1 00:00:10,724 --> 00:00:13,616 Most of the forest lives in the shadow of the giants 2 00:00:13,616 --> 00:00:15,757 that make up the highest canopy. 3 00:00:15,757 --> 00:00:17,376 These are the oldest trees, 4 00:00:17,376 --> 00:00:21,003 with hundreds of children and thousands of grandchildren. 5 00:00:21,003 --> 00:00:24,325 They check in with their neighbors, sharing food, supplies, 6 00:00:24,325 --> 00:00:27,468 and wisdom gained over their long lives. 7 00:00:27,468 --> 00:00:33,564 They do all this rooted in place, unable to speak, reach out, or move around. 8 00:00:33,564 --> 00:00:37,132 The secret to their success lies under the forest floor, 9 00:00:37,132 --> 00:00:41,134 where vast root systems support the towering trunks above. 10 00:00:41,134 --> 00:00:45,937 Partnering with these roots are symbiotic fungi called mycorrhizae. 11 00:00:45,937 --> 00:00:49,965 These fungi have countless branching, thread-like hyphae 12 00:00:49,965 --> 00:00:52,965 that together make up the mycelium. 13 00:00:52,965 --> 00:00:57,329 The mycelium spreads across a much larger area than the tree root system 14 00:00:57,329 --> 00:01:00,425 and connect the roots of different trees together. 15 00:01:00,425 --> 00:01:03,517 These connections form mycorrhizal networks. 16 00:01:03,517 --> 00:01:05,158 Through mycorrhizal networks, 17 00:01:05,158 --> 00:01:10,648 fungi can pass resources and signaling molecules between trees. 18 00:01:10,648 --> 00:01:13,834 We know the oldest trees have the largest mycorrhizal networks 19 00:01:13,834 --> 00:01:16,056 with the most connections to other trees, 20 00:01:16,056 --> 00:01:19,438 but these connections are incredibly complicated to trace. 21 00:01:19,438 --> 00:01:23,782 That’s because there are about a hundred species of mycorrhizal fungi– 22 00:01:23,782 --> 00:01:28,482 and an individual tree might be colonized by dozens of different fungal organisms, 23 00:01:28,482 --> 00:01:31,675 each of which connects to a unique set of other trees, 24 00:01:31,675 --> 00:01:36,101 which in turn each have their own unique set of fungal associations. 25 00:01:36,101 --> 00:01:39,293 To get a sense of how substances flow through this network, 26 00:01:39,293 --> 00:01:41,142 let’s zoom in on sugars, 27 00:01:41,142 --> 00:01:45,007 as they travel from a mature tree to a neighboring seedling. 28 00:01:45,007 --> 00:01:47,518 Sugar’s journey starts high above the ground, 29 00:01:47,518 --> 00:01:50,553 in the leaves of the tallest trees above the canopy. 30 00:01:50,553 --> 00:01:55,433 The leaves use the ample sunlight up there to create sugars through photosynthesis. 31 00:01:55,433 --> 00:01:58,016 This essential fuel then travels through the tree 32 00:01:58,016 --> 00:02:00,749 to the base of the trunk in the thick sap. 33 00:02:00,749 --> 00:02:04,069 From there, sugar flows down to the roots. 34 00:02:04,069 --> 00:02:06,717 Mycorrhizal fungi encounter the tips of the roots 35 00:02:06,717 --> 00:02:10,096 and either surround or penetrate the outer root cells, 36 00:02:10,096 --> 00:02:12,628 depending on the type of fungi. 37 00:02:12,628 --> 00:02:17,468 Fungi cannot produce sugars, though they need them for fuel just like trees do. 38 00:02:17,468 --> 00:02:18,477 They can, however, 39 00:02:18,477 --> 00:02:22,226 collect nutrients from the soil much more efficiently than tree roots— 40 00:02:22,226 --> 00:02:25,765 and pass these nutrients into the tree roots. 41 00:02:25,765 --> 00:02:26,571 In general, 42 00:02:26,571 --> 00:02:30,425 substances flow from where they are more abundant to where they are less abundant, 43 00:02:30,425 --> 00:02:32,322 or from source to sink. 44 00:02:32,322 --> 00:02:36,234 That means that the sugars flow from the tree roots into the fungal hyphae. 45 00:02:36,234 --> 00:02:38,040 Once the sugars enter the fungus, 46 00:02:38,040 --> 00:02:41,329 they travel along the hyphae through pores between cells 47 00:02:41,329 --> 00:02:44,412 or through special hollow transporter hyphae. 48 00:02:44,412 --> 00:02:46,592 The fungus absorbs some of the sugars, 49 00:02:46,592 --> 00:02:50,113 but some travels on and enters the roots of a neighboring tree, 50 00:02:50,113 --> 00:02:53,403 a seedling that grows in the shade and has less opportunity 51 00:02:53,403 --> 00:02:56,047 to photosynthesize sugars. 52 00:02:56,047 --> 00:02:59,536 But why does fungus transport resources from tree to tree? 53 00:02:59,536 --> 00:03:02,946 This is one of the mysteries of the mycorrhizal networks. 54 00:03:02,946 --> 00:03:07,255 It makes sense for fungus to exchange soil nutrients and sugar with a tree— 55 00:03:07,255 --> 00:03:09,346 both parties benefit. 56 00:03:09,346 --> 00:03:13,348 The fungus likely benefits in less obvious ways from being part of a network 57 00:03:13,348 --> 00:03:17,758 between trees, but the exact ways aren’t totally clear. 58 00:03:17,761 --> 00:03:20,048 Maybe the fungus benefits from having connections 59 00:03:20,048 --> 00:03:22,458 with as many different trees as possible, 60 00:03:22,458 --> 00:03:26,380 and maximizes its connections by shuttling molecules between trees. 61 00:03:26,380 --> 00:03:29,212 Or maybe plants reduce their contributions to fungi 62 00:03:29,212 --> 00:03:32,778 if the fungi don’t facilitate exchanges between trees. 63 00:03:32,778 --> 00:03:34,126 Whatever the reasons, 64 00:03:34,126 --> 00:03:38,576 these fungi pass an incredible amount of information between trees. 65 00:03:38,576 --> 00:03:42,964 Through the mycorrhizae, trees can tell when nutrients or signaling molecules 66 00:03:42,964 --> 00:03:46,305 are coming from a member of their own species or not. 67 00:03:46,305 --> 00:03:49,512 They can even tell when information is coming from a close relative 68 00:03:49,512 --> 00:03:51,722 like a sibling or parent. 69 00:03:51,722 --> 00:03:54,977 Trees can also share information about events like drought 70 00:03:54,977 --> 00:03:57,524 or insect attacks through their fungal networks, 71 00:03:57,524 --> 00:04:00,817 causing their neighbors to increase production of protective enzymes 72 00:04:00,817 --> 00:04:03,347 in anticipation of threats. 73 00:04:03,347 --> 00:04:08,037 The forest’s health relies on these intricate communications and exchanges. 74 00:04:08,040 --> 00:04:10,699 With everything so deeply interconnected, 75 00:04:10,699 --> 00:04:14,949 what impacts one species is bound to impact others.