Percent cover vs. species diversity: what your closure plan is actually measuring

Pull up almost any closure plan and the revegetation success criteria section will lean on one of two measurements, sometimes both. They get used interchangeably in casual conversation, which is a problem, because they answer different questions and a surety or regulator reading a self-reported annual report needs to know which one is actually on the table for a given milestone year.

Percent cover: how much ground is green

Percent vegetative cover is the simpler metric and the one most closure plans lead with. It's a measure of how much of the reclaimed surface is occupied by live vegetation when you look straight down at it, usually expressed against a reference area or a pre-mining baseline. A plan might set a target of 70% relative cover by year three, climbing toward 90% by final bond release. Field crews pull this with line-point intercept transects or Daubenmire frames, walking plots and recording hits.

Cover is popular as a success criterion because it's cheap to measure, easy to compare year over year, and it directly answers the question a surety cares about first: is the site holding soil, or is it bare ground waiting for the next storm to move sediment into a drainage. Erosion control, slope stability, and the visual "does this look reclaimed" test all ride on cover numbers.

The catch is that a monoculture of fast-growing annual grass can hit 90% cover and still be a failed reclamation if the closure plan's intent was a self-sustaining native plant community. Cover tells you coverage, not composition.

Species diversity: what's actually growing there

Species diversity standards show up in plans that care about long-term ecological function rather than short-term erosion control, which is most hard-rock and coal plans written in the last couple decades. These criteria usually specify a minimum number of species present, sometimes split by life form (woody, herbaceous, nitrogen-fixer), and increasingly reference a diversity index like Shannon-Wiener or Simpson's to capture both richness and evenness. A site with 15 species where one grass makes up 95% of cover scores very differently on a diversity index than a site where those 15 species are more evenly distributed.

Diversity is harder to field-verify. It requires a botanist or a trained tech walking plots and identifying plants to species, not just eyeballing green versus brown. That's part of why diversity data tends to be thinner in operator self-reports than cover data: it costs more to collect and it's easier to understate without someone checking.

Which one does your plan actually require

Most closure plans in practice use both, at different milestone years. A common pattern: cover is the gating criterion for early interim bond reductions, since the priority in years one through five is stopping erosion and establishing a living surface. Diversity becomes the gating criterion closer to final bond release, since that's the point where a regulator is deciding whether the site can be walked away from as a functioning plant community rather than a temporarily green one.

The practical problem for anyone reviewing a bond release request is that cover numbers are trivially easy to self-report in a way that looks good, and diversity numbers are rarely reported at the resolution needed to catch a monoculture passing as a success. An operator's annual compliance report can show 85% cover for three consecutive years and still be sitting on a stand that would fail a diversity count if anyone walked it.

This is the gap a remote vegetation-index record is built to close. A per-parcel reading taken independently, once a year, from wide-swath multispectral imagery gives a regulator or surety something to check a self-reported cover number against before a bond reduction gets signed off, without waiting on the next scheduled field audit. It won't replace a botanist's species count, but it catches the cases where cover is climbing on paper while the underlying plant community tells a different story. You can see how that tracking works against your own closure plan's milestones on the Revegetation Monitor landing page.

If your next bond decision hinges on a cover number you can't independently verify, that's worth fixing before you sign off on it.