Effects of lowered water table and agricultural practices on a remnant restiad bog over four decades

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Abstract
Peatlands are important contributors to global biodiversity, yet drainage and land management practices continue to affect many. We studied changes in a 114-ha raised bog remnant in northern New Zealand by assessing the vegetation and peat from 1974 to 2017, a period of agricultural conversion and intensification, and by comparing it with an intact bog. Over 43 years the remnant experienced significant water table drawdown, resulting in changes in vegetation composition and extirpation of species. However, the keystone peat-forming restiads (Restionaceae), Empodisma robustum and Sporadanthus ferrugineus, are still present. Between 1997 and 2017, concentrations in peat of N and K increased significantly but the concentration of P which, initially, was already higher than in the intact bog, did not increase. Foliar analysis of the ericoid mycorrhizal shrub Epacris pauciflora revealed significantly higher N and P, lower N/P and less depleted δ15N compared with the intact bog. Although raised bogs are inherently P-limited, the nature of the limitation appears to be changing at the remnant, with P becoming less limiting, a likely side effect of local fertiliser application. An ecological tipping point may be imminent, accompanied by a switch to higher productivity and expansion of woody species which are better adapted to lowered water table and increased nutrients.

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