Paleobotanical records provide opportunity to deepen an understanding of plant community ecology
by reconstructing the outcome of large-scale ecological ‘experiments’ in Earth’s past. However,
limited ability to describe ancient communities via plant functional traits and ecological strategies,
rather than (para)taxonomic composition, can hinder the relevance of constructed datasets. Many
functional traits are not measurable on fossil leaves and the link between leaf morphology and
ecological strategy are currently unresolved. To help fill this gap, we analyze leaf traits applicable to
fossil leaves (i.e., morphology, vein density, leaf mass per area) sampled at the community-scale
from modern plots spanning successional gradients, where plant function and ecological strategies
are expected to vary, in three different forest types: temperate deciduous forest (North Carolina,
USA), tropical rainforest (Malaysian Borneo), and a tropical dry forest (Minas Gerais, Brazil).
Preliminary results will be presented to draw empirical links between morphological leaf traits and
ecological strategy.
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Using Leaf δ13C to Assess Water Use Efficiency within Plant Communities Across a Successional Gradient in Temperate and Tropical Forests
Understanding how plant communities of the past have responded to disturbance events can
provide valuable insights when managing our natural resources and assessing human impacts
on ecosystems. The geologic record has the potential to reflect these responses through the
analysis of functional traits, which relate directly to plant function and ecosystem strategy.
There is currently little evidence of how functional traits measurable in fossil leaves vary
across succession in different forest types. Because of this, there is a limited ability to identify
disturbance as the primary driver of vegetation change within the fossil record. To improve
this ability, this study analyzes the carbon stable isotopic composition (δ 13C) of bulk organic
matter sampled at the community-scale across successional gradients in a temperate deciduous
forest (North Carolina, USA) and compares them against values from a previous study across
succession in a tropical evergreen forest (Malaysian Borneo). Leaf δ13C is representative of a plant's water use efficiency (WUE), an important axis of ecological strategy representing the carbon assimilated per water lost in a plant during photosynthesis. Leaf δ13C as a functional trait has the advantage that it is often preserved during leaf fossilization and, integrated across a plant community, can be informative about prevalent ecological strategies, functional diversity , and community assembly dynamics. In Borneo, the community-weighted mean of leaf δ13C to be highest in early-succession plots, indicative of a higher WUE in plant communities closely following a disturbance event. Old growth plots were found to have a lower δ13C, and thus a more conservative WUE. This study will further investigate if this
trend is followed within temperate forests, which is important as many mid-late Cenozoic
plant assemblages come from what would have been temperate regions. Developing a method of identifying disturbances within the geologic record, will improve the ability to discern drivers of plant community change in the past. This improved knowledge will help guide management decisions across a range of ecosystems.
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- Award ID(s):
- 1924390
- PAR ID:
- 10463438
- Date Published:
- Journal Name:
- Botany
- Page Range / eLocation ID:
- 726
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Under fire suppression, many tropical savannas transform into forests. Forest expansion entails changes in environmental variables and plant community structure. We hypothesized that forest expansion into savanna results in a shift in community‐weighted mean functional traits from stress tolerance to competitiveness, with generalist species having trait values intermediate between those of specialists of savanna and forest habitats.
We studied 30 plots distributed over three savanna–forest boundaries undergoing forest expansion in the Brazilian Cerrado, capturing a gradient from open savanna to recently formed forest. We measured functional traits of 116 woody species of savanna specialist, generalist and forest specialist functional groups and quantified changes in species composition and mean traits across the basal area gradient.
We identified two main axes of species traits. The first separated forest and generalist species from savanna specialists, with the latter possessing traits associated with resistance to disturbance and stress— such as thick leaves, thick bark, slower height growth and lower shade tolerance. Our second trait axis separated shrubs and understorey trees from pioneer species. Generalist species’ traits did not differ substantially from forest species, nor did they tend to have a typical pioneer strategy.
Community‐weighted trait means changed linearly with forest development. There was a steady increase in traits associated with competitive dominance rather than stress tolerance and fire resistance, indicating a wholesale shift in the selective environment. Several of these patterns—for example, increasing height and decreasing light requirements—are common in old‐field succession. In contrast to old‐field succession, we found that SLA increased, leaf thickness decreased and wood density stayed constant.
The assembly of forests appears to be shaped by environmental filters that contribute to a functional trajectory distinct from most other studied ecosystems. Our results highlight the importance of the functional composition of the early community and of the early colonizers of the open environment. Differences between savanna and forest specialists reflect the selective effects of the contrasting environments, while the traits of generalists—and their interaction with environmental filters—drive the dynamics of forest expansion.
A free
Plain Language Summary can be found within the Supporting Information of this article. -
Across the globe, the forest carbon sink is increasingly vulnerable to an expanding array of low- to moderate-severity disturbances. However, some forest ecosystems exhibit functional resistance (i.e., the capacity of ecosystems to continue functioning as usual) following disturbances such as extreme weather events and insect or fungal pathogen outbreaks. Unlike severe disturbances (e.g., stand-replacing wildfires), moderate severity disturbances do not always result in near-term declines in forest production because of the potential for compensatory growth, including enhanced subcanopy production. Community-wide shifts in subcanopy plant functional traits, prompted by disturbance-driven environmental change, may play a key mechanistic role in resisting declines in net primary production (NPP) up to thresholds of canopy loss. However, the temporal dynamics of these shifts, as well as the upper limits of disturbance for which subcanopy production can compensate, remain poorly characterized. In this study, we leverage a 4-year dataset from an experimental forest disturbance in northern Michigan to assess subcanopy community trait shifts as well as their utility in predicting ecosystem NPP resistance across a wide range of implemented disturbance severities. Through mechanical girdling of stems, we achieved a gradient of severity from 0% (i.e., control) to 45, 65, and 85% targeted gross canopy defoliation, replicated across four landscape ecosystems broadly representative of the Upper Great Lakes ecoregion. We found that three of four examined subcanopy community weighted mean (CWM) traits including leaf photosynthetic rate ( p = 0.04), stomatal conductance ( p = 0.07), and the red edge normalized difference vegetation index ( p < 0.0001) shifted rapidly following disturbance but before widespread changes in subcanopy light environment triggered by canopy tree mortality. Surprisingly, stimulated subcanopy production fully compensated for upper canopy losses across our gradient of experimental severities, achieving complete resistance (i.e., no significant interannual differences from control) of whole ecosystem NPP even in the 85% disturbance treatment. Additionally, we identified a probable mechanistic switch from nutrient-driven to light-driven trait shifts as disturbance progressed. Our findings suggest that remotely sensed traits such as the red edge normalized difference vegetation index (reNDVI) could be particularly sensitive and robust predictors of production response to disturbance, even across compositionally diverse forests. The potential of leaf spectral indices to predict post-disturbance functional resistance is promising given the capabilities of airborne to satellite remote sensing. We conclude that dynamic functional trait shifts following disturbance can be used to predict production response across a wide range of disturbance severities.more » « less
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The U.S. Pacific Northwest (PNW) hosts an extensive suite of Miocene-aged fossil plants sites, with the potential to document changes in plant community ecology in response to regional climatic change during the Miocene Climatic Optimum (MCO; 17-14 Ma) and the ensuing Middle Miocene Climatic Transition (MMCT; ~14 Ma). The MCO was the most recent period of sustained global warming and thus provides some analogy to anthropogenic climate change. An important component of characterizing plant community ecology is the diversity and prevalence of ecological strategies present within a community. Many previous paleoecology studies rely on a nearest living relative approach to infer components of ecological strategy (e.g., plant functional types) from fossil plant assemblages. In contrast, much work in neo-ecology stresses the importance of functional traits in elucidating prevalent ecological strategies and functional diversity within plant communities. Here we take advantage of exquisitely preserved leaf compression fossils from Clarkia, northern Idaho (~16.9 Ma), representing the height of the MCO, to measure leaf functional traits and elucidate ecological strategies of dominant species in this ancient temperate mixed conifer-deciduous-evergreen forest. We focus on 13 species, representing the most abundant angiosperm taxa in the assemblage, including Betula, Castanea, and Quercus. We reconstruct assimilation rates using gas exchange modeling, address leaf hydraulic efficiency by measuring leaf vein density, and reconstruct water use efficiency by accounting for the ratio of carbon assimilation to transpirational water loss. As these species are prevalent in many other Miocene floras of the PNW, this study provides a benchmark by which to interpret changes in the dominance or presence of these species through time and, by inference, how Miocene climatic changes impact the functional composition and diversity of this forest type. We are also providing an example of how present-day mixed deciduous forests may respond to current anthropogenic changes in CO 2.more » « less
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Abstract Background and Aims Understanding shifts in the demographic and functional composition of forests after major natural disturbances has become increasingly relevant given the accelerating rates of climate change and elevated frequency of natural disturbances. Although plant demographic strategies are often described across a slow–fast continuum, severe and frequent disturbance events influencing demographic processes may alter the demographic trade-offs and the functional composition of forests. We examined demographic trade-offs and the shifts in functional traits in a hurricane-disturbed forest using long-term data from the Luquillo Forest Dynamics Plot (LFPD) in Puerto Rico.
Methods We analysed information on growth, survival, seed rain and seedling recruitment for 30 woody species in the LFDP. In addition, we compiled data on leaf, seed and wood functional traits that capture the main ecological strategies for plants. We used this information to identify the main axes of demographic variation for this forest community and evaluate shifts in community-weighted means for traits from 2000 to 2016.
Key Results The previously identified growth–survival trade-off was not observed. Instead, we identified a fecundity–growth trade-off and an axis representing seedling-to-adult survival. Both axes formed dimensions independent of resprouting ability. Also, changes in tree species composition during the post-hurricane period reflected a directional shift from seedling and tree communities dominated by acquisitive towards conservative leaf economics traits and large seed mass. Wood specific gravity, however, did not show significant directional changes over time.
Conclusions Our study demonstrates that tree demographic strategies coping with frequent storms and hurricane disturbances deviate from strategies typically observed in undisturbed forests, yet the shifts in functional composition still conform to the expected changes from acquisitive to conservative resource-uptake strategies expected over succession. In the face of increased rates of natural and anthropogenic disturbance in tropical regions, our results anticipate shifts in species demographic trade-offs and different functional dimensions.
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Accepted Manuscript1.0
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