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1. Drought response in herbaceous plants: A test of the integrated framework of plant form and function

   https://doi.org/10.1111/1365-2435.14495  

   Funk, Jennifer L. ; Larson, Julie E. ; Blair, Megan D. ; Nguyen, Monica A. ; Rivera, Ben J. ( January 2024 , Functional Ecology)

   Multidimensional trait frameworks are increasingly used to understand plant strategies for growth and survival. However, it is unclear if frameworks developed at a global level can be applied in local communities and how well these frameworks—based largely on plant morphological traits—align with plant physiology and response to stress.

   We tested the ability of an integrated framework of plant form and function to characterise seedling trait variation and drought response among 22 grasses and forbs common in a semi‐arid grassland. We measured above‐ground and below‐ground traits, and survival to explore how drought response is linked to three trait dimensions (resource conservation, microbial collaboration, and plant size) associated with the framework as well as non‐morphological dimensions (e.g. physiological traits) that are under‐represented in global trait frameworks.

   We found support for three globally‐recognised axes representing trade‐offs in strategies associated with tissue investment (leaf nitrogen, leaf mass per area, root tissue density), below‐ground resource uptake (root diameter, specific root length), and size (shoot mass). However, in contrast to global patterns, above‐ground and below‐ground resource conservation gradients were oppositely aligned: root tissue density was positively correlated with leaf N rather than leaf mass per area. This likely reflects different investment strategies of annual and perennial herbaceous species, as fast‐growing annual species invested in lower density roots and less nitrogen‐rich leaves to maximise plant‐level carbon assimilation. Species with longer drought survival minimised water loss through small above‐ground size and low leaf‐level transpiration rates, and drought survival was best predicted by a principal component axis representing plant size.

   Contrary to our expectations, drought survival in seedlings did not align with the conservation or collaboration axes suggesting that seedlings with different functional strategies can achieve similar drought survival, as long as they minimise water loss. Our results also show that within local communities, expected trait relationships could be decoupled as some plant groups achieve similar performance through different trait combinations. The effectiveness of species mean trait values in predicting drought response highlights the value of trait‐based methods as a versatile tool for understanding ecological processes locally across various ecosystems.

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2. Ecological strategies begin at germination: Traits, plasticity and survival in the first 4 days of plant life

   https://doi.org/10.1111/1365-2435.13543  

   Larson, Julie E. ; Anacker, Brian L. ; Wanous, Sara ; Funk, Jennifer L. ; Baltzer, ed., Jennifer ( March 2020 , Functional Ecology)

   We commonly use trait variation to characterize plant function within and among species and understand how vegetation responds to the environment. Seedling emergence is an especially vulnerable window affecting population and community dynamics, yet trait‐based frameworks often bypass this earliest stage of plant life. Here we assess whether traits vary in ecologically meaningful ways when seedlings are just days old. How do shared evolutionary history and environmental conditions shape trait expression, and can traits explain which seedlings endure drought?

   We measured seedling traits in the first 4 days of life for 16 annual plant species under two water treatments, exploring trait trade‐offs, species‐level plasticity and the ability of traits to predict duration of survival under drought.

   Nearly half of traits showed the imprint of evolutionary history (i.e. significant phylogenetic signal), often reflecting differences between grasses and forbs, two groups separated by a deep evolutionary split. Water availability altered trait expression in most cases, though species‐level plastic responses also reflected evolutionary history.

   On average, new seedlings exhibited substantial trait variation structured as multiple trade‐offs like those found in mature plants. Some species invested in thick roots and shoots, whereas others invested in more efficient tissues. Separately, some invested in tougher roots and others in deeper roots. We also observed trade‐offs related to growth rates (fast or slow) and biomass allocation (above‐ or below‐ground). Drought survival time was correlated most strongly with seed mass, root construction and allocation traits, and phylogeny (grasses vs. forbs).

   Synthesis.Our results show that seed and seedling trait variation among annual species is substantial, and that a few attributes could capture major dimensions of ecological strategies during emergence. With seedling survival times ranging twofold among annuals (from 7.5 to 14.5 days), these strategies could mitigate recruitment responses to more frequent or longer dry spells. Multivariate trait and plasticity strategies should be further explored in studies designed to assess trait‐fitness linkages during recruitment.

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3. Hydraulic traits are not robust predictors of tree species stem growth during a severe drought in a wet tropical forest

   https://doi.org/10.1111/1365-2435.14235  

   Smith‐Martin, Chris M. ; Muscarella, Robert ; Ankori‐Karlinsky, Roi ; Delzon, Sylvain ; Farrar, Samuel L. ; Salva‐Sauri, Melissa ; Thompson, Jill ; Zimmerman, Jess K. ; Uriarte, María ( December 2022 , Functional Ecology)

   Severe droughts have led to lower plant growth and high mortality in many ecosystems worldwide, including tropical forests. Drought vulnerability differs among species, but there is limited consensus on the nature and degree of this variation in tropical forest communities. Understanding species‐level vulnerability to drought requires examination of hydraulic traits since these reflect the different strategies species employ for surviving drought.

   Here, we examined hydraulic traits and growth reductions during a severe drought for 12 common woody species in a wet tropical forest community in Puerto Rico to ask: Q1. To what extent can hydraulic traits predict growth declines during drought? We expected that species with more hydraulically vulnerable xylem and narrower safety margins (SM_P50) would grow less during drought. Q2. How does species successional association relate to the levels of vulnerability to drought and hydraulic strategies? We predicted that early‐ and mid‐successional species would exhibit more acquisitive strategies, making them more susceptible to drought than shade‐tolerant species. Q3. What are the different hydraulic strategies employed by species and are there trade‐offs between drought avoidance and drought tolerance? We anticipated that species with greater water storage capacity would have leaves that lose turgor at higher xylem water potential and be less resistant to embolism forming in their xylem (P₅₀).

   We found a large range of variation in hydraulic traits across species; however, they did not closely capture the magnitude of growth declines during drought. Among larger trees (≥10 cm diameter at breast height—DBH), some tree species with high xylem embolism vulnerability (P₅₀) and risk of hydraulic failure (SM_P50) experienced substantial growth declines during drought, but this pattern was not consistent across species. We found a trade‐off among species between drought avoidance (capacitance) and drought tolerating (P₅₀) in this tropical forest community. Hydraulic strategies did not align with successional associations. Instead, some of the more drought‐vulnerable species were shade‐tolerant dominants in the community, suggesting that a drying climate could lead to shifts in long‐term forest composition and function in Puerto Rico and the Caribbean.

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4. Trade‐offs in above‐ and below‐ground biomass allocation influencing seedling growth in a tropical forest

   https://doi.org/10.1111/1365-2745.13543  

   Umaña, María Natalia ; Cao, Min ; Lin, Luxiang ; Swenson, Nathan G. ; Zhang, Caicai ; Liu, ed., Xiaojuan ( November 2020 , Journal of Ecology)

   Plants allocate biomass to different organs in response to resource variation for maximizing performance, yet we lack a framework that adequately integrates plant responses to the simultaneous variation in above‐ and below‐ground resources. Although traditionally, the optimal partition theory (OPT) has explained patterns of biomass allocation in response to a single limiting resource, it is well‐known that in natural communities multiple resources limit growth. We study trade‐offs involved in plant biomass allocation patterns and their effects on plant growth under variable below‐ and above‐ground resources—light, soil N and P—for seedling communities.

   We collected information on leaf, stem and root mass fractions for more than 1,900 seedlings of 97 species paired with growth data and local‐scale variation in abiotic resources from a tropical forest in China.

   We identified two trade‐off axes that define the mass allocation strategies for seedlings—allocation to photosynthetic versus non‐photosynthetic tissues and allocation to roots over stems—that responded to the variation in soil P and N and light. Yet, the allocation patterns did not always follow predictions of OPT in which plants should allocate biomass to the organ that acquires the most limiting resource. Limited soil N resulted in high allocation to leaves at the expense of non‐photosynthetic tissues, while the opposite trend was found in response to limited soil P. Also, co‐limitation in above‐ and below‐ground resources (light and soil P) led to mass allocation to stems at the expense of roots. Finally, we found that growth increased under high‐light availability and soil P for seedlings that invested more in photosynthetic over non‐photosynthetic tissues or/and that allocated mass to roots at the expense of stem.

   Synthesis. Biomass allocation patterns to above‐ and below‐ground tissues are described by two independent trade‐offs that allow plants to have divergent allocation strategies (e.g. high root allocation at the expense of stem or high leaf allocation at the expense of allocation to non‐photosynthetic tissues) and enhance growth under different limiting resources. Identifying the trade‐offs driving biomass allocation is important to disentangle plant responses to the simultaneous variation in resources in diverse forest communities.

    

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5. Evaluating the roles of microbial functional breadth and home‐field advantage in leaf litter decomposition

   https://doi.org/10.1111/1365-2435.14026  

   Osburn, Ernest D. ; Hoch, Peter J. ; Lucas, Jane M. ; McBride, Steven G. ; Strickland, Michael S. ( March 2022 , Functional Ecology)

   Soil biota are increasingly recognized as a primary control on litter decomposition at both local and regional scales, but the precise mechanisms by which biota influence litter decomposition have yet to be identified.

   There are multiple hypothesized mechanisms by which biotic communities may influence litter decomposition—for example, decomposer communities may be specially adapted to local litter inputs and therefore decompose litter from their home ecosystem at elevated rates. This mechanism is known as the home‐field advantage (HFA) hypothesis. Alternatively, litter decomposition rates may simply depend upon the range of metabolic functions present within a decomposer community. This mechanism is known as the functional breadth (FB) hypothesis. However, the relative importance of HFA and FB in litter decomposition is unknown, as are the microbial community drivers of HFA and FB. Potential relationships/trade‐offs between microbial HFA and FB are also unknown.

   To investigate the roles of HFA and FB in litter decomposition, we collected litter and soil from six different ecosystems across the continental US and conducted a full factorial litter × soil inoculum experiment. We measured litter decomposition (i.e. cumulative CO₂‐C respired) over 150 days and used an analytical model to calculate the HFA and FB of each microbial decomposer community.

   Our results indicated clear functional differences among decomposer communities, that is, litter sources were decomposed differently by different decomposer communities. These differences were primarily due to differences in FB between different communities, while HFA effects were less evident.

   We observed a positive relationship between HFA and the disturbance‐sensitive bacterial phylum Verruomicrobia, suggesting that HFA may be an important mechanism in undisturbed environments. We also observed a negative relationship between bacterial r versus K strategists and FB, suggesting an important link between microbial life‐history strategies and litter decomposition functions.

   Microbial FB and HFA exhibited a strong unimodal relationship, where high HFA was observed at intermediate FB values, while low HFA was associated with both low and high FB. This suggests that adaptation of decomposers to local plant inputs (i.e. high HFA) constrains FB, which requires broad rather than specialized functionality. Furthermore, this relationship suggests that HFA effects will not be apparent when communities exhibit high FB and therefore decompose all litters well and also when FB is low and communities decompose all litters poorly. Overall, our study provides new insights into the mechanisms by which microbial communities influence the decomposition of leaf litter.

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