Simeon John Smaill

Indoor plant phenotyping facilities provide high‐resolution, dynamic trait data not achievable with other systems, but are rarely applied to trees due to size constraints. Using the model conifer Pinus radiata , this study evaluated whether early growth variation among three clonal genotypes can be detected and reliably resolved using indoor phenotyping. Clonal seedlings were grown in three growth media under controlled conditions and phenotyped over 84 days using a LemnaTec RGB (red‐green‐blue) imaging system. Smoothed projected shoot area (sPSA) was derived from multi‐angle images, with absolute growth rate (sAGR) and relative growth rate (sRGR) calculated to quantify growth dynamics. Phenotyping‐derived traits consistently discriminated clones ( p < 0.05) but not growth media ( p > 0.05), and no clone × media interaction was detected ( p > 0.1). Clone 2 exhibited greater sPSA than Clones 1 and 3 from 48 days after planting onwards. Absolute growth rates differentiated treatments by overall growth magnitude, with media effects present but clonal effects dominant ( p < 0.001). Relative growth rates showed similar patterns, with Clone 2 exhibiting the highest intrinsic growth ( p < 0.001). Using six to seven replicates per clone‐soil treatment, indoor phenotyping enabled clear separation of clonal differences in early growth. These results demonstrate that juvenile conifers express measurable phenotypic variation that can be reliably captured under controlled conditions, supporting the application of indoor phenotyping for high‐resolution early‐stage tree research.

Soil conditioning can generate persistent plant–soil feedbacks (PSF) that influence plant performance under subsequent growth conditions, yet the role of soil inoculum volume in mediating these effects remains poorly understood. Here, we tested how inoculum volume influences the relative strength of a known positive PSF effect. We performed a plant–soil feedback experiment with Pinus radiata D. Don in two phases: one, a “conditioning phase”, and two, a “feedback phase”, where inoculum from the first phase was used in different dilutions to test the growth differences resulting from conditioning. To understand how inoculum volume affects subsequent growth in the feedback phase, seedlings (n = 12 per treatment) were grown in soil from phase one using different volumetric dilutions; 100% conditioned soil, 50% conditioned soil + 50% inert media, or 25% conditioned soil + 75% inert media. Positive plant–soil feedbacks were observed in undiluted soils: seedlings produced 40–65% greater biomass and experienced 50–70% lower mortality compared to the lowest inoculum treatment. However, this response varied with dilution; the strength of plant–soil feedbacks decreased with increasing dilution of inoculum. These findings highlight soil inoculum volume as an important, but often overlooked, factor in plant–soil feedback experiments and applied soil management. Our study provides experimental evidence that effective soil conditioning depends on both conditioning and a required minimum inoculum volume to confer measurable benefits to future plantings.

This study investigates the soil respiration (Rs) dynamics in Pinus radiata D. Don plantation forests, which are crucial for understanding carbon cycling and ecosystem functioning of these common plantation forests of New Zealand. However, the combined effects of stand development, microbial diversity, and seasonal variability on Rs remain poorly understood in managed short-rotation conifer plantations. Here, we address this gap by quantifying Rs across a chronosequence of stand ages and genotypes, alongside measurements of soil temperature, moisture, and bacterial and fungal communities. As expected, Rs rates exhibited significant seasonal variation between summer (2.55 ± 0.21 µmol CO2 m-2 s-1, mean ± S.E.) and winter (1.69 ± 0.15 µmol CO2 m-2 s-1). Stand age had a significant effect on Rs, with the highest rates observed in the 6- and 12-year-old stands and the lowest rates in the 0- and 22-year-old stands, while other ages showed intermediate values. Most importantly, stand age had a profound effect on the soil microbiome, reflected in a decline in bacterial diversity and a concomitant increase in fungal diversity with increasing stand age. Further, stand age was associated with shifts in bacterial and fungal community structure at the phylum and family level. Overall, our findings underscore the importance of considering temporal, environmental, and soil microbial factors in assessing Rs dynamics in short-rotation Pinus radiata plantations, contributing valuable information to forest management, carbon cycling and model parameterisation.

Microbiomes play critical roles in host functioning and, therefore, there is increasing interest in the microbiome assembly of plants. However, sampling strategies for long-lived perennial trees need to be standardized to produce robust data that accurately represent the microbiome over time. This issue is currently unresolved because there is little evidence indicating which portion of perennial tree species (e.g., root region or surrounding soil) is the best to sample to produce the most accurate measure of microbiome communities. Our aim was to sample different compartments of a plant's belowground microbiome to identify the optimal sampling strategy to account for the microbial community present. We found that the structure of the microbial community depends most strongly on the environment (site) and compartment of sample collected (bulk soil, rhizosphere, or rhizoplane), rather than the depth or cardinal direction of the sample. We also found that the microbial community increased in diversity with increased distance from the tree within the rhizoplane and rhizosphere. The data presented here provide systematic evidence for a pragmatic and robust sampling regime that was tested and validated across different environments and soil types while controlling for host genotype. This sampling regime will enable effective partitioning of root compartments when studying the microbiome associated with perennial tree species, allowing targeted questions about the microbiome to be explored with greater accuracy.