University of Maine, School of Forest Resources, 5755 Nutting Hall, Orono, ME 04469, USA.
US Forest Service, Northern Research Station, 54 Government Road, Bradley, ME 04411, USA.
Tree Physiol. 2024 Oct 3;44(10). doi: 10.1093/treephys/tpae117.
Lowland northern white-cedar (Thuja occidentalis L.) forests are increasingly exposed to extreme droughts and floods that cause tree mortality. However, it is not clear the extent to which these events may differentially affect regeneration of cedar and its increasingly common associate, balsam fir (Abies balsamea (L.) Mill.). To test this, we measured how seedlings of cedar and fir were able to avoid, resist and recover from experimental drought and flood treatments of different lengths (8 to 66 days). Overall, we found that cedar exhibited a strategy of stress resistance and growth recovery (resilience) from moderate drought and flood stress. Fir, on the other hand, appears to be adapted to avoid drought and flood stress and exhibited overall lower growth resilience. In drought treatments, we found evidence of different stomatal behaviors. Cedar used available water quickly and therefore experienced more drought stress than fir, but cedar was able to survive at water potentials > 3 MPa below key hydraulic thresholds. On the other hand, fir employed a more conservative water-use strategy and therefore avoided extremely low water potential. In response to flood treatments, cedar survival was higher and only reached 50% if exposed to 23.1 days of flooding in contrast to only 7.4 days to reach 50% mortality for fir. In both droughts and floods, many stressed cedar were able to maintain partially brown canopies and often survived the stress, albeit with reduced growth, suggesting a strategy of resistance and resilience. In contrast, fir that experienced drought or flood stress had a threshold-type responses and they either had full live canopies with little effect on growth or they died suggesting reliance on a strategy of drought avoidance. Combined with increasingly variable precipitation regimes, seasonal flooding and complex microtopography that can provide safe sites in these forests, these results inform conservation and management of lowland cedar stands.
低地北美香柏（Thuja occidentalis L.）林越来越容易受到极端干旱和洪水的影响，这些灾害会导致树木死亡。然而，目前尚不清楚这些事件会在何种程度上对香柏及其日益常见的伴生种——香脂冷杉（Abies balsamea (L.) Mill.）的更新产生不同的影响。为了验证这一点，我们测量了香柏和冷杉幼苗在不同时长（8 至 66 天）的干旱和洪水处理实验中，是如何避免、抵抗和从胁迫中恢复的。总的来说，我们发现香柏表现出对中度干旱和洪水胁迫的抵抗和生长恢复（弹性）策略。另一方面，冷杉似乎适应了避免干旱和洪水胁迫，并且表现出整体较低的生长弹性。在干旱处理中，我们发现了不同的气孔行为的证据。香柏迅速利用可用水，因此比冷杉经历了更多的干旱胁迫，但香柏能够在水势低于关键水力阈值 3 MPa 以下的情况下存活。另一方面，冷杉采用了更为保守的用水策略，因此避免了极低的水势。在洪水处理中，香柏的存活率较高，只有在暴露于 23.1 天的洪水中时才达到 50%，而冷杉则仅需 7.4 天就达到 50%的死亡率。在干旱和洪水两种情况下，许多受到胁迫的香柏能够维持部分棕色树冠，并且通常能够在胁迫下存活下来，尽管生长受到抑制，这表明其具有抵抗和恢复的策略。相比之下，经历干旱或洪水胁迫的冷杉具有阈值型响应，它们要么树冠完全保持绿色，对生长影响很小，要么死亡，这表明它们依赖于避免干旱的策略。再加上不断变化的降水模式、季节性洪水以及复杂的微地形，这些地形在这些森林中可以提供安全的生境，这些结果为低地香柏林的保护和管理提供了信息。
