Jesse B. Nippert’s research while affiliated with Kansas State University and other places

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Publications (169)


Open letter: There are more than just trees and forests to be conserved and restored
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January 2025

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Conceptual figure depicting the growth forms and traits that distinguish growth strategies among encroaching woody species in tallgrass prairie. (a) Four growth forms of encroaching woody species at Konza Prairie Biological Station. Fire filters out woody species that cannot resprout in areas with a fire return interval of ≤4 years. (b) The most abundant encroaching species may have similar traits that increase their competitive ability or stress tolerance. This scenario (left) suggests the environment and/or competition with herbaceous species drives the community assembly of encroaching species with common traits that confer dominance. Alternatively, the most abundant encroaching species may have different traits. This scenario (right) suggests that niche differences among species s multiple species to coexist and become highly abundant at a single site. (c) Analyses that integrate leaf‐level traits that reflect interspecific carbon and water use strategies can be used to infer whether the most abundant encroaching species are similar (left panel) or have unique growth strategies (middle and right panels). Species may vary along Axis 1 (middle panel), Axis 2 (not shown) or both (right panel).
Relative cover (a, b), frequency (c, d) and species dominance index (e, f) of for the focal woody species from 1989 to 2022. Relative cover of each species was calculated using all species, woody and herbaceous, present in the plots (n = 20 plots per ungrazed and grazed areas). The tan line in panels (a, b) is the total relative cover of all woody species, including all trees and shrubs that grow above the grass canopy. Frequency was calculated as the percentage of plots where each species occurred. Species dominance index is calculated as the average relative cover and frequency, where an index closer to 1 means the species has high cover and is found in a high number of plots (Avolio et al., 2019).
Principal components analysis of the seven focal species and measured physiological and structural traits (n = 8 per species). Colours represent species and shapes represent the two locations sampled, one grazed (triangles) and one ungrazed (circles). Species names in the legend are ordered from most to least abundant. Physiological traits: Asat, light‐saturated photosynthetic rate; Jmax, maximum electron transport rate; LCP, light compensation point; ɸ, apparent quantum yield; Vcmax, maximum carboxylation rate; δ¹³C, integrated leaf water use efficiency; πtlp, turgor loss point. Structural traits: %C, leaf carbon content; %N, leaf nitrogen content; LDMC, leaf dry matter content; SLA, specific leaf area; wd, woody density.
Parameters from light response (A–C) and CO2 response curves (D, E; mean ± 1 SE) of the seven focal shrub species (n = 8 curves per species). (a) Light‐saturated photosynthetic rates, (b) light compensation point, (c) apparent quantum yield, (d) maximum electron transport rate and (e) maximum rate of carboxylation. Species are ordered from most abundant to least abundant. Letters represent significant differences among species (p < 0.05).
Traits related to water use and drought tolerance of the seven focal woody species. (a) Turgor loss point, (b) integrated leaf water use efficiency and (c) wood density. Species are ordered from most abundant to least abundant. Letters represent significant differences among species (p < 0.05).

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Divergent resource‐use strategies of encroaching shrubs: Can traits predict encroachment success in tallgrass prairie?

December 2024

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75 Reads

Changes in climate and land management over the last half‐century have favoured woody plants native to grasslands and led to the rapid expansion of woody species. Despite this being a global phenomenon, it is unclear why some woody species have rapidly expanded while others have not. We assessed whether the most abundant woody encroaching species in tallgrass prairie have common growth forms and physiology or unique traits that differentiate their resource‐use strategies. We characterized the abundance, above‐ground carbon allocation, and leaf‐level physiological and structural traits of seven woody encroaching species in tallgrass prairie that span an order of magnitude in abundance. To identify species‐specific increases in abundance, we used a 34‐year species composition dataset at Konza Prairie Biological Station (Central Great Plains, USA). We then compared biomass allocation and leaf‐level traits to determine differences in carbon and water use strategies among species. While all focal species increased in abundance over time, encroachment in this system is primarily driven by three species: Cornus drummondii, Prunus americana and Rhus glabra. The most dominant species, Cornus drummondii, had the most extreme values for several traits, including the lowest leaf:stem mass ratios, lowest photosynthetic capacity and highest turgor loss point. Two of the most abundant species, Cornus drummondii and Rhus glabra, had opposing growth forms and resource‐use strategies. These species had significantly different above‐ground carbon allocation, leaf‐level drought tolerance and photosynthetic capacity. There were surprisingly few interspecific differences in specific leaf area and leaf dry matter content, suggesting these traits were poor predictors of species‐level encroachment. Synthesis. Woody encroaching species in tallgrass prairie encompass a spectrum of growth forms and leaf physiology. Two of the most abundant woody species fell at opposite ends of this spectrum. Our results suggest niche differences among a community of woody species facilitate the rapid encroachment by a few species. This study shows that woody encroaching species do not conform to a ‘one‐size‐fits‐all’ strategy, and a diversity of growth forms and physiological strategies may make it more challenging to reach management goals that aim to conserve or restore grassland communities.


The CZ as an Ecological Problem: How the Interplay of Biotic and Abiotic Actors Determines the Functioning of Earth’s Living Skin

November 2024

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1 Citation

Long before the term ‘critical zone’ (CZ) was coined to encompass Earth’s biological and geological features from the top of the vegetative canopy to the depths of circulating groundwater, many scientists have recognized that both biotic and abiotic actors are centrally important for understanding many of Earth’s most fundamental processes. Contemporary CZ scientists continue this legacy. We describe findings that emphasize how life, emphasizing vegetation and microbes, responds to and shapes the physical environment in which it persists, yielding feedbacks for Earth’s climate, primarily through modifications to hydrologic functioning. We focus on the interactions of biota and the physical and chemical features of soil pedons and landscapes as they drive ecosystem-scale hydrologic fluxes. We focus on hydrologically-relevant features because of the long history of individual disciplines telling us about the large-scale importance of these processes, and because of emerging research highlighting the importance of the intersection of these disciplines for projecting future ecosystem functioning on a rapidly changing Earth. The knowledge we spotlight reveals Earth’s CZ as a fundamentally ecological problem.


Woody Encroachment Modifies Subsurface Structure and Hydrological Function

November 2024

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1 Citation

Ecohydrology

Woody encroachment—the expansion of woody shrubs into grasslands—is a widely documented phenomenon with global significance for the water cycle. However, its effects on watershed hydrology, including streamflow and groundwater recharge, remain poorly understood. A key challenge is the limited understanding of how changes to root abundance, size and distribution across soil depths influence infiltration and preferential flow. We hypothesised that woody shrubs would increase and deepen coarse‐root abundance and effective soil porosity, thus promoting deeper soil water infiltration and increasing soil water flow velocities. To test this hypothesis, we conducted a study at the Konza Prairie Biological Station in Kansas, where roughleaf dogwood ( Cornus drummondii ) is the predominant woody shrub encroaching into native tallgrass prairie. We quantified the distribution of coarse and fine roots and leveraged soil moisture time series and electrical resistivity imaging to analyse soil water flow beneath shrubs and grasses. We observed a greater fraction of coarse roots beneath shrubs compared to grasses, which was concurrent with greater saturated hydraulic conductivity and effective porosity. Half‐hourly rainfall and soil moisture data show that the average soil water flow through macropores was 135% greater beneath shrubs than grasses at the deepest B horizon, consistent with greater saturated hydraulic conductivity. Soil‐moisture time series and electrical resistivity imaging also indicated that large rainfall events and greater antecedent wetness promoted more flow in the deeper layers beneath shrubs than beneath grasses. These findings suggest that woody encroachment alters soil hydrologic processes with cascading consequences for ecohydrological processes, including increased vertical connectivity and potential groundwater recharge.



Grass leaf structural and stomatal trait responses to climate gradients assessed over the 20th century and across the Great Plains, USA

September 2024

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11 Reads

AoB Plants

Using herbarium specimens spanning 133 years and field-collected measurements, we assessed intraspecific trait (leaf structural and stomatal) variability from grass species in the Great Plains of North America. We focused on two widespread, closely related grasses from the tribe Paniceae: Dichanthelium oligosanthes subsp. scribnerianum (C3) and Panicum virgatum (C4). Thirty-one specimens per taxon were sampled from local herbaria from the years 1887 to 2013 to assess trait responses across time to changes in atmospheric [CO2] and growing season precipitation and temperature. In 2021 and 2022, the species were measured from eight grasslands sites to explore how traits vary spatially across natural continental precipitation and temperature gradients. Δ13C increased with atmospheric [CO2] for D. oligosanthes but decreased for P. virgatum, likely linked to increases in precipitation in the study region over the past century. Notably, this is the first record of decreasing Δ13C over time for a C4 species illustrating 13C linkages to climate. As atmospheric [CO2] increased, C:N increased and δ15N decreased for both species and %N decreased for D. oligosanthes. Across a large precipitation gradient, D. oligosanthes leaf traits were more responsive to changes in precipitation than those of P. virgatum. In contrast, only two traits of P. virgatum responded to increases in temperature across a gradient: specific leaf area (increase) and leaf dry matter content (decrease). The only shared significant trend between species was increased C:N with precipitation. Our work demonstrates that these closely related grass species with different photosynthetic pathways exhibited various trait responses across temporal and spatial scales, illustrating the key role of scale of inquiry for forecasting leaf trait responses to future environmental change.



PSInet: A new global water potential network

August 2024

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350 Reads

Tree Physiology

Given the pressing challenges posed by climate change, it is crucial to develop a deeper understanding of the impacts of escalating drought and heat stress on terrestrial ecosystems and the vital services they offer. Soil and plant water potential play a pivotal role in governing the dynamics of water within ecosystems and exert direct control over plant function and mortality risk during periods of ecological stress. However, existing observations of water potential suffer from significant limitations, including their sporadic and discontinuous nature, inconsistent representation of relevant spatio-temporal scales, and numerous methodological challenges. These limitations hinder the comprehensive and synthetic research needed to enhance our conceptual understanding and predictive models of plant function and survival under limited moisture availability. In this article, we present PSInet (PSI—for the Greek letter Ψ used to denote water potential), a novel collaborative network of researchers and data, designed to bridge the current critical information gap in water potential data. The primary objectives of PSInet are: (1) Establishing the first openly accessible global database for time series of plant and soil water potential measurements, while providing important linkages with other relevant observation networks. (2) Fostering an inclusive and diverse collaborative environment for all scientists studying water potential in various stages of their careers. (3) Standardizing methodologies, processing, and interpretation of water potential data through the engagement of a global community of scientists, facilitated by the dissemination of standardized protocols, best practices, and early career training opportunities. (4) Facilitating the use of the PSInet database for synthesizing knowledge and addressing prominent gaps in our understanding of plants’ physiological responses to various environmental stressors. The PSInet initiative is integral to meeting the fundamental research challenge of discerning which plant species will thrive and which will be vulnerable in a world undergoing rapid warming and increasing aridification.


Repeated clearing as a mechanism for savanna recovery following bush encroachment

Many savannas are experiencing increased cover of trees and shrubs, resulting in reduced herbaceous productivity, shifts in savanna functional structure and potential reductions in ecotourism. Clearing woody plants has been suggested as an effective management strategy to mitigate these effects and restore these systems to an open state with higher rates of grass production and herbivory. This study investigated the effectiveness of repeated shrub clearing as a tool to mitigate bush encroachment in a semi‐arid savanna in southern Africa. We present data from a 7‐year experiment in the Mthimkhulu Game Reserve bordering Kruger National Park, South Africa. Colophospermum mopane stems and resprouting shoots were basally cut 2–3 times per year (2015–2022) in three pairs of treatment and control plots of 60 × 60 m. We monitored changes in soil moisture, grass biomass and herbivore activity via dung counts. We assessed C. mopane physiological responses to repeated cutting using non‐structural carbohydrates and stable water isotopes to infer changes to energy storage and functional rooting depth, respectively. The cleared treatment had higher soil moisture and grass biomass than the control treatment. Dung counts showed impala and buffalo visited the cleared treatment more frequently than the control treatment. Repeated cutting had limited effects on C. mopane survival in the first 2–3 years after initial clearing, but 80% of individuals were dead after 7 years. Repeatedly cut C. mopane had lower belowground starch concentrations and used water from shallower soil depths than C. mopane in control plots. Synthesis and applications. Repeated cutting increased soil moisture availability and grass biomass, and attracted charismatic grazing herbivores. While more costly than once‐off clearing methods, this practice created more employment opportunities for a neighbouring rural community. Transforming portions of the ecosystem to a grass‐dominated state may increase ecotourism potential through improved game viewing in open systems.


Contrasting shrub and grass hydraulic responses to experimental drought

Oecologia

Whole-plant hydraulics provide important information about responses to water limitation and can be used to understand how plant communities may change in a drier climate when measured on multiple species. Here, we measured above- and belowground hydraulic traits in Cornus drummondii, an encroaching shrub within North American tallgrass prairies, and Andropogon gerardii, a dominant C4 grass, to assess the potential hydraulic responses to future drought as this region undergoes woody expansion. Shelters that reduced precipitation by 50% and 0% were built over shrubs and grasses growing in sites that are burned at 1-year and 4-year frequencies. We then measured aboveground (Kshoot), belowground (Kroot), and whole-plant maximum hydraulic conductance (Kplant) in C. drummondii and Kroot in A. gerardii. We also measured vulnerability to embolism (P50) in C. drummondii stems. Overall, we show that: (1) A. gerardii had substantially greater Kroot than C. drummondii; (2) belowground hydraulic functioning was linked with aboveground processes; (3) above- and belowground C. drummondii hydraulics were not negatively impacted by the rainfall reductions imposed here. These results suggest that a multi-year drought will not ameliorate rates of woody expansion and highlight key differences in aboveground and belowground hydraulics for dominant species within the same ecosystem.


Citations (71)


... Key factors in these observed differences include climate, bedrock composition and vadose zone characteristics. However, the mechanisms by which woody encroachment modifies subsurface hydrologic processes across these factors are not fully understood (Huxman et al. 2005;Keen et al. 2024). ...

Reference:

Woody Encroachment Modifies Subsurface Structure and Hydrological Function
Unexpected hydrologic response to ecosystem state change in tallgrass prairie
  • Citing Article
  • September 2024

Journal of Hydrology

... resources, and soil conditions (Qian et al., 2020). The leaf area index (LAI) of alfalfa is a crucial indicator for defining the canopy structure of alfalfa, statistically describing the growth and density changes of alfalfa leaf populations (Tooley et al., 2024). It can be used to assess the dynamics of leaf growth and yield in alfalfa (Tripathi et al., 2018). ...

Evaluating methods for measuring the leaf area index of encroaching shrubs in grasslands: From leaves to optical methods, 3-D scanning, and airborne observation
  • Citing Article
  • April 2024

Agricultural and Forest Meteorology

... showing this species has low instantaneous photosynthetic rates (McCarron & Knapp, 2001;Wedel et al., 2021). This seemingly low leaf-level assimilation capacity is compensated for by dense canopies (Tooley et al., 2022), high apparent quantum yield ( Figure 4C) and deep root systems that decrease the likelihood of stems and leaves facing drought (Keen et al., 2024;Ratajczak et al., 2011). ...

Save or spend? Diverging water‐use strategies of grasses and encroaching clonal shrubs

... HBV can be calibrated either manually or using automatic methods like genetic algorithm optimization (Seibert, 2000;Seibert & Vis, 2012;Vis et al., 2015). Monte Carlo simulations have also been used to identify cases that reproduce streamflow data (Sadayappan et al., 2023). Modeled water fluxes and storage from HBV along with its parameters are used as hydrological inputs for BioRT-HBV. ...

Drier streams despite a wetter climate in woody-encroached grasslands
  • Citing Article
  • November 2023

Journal of Hydrology

... Understanding nesting distributions and the related elephant impact on these trees helps inform conservation strategies for both white-backed vultures and the broader ecosystem. If vultures show a strong preference for one type of habitat and that habitat is under greater threat from elephants and other environmental factors, conservationists can better prioritise habitat protection and restoration efforts (Swemmer et al. 2023). ...

The effects of floods, droughts and elephants on riparian tree mortality in a semi-arid savanna
  • Citing Article
  • October 2023

Forest Ecology and Management

... C 4 origins are not distributed uniformly across plant lineages, but are clustered in the specific 'PACMAD' clade (Edwards & Smith, 2010;Christin et al., 2013). Any physiological differences between distantly related C 3 and C 4 species could result from differences in selective pressures and/or evolutionary trajectories within a specific clade having little to do specifically with photosynthetic pathway variation (Donnelly et al., 2023). Thus, in order to establish the specific impact of C 4 evolution on the relationship between K leaf and A max , phylogenetically informed studies considering closely related C 3 and C 4 grasses are necessary (Edwards & Still, 2008;Edwards & Smith, 2010). ...

Evolutionary lineage explains trait variation among 75 coexisting grass species

... Biodiversity decline in temperate hornbeam-beech forests has also been reported to extend up to 60 m from roadsides (Deljouei et al., 2017). The shorter range of influence observed in our study can be attributed to the fact that grasses exhibit higher resistance to cold and arid conditions compared to trees (Belovitch et al., 2023;Hjelm and Ö gren, 2003). ...

African savanna grasses outperform trees across the full spectrum of soil moisture availability

... The spillover hypothesis had moderate support (59% of tests found evidence for spillover). For example, experimental removal of forest cover surrounding riparian zones altered stream flow and increased nutrient inputs (Dodds et al. 2023). Tests of the ecosystem decay hypothesis had moderate support (68%), but the tests were largely indirect, in that occupancy, abundance, or richness was measured in patches rather than demographic rates or ecosystem changes, which can arise from ecosystem decay (Lindenmayer et al. 2009). ...

Trajectories and state changes of a grassland stream and riparian zone after a decade of woody vegetation removal

... This is because vegetation water content directly influences the contribution of vegetation to backscattering; an inaccurate estimate of VWC complicates the retrieval of soil moisture. The adequate penetration depth of the L band can reach 10 cm or more, surpassing that of the optical band [72]. This likely accounts for the higher accuracy of underlying soil moisture retrieval using RVI. ...

Remotely Sensed Soil Moisture Can Capture Dynamics Relevant to Plant Water Uptake

... Effective water resource management policies can help regulate water use, allocation, and conservation practices to maintain streamflow levels in headwater catchments (Gaertner et al. 2020). Enforcing environmental protection regulations can safeguard headwater catchments from degradation due to human activities, preserving natural hydrological processes and streamflow dynamics (Hatley et al. 2023). Policies aimed at climate change adaptation can also help address the impacts of changing climatic conditions on streamflow regimes in headwater catchments, ensuring resilience to future hydrological challenges (Biederman et al. 2015). ...

Intermittent streamflow generation in a merokarst headwater catchment