The Strangler and the Survivor

The strangler habit of Banyan is the key to the presence of SWTs. In nature, the seeds of Banyan are dropped on the crotches of host trees, usually by birds or bats. Seedlings will send down aerial roots to the soil, develop roots and quickly become lignified. When more aerial roots are developed, they will fuse together to form crisscrossing network and tighten the grip on the host tree. In urban setting, dry-packed masonry wall with large joints is the best “host”. The negatively-phototropic aerial roots immerse through the gaps between masonry, reach the soil and ground water seepage behind the wall, and begin to establish on the vertical surface.

There are a few arguments on how SWTs have survived from World War II. During Japanese Occupation, trees in the Central and Western area were chopped down massively for wood fuel. Some said the residents protected the trees from being removed, while others claimed that the Japanese kept them just because there were literally no more trees in the area. A more reasonable guess is that the SWTs had time to grow due to the lack of maintenance of the masonry walls during war time.

既是「絞殺者」，亦是「倖存者」

榕樹的絞殺習性是石牆樹存在的關鍵。在自然界中，榕樹的種子通常透過鳥類或蝙蝠，落在寄主樹上。幼苗會長出氣根直到接觸到土壤，發展出樹根並迅速木質化。當長出更多的氣根時，它們將融合並形成交叉網絡，加強對寄主樹的絞盤。在城市環境中，帶有縫隙的石砌擋土牆是最好的「寄主」。背光性的氣根穿過石塊之間的縫隙，到達牆後的土壤和滲流的地下水，並開始在垂直表面上成長。

關於石牆樹如何從第二次世界大戰中倖存下來，存在一些爭論。在日本佔領期間，中西區的樹木被大量砍伐以用作木材燃料。有些說法指，是當區居民挺身保護樹木，石牆樹才免遭移走，而另一說法是，因為該區當時已經沒有任何綠化，日本人才決定保留它們。不過，更合理的猜測是由於戰時並沒有人留意對石牆進行維護，石牆樹才會僥倖存活下來。