Wednesday, May 13, 2009

Why do we use first leaf rather than budburst in Project BudBurst?

It might strike you as a bit odd that in Project Budburst we generally do not record the date of budburst (!). It all has to do with keeping things simple and also making things as accurate as possible. Budburst, when you see leaf buds swelling up then “bursting” open, unfurling tiny rolled up leaves is the very symbol of spring in most northern climates. How plants actually do this, and the pattern that unfolds is quite variable, reflecting clever adaptations of plants to many different climates, habitats and stresses in the environment. Some plants such as Antelope bitterbrush, and most dogwood species, such as red osier dogwood don’t budburst at all, since their leaves are not inside buds in the winter.

They produce tiny leaves in the late summer or fall which turn red or brown in winter (and change their chemistry to protect against frost). In the spring they turn green and then expand into the mature form. Other plants such as western serviceberry, have leaves tightly rolled up, which expand out of the winter buds then later begin to unroll. Some plants have leaves which expand out of bud green, for others the leaves are various shades of red or brown when they come out of the buds (making them a little hard to see), then they turn green and unroll, or unroll first then turn green. For others there is a distinctive stage where the tip of the bud turns green, just before budburst occurs (such as American linden and black locust) . So budburst is not as simple as you might think! But this makes sense in terms of plant adaptations. Plants that begin growing when heavy frost is common such as bitterbrush, western serviceberry and chokecherry, and apples should have adaptions to frost that are less critical for species which emerge later in spring such as oaks, locusts, and elderberries.

First leaf on the other hand seems a bit more straight-forward to identify. When a leaf emerges from the bud you can clearly see the widest part of the leaf emerge, and usually see the stem (petiole) at the base. Usually the leaf then unfolds or unrolls and becomes flat, finally reaching our “first leaf” stage. How a leaf reaches this stage still can vary depending on the species. Lilacs and aspen for example first produce thick stiff leaves which emerge flat, so reach the first leaf stage quickly (and take a while to reach the all leaf stage). Red maple emerges in a droopy, limp state and takes a while to turn green and become flat.

Others such as chokecherry have leaves which emerge folded and take a while to become flat as well. It is important that we all agree on these definitions since it can effect the dates at which a plant reaches a given stage. This is also why we are working towards having better photos to document first leaf and the other stages in our phenophase field guides. So please send us your photos of first leaf and other stages as you observe them this spring. It should be a great help it making sure we are all making comparable observations.

Figure captions:

  1. Droopy new leaves of red maple
  2. Early stages of budburst for western serviceberry. Note the new leaves are thickly covered with hairs, helping protect them from severe frost.
  3. Red osier dogwood with tiny winter leaves (no bud scales)
  4. Stiff young leaves of common lilac – well adapted to heavy frosts that may occur at this time of year.

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Here in the northern Rockies this week spring is still developing slowly. We had a winter storm advisory the other night again, and have snow above about 4,000’ today. Balsamroot is in full bloom now, turning some grassy south-facing hillslopes bright yellow. Most of our shrubs are at the first leaf stage, (some for weeks), and you can see many swelling flower buds. Soon as we get some warm days things may change fast!

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