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Institute
Massive historical land cover changes in the Central European lowlands have resulted in a forest distribution that now comprises small remnants of ancient forests and more recently established post-agricultural forests. Here, land-use history is considered a key driver of recent herb-layer community changes, where an extinction debt in ancient forest remnants and/or a colonization credit in post-agricultural forests are being paid over time. On a regional scale, these payments should in theory lead toward a convergence in species richness between ancient and post-agricultural forests over time. In this study, we tested this assumption with a resurvey of 117 semi-permanent plots in the well-studied deciduous forests of the Prignitz region (Brandenburg, NE Germany), where we knew that the plant communities of post-agricultural stands exhibit a colonization credit while the extinction debt in ancient stands has largely been paid. We compared changes in the species richness of all herb layer species, forest specialists and ancient forest indicator species between ancient and post-agricultural stands with linear mixed effect models and determined the influence of patch connectivity on the magnitude of species richness changes. Species richness increased overall, but the richness of forest specialists increased significantly more in post-agricultural stands and was positively influenced by higher patch connectivity, indicating a convergence in species richness between the ancient and postagricultural stands. Furthermore, the richness of ancient forest indicator species only increased significantly in post-agricultural stands. For the first time, we were able to verify a gradual payment of the colonization credit in post-agricultural forest stands using a comparison of actual changes in temporal species richness.
Plant community assembly in temperate forests along gradients of soil fertility and disturbance
(2012)
Plant community assembly from a regional pool is largely driven by two mechanisms: environmental filtering and niche partitioning, which result in trait convergence or divergence, respectively. Although empirical evidence for both assembly mechanisms exists, the environmental conditions and traits where each of the two assembly patterns is prevalent remain unclear. We studied community assembly mechanisms in herb layer communities of temperate forest patches in NW Germany, looking at distributions of competitive and reproductive traits along gradients of soil fertility and disturbance. We also examined how community assembly patterns changed over a time span of two decades. Canopy height converged toward taller species with increasing soil fertility and increasing light availability. Most reproductive traits diverged with an increasing degree of disturbance and with increasing fertility. Comparisons over time indicated that disturbance events induced the coexistence of species with different reproductive strategies and also selected for tall species as a result of enhanced competitive pressure. Our study demonstrates that in accordance with existing hypotheses, competitive traits (e.g., canopy height) can be convergent in favorable environments. However, this convergence is associated with a divergence of traits related to other challenges (e.g., reproduction), indicating that true functional redundancy within communities does not exist. Moreover, our study shows that the expected divergence of reproductive traits at disturbed sites can be accompanied by a convergence of other traits (e.g., canopy height), indicating that several assembly mechanisms can operate simultaneously.
Understorey plant communities play a key role in the functioning of forest ecosystems. Under favourable environmental conditions, competitive understorey species may develop high abundances and influence important ecosystem processes such as tree regeneration. Thus, understanding and predicting the response of competitive understorey species as a function of changing environmental conditions is important for forest managers. In the absence of sufficient temporal data to quantify actual vegetation changes, space-for-time (SFT) substitution is often used, i.e. studies that use environmental gradients across space to infer vegetation responses to environmental change over time. Here we assess the validity of such SFT approaches and analysed 36 resurvey studies from ancient forests with low levels of recent disturbances across temperate Europe to assess how six competitive understorey plant species respond to gradients of overstorey cover, soil conditions, atmospheric N deposition and climatic conditions over space and time. The combination of historical and contemporary surveys allows (i) to test if observed contemporary patterns across space are consistent at the time of the historical survey, and, crucially, (ii) to assess whether changes in abundance over time given recorded environmental change match expectations from patterns recorded along environmental gradients in space. We found consistent spatial relationships at the two periods: local variation in soil variables and overstorey cover were the best predictors of individual species’ cover while interregional variation in coarse-scale variables, i.e. N deposition and climate, was less important. However, we found that our SFT approach could not accurately explain the large variation in abundance changes over time. We thus recommend to be cautious when using SFT substitution to infer species responses to temporal changes.
Questions
What are the most likely environmental drivers for compositional herb layer changes as indicated by trait differences between winner and loser species?
Location
Weser-Elbe region (NW Germany).
Methods
We resurveyed the herb layer communities of ancient forest patches on base-rich sites of 175 semi-permanent plots. Species traits were tested for their ability to discriminate between winner and loser species using logistic regression analyses and deviance partitioning.
Results
Of 115 species tested, 31 were identified as winner species and 30 as loser species. Winner species had higher seed longevity, flowered later in the season and more often had an oceanic distribution compared to loser species. Loser species tended to have a higher specific leaf area, were more susceptible to deer browsing and had a performance optimum at higher soil pH compared to winner species. The loser species also represented several ancient forest and threatened species. Deviance partitioning indicated that local drivers (i.e. disturbance due to forest management) were primarily responsible for the species shifts, while regional drivers (i.e. browsing pressure and acidification from atmospheric deposition) and global drivers (i.e. climate warming) had moderate effects. There was no evidence that canopy closure, drainage or eutrophication contributed to herb layer changes.
Conclusions
The relative importance of the different drivers as indicated by the winner and loser species differs from that found in previous long-term studies. Relating species traits to species performance is a valuable tool that provides insight into the environmental drivers that are most likely responsible for herb layer changes.
QuestionBelow-ground processes are key determinants of above-ground plant population and community dynamics. Still, our understanding of how environmental drivers shape plant communities is mostly based on above-ground diversity patterns, bypassing below-ground plant diversity stored in seed banks. As seed banks may shape above-ground plant communities, we question whether concurrently analysing the above- and below-ground species assemblages may potentially enhance our understanding of community responses to environmental variation. LocationTemperate deciduous forests along a 2000km latitudinal gradient in NW Europe. MethodsHerb layer, seed bank and local environmental data including soil pH, canopy cover, forest cover continuity and time since last canopy disturbance were collected in 129 temperate deciduous forest plots. We quantified herb layer and seed bank diversity per plot and evaluated how environmental variation structured community diversity in the herb layer, seed bank and the combined herb layer-seed bank community. ResultsSeed banks consistently held more plant species than the herb layer. How local plot diversity was partitioned across the herb layer and seed bank was mediated by environmental variation in drivers serving as proxies of light availability. The herb layer and seed bank contained an ever smaller and ever larger share of local diversity, respectively, as both canopy cover and time since last canopy disturbance decreased. Species richness and -diversity of the combined herb layer-seed bank community responded distinctly differently compared to the separate assemblages in response to environmental variation in, e.g. forest cover continuity and canopy cover. ConclusionsThe seed bank is a below-ground diversity reservoir of the herbaceous forest community, which interacts with the herb layer, although constrained by environmental variation in e.g. light availability. The herb layer and seed bank co-exist as a single community by means of the so-called storage effect, resulting in distinct responses to environmental variation not necessarily recorded in the individual herb layer or seed bank assemblages. Thus, concurrently analysing above- and below-ground diversity will improve our ecological understanding of how understorey plant communities respond to environmental variation.