Sack Lawren, Grubb Peter J
Department of Plant Sciences, University of Cambridge, Downing Street, Cambridge, CB2 3EA, UK.
Oecologia. 2002 Apr;131(2):175-185. doi: 10.1007/s00442-002-0873-0. Epub 2002 Apr 1.
To test whether the impact of drought on the growth and biomass allocation of first-season shade-tolerant woody seedlings in low irradiance differs from that in high irradiance, seedlings of Viburnum lantana, V. opulus, V. tinus and Hedera helix were grown in pots at two watering frequencies × three irradiances. Hypotheses in the recent literature variously predict that drought will have a stronger, weaker or equal impact on seedling relative growth rate (RGR) in deep shade relative to that in moderate shade. Experimental irradiance levels were selected in the typical range for temperate deciduous forest seedlings in either understorey or clearings: 3-4% daylight (low red: far-red shade), 3-4% daylight (neutral shade), and 30-40% daylight (neutral shade). Watering was 'frequent' (every 3-4 days) or 'infrequent' (five times during the 8-week experiment), producing soil matric potentials as low as -0.03 MPa, and -2 MPa. To prevent the interaction of irradiance and watering treatments, each seedling was grown in a 'shade tower' that was surrounded by an uncovered sward of grass (Festuca rubra), which depleted pot water at the same rate regardless of the species of seedling, or its irradiance treatment. Shading affected all species: seedlings in 3.5% daylight grew at 56-73% of their dry-mass RGR in 35% daylight. Low red: far-red shade reduced the RGR of Hedera to 68% of its value in neutral shade. Infrequent watering significantly reduced the RGR of only V. lantana and V. opulus, by approximately the same proportion across irradiance treatments. Infrequent watering did not significantly alter any species' biomass allocation across irradiance treatments. Such orthogonal impacts of deep shade and drought on seedling growth and biomass allocation indicate a large potential for niche differentiation at combinations of irradiance and water supply for species of forest seedlings, and suggest a multiplicative-effects approach for modelling seedling performance in microsites with different combinations of irradiance and water supply.
为了测试干旱对低光照条件下第一季耐荫木本幼苗生长和生物量分配的影响是否与高光照条件下不同，将欧洲荚蒾、欧洲花楸、意大利荚蒾和常春藤的幼苗种植在花盆中，设置两种浇水频率×三种光照强度。近期文献中的假设预测各不相同，有的认为干旱对深荫下幼苗相对生长速率（RGR）的影响比对中度荫蔽下的影响更强，有的认为更弱，还有的认为相等。实验光照水平选取了温带落叶林幼苗在林下或林中空地的典型范围：3 - 4%日光（低红：远红荫蔽）、3 - 4%日光（中性荫蔽）和30 - 40%日光（中性荫蔽）。浇水分为“频繁”（每3 - 4天一次）或“不频繁”（在8周的实验中浇5次水），产生的土壤基质势分别低至 - 0.03兆帕和 - 2兆帕。为防止光照和浇水处理的相互作用，每株幼苗种植在一个“荫蔽塔”中，其周围是未覆盖的草地（紫羊茅），无论幼苗种类或光照处理如何，草地消耗花盆水分的速率相同。遮荫影响所有物种：3.5%日光下的幼苗干质量RGR是35%日光下的56 - 73%。低红：远红荫蔽使常春藤的RGR降至中性荫蔽下的68%。不频繁浇水仅显著降低了欧洲荚蒾和欧洲花楸的RGR，且在各光照处理下降低比例大致相同。不频繁浇水在各光照处理下未显著改变任何物种的生物量分配。深荫和干旱对幼苗生长和生物量分配的这种正交影响表明，森林幼苗物种在光照和水分供应组合方面具有很大的生态位分化潜力，并建议采用乘法效应方法来模拟不同光照和水分供应组合的微生境中幼苗的表现。
