0009479: Growth of floating tree causes random collapses on clear cut area.

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ID	Project	Category	View Status	Date Submitted	Last Update
0009479	Dwarf Fortress	Dwarf Mode -- Environment	public	2016-01-16 03:24	2023-01-01 17:32

Reporter	Superrob
Assigned To	Loci
Priority	normal	Severity	minor	Reproducibility	random
Status	acknowledged	Resolution	open
Platform	Windows 8	OS		OS Version
Product Version	0.42.04
Target Version		Fixed in Version

Summary	0009479: Growth of floating tree causes random collapses on clear cut area.
Description	I have a clear cut area just outside a statue that I am making. Seemingly at random, something in the clear cut area will collapse, gouging out large sections of land, and throwing dust everywhere. However, as far as I can tell, there are no trees, except for one that seems to be floating. There is another floating tree as well, that seems to resist all attempts to designate it for cutting.
Steps To Reproduce	To find a collapse, goto the announcements, and zoom to the collapse announcement. You will find that it occurs 2 z levels above the ground, and is filed with loamy, and black sand. Scroll up once to see a tree. jump down 2 z levels, and find that there are roots at the surface, but the tree trunk doesn't connect to it at all. Hit F8 and jump to a Tree that seems ground level, but cannot be designated.
Additional Information	http://dffd.bay12games.com/file.php?id=11664 [^] Game save is here.
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Issue History
Date Modified	Username	Field	Change
2016-01-16 03:24	Superrob	New Issue
2016-01-18 13:26	Detros	Note Added: 0034457
2016-01-19 02:52	Superrob	Note Added: 0034468
2016-01-19 11:04	Detros	Note Added: 0034476
2016-01-23 11:53	Superrob	Note Added: 0034528
2016-01-23 16:43	Detros	Note Added: 0034535
2017-05-20 01:20	PatrikLundell	Note Added: 0036564
2017-05-20 07:12	PatrikLundell	Note Edited: 0036564	bug_revision_view_page.php?bugnote_id=0036564#r14742
2017-05-23 10:37	lethosor	Assigned To	=> lethosor
2017-05-23 10:37	lethosor	Status	new => acknowledged
2017-05-23 10:37	lethosor	Assigned To	lethosor =>
2017-05-23 10:37	lethosor	Status	acknowledged => new
2017-05-23 10:38	lethosor	Assigned To	=> lethosor
2017-05-23 10:38	lethosor	Status	new => acknowledged
2017-05-23 10:38	lethosor	Assigned To	lethosor =>
2017-05-23 10:38	lethosor	Status	acknowledged => new
2017-05-31 21:16	Loci	Assigned To	=> Loci
2017-05-31 21:16	Loci	Status	new => acknowledged
2017-05-31 21:19	Loci	Summary	Random collapses on clear cut area. => Growth of floating tree causes random collapses on clear cut area.
2017-06-01 02:26	PatrikLundell	Note Edited: 0036564	bug_revision_view_page.php?bugnote_id=0036564#r14755
2017-06-08 18:40	Huntthetroll	Issue Monitored: Huntthetroll
2023-01-01 17:32	MrDigger	Note Added: 0041363

Notes

This report is the closest one I could find for the issue of trees spawning and falling, creating cave-ins, so I add my findings to this report.

I dug away the level below the surface under a large area (about 40*40) with numerous trees, built a support, channeled away the sides of the surface area, cut down the trees outside the area whose branches tangled with those of trees on the inside, hooked a lever to the support, and pulled the lever. The area caved in, as intended. It was also noted that roots of trees that had been plunged to destruction still remained in the soil outside of the area, indicating the fallen trees weren't removed properly (when cutting a tree the roots magically revert to soil).
Shortly after that the fortress was interrupted by reports of cave-ins in the area, as trees came crashing down.
Using DFHack to investigate the issue, it was found that one difference between a standing ordinary tree and the same tree after it has been cut down is that df.global.world.plants.all[<index>].tree_info (in DFHack terms) points to a struct describing the tree before it's cut, but the pointer is nil after cutting.
Quite a few trees were found within the cave-in area that did have pointers to tree descriptions. In further testing the trees in half the cave-in areas had their tree_info pointers set to nil, while half were kept. This caused cave-ins to happen only in the area where the pointers were kept. Reverting to a save before pointer manipulation and removing the pointers on the other half instead cause cave-ins to switch side.
A further indication of something being amiss is that in the half of the area where the pointers were retained, the ground (one level down) showed patches of leaves, flowers, and fruit in the same way as those "contaminants" appear under the trunk of a tree if you dig out the tile beneath the trunk.

Conclusion: DF does not check cave-ins for trees to be removed as a result of falling, so those trees remain and have effects (leaves, roots,...). I haven't confirmed this, but my guess is that collapses happen when trees grow.

Edit: Further testing has confirmed that the tree most likely to have causing a cave-in has morphed from after the previous cave-in to the pause caused by the latest one. Twigs and a trunk appeared, and a number of tiles were marked as blocked.

Edit 2: Building a floor over parts of the area where tree cave-ins were induced caused trees to gradually appear on top of the floor, including 4 trunk tile (i.e. mature) ginko trees. In addition to that, one tree appeared suspended next to the floored over area, but stopped from falling by having the branches tangled with those of a tree standing on the floor.