Regeneration Following Clearcutting Study at Harvard Forest since 1991

Measurements of regeneration following removal in 1990 of a 64-year old red pine plantation on the Prospect Hill tract were continued for the twelfth year in 2001. Browsing in 2001 remained at very low levels (less than 2% of stems). As mean tree height continues to increase both the amount of browsing and the impact of browsing on future stand characteristics should remain low. Overall, our observations show that browsing has had little long-term impact during the regeneration of this stand. White ash, the most heavily browsed species, remains the most common species in the plots. After remaining quite stable over the past five years, in 2001 the overall stem density of tree species declined to 17,883 stems/ha, compared with 19,464 stems/ha in 2000, 19,414 stems/ha in 1999, 19,958 stems/ha in 1998, 19,414 stems/ha in 1997, and 20,696 stems/ha in 1996. The relative importance of major species has remained the same over the past six years. In 2001, white ash (36.5%) remained the most numerous tree species, followed by red maple (26.9%), sugar maple (14.4%) and black cherry (9.4%). These percentages changed little from 2000. After a slight decrease in 2000, red oak increased slightly to 7.5% of tree stems in 2001, the majority of which were small seedlings. Overall, the percentage of stems that originated as seedlings rather than sprouts decreased to 19.3%, down from 23.1% in 2000, 23.4% in 1999, 25.4% in 1998, and 23.7% in 1997. The majority of these seedlings (55.7%) were white ash, most less than 0.5 m tall. Mean stem height rose to 3.46 m, compared to 3.20 m in 2000, 3.24 m in 1999, 3.01 m in 1998, 2.92 m in 1997, 2.87 m in 1996 and 2.67 m in 1995. The resumption in mean height growth over the past year probably reflects low seedling establishment and mortality of seedlings and young sprouts less than 0.5 m tall along with continued growth of the taller stems. The tallest stems were 20 white ash, 15 red maples, 15 sugar maples, 6 black cherries, 5 pin cherries, 3 paper birches, and 1 trembling aspen greater than 7 m tall. Diameter at breast height (dbh) is now being recorded for all stems taller than seven meters. Of the five most common species, sugar maple had the tallest mean height (4.75 m), followed by red maple (3.98 m), black cherry (3.69m), and white ash (2.99 m). Because of the preponderance of small seedlings, red oak mean height was only 0.67 m. It remains to be seen how many seedlings will survive to play a role in the developing stand. Our next sampling will be done in year 15.

2001年，本研究对1990年在展望山地块（Prospect Hill tract）采伐的64年生红松人工林的更新状况开展了第12年的跟踪监测。2001年的植食性动物啃食率仍维持在极低水平（不足茎秆总数的2%）。随着林木平均高度持续增长，啃食发生率及其对林分未来特征的影响都将维持低位。整体观测结果表明，在本次林分更新过程中，啃食并未产生显著长期影响。受啃食最为严重的白蜡木（white ash）仍是样地内最常见的树种。在过去五年保持相对稳定后，2001年林木总茎密度降至17883株/公顷，对比2000年的19464株/公顷、1999年的19414株/公顷、1998年的19958株/公顷、1997年的19414株/公顷及1996年的20696株/公顷。主要树种的相对重要性在过去六年中未发生明显变化。2001年，白蜡木（占比36.5%）仍是数量最多的树种，依次为红枫（26.9%）、糖枫（14.4%）与黑樱桃（9.4%），该占比与2000年相比变化极小。红橡树占比在2000年小幅下降后，于2001年回升至7.5%，其中绝大多数为小型实生苗。以实生苗而非萌条形式起源的茎秆占比降至19.3%，低于2000年的23.1%、1999年的23.4%、1998年的25.4%及1997年的23.7%。其中大部分实生苗（55.7%）为白蜡木，且多数高度不足0.5米。平均茎高升至3.46米，对比2000年的3.20米、1999年的3.24米、1998年的3.01米、1997年的2.92米、1996年的2.87米及1995年的2.67米。本年度平均树高生长恢复，可能源于高度不足0.5米的实生苗与幼萌条定植率偏低、死亡率较高，同时较高茎秆的生长持续进行。高度超过7米的最高茎秆包括20株白蜡木、15株红枫、15株糖枫、6株黑樱桃、5株美洲山樱桃（pin cherry）、3株纸皮桦（paper birch）以及1株颤杨（trembling aspen）。胸径（diameter at breast height, dbh）现已开始对所有高度超过7米的茎秆进行测量。在5种最常见的树种中，糖枫的平均高度最高（4.75米），依次为红枫（3.98米）、黑樱桃（3.69米）及白蜡木（2.99米）。受小型实生苗占比偏高影响，红橡树的平均高度仅为0.67米。目前尚不清楚有多少实生苗能够存活并在发育中的林分中发挥作用。我们的下一次采样将在第15年进行。