Recipient Organization
CORNELL UNIVERSITY
(N/A)
ITHACA,NY 14853
Performing Department
Horticulture
Non Technical Summary
The urban forest will face a fundamental challenge in a changing climate. Stakeholders, including the green industry, land owners and urban foresters have identified significant barriers to tree growth and survival that require advancements in the area of tree response to predicted frequency in drought. Urban trees must survive in unique growing conditions including limited rooting volumes and poor soils that exacerbate abiotic stresses such as water limitation. This project contributes to long-term research on understanding tree processes below-ground and leverages recent work on furthering the green industry tree species palette and tree species root exudate diversity. Characterizing the soil-root interface (rhizosphere) of urban trees that represent different water use strategies allows for a better understanding of water limitation to tree productivity and potential strategies for mitigation.
Animal Health Component
20%
Research Effort Categories
Basic
80%
Applied
20%
Developmental
(N/A)
Goals / Objectives
Urban environments tend to be inhospitable to tree growth. Poor soils with low water holding capacity and low soil organic matter can alter the growth and physiology of tree root systems. The aim of this research is promote tree productivity by analyzing the effect of quantitative and qualitative root exudate characteristics on the rhizosphere across tree species that represent diverse water use strategies. This research will lead to an understanding plant-rhizosphere interactions by 1) characterizing how root exudates of urban tree species representing diverse water use strategies influence root and rhizosphere hydrology, and by 2) evaluating how the ability of a root system to alter root exudation may function as a mechanism to manipulate the rhizosphere microbiome during dry-wet cycles. Long-term aims are to increase urban tree health and productivity through potential management of the rhizosphere in the face of predicted increases in soil moisture limitation due to climate change.
Project Methods
A Long-term field experiment that mimics reduced rooting volumes via 48 in-ground, hydraulically isolated, tree boxes (1.83m x 1.83m x 1.5m, w x h x d) were recently established at the Cornell Orchards. Boxes were lined and sealed with 40 mil root barrier to prevent water from getting into the sampling area of the root box. Two sides of each planting box were constructed of 2.3 mm thick, 1.2m x 1.2 m clear polycarbonate sheeting to allow for easy viewing of the tree root system and ease of cutting of the material for root access. Subsurface drainage was established during tree box instillation and soil from the site was amended with compost and back filled to attempt to meet city planting recommendations. Researcher/public access boxes with installed ladders in-between each tree allow individuals to view the trees belowground. When the access areas are not in use they are covered with an insulated lid to prevent large temperature fluctuations.We have selected seven tree species based on their water use strategies, i.e. insensitive to water stress, anisohydric and sensitive to water stress isohydric. We have already evaluated several tree species water use strategies in the lab including Acer rubrum, Quercus bicolor, and Tilia cordata. We plan to additionally screen Magnolia, Koelreuteria paniculata, Ostrya virginiana, and Cercidephyllum japonicum. After tree establishment, drought stress will be imposed via precipitation exclusion shelters. These consist of sub-canopy halved pvc troughs covering 50% of the soil surface area that divert precipitation from reaching the soil surface. These methods align with current precipitation reduction experiments at Hubbard Brook (Asbjornsen personal comm.) and are also in line with current precipitation reduction experiments (Asbjornsen et al., in press, Drought- Net, NSF program). These protocol for inducing drought follow recommendations from the NSF funded Drought-NET IDE program which allows for study systems to be compared on a world-wide basis.Minirhizotron tubes will be installed with each tree to allow for root observations during tree establishment and to quantify root life span and root interactions in situ, tubes will be installed at a 30° angle to a depth of 1 m, 50 cm from the base of the tree. Root development will be monitored by measurements taken every other week by a specialized laparoscopic camera (Bartz, Inc, Santa Barbara, CA, USA) during the growing season and once a month during plant dormancy.These sampling times follow the common protocol for minirhizotron imaging (Bauerle et al. 2008). If needed root imaging can be increased to a higher frequency in order to capture particular growth or death events of interest. The acquired pictures will be analyzed using specialized root analysis software (WinRhizo Tron MF, Regents Inc. Quebec, Canada).Time domain reflectometry (TDR) probes will be established through the mylar sheets at three depths monitor soil moisture and temperature. Water retention and conductance of soil samples collected from the rhizosphere will be measured and compared with those of the associated bulk soil. The multi-probe Wind evaporative method will be used in a custom soil column for small soil volumes (~5 ml) with a pair of microtensiometers placed at 1.5 and 3 cm from the exposed surface (Tamari 1993); following Šim?nek et al., water retention and conductivity will be extracted by non-linear parameter optimization coupled with numerical simulations of Richards equation (Šim?nek 1998).After tree establishment tree branches and roots will be sampled for hydraulic conductivity at declining water potentials to construct vulnerability curves. These curves estimate the "vulnerability" of the tree organs or in other words when the organ is likely to form embolisms or air bubble formation in its xylem that potentially reduces the flow of water within the tree. Tree height and shoot length as a measure of plant growth will be collected monthly.In collaboration with the Kessler lab at Cornell, we have established a method for phenolic root exudate analysis. Preliminary results confirm that different tree species show great variation in the quantity and quality of phenolic exudates. Fine roots will be accessed through the polycarbonate windows, cleaned and put in a syringe containing glass beads and a carbon-free nutrient solution. Exudates are collected from the flushed root solution following Phillips 2008. In order to confirm that root exudate quality directly selects for microbial community composition and activity, we will expose native microbial communities to exudates collected from the tree species that were grown under either ideal conditions or water stress in the experiment. To simulate a rhizosphere environment, plastic cylinders (50 mm x 20 mm) with stainless steel mesh bottoms will be filled with 10 g of field soil and covered with a microfiber-glass filter. The membrane filter will allow for even application of exudates to the soil (Falchini et al. 2003).Additional roots will be collected for rhizosphere microbial analysis. For all of the experiments, the rhizosphere will be defined as the soil adhering to the root after gently shaking off bulk soil (Yergeau et al. 2015). Total DNA will be extracted using the NucleoSpin 96 Soil kit (Macherey-Nagel, Bethlehem, PA). We will amplify the 16S rRNA gene region to examine bacterial composition, as well as the fungal internal transcribed spacer (ITS) region, using universal primers recommended by the Joint Genome Institute (Daum 2016). Sequences will be processed following the Brazilian Microbiome Project (http://www.brmicrobiome.org/). Droughted microbiome communities will be compared to community composition in the control treatments (e.g. wet-adapted microbes only) to determine whether current soil conditions, or relative microbiome proportions, are more important for determining tree root microbiome recruitment. We will look at this by comparing both global community distance (e.g. Bray-Curtis distance) and by looking at the relative abundance of individual OTUs, to determine whether specific taxa are disproportionately impacted, regardless of their initial abundance.