Competition and tolerance of low soil water favor Carex dominance over establishing Acer seedlings in managed temperate mesic forests

Abstract

Carex pensylvanica (Pennsylvania sedge) dominance of managed temperate mesic forest understories is associated with high deer browsing pressure, frequent partial timber harvests, invasive earthworms, and low seedling-to-sapling recruitment rates of canopy-dominant Acer saccharum (sugar maple). There is concern that Carex dominance may be difficult to reverse because Carex may decrease soil resource availability and tolerate these conditions better than Acer seedlings. However, evaluation of this notion and of management aimed at reducing possible Carex impacts on Acer seedlings is lacking. Here, we examine the effects of (1) removing Carex on soil water and the growth and survival of naturally-established Acer seedlings over a forest understory light gradient, (2) drought on growth, survival, water relations, and soil mineral N pools of potted monocultures and mixtures of Carex and Acer seedlings, and (3) summer vs. autumn glyphosate herbicide treatments on Carex and Acer seedling dynamics in group-selection harvest openings. Carex removal increased soil water and growth of naturally established Acer seedlings but only in harvest gaps >8% light. Carex exhibited greater tolerance of drought than Acer by closing stomates at lower pre-dawn water potentials (−4.2 MPa vs. −3.0 MPa), and incurring lower post-drought mortality (~0% vs. 50%). In well-watered Carex-Acer mixtures, Carex reduced soil NH4+ but not Acer growth or survival relative to Acer monocultures. Glyphosate treatments reduced Carex by >90%, but possible benefits to young Acer seedlings may have been offset by greater deer herbivory on Acer in glyphosate treatments. Our results indicate that Carex reinforces its dominance of mesic hardwood forest understories via water competition and drought tolerance mechanisms that slow the establishment and recruitment of canopy dominant Acer. These effects could be exacerbated by increased intensity and frequency of droughts in the future, such that Carex dominance of understories could accelerate the xerophication of these systems in the face of climate change. Herbicides can be used to reduce Carex but may need to be combined with deer population reductions to promote adequate Acer seedling to sapling recruitment for sustainable management by selection silviculture. The sustainability of other temperate mesic forests globally characterized by high ungulate populations and associated high graminoid density may be similarly challenged.