whether accurately simulating vertical air temperature (AT) distribution within forest canopies is essential for predicting vertical photosynthesis and transpiration distribution using multilayer canopy models. Inspecting earlier observational studies that reported vertical AT distribution within forest canopies, we showed that the common vertical AT difference within forest canopies was lower than 3.0℃. We showed, using a leaf-scale transpiration-photosynthesis model, that a 3.0℃ AT difference caused smaller differences in leaf- scale photosynthesis and transpiration rates than a common vertical difference in photosynthetic active radiation (PAR) intensity within forest canopies when AT was higher than ca. 15℃. While, the AT difference caused larger differences in leaf-scale photosynthetic and transpiration rates than the PAR difference when AT was lower than ca. 10℃. However, the ranges in the rates with changing AT by 3.0℃ were comparable with predictability of a leaf-scale transpiration-photosynthesis model. Thus, we conclude that accurately simulating AT distribution is not essential at this stage for calculating vertical photosynthesis and transpiration distribution using multilayer canopy models.本研究では、林内気温鉛直分布を正確に再現することが、林内光合成・蒸散鉛直分布を多層モデルで推定するのに不可欠かどうかを調べた。筆者らは既存文献を踏査して、林内で計測される鉛直方向の気温差が通常3.0℃以下であることを示した。つづいて、単葉スケールの蒸散・光合成モデルによる計算によってつぎのことを示した。気温が約15℃以上のとき、3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、林内で通常計測される光合成有効放射量の鉛直方向の差が引き起こす光合成・蒸散量の違いよりも小さい。一方、気温が約10℃以下のとき、3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、林内で通常計測される光合成有効放射量の鉛直方向の差が引き起こす光合成・蒸散量の違いよりも大きいが3.0℃の気温の差が引き起こす光合成・蒸散量の違いは、単葉スケールの蒸散・光合成モデルの予測精度と同程度である。したがって、現時点において、林内気温鉛直分布を正確に再現することは、林内光合成・蒸散鉛直分布を多層モデルで推定するのに不可欠ではないと結論した

シカ害のある流域における土壌の性質と渓流水質

本研究では，九州大学農学部附属宮崎演習林内の，シカによる下層植生の食害がある流域における土壌の理化学性と渓流水質について現状を報告する．シカによる下層植生の食害のあるB流域と，シカ害が有り将来的にシカ防除柵による対策を予定しているC流域において不撹乱土壌を採取し，乾燥密度，間隙率，飽和体積含水率，飽和透水性係数といった土壌の物理性を測定した．また，土壌の化学性として土壌pHおよびECを測定した．B流域の方がC流域と比較して土壌の間隙率と土壌ECが高く，植物が水分・養分を利用しやすい土壌であることが示唆された．また，C流域のとくに斜面下部に砂礫が多く堆積していた．シカ害の無いA流域も含め3流域での渓流水質の比較を行ったところ，現段階ではシカ害による林床の状態の違いが渓流水質に顕著な差を生じさせてはいないことが明らかとなった．We report the soil physico-chemical properties and water quality of stream in two watersheds, which were damaged by deer feeding in Shiiba Research Forest. Bulk densities, volumetric contents of total soil pores, saturated volumetric water contents and saturated hydraulic conductivities were determined as physical characteristics, and water-soluble pH and EC were measured as chemical indicators in soil samples taken from three positions in the slope. Compared in site C, the volumetric contents of total soil pores and EC in site B were higher. This suggests that plants in site B have a greater tendency to use soil water and nutrients. In site C, especially in under position of the slope, a large amount of sand gravels were observed. In comparison of water quality of stream in 3 sites including site A, which has not damaged by deer, there is no difference in three sites, which have different condition of forest floor by deer damage. In the future plan, site C is surrounded by protecting net to avoid deer damage, and we continue to monitor the process of vegetation restoration and its effects on soil characteristics, comparing the control watershed, site B