Abstract
Alterations in under-canopy resource availability associated with elevated ponderosa pine (Pinus ponderosa Dougl.) abundance can negatively influence understory vegetation. Experimental evidence linking under-canopy resource availability and understory vegetation is scarce. Yet this information would be beneficial in developing management strategies to recover desired understory species. We tested the effects of varying nitrogen (N) and light availability on Idaho fescue (Festuca idahoensis Elmer), the dominant understory species in ponderosa pine/Idaho fescue plant associations in eastern Oregon. In a greenhouse experiment, two levels of N (50 kg∙N∙ha−1 and 0 kg∙N∙ha−1) and shade (80% shade and 0% shade) were applied in a split-plot design to individual potted plants grown in soil collected from high abundance pine stands. Plants grown in unshaded conditions produced greater root (p = 0.0027) and shoot (p = 0.0017) biomass and higher cover values (p = 0.0378) compared to those in the shaded treatments. The addition of N had little effect on plant growth (p = 0.1602, 0.5129, and 0.0853 for shoot biomass, root biomass, and cover, respectively), suggesting that soils in high-density ponderosa pine stands that lack understory vegetation were not N deficient and Idaho fescue plants grown in these soils were not N limited. Management activities that increase under-canopy light availability will promote the conditions necessary for Idaho fescue recovery. However, successful restoration may be constrained by a lack of residual fescue or the invasion of more competitive understory vegetation.
Highlights
In ponderosa pine (Pinus ponderosa Dougl.) forests, understory herbaceous vegetation is important for maintaining soil stability, site hydrology, and nutrient dynamics, it provides important wildlife habitat and livestock forage, and it is a major component of the fire regime, natural variability, and aesthetic value of these ecosystems
Carr and Krueger [3] observed reductions in under-canopy light and nitrogen availability associated with increasing tree abundance in a case study of a ponderosa pine forest in the Pacific Northwest, no experimental evidence linking under-canopy resource availability to understory vegetation dynamics was available
It was apparent that Idaho fescue plants grown in shaded conditions were less vigorous; exhibiting lower root biomass, shoot biomass, and foliar cover values
Summary
In ponderosa pine (Pinus ponderosa Dougl.) forests, understory herbaceous vegetation is important for maintaining soil stability, site hydrology, and nutrient dynamics, it provides important wildlife habitat and livestock forage, and it is a major component of the fire regime, natural variability, and aesthetic value of these ecosystems. Existing research has provided insight into the mechanisms associated with understory vegetation response to altered pine abundance [6,10,11,12,13], the results have been inconsistent and have not addressed climax Pacific Northwest ponderosa pine stands, a complete understanding of the management needs in these forests is lacking. Carr and Krueger [3] observed reductions in under-canopy light and nitrogen availability associated with increasing tree abundance in a case study of a ponderosa pine forest in the Pacific Northwest, no experimental evidence linking under-canopy resource availability to understory vegetation dynamics was available. We initiated an experiment designed to corroborate observational data and evaluate if under-canopy light and nitrogen availability influence the growth and vigor of understory vegetation and to consider the implications for the management of Pacific Northwest ponderosa pine forests. We selected the perennial bunchgrass Idaho fescue (Festuca idahoensis Elmer) for this study as it is the dominant understory species in ponderosa pine/Idaho fescue plant associations in eastern Oregon [14] and has been shown to decline significantly under conditions of high ponderosa pine abundance [3]
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