Seed diversity and microhabitat heterogeneity effects on plant establishment

Paul Richardson and colleagues have a paper in Early View in the Journal titled “Fine-scale spatial heterogeneity and incoming seed diversity additively determine plant establishment“.  Read it here.

The authors have provided a very thorough description of their study with many beautiful pictures of their study sites.  Enjoy.

Plant establishment has important consequences for populations, communities, and goods and services provided by ecosystems. It appears to be regulated by multiple interacting factors, including the fine-scale spatial variability of the environment and the variability of ecological traits expressed by arriving individuals. Furthermore, interactions between biodiversity and heterogeneity may be critical, if neighbour differences are more strongly expressed in patchier environments. Trying to figure out the nature and importance of each factor can be overwhelming, and the control required usually confines empirical work to the greenhouse. However, managers attempting to restore or create new ecosystems after major land use changes need to know what manipulations of species pools and environmental conditions will maximize plant establishment under harsh and variable “real world” conditions. We tackled this problem by conducting a field experiment in an environment that is on the one hand typical of degraded ecosystems requiring restoration, but on the other hand uniquely suited for investigating interrelationships among diversity, heterogeneity, and establishment: dry limestone quarry floors.

Abandoned limestone quarries represent a common environmental problem in the sense that the extremity of environmental change generally prevents pre-disturbance ecosystems from ever fully returning. However, dry quarry floors also present exciting opportunities for extending the habitat of diverse and stress-tolerant native herb communities typically confined to naturally-occurring limestone pavements (“alvars”). These communities provide important vegetation cover despite the severe environment, though cover can range from low to high along environmental gradients primarily related to the accumulation of soil components within the cracks, crevices and depressions that sporadically punctuate the otherwise barren bedrock. At patch scales < 0.2 m2 in area, plant communities may contain from 1 to nearly 30 vascular species, while ground conditions can vary from homogeneous surface cover by any one “microhabitat types”  (bedrock, gravel, till, soil, litter, etc.) to mosaics incorporating all types within a single patch. In limestone quarries undergoing restoration to alvar habitat, manipulating which seed assemblages are sown to which patches is a relatively easy means to test whether establishment is more strongly regulated by seed diversity or microhabitat heterogeneity, and the degree to which these factors interactively determine plant establishment.

We added a constant number of seeds to numerous small patches of quarry floor while varying the number and identities of species sown. One fifth of the patches received the entire pool of 12 species. One fifth received six of these species, and one fifth received the other six. One of the 6-speceis groups was further partitioned into two 3-species assemblages, each of which was applied to one fifth of the patches. We effectively crossed seed diversity with environmental gradients encompassing both average patch conditions and within-patch variability by replicating these compositions 40 times across a wide but natural range of environmental conditions. After following seedling establishment and growth over two years, we analyzed how the number of established plants depended on seed composition, average microhabitat conditions, microhabitat heterogeneity, and interactions. We tested for seed diversity effects by contrasting sets of composition treatments that differed in species number but not overall identity. We also used this approach to test whether environmental variables influenced plant establishment differently given high vs. low diversity of incoming seeds.

Limestone herb communities established with greater density when sown with more species, and when sown to more heterogeneous microhabitats. In contrast with some theoretical predictions, these effects were independent: heterogeneity was equally as helpful at patches receiving 6 species as at those receiving 12, and optimum establishment occurred where both types of variability were greatest. These factors explained establishment even when average conditions were accounted for. Interestingly, the benefit of doubling seed diversity was less than that of exploiting maximally heterogeneous patches. Although heterogeneity’s impact was independent of seed diversity, the mechanism may nevertheless be closely tied to another aspect of diversity – established diversity. We found that environmental conditions sorted incoming species, with certain species favouring certain microhabitat types such that patches containing multiple microhabitat types supported establishment of multiple species. This process likely played out early on but had longer-term effects due to better stress-dampening where more species established initially. Fine-scale heterogeneity within high-stress ecosystems thus seems to provide important opportunities at the establishment stage for species differences to become expressed, eventually enhancing plant density by supporting assemblages prone to facilitative interactions.

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