13 research outputs found
Non-Neutral Vegetation Dynamics
The neutral theory of biodiversity constitutes a reference null hypothesis for the interpretation of ecosystem dynamics and produces relatively simple analytical descriptions of basic system properties, which can be easily compared to observations. On the contrary, investigations in non-neutral dynamics have in the past been limited by the complexity arising from heterogeneous demographic behaviours and by the relative paucity of detailed observations of the spatial distribution of species diversity (beta-diversity): These circumstances prevented the development and testing of explicit non-neutral mathematical descriptions linking competitive strategies and observable ecosystem properties. Here we introduce an exact non-neutral model of vegetation dynamics, based on cloning and seed dispersal, which yields closed-form characterizations of beta-diversity. The predictions of the non-neutral model are validated using new high-resolution remote-sensing observations of salt-marsh vegetation in the Venice Lagoon (Italy). Model expressions of beta-diversity show a remarkable agreement with observed distributions within the wide observational range of scales explored (5⋅10(−1) m÷10(3) m). We also consider a neutral version of the model and find its predictions to be in agreement with the more limited characterization of beta-diversity typical of the neutral theory (based on the likelihood that two sites be conspecific or heterospecific, irrespective of the species). However, such an agreement proves to be misleading as the recruitment rates by propagules and by seed dispersal assumed by the neutral model do not reflect known species characteristics and correspond to averages of those obtained under the more general non-neutral hypothesis. We conclude that non-neutral beta-diversity characterizations are required to describe ecosystem dynamics in the presence of species-dependent properties and to successfully relate the observed patterns to the underlying processes
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Continental-scale impacts of intra-seasonal rainfall variability on simulated ecosystem responses in Africa
Climate change is expected to modify intra-seasonal rainfall variability,
arising from shifts in rainfall frequency, intensity and seasonality. These
intra-seasonal changes are likely to have important ecological impacts on
terrestrial ecosystems. Yet, quantifying these impacts across biomes and
large climate gradients is largely missing. This gap hinders our ability to
better predict ecosystem services and their responses to climate change, especially
for arid and semi-arid ecosystems. Here we use a synthetic weather generator
and an independently validated vegetation dynamic model (SEIB-Dynamic Global Vegetation Model, DGVM) to
virtually conduct a series of "rainfall manipulation experiments" to study
how changes in the intra-seasonal rainfall variability affect continent-scale
ecosystem responses across Africa. We generate different rainfall scenarios
with fixed total annual rainfall but shifts in (i) frequency vs. intensity,
(ii) rainy season length vs. frequency, (iii) intensity vs. rainy season
length. These scenarios are fed into SEIB-DGVM to investigate changes in
biome distributions and ecosystem productivity. We find a loss of ecosystem
productivity with increased rainfall frequency and decreased intensity at
very low rainfall regimes (<400 mm year<sup>−1</sup>) and low frequency
(<0.3 event day<sup>−1</sup>); beyond these very dry regimes, most ecosystems
benefit from increased frequency and decreased intensity, except in the wet
tropics (>1800 mm year<sup>−1</sup>) where radiation limitation prevents
further productivity gains. This result reconciles seemingly contradictory
findings in previous field studies on the impact of rainfall
frequency/intensity on ecosystem productivity. We also find that changes in
rainy season length can yield more dramatic ecosystem responses compared with
similar percentage changes in rainfall frequency or intensity, with the
largest impacts in semi-arid woodlands. This study demonstrates that
intra-seasonal rainfall characteristics play a significant role in
influencing ecosystem function and structure through controls on
ecohydrological processes. Our results suggest that shifts in rainfall
seasonality have potentially large impacts on terrestrial ecosystems, and
these understudied impacts should be explicitly examined in future studies of
climate impacts
An analysis of structure: Biomass structure relationships for characteristic species of the western Kalahari, Botswana
Savannah ecosystems are important carbon stocks on the Earth, and their quantification is crucial for understanding the global impact of climate and land-use changes in savannahs. The estimation of aboveground/belowground plant biomass requires tested allometric relationships that can be used to determine total plant biomass as a function of easy-to-measure morphological indicators. Despite recent advances in savannah ecology, research on allometric relations in savannahs remains confined to a few site-specific studies where basal area is typically used as the main morphometric parameter with plant biomass. We investigate allometric relations at four sites along a 950-km transect in the Kalahari across mean rainfall gradient 170 mm yr-1-550 mm yr-1. Using data from 342 harvested trees/shrubs, we relate basal area, height and crown diameter to aboveground biomass. These relationships are strongest in trees and weakest in small shrubs. Strong allometric relationships are also determined for morphologically similar groups of woody vegetation. We show that crown diameter can be used as an alternative to basal area in allometric relationships with plant biomass. This finding may enhance the ability to determine aboveground biomass over large areas using high-resolution aerial or satellite imagery without requiring ground-based measurements of basal area. © 2013 John Wiley & Sons Ltd