Hydrological niche segregation of plant functional traits

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Water is one of the major drivers determining distribution and abundance of plant species. Namely, plant species’ presence and location in the landscape can be explained using metrics of soil water because plant species are restricted to a species-specific range of soil water conditions, i.e. their hydrological niche.
However, little is known about the specific traits that determine the hydrological niche of a plant species.
To investigate the relationship between plant functional traits, community structure and hydrological niche segregation, we developed a new generic individual-based model PLANTHeR which describes plant functional trait abundance as a function solely of soil water potentials and individual behavior. An important
innovation is that there are no a priori defined trade-offs so that the model is neither restricted to a certain set of species nor scaled to a specific ecosystem.
We show that PLANTHeR(large plant stand) is able to reproduce well-known ecological rules such as the self-thinning law. We found that plant functional traits and their combinations (plant functional types − PFTs) were restricted to specific ranges of soil water potentials. Furthermore, the existence of functional trait trade-offs and correlations was determined by environmental conditions. Most interestingly, the correlation intensity between traits representing competitive ability and traits promoting colonization ability changed with water stress intensities in a unimodal fashion.
Our results suggest that soil water largely governs the functional composition, diversity and structure of plant communities. This has consequences for predicting plant species’ response to changes in the hydrological cycle due to global change. We suggest that PLANTHeR is a flexible tool that can be easily
adapted for further ecological-modelling studies.

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Introduction
Water is one of the major forces governing vegetation patterns in time and space across a wide range of scales (Manfreda et al., 2010). In contrast to other abiotic factors such as nutrients, water is particularly interesting due to its dual effect on plant performances such as growth, survival or fecundity. On the one hand,
water is a key resource, and a shortage in supply limits plant performance
(Silvertown et al., 2015). On the other hand, water acts as a disturbance agent (flooding and drought) and can drastically damage individual plants. Furthermore, water also mediates other soil conditions, e.g., oxygen concentration and nutrient availability (Yang and Jong 1971; Mustroph et al., 2016). timber plant stand,Thus, both excessive
and insufficient water availability impedes individual plant performances
and may consequently determine the distribution and ∗ Corresponding author.
E-mail address: maximiliane.herberich@uni-tuebingen.de (M.M. Herberich).
abundance of plant species (Feddes et al., 1978; Moeslund et al.,
2013).
The influence of water availability on the composition and distribution of local vegetation is especially evident in the increasing number of field observations related to so-called hydrological niche segregation (Silvertown et al., 2015). Plant species can be defined by species-specific ranges of soil water conditions, i.e. their hydrological niche, such that the species’ presence and location
in the landscape can be described using functions of soil water (Silvertown et al., 1999). However, the mere description of distribution patterns is not sufficient to predict species’ response to changes in soil water; we also need to know the specific plant characteristics which underlie these patterns.
Functional traits can be a direct link between species response and  environmental factors,large plant stand (Cornelissen et al., 2003). These relate environmental factors to individual fitness via their effects on growth, reproduction, and survival (Laughlin and Laughlin 2013).
Thus, functional traits enable us to understand how changing environmental
conditions affect vegetation composition and structure across different scales of ecological organization (Cornelissen et al., 2003). Non-phylogenetic groups that share a set of key functional traits, i.e. respond in a similar way based on a shared response mechanism to a syndrome of environmental factors, are called plant functional types (PFTs, Gitay and Noble 1997; Cornelissen et al.,
2003).
For a large number of species, the functional traits which accurately indicate how species respond to changing environmental conditions are difficult to quantify (Cornelissen et al., 2003). Instead ‘soft functional traits’ are frequently used which
are only indirect measures of the actual plant function but which
are relatively quick and easy to quantify (Hodgson et al., 1999).
Several soft functional traits have been associated with species
response to water as a resource (e.g., spinescence, leaf size, leaf
phenology, bark thickness, seed mass) or as a disturbance agent
(e.g., resprouting ability, plant height) (Cornelissen et al., 2003;
Violle et al., 2011; Kukowski et al., 2013). However, little is known
about the specific traits that may explain the hydrological niche
of a plant species (Silvertown et al., 2015). One reason may be
the above-mentioned function of water simultaneously acting
as a resource, a disturbance agent and a measure of other soil
conditions, each of which selects for partly conflicting adaptations.
To make it even more complicated, water is highly dynamic in time
and space on a fine scale, so that hydrologically heterogeneous
habitats such as wet heathlands or temporarily flooded meadows
may even exhibit opposing types of water stress, i.e. seasonal alternation
between waterlogging and drought in the same location in
the course of a growing season (Oddershede et al., 2015).
Classical niche theories emphasize trait trade-offs as a mechanism
underlying species segregation along environmental
gradients (Chesson 2000; Tilman 2004). For example, species
segregation along soil water gradients in English meadows was
caused by a trade-off between species’ tolerances to aeration
stress and soil-drying stress (Silvertown et al., 1999). Another
trade-off possibly associated with hydrological niche segregation
exists between water-use efficiency and relative growth rate, i.e.
fast-growing species are rarely drought resistant and vice-versa
(Angert et al., 2009).
However, trade-off-based niche theories fail to provide a general
explanation for species’ relative abundance and vegetation structure
because they assume a priori that species that are better at
dealing with one environmental constraint are necessarily worse
at dealing with another (Tilman 2004). Also, while trade-offs cause
species to segregate along environmental gradients, the environmental
factor itself could affect postulated classical trade-offs such
as the competition-colonization trade-off (Tilman 1994). Besides,
a dichotomous trait-space with a singular pre-defined trade-off
is insufficient to approximate the complexity and spatiotemporal
variability of multi-species natural ecosystems, especially when it
comes to hydrological niches (Kukowski et al., 2013; Silvertown
et al., 2015). Therefore, a multi-trait modeling approach should be
favored over a dichotomous one when trying to theorize the role
of water availability in structuring natural plant communities.
A novel modeling approach that has relaxed classical assumptions
about dichotomous trade-offs has yielded highly unexpected,
yet plausible, trait combinations for plants adapting to disturbance
(Seifan et al., 2012). These authors showed that disturbance
promoted trade-offs between different colonization modes and
between dormancy and disturbance-tolerance, while surprisingly,
the classical competition-colonization trade-off was not generated.
Instead, competition strength varied in a consistent manner with
changes in disturbance intensity, while dispersal distance varied
in a consistent manner with changes in disturbance predictability.
These results indicate that an unrestricted modeling approach that
does not define a priori trade-offs among plant traits is very useful
for identifying the relationship between traits and environmental
gradients. Also, a trade-off unrestricted approach is sufficiently
flexible to include different types of ecosystems and plant strategies
ranging from short-lived herbaceous plants to long-lived trees. This
represents important progress compared to classical geo-biosphere
models. These models tried to capture the essential dynamics of an
ecosystem by modelling few plant functional types or life forms
defined a priori by a small set of postulated characteristics with
limited explanatory power (Bonan et al., 2002; Verant et al., 2004;
Lapola et al., 2008).


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