Synchronised and fluctuating reproduction by plant populations, called masting, is widespread in diverse taxonomic groups. Here, we propose a new method to explore the proximate mechanism of masting by combining spatiotemporal flowering data, biochemical analysis of resource allocation and mathematical modelling. Flowering data of 170 trees over 13 years showed the emergence of clustering with trees in a given cluster mutually synchronised in reproduction, which was successfully explained by resource budget models. Analysis of resources invested in the development of reproductive organs showed that parametric values used in the model are significantly different between nitrogen and carbon. Using a fully parameterised model, we showed that the observed flowering pattern is explained only when the interplay between nitrogen dynamics and climatic cues was considered. This result indicates that our approach successfully identified resource type-specific roles on masting and that the method is suitable for a wide range of plant species.

In forested streams, surrounding riparian forests provide essential supplies of organic matter to aquatic ecosystems. We focused on two pathways of particulate organic matter inputs: direct input from upper riparian forests and indirect lateral input from bank slopes, for which there are limited quantitative data. We investigated the inputs of coarse particulate organic matter (CPOM) and carbon and nitrogen in the CPOM into the uppermost reaches of a headwater stream with steep bank slopes in Hokkaido, Japan. CPOM collected by litter traps was divided into categories (e.g., leaves, twigs) and weighed. Monthly nitrogen and carbon inputs were also estimated. The annual direct input of CPOM (ash-free dry mass) was 472 g m -2 , a common value for temperate riparian forests. The annual lateral CPOM input was 353 g m -1 and 941 g m -2 when they were converted to area base. This value surpassed the direct input. Organic matter that we could not separate from inorganic sediments contributed to the total lateral input from the bank slopes (124 g m -1 ); this organic matter contained relatively high amounts of nitrogen and carbon. At uppermost stream reaches, the bank slope would be a key factor to understanding the carbon and nitrogen pathways from the surrounding terrestrial ecosystem to the aquatic ecosystem.

No abstract

Understanding how groundwater contribution changes with increasing drainage area is an important research challenge for catchment hydrology (Condon et al., 2020;Fan, 2019). Although hydrological and biogeochemical studies of headwater catchments have often assumed that the underlying bedrock was impermeable, many studies have revealed that non-negligible amounts of water and solutes infiltrate into bedrock and leave headwater catchments not detected at the weir (e.g., Aishlin & McNamara, 2011;Frisbee et al., 2016;Iwasaki et al., 2016;Schaller & Fan, 2009). On the other hand, hydrogeology studies have revealed the existence of regional groundwater flows with larger spatial scales than local groundwater flows (e.g., Ono et al., 2019;Tóth, 1962). In the current study, local groundwater contributing to headwater streams and regional groundwater not contributing to them were referred to as shallow and deep groundwater, respectively. Small headwater catchments are perched on top of regional groundwater systems, and deep groundwater usually discharges to lower basins (Fan, 2019;Smerdon et al., 2012). Thus, understanding the scaling relationships of groundwater contributions is essential when applying small headwater catchment study findings to larger catchments.The role of groundwater flow in scaling relationships has been examined by observing specific discharge (discharge per unit drainage area) and stream water chemistry during baseflow periods at multiple points with different drainage areas in a watershed. Shaman et al. (2004), investigating low-flow specific discharge in the Neversink River watershed, USA, which is underlain by sedimentary bedrock, suggested that deep groundwater contributions increase with increasing drainage area in subcatchments smaller than a representative elementary area (REA) of 8-21 km 2 but become constant in subcatchments larger than the REA. Asano et al. (2020) found an increase in baseflow-specific discharge with drainage area in a 93.58-km 2 sedimentary watershed of the Arakawa River, Japan. In addition to baseflow-specific discharge, stream and spring baseflow chemistry have been commonly used to understand groundwater dynamics across spatial scales by separating stream water into shallow and deep groundwater. Two-source separation has shown an increasing ratio of deep groundwater with drainage area in a 5-km 2 sedimentary Inokawa catchment,

We examined the relationship between the annual escapement of salmon and the d 15 N of willow (Salix spp.) leaves to evaluate the contribution of marine-derived nutrients (MDN) to riparian vegetation around the Pacific Northwest and Northeast regions. Foliar d 15 N values ranged from −3.42‰ to 4.65‰. The value increased with increasing density of carcasses up to 500 fish/km and 1500 fish/km. d 15 N values were variable at carcass densities below 500 fish/km. Possible factors affecting the fluctuation of d 15 N at reference sites are: (1) denitrification; (2) the presence of N 2 -fixing trees, such as alder; and (3) agricultural runoff. d 15 N values at the sites with carcass densities over 500 fish/km were consistently high, while a value of d 15 N below zero was observed at only one site (Rusha River; d 15 N = −1.87‰). At this site, most adult pink salmon returned to limited locations near the estuary because steeper channel gradients acted as a migration barrier, resulting in the negative d 15 N value. Nevertheless, we concluded that our results showed evidence of the feedback of MDN to terrestrial vegetation, although the use of the d 15 N value as a terrestrial end member at spawning sites is limited. If the relationship between the enrichment index, which is expressed as the values using a mixing model, and salmon abundance was estimated, the availability of MDN in riparian ecosystems could possibly be evaluated and will lead to the establishment of escapement goals.